Actinide XAS 2014

20 May 2014, 07:30 → 22 May 2014, 17:00 Europe/Zurich

Festsaal - Plenum (Schloss Böttstein)

Claude Degueldre (Paul Scherrer Institut), Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source), Rainer Dähn (Paul Scherrer Institut)

7th Workshop on Speciation, Techniques, and Facilities
for Radioactive Materials at Synchrotron Light Sources
 

Actinide XAS is a series of international workshops which focus on the basic- and applied research of radioactive materials using synchrotron-based techniques.

Organized by:

    Paul Scherrer Institut (PSI)
    Laboratory for Waste Management (LES)
    5232 Villigen PSI
    Switzerland

Group picture of the participants in front of Schloss Böttstein
 

Flyer flyer-anxas.pdf

Travelling Information PSI access.pdf

Tuesday 20 May

07:45 → 08:20 Registration

08:20 → 08:40 Welcome

Welcome

Convener: Dr Rainer Daehn (Paul Scherrer Institut)

08:40 → 10:15 Solution and Coordination Chemistry of the Actinides

Convener: Claude Degueldre (Paul Scherrer Institut)

08:40 Recent soft X-ray science with actinides and new opportunities for actinide science with synchrotron radiation

Synchrotron radiation (SR) methods have been employed to investigate the chemistry and physics of a wide range of topics in actinide science for several decades. Over this period, the number of SR techniques brought to bear on critical issues in the actinide science field has grown greatly. Several of the results from actinide SR studies have yielded truly unique information that has fundamentally improved the understanding of actinide chemistry and physics. Recently, there have been increasingly significant activities in the soft X-ray region of the SR spectrum to probe the electronic structure of actinide materials. A key aspect leading to the recent impact and success of the soft X-ray approaches is the invaluable contribution of theory and simulation not only to understand, but also to predict results from studies based on the absorption and emission methodologies. This presentation will briefly recount contemporary progress in the soft X-ray region using absorption and emission techniques. Recent investigations of electronic structure and chemical bonding in a range of actinide materials that includes fundamental materials as well as complexes relevant to separations and catalysis will be highlighted. The soft X-ray studies will be used to set the stage for the prospects of new, near-term scientific opportunities for actinide science with SR. The advent of new developments in beamline instrumentation, light sources, and detector technologies presents unique opportunities for actinide science with SR that will be highlighted.

Speaker: Dr David Shuh (LBNL)

09:05 Hierarchical structure assembled by coordination species in biphasic solvent extraction

The aim of this study is to elucidate a hierarchical structure of Zr(NO3)4･2TBP in octane, formed in an organic phase of the biphasic solvent extraction system, where TBP is tri-n-butyl phosphate.[1] Toward this end, the methods of extended X-ray adsorption fine structure (EXAFS) and small-angle scattering (SAS), exhibiting a good compatibility in the viewpoint of observing length scale, were employed in this study. EXAFS and SAS are capable of covering the length scales from 0.1–1 nm and 1—100 nm, respectively, reflecting a single coordination structure of Zr(NO3)4･2TBP complex and its self-assembly. Here, note that we need to combine the data obtained for both methods in order to proceed with our understandings of the overall hierarchical structure. First, EXAFS profiles were numerically analyzed in detail in conjunction with a density functional theory calculation, successfully yielding the averaged x-y-z coordinates of Zr(NO3)4･2TBP single complex as a fundamental unit of the self-assembly. We next estimated the contribution of the SAS profile from Zr(NO3)4･2TBP single complex on the basis of the x-y-z coordinates by using Debye function for scattering on an absolute scatter intensity scale. The resultant SAS profiles calculated here exhibited a large difference with the small-angle neutron scattering (SANS) profiles obtained experimentally at q < 1.0 nm-1 (q is the magnitude of the scattering vector), that is, the excess scattering component due to the self-assembling, deviating from that of a single complex, was observed in SANS profiles. In this paper, we would like to report the overall picture of the self-assembled structure into a wide length scale and introduce an original insight into the treatment of EXAFS and SAS profiles. [1] R. Motokawa et al., J. Phys. Chem. B, vol.116, pp1319-1327, (2012).

Speaker: Dr Ryuhei MOTOKAWA (Japan Atomic Energy Agency)

09:40 Covalency in actinide extractants

Nuclear energy is the most viable alternative to fossil fuel combustion until large-scale renewable energy technologies can be implemented. However, use of nuclear energy is limited by the difficulties in remediation, management, and disposal of nuclear waste and the significant hazard posed by the minor actinides present in spent fuel. Efficient separation of trivalent minor actinides from lanthanide ions represents a long-standing challenge because of the similar chemical properties shared by 4f- and 5f-elements. For reasons not well understood, some soft-donor ligands (e.g., dithiophosphinic acids, R2PS2H) are known to selectively extract actinides from lanthanides, with Am/Eu separation factors on the order of 100,000 observed (Klaehn, J. R. et al. Inorg. Chim. Acta 2008, 361, 2522). It is likely that this phenomenon results from increased covalency with actinides over lanthanides; however, a clear understanding of f-element participation in covalent bonding has not been established. Recent pioneering work of Solomon, Hedman, and Hodgson significantly advanced capabilities in ligand K-edge X-ray absorption spectroscopy (XAS) to directly measure covalency in bonding (Solomon, E. I. et al. Coord. Chem. Rev. 2005, 249, 97). We will discuss the use of ligand K-edge XAS and time-dependent density functional theory (TDDFT) to evaluate a novel family of dithiophosphinate extractants. We investigate M–S2PR2 orbital mixing for a carefully selected series of dithiophosphinate complexes by comparing the solid-state and solution-phase sulfur K-edge X-ray absorption spectra, in conjunction with TDDFT. The conclusions derived from solid-state S K-edge measurements are validated through comparison with extractants in solution, providing a foundation for in situ studies of liquid-liquid extraction systems. Furthermore, we investigate the relative roles of 3d, 4d, 5d, and 6d/5f-orbitals in M–S covalent bonding and provide preliminary insight into the structure-to-function relationships that may correlate with the selective extraction of trivalent minor actinides from trivalent lanthanides.

Speaker: Dr Stosh Kozimor (Los Alamos National Laboratory)

10:15 → 10:45 Coffee break

Coffee break

10:45 → 12:45 Solution and Coordination Chemistry of the Actinides

Convener: Claude Degueldre (Paul Scherrer Institut)

10:45 In situ XAFS measurement of uranyl-amide complexes under Xe-lamp irradiation

In our group, an innovative chemical separation process is proposed for the recovery of all actinide from spent nuclear fuel. One of the main purposes of this process is selective isolation of uranium with branched alkyl type N,N-dialkyl-monoamide(BAMA) and the other is all actinide recovery with hetero donor ligand, phenathroline amide, which composed oxygen and nitrogen atoms. Since BAMA has the steric hindrance on the complexation with metal cations, BAMA can be used to separate An(VI) from An(IV). One of BAMA, N,N-di-(2-ethyl)hexyl-(2,2-dimethyl)propanamide (D2EHDMPA), can recover U(VI) selectively without accumulating Pu(IV) in uranium isolation process. From the results of uranium extraction in the presence of neptunium, D2EHDMPA can extract and separate U(VI) from Np(VI) without reduction from Np(VI) to Np(V) or Np(IV). Furthermore, D2EHDMPA could not extract fission products like technetium in the presence macro amount of uranium. The structural parameters of uranium(VI)-BAMA were determined by EXAFS to elucidate the mechanism of selective separation of uranium(VI). EXAFS measurements of uranium(VI) and thorium(IV)-BAMA complexes were performed at the BL-11XU of SPring-8. Uranium(VI) and thorium(IV)-BAMA complexes were measured by transmission mode. Ligands used structural analysis were N,N-dioctylbutanamide(DOBA) and D2EHDMPA for U(VI) and Th(IV). Uranium(VI) and thorim(IV)-BAMA samples for EXAFS measurement were prepared by solvent extraction method using n-dodecane as diluent and by synthesis in ethanol solution. The obtained bond distance between uranium(VI) and oxygen of N,N-dialkyl monoamide are 2.39 Å for DOBA, and 2.31 Å for D2EHDMPA. Bond distances of U-O (O=C; amides) were changed dependence on amides structure, especially carbonyl alkyl group branching on α-position. In case of Th(IV)-D2EHDMPA complex in ethanol, D2EHDMPA could not coordinate with Th(IV). The components of first coordinate sphere of Th were oxygen atoms of four nitrate anions and water molecules. The U(VI) isolation manifested only in branched alkyl amide (BAMA) extraction system. The bulky D2EHDMPA extractant could not solvate tetra valence cation like Th and Pu because of steric effects of nitrate ions which already coordinated Th and Pu cation. In the presentation, we will show the results of in situ XAFS measurement of U-amide (and/or TBP) complexes on uranium valence change by Xe-lamp irradiation in order to evaluate U(VI) selectivity.

Speaker: Dr Shinichi SUZUKI (Japan Atomic Energy Agency)

11:20 Defining orbital mixing standards for quantitative studies of covalency in actinide ligand bonds

The exact nature of chemical bonds between actinides and light atoms such as carbon, nitrogen, oxygen, and fluorine is of widespread interest because these interactions control the physics and chemistry of many technologically important processes. Among approaches explored previously, ligand K-edge X-ray absorption spectroscopy (XAS) has emerged as an effective method for quantitatively probing electronic structure and orbital mixing. The presence of covalent mixing is observed as a pre-edge feature in the ligand K-edge XAS, which only has transition intensity if the final state metal orbital contains a component of ligand p orbital character. For actinide–chloride bonds, the Cl K-edge pre-edge intensity is related to percentage covalency by using a copper tetrachloride intensity standard. Recent advances have shown that quantitative pre-edge peak intensities can be measured at the K-edges for light atoms through a combination of XAS with a scanning transmission X-ray microscope (STXM), non-resonant inelastic X-ray scattering (NIXS), and hybrid density functional theory calculations (DFT). However, corresponding intensity standards have not been developed. Herein, a new effort is discussed that employs these techniques to define experimental intensity standards and garner a quantitative understanding of bonding interactions with d- and f-block coordination compounds. Oxygen K-edge XAS measurements and DFT studies began with the lanthanide dioxides and sesquioxides, LnO2 and Ln2O3 (Ln = Ce, Pr, Tb), which were chosen because 4f covalency in the Ln–O bonds is already well-established from L3-edge X-ray spectroscopies. Features in the O K-edge XAS increase in intensity in analogy to the L3-edge measurements, suggesting that both techniques provide equivalent information regarding covalency. Results from an ongoing collaboration with theorists tie these experimental trends to changes in orbital mixing with the σ and π-bonding 4f orbitals. Following a similar approach, ytterbocene, (C5H5)2Yb, was chosen to standardize intensities at the carbon K-edge. Preliminary efforts to use these benchmarks in quantitative determinations of covalency for a variety of actinide molecules and materials will be discussed.

Speaker: Dr Stefan Minasian (Lawrence Berkeley National Laboratory)

11:45 Formation and structure of polynuclear tetravalent actinide and lanthanide carboxylates in aqueous solution and solid state

Tetravalent actinides and lanthanides form strong complexes with carboxyl containing ligands. Such complexes play an important role in technological processes as well as in biological and environmental systems. So far, most of the thermodynamic data of actinide(IV) and lanthanide(IV) carboxylates are estimated by assuming mononuclear solution species [1]. We applied comprehensive studies with EXAFS, UV-Vis-NIR spectroscopy and X-ray diffraction on Th4+, U4+, Np4+ and Ce4+ carboxylates (RCOO–; R = H, CH3, CHR’NH2; R’ = H, CH3, CH2SH) in aqueous solution and solid state [2-5]. Our studies reveal that in all of the investigated systems hexanuclear complexes appear, which become predominant with increasing metal and ligand concentration and increasing pH, and dominate finally the species distribution. We present here their structure, stability constants, and the mechanisms of complex formation. The appearance of hexanuclear complexes in aqueous solution coincides with the onset of the An(IV) hydrolysis on the one hand, and the deprotonation of the carboxylic function on the other hand. This results in a competing reaction between hydrolysis and ligation. The hydrolysis induces a polymerization of the metal ions via oxo and hydroxo bonds, whereas 12 carboxylic ligands provide charge neutrality of the hexanuclear core and prevent further polymerization. Our studies indicate that future work on tetravalent actinide carboxylates in aqueous solution requires consideration of these hexanuclear species. REFERENCES [1] Casellato, U.; Vigato, P. A.; Vidali, M. Coord. Chem. Rev. 1978, 26, 85-159. [2] Takao, S.; Takao, K.; Kraus, W.; Emmerling, F.; Scheinost, A.C.; Bernhard, G.; Hennig, C. Eur. J. Inorg. Chem. 2009, 4771-4775. [3] Takao, K.; Takao, S.; Scheinost, A.C.; Bernhard, G.; Hennig, C. Inorg. Chem. 2012, 51, 1336-1344. [4] Hennig, C.; Takao, S.; Takao, K.; Weiss, S.; Kraus, W.; Emmerling, F.; Scheinost, A.C. Dalton Trans. 2012, 41, 12818-12823. [5] Hennig, C.; Ikeda-Ohno, A.; Kraus, W.; Weiss, S.; Pattison, P.; Emerich, H.; Abdala, P.M.; Scheinost, A.C. Inorg. Chem. 2013, 52, 11734-11743.

Speaker: Dr Christoph Hennig (The Rossendorf Beamline at ESRF, BP 220, F-38043 Grenoble, France)

12:10 Redox chemistry, solubility and hydrolysis of Np: XAFS contribution to thermodynamics

Actinide aqueous chemistry is a relevant, multifold and challenging research field of inorganic chemistry. In the context of nuclear waste disposal, actinides arise as potentially relevant dose contributors in the long term. Because of their specific electronic configuration, several oxidation states of actinides (+III to +VII) can exist in aqueous solution. This imposes a differential chemical behaviour as a function of the boundary redox conditions, which is of special concern for the safety case of repositories for nuclear waste disposal. In this framework, it is essential to continuously reduce experimental and systematic uncertainties, fill existing gaps in thermodynamic databases and improve knowledge and available data for more complex and relevant (geo)chemical systems. This presentation focuses on neptunium as a relevant and challenging case of actinide science and good example of the experimental and conceptual approaches employed at KIT–INE for investigating actinide aquatic chemistry and thermodynamics. Based upon experimental studies characterizing neptunium redox transformation processes, separate series of solubility experiments were performed involving Np(IV), Np(V), Np(VI) and Np(VII) in dilute to concentrated saline systems. Investigations with other actinides (Th, U, Pu) are discussed in order to support the interpretation of the Np studies. In this context, XAFS techniques are envisaged as key tools complementing solubility experiments and contributing to gain insight on the chemical models governing the investigated systems. Selected examples will highlight the existing synergies between solubility studies and XAFS techniques in the development of thermodynamic models: - Np(IV) and Th(IV) solubility and hydrolysis in CaCl2 solutions - Np(V) solubility in NaCl solutions: formation of ternary Na–Np(V)–OH solid phases - Redox chemistry of Np(V/VI) under near-neutral to hyperalkaline pH conditions - Np(VI) solubility in alkaline NaCl media: analogies with U(VI) and Pu(VI) - Formation of Np(VII) under hyperalkaline oxidizing conditions All studies included in this presentation aim at deriving comprehensive thermodynamic models of the investigated systems, which can be implemented in thermodynamic databases for geochemical model calculations.

Speaker: Dr Xavier Gaona (KIT-INE)

12:45 → 13:45 Lunch

Lunch

13:45 → 15:20 Solid State Chemistry and Physics of the Actinides

Convener: Dr Andreas Scheinost (HZDR Institute of Resource Ecology)

13:45 Delocalization and occupancy effects of 5f orbitals in plutonium intermetallics using L3-edge resonant x-ray emission spectroscopy

Although actinide (An) L3-edge x-ray absorption near-edge structure (XANES) spectroscopy has been very effective in determining An oxidation states in insulating, ionically-bonded materials, such as in certain coordination compounds and mineral systems, the technique fails in systems featuring more delocalized 5f orbitals, especially in metals. Recently, actinide L3-edge resonant x-ray emission spectroscopy (RXES) has been shown to be an effective alternative. This technique is further demonstrated here using a parametrized partial unoccupied density of states method to quantify both occupancy and delocalization of the 5f orbital in alpha-Pu, delta-Pu, PuCoGa5, PuCoIn5, and PuSb2. These new results, supported by FEFF calculations, highlight the effects of strong correlations on RXES spectra and the technique's ability to differentiate between f-orbital occupation and delocalization.

Speaker: C. H. Booth (LBNL)

14:20 The absence of chemical sensitivity in the 4d and 5d X-ray absorption spectroscopy of uranium compounds

Recently, X-ray absorption spectroscopy (XAS) and related derivative measurements have been used to demonstrate the Pu 5f states are strongly relativistic and have a 5f occupation number near 5. [1] Owing to the success in this regime, it has been argued that the XAS measurements should be a powerful tool to probe 5f occupation variation, both as a function of elemental nature (actinide atomic number) and as a function of physical and chemical perturbation, e.g. oxidation state. It will be shown here that XAS and its related measurements fail in this latter aspect for a wide variety of uranium compounds and materials. [2-9] Possible causes will be discussed. Acknowledgements Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work was supported by the DOE Office of Science, Office of Basic Energy Science, Division of Materials Science and Engineering. JGT wishes to thank Professor Guenter Kaindl for his critical reading of the manuscript and use of his XAS data. References 1. J.G. Tobin, P. Söderlind, A. Landa, K.T. Moore, A.J. Schwartz, B.W. Chung, M.A. Wall, J.M. Wills, R.G. Haire, and A.L. Kutepov, J. Phys. Cond. Matter 20, 125204 (2008), and references therein. 2. Kalkowski, G. K. Kaindl, W. D. Brewer, and W. Krone, Phys. Rev. B 35, 2667 (1987). 3. B.W. Veal and D. J. Lam, Phys. Lett. A 49, 466 (1974). 4. B.W. Veal and D. J. Lam, Phys. Rev. B 10, 4902 (1974). 5. B.W. Veal, D. J. Lam, W.T. Carnall and H.R. Hoekstra, Phys. Rev. B 12, 5651 (1974). 6. B.W. Veal, D. J. Lam, H. Diamond and H.R. Hoekstra, Phys. Rev. B 15, 2929 (1974). 7. Elisabeth Thibaut, Jean-Pol Boutique, Jacques J. Verbist, Jean-Claude Levet and Henri Noel, J. Am. Chem. Soc. 104, 5266-5273 (1982). 8. Yu.A. Teterin, V.A. Terehov, M.V. Ryzhkov, I.O. Utkin, K.E. Ivanov, A.Yu. Teterin A.S. Nikitin, Journal of Electron Spectroscopy and Related Phenomena 114–116, 915–923 (2001). 9. A. Yu. Teterin, Yu. A. Teterin, K. I. Maslakov, A. D. Panov, M. V. Ryzhkov,L. Vukcevic, Phys. Rev. 74, 045101 (2006).

Speaker: Dr JG Tobin (Lawrence Livermore National Lab)

14:55 Actinides in irradiated fuel: A shooting on their capricious redox properties

Oxide fuels (UOx, MOx) are currently used in the nuclear reactors. The study of the actinides atomic environment in homogeneous non-irradiated/irradiated fuel samples was performed employing x-ray fluorescence (XRF) and x-ray absorption fine structure (XAFS) spectroscopy. This investigation is important from a safety view because the actinides can undergo various redox states going from III to VII in the beginning of the family. Chemical bounds, valences and stoichiometries of the actinides (Th, U, Np, Pu, Am, Cm) are determined from the experimental data gained for the irradiated fuel material examined in its center location and peripheral zone of the fuel as well as when possible for the non-irradiated fuel e.g. [1-6]. Their data are compared with those recorded for AnO2 compounds. The speciation of these actinides in the oxide fuels sample has also been investigated by thermodynamic into account recent chemical data e.g. [7]. The study is completed with recent EELS data e.g. [8,9] offering a zoom in the actinide electronic structures in the dioxide. References [1] C. Cozzo, A. Orlov, C. Borca, C. Degueldre, X-ray absorption in plutonium uranium mixed oxide fuel: Thorium characterization, ,Progress in Nuclear Energy, In Press, Corrected Proof, Available online 2 October 2013 [2] C. Mieszczynski, C. Degueldre, G. Kuri, J. Bertsch, C.N. Borca, Investigation of irradiated uranium-plutonium mixed oxide fuel by synchrotron based micro X-ray diffraction, Progress in Nuclear Energy, Volume 57, May 2012, Pages 130-137 [3] C. Degueldre, C. Cozzo, M. Martin, D. Grolimund, C. Mieszczynski , Americium characterization by X-ray fluorescence and absorption spectroscopy in plutonium uranium mixed oxide, Journal of Solid State Chemistry, Volume 202, June 2013, Pages 315-319 [4] C. Degueldre, C. Borca, C. Cozzo, Curium analysis in plutonium uranium mixed oxide by x-ray fluorescence and absorption fine structure spectroscopy, Talanta, Volume 115, 15 October 2013, Pages 986-991 [5] C. Degueldre, M. Martin, G. Kuri, D. Grolimund, C. Borca, Plutonium–uranium mixed oxide characterization by coupling micro-X-ray diffraction and absorption investigations, Journal of Nuclear Materials, Volume 416, Issues 1–2, 1 September 2011, Pages 142-150 [6] C. Degueldre, S. Pin, J. Poonoosamy, D.A. Kulik, Redox state of plutonium in irradiated mixed oxide fuels, Journal of Physics and Chemistry of Solids, Volume 75, Issue 3, March 2014, Pages 358-365 [7] C. Degueldre, R. Schaeublin, J. Krbanjevic, E. Minikus, Electron energy loss spectroscopy investigation through a nano ablated uranium dioxide sample, Talanta, Volume 106, 15 March 2013, Pages 408-413 [8] C. Degueldre, L. Veleva, Electron energy loss spectroscopy investigations through nano-ablated actinide dioxide samples, Progress in Nuclear Energy, In Press, Corrected Proof, Available online 17 December 2013

Speaker: Claude Degueldre (Paul Scherrer Institut)

15:20 → 15:50 Coffee

15:50 → 17:05 Solid State Chemistry and Physics of the Actinides

Convener: Dr Andreas Scheinost (HZDR Institute of Resource Ecology)

15:50 XAS characterization of U, AM oxides obtained using co-conversion and CRMP processes

U1-xAmxO2±δ compounds (known as AmBB: americium-bearing blankets) are considered promising materials for americium transmutation in fast neutron reactors.1 The fabrication of these mixed-oxide pellets generally follows a powder metallurgy route that generates significant amounts of highly contaminant fine particles. Dustless processes which can avoid the dispersion of such fine powder in the fabrication line are thus recommended due to the high radiotoxicity of americium. In this aim, the development of innovative concepts has been initiated and two new routes are currently studied. The first one is based on the co-conversion of oxalates precursors.2 It enables obtaining an oxalate complex with a chosen U/Am ratio that can be mineralized to form a solid solution without additional milling or pelletizing step. Green pellets can then directly be pressed from the obtained oxide powder. The second one is Calcined Resin Microsphere Pelletizing (CRMP) process; its general approach consists of elaborating mixed-oxide microsphere precursors through an adaptation of the weak acid resin (WAR) process. The synthesis is based on the fixation of americium and uranium cations into ion exchange resin microspheres. As-loaded amorphous microspheres are then mineralized during a dedicated thermal treatment leading to the formation of porous oxide microspheres employed as pelletizing precursors.3,4 In the present work, samples at different steps of these innovative processes are studied through XANES and EXAFS measurements performed at ULIII, ULII and AmLIII edges on ROBL beamline at the ESRF, coupled with XRD. Concerning the oxalate route, XAS measurements were recorded after co-conversion of oxalates under argon and pellet sintering. For CRMP process, oxide microspheres obtained from loaded resin with uranium and americium were characterized. More precisely, spectra were collected on AmBB-type samples after the first thermal treatment under air, the second thermal treatment under a reducing atmosphere and pellet sintering. Regarding XRD, result interpretation remains however difficult as oxides obtained from starting materials are both low-ordered, preventing structural signal for the first steps. XANES results showed that after mineralization uranium is oxidized compared to U(+IV) and is then reduced during the thermal treatments under a reducing atmosphere (Ar-H2) to a mixture of U(+V) and U(+IV). In the meantime, americium is only present as Am(+III), whatever the thermal treatment conditions. Previous XAS studies on uranium-americium mixed oxides prepared by solid state reaction revealed a similar cationic charge distribution with the presence of reduced Am(+III) but partially oxidized uranium(+IV/+V).5,6 The present results indicate electronic charge transfer between uranium and americium in the compounds and thus their chemical homogeneity. The presentation will give an emphasis on the influence of the synthesis process on the cationic charge distribution and the local structure of these compounds through the presentation of correlated results from XAS, and XRD. References: (1) Warin, D. J. Nucl. Sci. Technol. 2007, 44, 410–414. (2) Horlait, D.; Lebreton, F.; Gauthé, A.; Caisso, M.; Arab-Chapelet, B.; Picart, S.; Delahaye, T. J. Nucl. Mater. 2014, 444, 181–185. (3) Remy, E.; Picart, S.; Grandjean, S.; Delahaye, T.; Herlet, N.; Allegri, P.; Dugne, O.; Podor, R.; Clavier, N.; Blanchart, P.; Ayral, A. J. Eur. Ceram. Soc. 2012, 32, 3199–3209. (4) Remy, E.; Picart, S.; Delahaye, T.; Jobelin, I.; Dugne, O.; Bisel, I.; Blanchart, P.; Ayral, A. J. Nucl. Mater. 2014, 448, 80–86. (5) Prieur, D.; Martin, P. M.; Jankowiak, A.; Gavilan, E.; Scheinost, A. C.; Herlet, N.; Dehaudt, P.; Blanchart, P. Inorg. Chem. 2011, 50, 12437–12445. (6) Prieur, D.; Martin, P.; Lebreton, F.; Delahaye, T.; Banerjee, D.; Scheinost, A. C.; Jankowiak, A. J. Nucl. Mater. 2013, 434, 7–16.

Speaker: Marie Caisso (CEA Marcoule DEN/MAR/DTEC/SDTC/LEMA)

16:15 Influence of high americium contents in the local structure and charge distribution of mixed uranium-americium oxides

Mixed uranium-americium oxides are regarded as promising compounds for transmutation in order to reduce americium contribution to radiotoxicity and heat load of ultimate nuclear waste [1]. These materials are thus currently studied, not only in terms of fabrication processes [2–5] or during experimental irradiations [3,6,7] but also through more fundamental research to understand their thermodynamic and physical properties [8–11]. In this context, XAS experiments reported by Prieur et al. on samples with americium content of 10 to 20 at.% evidenced a peculiar behaviour. Americium was present as Am(+III) state whereas uranium was oxidized to a mixed valence, U(+IV/+V), with similar Am(+III) and U(+V) mole fractions hence O/M (oxygen to metal) ratios close to 2.00 [8,11]. Despite the presence of cations at three different oxidation states, these compounds remain monophasic, presenting the expected fluorite-type structure without any major structural distortions or disorder. These results remain however limited to relatively low americium contents. In this work, we combine XRD and XAS at U and Am LII/LIII edges to study the influence of such cationic charge distribution in mixed uranium-americium oxides presenting higher americium contents: 40 and 50 at.%. These compounds exhibit a single fluorite-type structure [12,13]. XANES results show that the same behaviour is observed for these samples, i.e., americium remain Am(+III) while uranium is oxidized even though this oxidation does not compensate for Am(+III) in the 50%-Am sample which thus presents an O/M ratio close to 1.93. Based on linear combination of reference compound spectra (U(+IV)O2, U(+IV/+V)4O9, U(+V+VI)3O8), the average oxidation state of uranium is estimated around 4.67, which corresponds to the upper limit of existence of a fluorite-type-derived cubic structure in the U-O system (U3O7). EXAFS spectrum analyses show that the local structure around americium remains fairly close to that found for lower americium contents, with only a limited increase of structural disorder and the possible presence of vacancies in the oxygen sublattice. Around uranium, EXAFS spectra reveal significant distortions of the first coordination shell that consists of several U-O distances whereas only one U-(U/Am) distance is present in the second coordination shell. The presentation will focus on the relationship between the charge distribution and the local structure, with an emphasis on the apparent limitation of uranium oxidation states similar to that observed in the U-O system. References: [1] D. Warin, J. Nucl. Sci. Technol. 44 (2007) 410. [2] D. Prieur, F. Lebreton, P.M. Martin, et al., J. Eur. Ceram. Soc. 32 (2012) 1585. [3] T. Delahaye, F. Lebreton, D. Horlait, et al., J. Nucl. Mater. 432 (2013) 305. [4] D. Horlait, A. Feledziak, F. Lebreton, et al., J. Nucl. Mater. 441 (2013) 40. [5] D. Horlait, F. Lebreton, A. Gauthé, et al., J. Nucl. Mater. 444 (2014) 181. [6] E. D’Agata, P.R. Hania, S. Bejaoui, et al., Nucl. Eng. Des. 242 (2012) 413. [7] D. Prieur, A. Jankowiah, T. Delahaye, et al., J. Nucl. Mater. 414 (2011) 503. [8] D. Prieur, P.M. Martin, A. Jankowiak, et al., Inorg. Chem. 50 (2011) 12437. [9] D. Prieur, P.M. Martin, F. Lebreton, et al., J. Solid State Chem. 194 (2012) 206. [10] F. Lebreton, R.C. Belin, D. Prieur, et al., Inorg. Chem. 51 (2012) 9369. [11] D. Prieur, P. Martin, F. Lebreton, et al., J. Nucl. Mater. 434 (2013) 7. [12] F. Lebreton, D. Horlait, T. Delahaye, P. Blanchart, J. Nucl. Mater. 439 (2013) 99. [13] D. Horlait, F. Lebreton, P. Roussel, T. Delahaye, Inorg. Chem. 52 (2013) 14196.

Speaker: Mr Florent Lebreton (CEA, DEN, DTEC/SDTC/LEMA – 30207 Bagnols-sur-Cèze, France)

16:40 Chemistry of fission products and actinides on nuclear fuel using high energy resolution XAS

One of the most challenging research in nuclear energy concerns nuclear fuel and its behavior under irradiation during both normal and off-normal conditions in nuclear power plants. In each case, nuclear fuel behavior and its interaction with cladding materials strongly depend on the chemistry of fission products and actinides during corresponding process. In particular, speciation data are key parameters to establish accurate thermodynamics modeling. Such information can be supplied using X-ray Absorption Spectroscopy (XAS). But, due to the very high radioactivity of the sample (>200 Mbq/mg), and the impossibility to isolate the fluorescence line of one element (background interference of radioactivity, Bragg peaks due to the actinide matrix and fluorescence from other nearer Z elements), implementation of this analysis is impossible. To overcome these difficulties, the XAS experiment can be performed by using a multi-crystal analyser spectrometer which is based on the use of silicon or germanium crystals in the Rowland circle geometry. By diffracting the emitted X-rays, this instrument allows simultaneous focusing and energy discrimination with higher resolution than a conventional solid detector. The comparison between High Energy Fluorescence Detected XANES (HERFD-XANES) and Total Fluorescence Yield XANES (TFY-XANES) shows large benefits of this approach to collect good spectra, especially in diluted radioactive materials. Some latest results obtained on uranium based materials such as virgin nuclear fuel UO2 will be discussed.

Speaker: Dr RENE BES (Synchrotron SOLEIL, MARS beamline &amp; CEA DEN/DEC/SESC/LLCC)

17:30 → 22:00 Poster

Convener: Dr Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source)

17:30 Application of positive matrix factorization and microscopic X-ray fluorescence spectroscopy in the examination of argillaceous rocks

Argillaceous rocks are considered in most European countries as suitable host rock formations for the deep geological disposal of high-level radioactive waste (HLW). The most important chemical characteristic in this respect is their generally strong radionuclide retention property due to the high sorption capacity. Consequently, the physico-chemical parameters of these processes have to be studied in great detail. Synchrotron radiation microscopic X-ray fluorescence (SR micro-XRF) has sufficient sensitivity to study these processes on the microscale without the necessity of the application of radioactive substances. The studies at the Environmental Physics Department of Centre for Energy Research of Hungarian Academy of Sciences focus on the interaction between the escaped ions and the host-rock surrounding the planned HLW repository. SR micro-XRF measurements were performed on thin sections subjected to sorption experiments using 5 μm spatial resolutions. Inactive Cs(I), Ni(II), Nd(III) and natural U(VI) were selected for the experiments chemically representing key radionuclides. The thin sections were prepared on high-purity silicon wafers from geochemically characterized cores of Boda Claystone Formation, Hungary. Samples were subjected to 72-hour sorption experiments with one ion of interest added. The micro-XRF elemental maps taken usually on several thousand pixels indicate a correlation of Cs and Ni with Fe- and K-rich regions suggesting that these elements are predominantly taken up by clay-rich phases. U and Nd was found to be bound not only to the clayey matrix, but the cavity filling minerals also played important role in the uptake. Multivariate methods were found to be efficient tools for extracting information from the elemental distribution maps even when the clayey matrix and fracture infilling regions were examined in the same measured area. By using positive matrix factorization as a new approach the factors with higher sorption capacity could be identified and with additional mineralogical information the uptake capacity of the different mineral phases could be quantified. The results were compared with cluster analysis when the regions dominated by different mineral phases are segmented. The multivariate approach based on micro-XRF to identify the minerals was validated using microscopic X-ray diffraction.

Speaker: Ms Annamária Kéri (Hungarian Academy of Sciences Centre for Energy Research)

17:30 Comparative investigation of N donor ligand and lanthanide/actinide partitioning complexes from the metal and ligand point of view

Comparative investigation of N donor ligand and lanthanide/actinide partitioning complexes from the metal and ligand point of view T Prüßmann1, MA Denecke1,4, A Geist1, J Rothe1, P Lindqvist-Reis1, NL Banik1, B Schimmelpfennig1, D Fellhauer1, C Apostolidis3, O Walter3, DR Batchelor1, P Nagel1, S Schuppler1, K Kvashnina2, T Vitova1 1 Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany; 2 European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France; 3 Institute for Transuranium Elements, European Commission, Joint Research Center, D-76125 Karlsruhe, Germany 4 Dalton Nuclear Institute, The University of Manchester, Manchester, UK, M13 9PL Abstract: N-donor ligands such as n-Pr-BTP (2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine) [1] studied here preferentially bind trivalent actinides (An(III)) over trivalent lanthanides (Ln(III)) in liquid-liquid separation of An(III) from spent nuclear fuel [2]. However, the chemical and physical processes responsible for this selectivity are not yet well understood [3]. We present systematic comparative X-ray absorption spectroscopy (XAS) investigations at the An/Ln L3 edge and the N K edge of [An/Ln(n-Pr-BTP)3](NO3)3 [An/Ln(n-Pr-BTP)3](CF3SO3)3 and [Ln(n-Pr-BTP)3](ClO4)3 complexes. These studies will potentially help to differentiate bonding mechanisms between An and Ln bound to different ligands and counter ions by determining orbital energy differences and relative electronic populations. We find that the energy position of the pre-edge in N K edge XAS spectra for all studied Ln metal complexes is inversely correlated with the distribution ratios for Ln extraction into the organic phase. The origin of the pre-edge is explained by TDDFT (ORCA, ADF, TURBOMOLE) calculations [4]. It consists of excitations to several frontier orbitals, each containing contributions from several chemically non-equivalent N atoms. Analyzing the origin of the 1s electronic transitions allows to identify the contributions of non-equivalent N atoms of the BTP molecule to the spectra. Calculations of the La(BTP)3 complex show a different transition sequence, suggesting higher charge density in the vicinity of the bonding N atoms. The lower oscillatory strength of transitions from bonding N atoms compared to non-bonding N atoms for the complex compared to the free ligand might be interpreted as evidence for lower contributions of N p states to π* molecular orbitals, suggesting strong admixtures of metal orbitals in the complex. We will compare calculations for La/Eu(BTP)3 and Am(BTP)3. High-energy resolution An/Ln L3 edge XANES (HR-XANES) shows that complexation with n-Pr-BTP influences the electronic structure in terms of higher charge density on the An/Ln cations. The pre-edge observed in [Ln(n-Pr-BTP)3](OTf)3 originates from 2p → 4f electronic transitions as shown by ab initio quantum chemical calculations (FEFF9.5 code [5]). 1. Kolarik, Z., U. Mullich, and F. Gassner, Extraction of Am(III) and Eu(III) nitrates by 2,6-di-(5,6-dipropyl-1,2,4-triazin-3-yl)pyridines. Solvent Extraction and Ion Exchange, 1999. 17(5): p. 1155-1170. 2. Geist, A., et al., 6,6 '-bis (5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo 1,2,4 triazin-3-yl) 2,2 ' bipyridine, an effective extracting agent for the separation of americium(III) and curium(III) from the lanthanides. Solvent Extraction and Ion Exchange, 2006. 24(4): p. 463-483. 3. Banik, N.L., et al., Characterization of redox sensitive plutonium(iii) complexed with alkylated 2,6-ditriazinylpyridine (BTP) in organic solution. Dalton Transactions, 2010. 39: p. 5117-5122. 4. Prüßmann, T., et al., Comparative investigation of N donor ligand-lanthanide complexes from the metal and ligand point of view. Journal of Physics: Conference Series, 2013. 430(1): p. 012115. 5. Rehr, J.J., et al., Ab initio theory and calculations of X-ray spectra. Comptes Rendus Physique, 2009. 10(6): p. 548-559.

Speaker: Mr Tim Pruessmann (Karlruhe Institute of Technology)

17:30 Complexation of actinides by ramified N-macrocycle DOTA

The comprehension of the actinides affinity for interest ligands has been a pivotal issue. The difference in interaction between N-donors ligands and O-donor ligands is a hint on the selectivity difference between trivalent and tetravalent actinides. The study of molecules featuring both N and O-donor functions may be a way to reach a better understanding. Thus, the poly-amino-carboxylate ligands form an interesting family of f-element cations chelating agents. One specific ligand has been selected for this study: the DOTA macrocycle (1,4,7,10-tetraazacyclododecane-tetraacetic acid. EXAFS experiments proved DOTA forms complexes with both AnIII and AnIV. Complexation of AnIII is similar to what has been observed with LnIII, the system slowly evolves towards the final species, as shown on the UV-Vis spectra. In equivalent chemical conditions, Np(IV) and Pu(IV) behaves as the AnIII. Theoretical structures (DFT) were used to fit the experimental data, leading to the conclusion of a final complex is the (1:1) species: the cation gets inside the cavity formed by the N-cycle and the carboxylate arms and one water molecule is completing the coordination sphere. An-O and An-N distances are discussed regarding equivalent lanthanides species.

Speaker: Dr Thomas Dumas (CEA)

17:30 Complexation of Americium(III) with chloride – A high-temperature EXAFS study

The long-term storage of high-level nuclear waste will be applied in deep geological formations, different host rock formations (salt, clay, crystalline formations) are investigated throughout Europe [1]. Due to their long half- lives, the long-term radiotoxicity will be determined by the transuranium elements (Np, Pu, Am) included in the waste materials. Therefore, a well-funded knowledge of the geochemical behaviour of these long-lived radioelements is of high relevance for a reliable safety assessment of a potential storage site. Due to the radioactive decay of the stored radionuclides, the temperature in the near-field of the repository will heat up significantly (Tmax = 200 °C (salt) [2]). As reducing conditions are expected in the near-field, +III will be the stable oxidation state of Am (and partly Pu). The strong increase in temperature will have a significant effect on the chemical properties of the trivalent actinides and therefore speciation studies at elevated temperatures are of particular interest. As chloride will be the most abundant anion available for complexation reactions aim of the present work was to determine the structural parameters of Am(III)-chloride complexes as a function of the temperature up to 200 °C. The Am LIII-edge EXAFS measurements were performed at the INE-Beamline (ANKA, Karlsruhe, Germany) using a newly developed high-temperature EXAFS cell [3]. No differences in the EXAFS spectra and Fourier Transforms are observed for T ≤ 90 °C. At T = 200 °C, the peak in the pseudo-radial distribution (Fig. 1, right) is visibly broadened which is attributed to a coordination of Am(III) by chloride, revealing that Am(III) is coordinated by 2.4 chloride ligands [3]. At 200 °C, a chloride concentration of 3 M is sufficient to cause a visible complex formation with Am(III). In contrast to this, room temperature studies showed that no chloride complexes are formed for [Cl-] < 8 M [4]. The present study points out that the impact of chloride on the speciation of trivalent actinides will increase significantly with increasing temperature. Furthermore, our results provide a new insight into the complex formation properties of actinides at elevated temperatures which is a valuable contribution to improve the molecular-level understanding of the related geochemical processes. *Author for correspondence: daniel.froehlich@partner.kit.edu. [1] W. Kickmaier, et al., Nucl. Eng. Des. 176, 75 (1997). [2] T. Brasser, et al., GRS report, GRS-247 (2008). [3] A. Skerencak-Frech et al., Inorg. Chem, 53, 1062, (2014). [4] P. G. Allen, et al., Inorg. Chem. 39, 595 (2000).

Speaker: Dr Daniel Fröhlich (Physikalisch-Chemisches Institut, Universität Heidelberg, Germany)

17:30 Fingerprinting uranyl minerals with oxygen K-edge NEXAFS

There is a great variety of secondary uranium minerals. In spite of water molecules associated with these minerals, they can be classified by their oxygen K-edge x-ray absorption spectra, opening the door for advanced molecular and isotopic uranium analysis.

Speaker: Dr Andrew Duffin (Pacific Northwest National Laboratory)

17:30 Impact of increasing MoO3 loading on the composition of multicomponent borosilicate glass

One option to dispose solid Mo(VI)- and Cs(I)-rich residual material sedimenting in storage tanks for high level liquid waste (HLLW) is immobilisation in borosilicate glass. Due to its low solubility in glass melts, Mo(VI) tends to form separate molybdate-rich phases during the vitrification process [1]. These phases may crystallise during melt cooling and are able to incorporate radionuclides within their crystal structure. If water-soluble alkali molybdates form, the release of radionuclides will be facilitated in case of water intrusion into a deep geological repository. The chemical composition of the separate molybdate phases depends strongly on the vitrification temperature, the waste concentration and the borosilicate glass composition. Understanding factors favouring the formation of chemically stable crystalline Mo(VI) phases in borosilicate glasses allows development of glass compositions capable of incorporating high Mo loadings by simultaneously ensuring durability under repository conditions. In this work, a multi-component borosilicate glass [2] with varying Mo-rich nuclear waste simulate loadings was prepared and characterised by several techniques. Powder X-ray diffraction (XRD) and Raman spectroscopy studies confirm formation of crystalline CaMoO4 and BaMoO4 phases at total MoO3-concentrations above 5.3 wt% and at 1300 °C melting temperature. Linear combination least squares (LCLS) fit analyses of Ba L3 edge X-ray absorption near edge structure (Ba-L-XANES) and Raman spectra permit a quantitative estimation of the degree of crystallinity as a function of the MoO3-content. Mo K edge high energy resolution XANES (Mo K-HR-XANES) reveals variations in the chemical environment of the MoO4- anion. LCLS fit analysis shows that Na+ cations preferably compensate molybdate charge at low Mo loading (0.5 wt%). Ca/BaMoO4 species are increasingly observed at higher loading reaching 65% at 12 wt% MoO3 content. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) show formation of spherical structures enriched in Ba, Ca and Mo. In addition, µ-XRF, µ-Mo-K-XANES and µ-XRD studies give access to the elemental Mo speciation and distribution of crystalline phases in the glass products, which are confirmed by transmission electron microscopy (TEM). TEM distinguishes between spatially separate CaMoO4 and BaMoO4 crystalline clusters with very similar lattice parameters. We demonstrate that the combination of advanced spectroscopy and microscopy techniques allows characterising crystalline regions with high Mo, Ba and Ca content. No evidence for formation of water-soluble alkali crystalline molybdates is found, indicating that the chemical composition of the glass used is favourable for immobilisation of Mo-rich nuclear waste with this specific chemical composition [3].

Speaker: Mr Sebastian Bahl (Karlsruhe Institute of Technology (KIT))

17:30 Improved corrosion resistance of zirconium alloys: Enlightening the mode of action of Nb doping by complementary hard x-ray chemical microscopy techniques

Zirconium alloys are extensively used as materials in a broad range of technical applications in various types of nuclear facilities. The mechanical and physico-chemical properties of these alloys can be tuned by modifying the chemical composition, mainly by the addition of dopants. Empirical studies have revealed that one of the key properties, the corrosion resistance, can be significantly enhanced by doping the alloy with niobium. However, the processes induced by the addition of niobium being responsible for limiting the corrosion rate are not yet fully understood on a mechanistic level. Furthermore, in the case of neutron irradiation and a corrosive environment, the system gains one additional dimension of complexity and corresponding limitation in terms of detailed process understanding. In order to elucidate fundamental aspects of such corrosion processes, complementary synchrotron-based microscopic X-ray techniques have been employed to investigate a neutron irradiated zirconium alloy with 2.5%Nb doping and a corresponding non-irradiated (autoclaved) reference sample. Extending a pilot study [Froideval et al, 2009], X-ray fluorescence (XRF), X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques has been applied simultaneously in two-dimensional, microscopic imaging mode using micro-focused X-rays. The resulting chemical images with high spatial resolution provide detailed insights into the chemical nature of the pristine metal, the metal-oxide interface, as well as the corrosion layer formed. As the most basic chemical imaging tool, micro-XRF produces pictures of the distribution of chemical elements of interest. This allows tracing down leaching, segregation and re-distribution phenomena occurring during the build-up of corrosion layers. Using the electronic state contrast expressed at the element-specific absorption edges, oxidation state mappings reveal the variations in valencies – in the present case of zirconium and niobium – across the metal/interface/corrosion-layer composite system. Chemical images illustrating the distribution (as well as the orientation and the state) of crystalline phases formed during corrosion are elaborated by means of two-dimensional micro-XRD. Finally, spatially highly localized micro-XAS can provide information about the molecular-level coordination of a specific element, even in the case of non-crystalline (‘x-ray amorphous’) material domains. The different kind of results obtained simultaneously from an identical sample voxel allows gaining new insights into the fundamental processes occurring during the corrosion of niobium-doped zirconium alloys under neutron irradiation. References: Froideval, A., Abolhassani, S., Gavillet, D., Grolimund, D., Borca, C., Krbanjevic, J., Degueldre, C., J. Nucl. Mater. 385 (346-350), 2009.

Speaker: Dr Vallerie Ann Samson (PSI)

17:30 In-situ high temperature XAS experiments on oxide nuclear fuels and precursors

Over the last decades, the application of XAS (X-ray Absorption Spectroscopy) to actinide-bearing materials has proven to be a very reliable method to obtain valuable information on these oxides, through a coupled insight into oxidation states and local structure. Experiments were notably performed on UO2+x, U1 yPuyO2±x and U1-yAmyO2±x compounds [1-4]. Concerning the latter for instance, an unexpected charge distribution was identified over a large range of composition (0.05 ≤ x ≤ 0.50), with the complete reduction of Am to the trivalent state whereas uranium is partially oxidized to pentavalent state, a result which was not available via other characterization methods [3,4]. Such data remain however limited to measurements performed after fabrication (i.e. ex-situ) at RT (room temperature) or below (15 or 77 K). Even if they are very valuable, the same information must be obtained under thermodynamic conditions where elemental mechanisms are taking place. To perform the in-situ XAS experiment a new experimental set-up dedicated to actinide-based materials has been developed. It consists of a furnace based on the heating wire technology [5] embedded in a dedicated second confinement barrier. The aim of this experimental set-up is to cover thermodynamic conditions encountered during fuel fabrication (precursor calcination, sintering), irradiation in reactor (LWR, Na-FNR…) or severe nuclear accidents. The existing version can heat the sample up to 2000 K under various atmospheres by flushing any gases around the heating wire: air, neutral (Ar, N2) and reductive (Ar+H2+O2). During the thermal sequence, the oxygen potential can be dynamically modified by mixing the gases and is measured at the furnace entrance. The first in-situ high temperature measurements on U-bearing samples were performed at INE-ANKA beamline. They were focused on U/Ln oxide behaviours and on the conversion into oxides of several U/Ln precursors for nuclear fuel fabrications. The results obtained notably showed, through XANES spectra, the successive variations of oxidation states of the cations occurring during these conversions, as well as some changes in local symmetry and cristallinity. Concerning the experimental setup, the experiment confirmed the feasibility of the measurements in a double-sealed environment under controlled conditions. It thus represents the first step of a bigger project, which aims at performing in-situ XAS measurements on transuranium (i.e., Pu, Am, Np)-bearing samples. [1] S.D. Conradson, B.D. Begg, et al., J. Solid State Chem. 178 (2005) 521-535. [2] P. M. Martin, S. Grandjean, et al., J. Alloys Comp. 444-445 (2007) 410-414. [3] F. Lebreton, P.M. Martin, et al., ANKA Experimental Report A2012-019-002808. [4] D. Prieur, P.M. Martin, et al., Inorg. Chem. 50 (2011) 12437-12445. [5] V. Magnien et al., Geochim. Cosmochim. Acta., 72(2008)2157 ; D.R. Neuville et al., Amer. Min. 93(2008)228.

Speaker: Mr Florent Lebreton (CEA Marcoule)

17:30 Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach

Because actinides (An) are typically accept various oxidation states, and because fuel redox conditions evolve with burn-up, others oxides than stoichiometric AnO2 are formed during irradiation. In this work, neptunium chemical state and local environment in stoichiometric and oxidized (U0.9Np0.1)O2(+x) fresh samples were obtained by X-ray absorption spectroscopy at the NpLIII edge (17610 eV) at the ROBL beamline (ESRF). No variation of the white line position and intensity relative to NpO2 reference was evidenced, suggesting that Np remains tetravalent in all samples. However, since XANES NpLIII analysis was found to be ambiguous relative to Np+IV/V determination in humic solution (e.g. Denecke et al. 2004), the results were completed by EXAFS analysis. The EXAFS spectra did not reveal a short Np-O distance which would indicate the neptunly unit O=Np+V=O and corroborates as such the exclusive presence of Np+IV. Independently, the oxidation state of both actinides was predicted by a thermodynamic approach first considering separate phases (heterogeneous fuel), then ideal and non-ideal solid solution. The minimization of Gibbs energy was obtained by GEM-Selektor code developed at LES, NES, PSI, based on Gibbs energy values and molar heat capacities available in the literature for actinide oxides of interest (i.e. UO2, U4O9, U3O8, UO3, NpO2 and Np2O5). Thermodynamic modelling predicts the full reduction of NpO2+x into NpO2 and oxidation of UO2 in UO2+x, thereby confirming XAS observations.

Speaker: Melanie Chollet

17:30 Plutonium oxidation state speciation in aqueous solution studied by Pu L and M edge high energy resolution XANES technique

In this work four electrochemically aqueous plutonium (Pu) species prepared in perchloric acid solution at different oxidation states (III, IV, V, VI) as well as Pu(IV) colloids are characterized for the first time by Pu L3 and M5 edge high energy resolution X-ray absorption near-edge structure spectroscopy (HR-XANES). A Johann type five-analyzer crystal spectrometer recently installed and commissioned at the INE-Beamline for actinide research at the ANKA synchrotron radiation facility, Karlsruhe, Germany was applied Different to conventional XANES several spectral features could be identified. The most intense absorption resonances (White Line, WL) have higher intensities for all Pu L3 HR-XANES spectra compared to the conventional XANES. Additionally, the Pu(V) and Pu(VI) L3 HR-XANES spectra exhibit better resolved post-edge features. The energy distance between the WL and this resonance is sensitive to the bond distance between the Pu and axial O atoms in Pu(V) and Pu(VI). Extended X-ray absorption fine structure (EXAFS) investigation is performed to correlate oxidation states with average Pu-O bonding distances. For the Pu(VI) M5 edge HR-XANES a feature at higher energy is well resolved, which might be sensitive to changes in Pu-O bond length and to the level of hybridization of metal and axial oxygen valence orbitals. A pre-edge ‘shoulder’ is detected in the Pu(III) spectrum. The origin of hitherto unresolved features is elucidated by quantum chemical calculations using the FEFF9.5 code. The HR-XANES experimental technique provides new insights into the actinides electronic structure and allows detection of minor contributions of Pu oxidation states in oxidation state mixtures.

Speaker: Mr Ivan Pidchenko (Institute for Nuclear Waste Disposal)

17:30 Real-time-resolved observation of complexation reaction of U Ions by using dispersive XAFS technique

Dispersive XAFS technique is unique approach for the real-time-resolved observation of chemical reaction. X rays with wide-band energy are created by a curved crystal and counted by a space-resolved detector after sample transmission. We can obtain XAFS spectra without any mechanical motion of crystal or detector. We have developed dispersive XAFS observation system at BL14B1 (Japan Atomic Energy Agency beamline) of SPring-8 and applied to some chemical reactions. Complexation reactions of U and other ions are targets of our interest. 1-10 Hz EXAFS observation for the complexation reaction reveals that fast electronic change of center atoms and slow local structural change due to complex formation. We want to show some applications of the dispersive XAFS technique which allows us to get the fast and stable EXAFS spectra.

Speaker: Dr Daiju Matsumura (Japan Atomic Energy Agency)

17:30 Se K-edge micro XANES on high-burnup UO2 spent nuclear fuel: First results in the framework of the FIRST-Nuclides project

In the context of the FIRST-Nuclides collaborative project (EURATOM FP7 programme) Paul Scherrer Institut is carrying out leaching experiments on high-burnup UO2 spent nuclear fuel (SNF) from Swiss nuclear power plants, in order to determine the short-term release of easily accessible nuclides (instant release fraction, shortly IRF). The focus is on nuclides such as 129I, 135Cs, 14C and 79Se. Previous experiments (Johnson et al., 2012) had shown that, contrary to expectations, no detectable Se was released to the aqueous phase even after 100 days of leaching. In order to understand the behaviour of 79Se during aqueous leaching of SNF in a geological repository, X-ray absorption experiments were carried out at the MicroXAS beamline (SLS) on high burnup UO2 spent fuel from the Leibstadt nuclear power plant. As the comparison of the Se K-edge micro XANES data obtained on SNF with the spectra of reference compounds yielded ambiguous results, the experimental spectra were modelled ab initio via FEFF or FDMNES calculations coupled with geometrical optimization procedures provided by the FitIt package. Best fits of the experimental SNF XANES spectra were obtained by assuming substitution of Se(-II) (selenide) in oxygen sites of the UO2 lattice, with Se-U distances very similar to those of crystalline U selenide compounds. These results suggest that 79Se may be stabilized as selenide in SNF, explaining the failure to detect aqueous selenium in leaching experiments. Release of 79Se to aqueous solutions under geological storage conditions would then proceed by matrix dissolution rather than via IRF.

Speaker: Dr Enzo CURTI (Paul Scherrer Institut)

17:30 Speciation and distribution of plutonium after uptake by Opalinus Clay as measured by synchrotron microbeam techniques

Due to its long half-life Pu has a significant contribution to the radiotoxicity of spent nuclear fuel in a repository after long periods of storage. Pu exhibits a complicated redox chemistry, where up to three oxidation states can coexist under environmentally relevant conditions [1]. In order to predict the migration behavior of Pu after its potential release from a repository of spent nuclear fuels into the environment, the interaction of Pu with the host rock formation has to be understood in detail. Argillaceous rocks are under consideration in several European countries as a potential host rock for high-level nuclear waste repositories. In clay formations diffusion and sorption are considered to be the main transport and retardation processes [2]. Accordingly, the interaction of 242Pu(VI) with Opalinus Clay (OPA) from Mont Terri, Switzerland, in particular, the sorption and migration behaviors were studied by batch and diffusion experiments, respectively. A combination of spatially-resolved synchrotron based techniques (micro X-ray absorption spectroscopy (µ-XAS), micro X-ray fluorescence spectroscopy (µ-XRF), and micro X-ray diffraction (µ-XRD) was used to study the distribution and speciation of Pu on Opalinus Clay (OPA) after sorption and diffusion processes. Several thin sections of OPA were contacted with 20 μM 242Pu(VI) in OPA pore water (pH 7.6, I=0.4 M) under aerobic conditions for at least three days. For comparison, a OPA bore core in a diffusion cell was contacted with 20 μM 242Pu(VI) under the same conditions for more than one month. The sorption and diffusion samples were investigated at the microXAS beamline at the Swiss Light Source, Paul Scherrer Institut, Switzerland. μ-XRF based chemical imaging has been used to determine the elemental distribution of Pu and other elements contained in OPA, e.g., Fe and Ca. Regions of high Pu concentrations were subsequently investigated by μ-XANES to identify the oxidation state of Pu sorbed onto the OPA clay material.. The results of Pu LIII-edge μ-XANES spectra on Pu hot spots showed that Pu(IV) is the dominating species within OPA, i.e., the highly soluble Pu(VI) was reduced within the clay rock material and Pu was retained by the Opalinus Clay rock in the reduced and less mobile tetravalent oxidation state. Simultaneously to chemical imaging and micro-spectroscopy, μ-XRD was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of the observed Pu enrichments. μ-XRD results indicated that Pu is localized on or in close vicinity of the Fe(II)-bearing mineral siderite and the clay mineral illite. Siderite is one of the redox-active mineral phases of OPA. Our results point towards a decisive role of siderite regarding the speciation on Pu after migration into Opalinus Clay rock. The obtained results and inferred geochemical understanding indicate that OPA is a suitable host rock for a high-level nuclear waste repository. Acknowledgement This work was financed by the Federal Ministry of Economics and Technology (BMWi) under contract no. 02E10981and Actinet-I3 under contract no. 232631. We are grateful to Camelia Borca at SLS for her support during the measurements. Further we thank Dr. Christian Marquardt (INE, KIT)) and Maik Biegler (Max-Planck-Institute for Chemistry, Mainz) for providing OPA samples and preparation of the OPA thin sections, respectively. __________________________________________________________________________________ [1] Runde, W.: The chemical interactions of actinides in the environment. Los Alamos Sci. 26, 392-411 (2000). [2] Van Loon, L. R., Soler, J. M., Bradbury, M. H.: Diffusion of HTO, 36Cl- and 125I- in Opalinus Clay samples from Mont Terri: Effect of confining pressure. J. Contam. Hydrol. 61, 73-83 (2003).

Speaker: Mr Ugras Kaplan (Karlsruhe Institute of Technology - INE)

17:30 Speciation of neptunium along diffusion pathways in Opalinus Clay using micro-XAFS and micro-XRF

For the long-term safety of nuclear waste repositories and a possible migration of the relevant transuranium actinide Np into the environment, the study of the interaction of Np with clays is of highest importance, since argillaceous rocks are under consideration as potential host rocks or at least as backfill material in high-level nuclear waste repositories in countries like Switzerland and Germany. In our previous SLS-experiment the Opalinus Clay from Mont Terri, Switzerland, was used as a natural reference clay mineral to study the sorption and diffusion behaviour of Np(V) in clay and to simulate the migration process under environmental related conditions. Several OPA bore cores were contacted with synthetic OPA pore water (pH 7.6) as mobile phase containing 8 µM Np(V) under aerobic conditions in self-made diffusion cells with a constant flow rate. Though they were started at three different diffusion times to see the diffusion and speciation in dependency of time. As we had success in the preparation of a similar filter-free cell of OPA contacted with Pu using parallel layered OPA (in diffusion direction), this concept was also used here. After the termination of the diffusion process we split each bore core along the axis of diffusion. In this way we obtained samples revealing Np diffusion profiles. The first step was the examination of the distance of the Np diffusion by µ-XRF. Therefore a large, smooth area must be chosen for the µ-XRF scans which have been implemented over several mm. The final mapping of Np was calculated by subtracting the µ-XRF-mapping of Sr with an excitation energy of 17592 eV from the same mapping using 17616 eV, above the Np L3-edge of 17610 eV. Large mappings of a size of 0.5 x 4 mm were realized with a step size of 10 µm. The µ-XRF profiles also provided besides Np and Sr a deeper look at several other elements as Mn, Co, Ti, Zn, Fe, Ca and Rb. At this point it can be mentioned that of those elements the most pronounced heterogeneity of distribution was observed for Zn, Fe and especially Ca. These mappings can even display whole particles for Ca, probably calcite, with an area of several ten thousand µm². However, the strongest effect might have differences in the concentrations of Fe, since it is reported to be under suspicion to reduce Np(V). Regarding the local diffusion and the local redox environment of Np and its speciation this heterogeneity of OPA must be taken into account. As mentioned before, these results were also connected with Np L3-edge µ-XANES along the diffusion pathways. They were analysed with the software package Athena using background and energy correction as well as normalization. Afterwards a linear combination fit analyse using reference spectres of Np(IV) and Np(V) aquo ions was performed. They provide certain evidence of redox processes taking place with the initially added Np(V) in OPA. This can be concluded from the basic trend of the Np(V)/Np(IV) ratios which exhibit that Np(V) is increasingly reduced to Np(IV) as it diffuses deeper through the OPA core. Matching to this, the spectra for the furthest diffusion pathways even show convincing signs for an almost exhaustively reduction, though the quality is impaired by the lower concentrations. Nevertheless, these new results fortify Np(IV) as the main species, even though more experiments investigating the time-dependency will be performed. As a last point we can already verify that the migration of the mobile Np(V) through OPA is considerable retarded as it gets significantly reduced to the highly sorbing Np(IV) on its diffusion through the OPA. These are positive results considering the safety assessment for a final disposal of nuclear waste in argillaceous rock.

Speaker: Mr Prashanta Jonathan Benjamin Rosemann (Institute of Nuclear Chemistry, Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany)

17:30 Speciation of uranium sorbed on magnetite/maghemite nanoparticles: HR-XANES and «ab-initio» calculations

Deep understanding of mobilization/immobilization mechanisms of uranium in the environment is essential for safety assessment of a long-term nuclear waste repository. For example the soluble in groundwater U(VI) can be immobilized by sorption or incorporation into minerals [1]. Iron oxide nanoparticles are abundant in the environment and play a major role in sorption and redox behavior of U. Magnetite (Fe3O4) is a ferrous bearing oxide and a corrosion product of steel used in storage containers of nuclear waste. High-energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy at the actinide (An) L3 and M4,5 edges is currently developing technique providing new insights into the An electronic and coordination structure [2,3]. We present the U L3/M4 HR-XANES experimental spectra of U sorbed on magnetite or maghemite (U-magnetite, U-maghemite) nanoparticles and U M4 HR-XANES spectra of UO3, meta-autunite (Ca(UO2)2PO4)210-12•(H2O)) and torbernite (Cu(UO2)2(PO4)2•12(H2O)). The HR-XANES spectra are analyzed on the basis of theoretical simulations to investigate local coordination structure of U in the magnetite/maghemite nanoparticles. Theoretical spectra are calculated by applying a self-consistent field (SCF) full multiple scattering (FMS) method implemented in the FEFF9.6 code [4]. Phase shifts of the photoelectron are calculated in the framework of self-consistent crystal muffin-tin (MT) potential scheme with 15% overlapping MT spheres. Best agreement of energy positions and intensities of spectral features between theoretical and experimental spectra is achieved for spectra calculated with the Hedin-Lundquist potential with non-screened core-hole (final state rule). FMS calculations of U L3-edge HR-XANES spectra of the U-magnetite and U-maghemite nanoparticles is performed for two types of substitution, i.e., uranium replaces iron in octahedral or tetrahedral sites. Comparison of experimental and theoretical spectra suggest that U is incorporated in octahedral magnetite sites for the specific experimental conditions. The FEFF9.6 program is found to be an adequate tool for analysis of U L3 –edge and M4,5 edges HR-XANES of U containing compounds. Acknowledgments This work is supported by the German-Russian Interdisciplinary Science Center (G-RISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DADD)(Project P-2014a-1). References [1] Riley, R. G.; Zachara, J. M. Chemical Contaminants on DOE Lands and Selection of Contaminant Mixtures for Subsurface Science Research; U.S. Department of Energy,Office of Energy Research:Washington, DC, 1992. [2] Vitova T et al. 2010 PRB 82 235118 [3] Kvashnina K O, Butorin S M and Glatzel P 2011 J. Anal. At. Spectrom. 26 1265 [4] JRehr J J, Kas J J, Prange M P, Sorini A P, Takimoto Y and Vila F 2009 C. R. Physique 10 548 59

Speaker: Yulia Podkovyrina (Southern Federal University)

17:30 Structural studies of the Eu(III) and U(VI) interactions with pentapeptides

In the wake of the Fukushima accident, it is of great importance to assess the mechanisms governing radionuclide impact on the environment (particularly the biosphere) and to unravel the molecular processes underlying actinide transport and deposition in tissues. Most data available on the interaction of actinides with biological systems are based on physiological or biokinetic measurements, with scarce information on the microscopic factors such as structure of the actinide coordination site within biological molecules (proteins, peptides...). These structural data are essential to understand structure, function and affinity interdependence, which governs the organ deposition of such elements. In this poster, we will present first results on the complexation of Eu3+, which we used as a surrogate of trivalent actinides, and UO22+ with peptides containing various, biologically relevant, geometrical constraints. Three pentapeptides have been synthesized, DGDGD, ADPDA and DPDPD. The latter two contain proline (P) residues that induce an angular strain that is absent while the first peptide contains two flexible glycin (G) residues. The expected complexing residues are in all cases the carboxylate function of the aspartate aminoacids (D). The number of carboxylic side chains was also varied between 2 and 3. Furthermore, the influence of the intrinsic geometry of the cation is also investigated by comparing the complexation of the linear uranyl(VI) oxo-cation versus the “spherical” europium(III). Among structural investigation techniques, X-ray Absorption Spectroscopy (XAS) was found adequate since it is an element specific local structural and electronic probe that is increasingly used to sense metal centres in biological systems. This technique coupled with other spectroscopic methods such as infra-red spectroscopy and SLRT enables us to draw a full understanding of the metal environment. To screen the affinity of peptides with both Eu(III) and U(VI), Isothermal Tiration Calorimetry (ITC) was also used. Additionally, theoretical tools such as molecular mechanics and dynamics were used to model the actinide complex stability.

Speaker: Aurelie Jeanson (CNRS)

17:30 X-Ray photoelectron spectral structure and chemical bonding in AmO2

The quantitative analysis of the x-ray photoelectron spectral (XPS) structure of americium dioxide AmO2 was done in the valence electrons binding energy range taking into account binding energies and spectral structures of the core electronic levels (~35 - 1250 eV) and the relativistic calculation data for the AmO812- (D4h) cluster reflecting americium close environment in AmO2. The experimental data suggest that the many-body processes and the multiplet splitting contribute to the valence XPS structure significantly less than the outer (0 - ~15 eV) and the inner (~15 - ~35 eV) valence molecular orbitals formation does. The filled Am 5f electronic states were shown to appear in the valence band of AmO2. The atomic Am 6p electronic orbitals were shown to participate in formation of both inner and outer valence molecular orbitals (bands). The most part in the inner valence molecular orbitals formation were found to take the filled Am 6p3/2 and O 2s atomic shells. The composition and the sequent order of such orbitals in AmO2 were established in the binding energy range 0 - ~35 eV. The obtained experimental and calculation data allowed for the first time a quantitative scheme of the molecular orbitals for AmO2. This scheme is essential and fundamental for understanding of the chemical bonding nature in AmO2 and for interpretation of the fine structures of other x-ray spectra. The work was supported by the RFBR grant #13-03-00214.

Speaker: Prof. Yury Teterin (National Research Center "Kurchatov Institute")

17:30 XAFS investigation of a HAWC glass fragment issued from the Karlsruhe Vitrification Plant (VEK)

The Karlsruhe Reprocessing Plant (WAK) was operated from 1971 to 1991 as a pilot facility for reprocessing of spent nuclear fuels from German pilot reactors and commercial power plants. Reprocessing activities resulted in ~60 m3 of highly active waste concentrates (HAWC) stored on-site in liquid form. An important step in the current decommissioning of the WAK was the HAWC vitrification in the Karlsruhe Vitrification Plant (VEK) constructed close to the HAWC storage facilities [1]. Sections of genuine HAWC glass rods were retained during the vitrification process and transferred to the INE shielded box line for later glass product characterization. In 2013 a mm sized fragment with a contact dose rate of ~590 µSv/h was selected and mounted in a specially designed sample holder for pilot XAS/XRF investigations at the INE-Beamline [2] located at the KIT synchrotron light source ANKA [3]. The experiment aimed at elucidating the potential of direct actinide / radionuclide speciation (with an emphasis on fission products) in highly active nuclear materials (e.g., waste glass, spent nuclear fuel) and assessing the possible influence of the γ-radiation field surrounding highly active samples on the XAS detection electronics. While the influence of the γ-radiation field turned out to be negligible, initial radionuclide speciation studies by XAS were most promising. Exemplarily, normalized Se K- (left) and Tc K-edge XANES measurements of the HAWC glass fragment and corresponding Se and Tc reference samples were performed. Edge position and simple spectral fingerprint analysis point to the presence of Se in the glass as selenite oxoanion (SeO32-) as in Na2SeO3. Pronounced dampening of the near-edge fine structure indicates dispersion of the SeO32− oxoanions in the glass matrix, where the crystalline ordering such as in the Na2SeO3 reference is lost. Tc is dispersed as pertechnetate oxoanion (TcO4-) in the glass matrix as in the aqueous Tc(VII)/HClO4 reference sample, which is unequivocally proven by the edge shift relative to Tc(IV)O2 and the pronounced pre-edge feature at ~21056 eV indicative of tetrahedral oxygen coordination. Elemental X-ray lines were identified in the XRF spectra of the HAWC glass fragment recorded for different excitation energies using a five element LEGe fluorescence detector. In addition to Se and Tc discussed above and the plethora of elements contained in the glass fragment, the focus of these initial XAFS investigations was on the possibility for direct actinide speciation by recording corresponding L3 edge XAFS data. Th was not expected to be contained in the glass and, hence, was not detectable in XAFS scans across the Th L3 edge. Only a very tiny absorption signature was detected by scanning the energy across the Cm L3 edge. This signal was not sufficient to obtain any useful XANES data. This is in full agreement with the known composition of the HAWC oxide residue [1], where the Cm content is expected to reach about 1/10 of the Pu content, which, in turn, amounts to ~1/30 of the U content of the HAWC glass. The concentrations of Am and Np are expected to reach twice the value obtained for Pu. For all actinide elements besides Th and Cm (U, Np, Pu, Am) as well as for Zr registration of high quality XANES data was possible - for some of these elements even reasonable EXAFS spectra were recorded. The XANES analysis reveals that uranium is present as U(VI), neptunium as Np(V), plutonium as Pu(IV), americium as Am(III) and zirconium as Zr(IV) in the HAWC glass. [1] G Roth, S Weisenburger, Vitrification of high-level liquid waste: glass chemistry, process chemistry and process technology, Nucl. Engineering and Design 202, 197-207 (2000) [2] J. Rothe, S. Butorin, K. Dardenne, M. A. Denecke, B. Kienzler, M. Löble, V. Metz, A. Seibert, M. Steppert, T. Vitova, C. Walther, H. Geckeis. The INE-Beamline for actinide science at ANKA. Rev. Sci. Instrum., 83, 043105 (2012) [3] www.anka.kit.edu

Speaker: Dr Kathy DARDENNE (Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (KIT-INE))

17:30 XPS structure and chemical bond nature in Сs2PuO2Cl4

The quantitative analysis of the x-ray photoelectron spectral (XPS) structure of Сs2PuO2Cl4 single crustal containing the plutonyl group PuO22+ was done in the valence electrons binding energy range 0 - ~35 eV taking into account binding energies and spectral structures of the core electronic levels (~35 - 1250 eV) and the relativistic calculation data for the PuO2Cl42- (D4h) cluster reflecting plutonium close environment in Сs2PuO2Cl4. The experimental data suggest that the many-body processes and the multiplet splitting contribute to the valence XPS structure significantly less than the outer (0 - ~15 eV) and the inner (~15 - ~35 eV) valence molecular orbitals formation does. The filled Pu 5f electronic states were shown to appear in the valence band of Сs2PuO2Cl4. The atomic Pu 6p electronic orbitals were shown to participate in formation of both inner and outer valence molecular orbitals (bands). The most part in the inner valence molecular orbitals formation were found to take the filled Pu 6p3/2 and O 2s, Cl 3s atomic shells. The composition and the sequent order of such orbitals in Сs2PuO2Cl4 were established in the binding energy range 0 - ~35 eV. The obtained experimental and calculation data allowed for the first time a quantitative scheme of the molecular orbitals for Сs2PuO2Cl4. This scheme is essential and fundamental for understanding of the chemical bonding nature in Сs2PuO2Cl4 and for interpretation of the fine structures of other x-ray spectra. The work was supported by the RFBR grant #13-03-00214.

Speaker: Prof. Yury Teterin (National Research Center "Kurchatov Institute")

Wednesday 21 May

08:40 → 09:50 Solid State Chemistry and Physics of the Actinides

Convener: Dr David Shuh (LBNL)

08:40 Advanced X-ray spectroscopy at M and N edges of actinides

Data obtained in the tender and soft x-ray ranges for the Th, U, Np and Pu systems are presented and discussed. The use of the so-called Hämäläinen scan at the actinide 3d edges by monitoring the 4f-to-3d x-ray emission transition drastically improves the energy resolution compared to conventional XANES and reveals multiple structures never seen before in contrast to a single line in conventional XANES spectra. This allows for an access in some cases to unique information which is difficult to obtain with other techniques. The data are compared with the results of calculations in the framework of the Anderson impurity model accounting for the Coulomb interaction (U) within the 5f shell and between 5f electrons and a core-hole. The combination of such experiment and theory enables the study of detailed properties of the electronic structure such as ligand field effects and spin interactions involving the 5f electrons as well as the degree of hybridization of the 5f states. The measurements at the actinide 4f edges reveal all the fine structure in the distribution of unoccupied 6d states in actinide systems which cannot be observed in commonly used XANES spectra at the actinide L edges due to their large 2p(3d) core-hole lifetime broadening. This enhances the sensitivity of the spectroscopy to fine changes in the electronic state distribution due to e.g. doping/non-stoichiometric effects. The data are compared with the results of LDA+U calculations which also provide the estimation of the U value by probing the delocalized electronic component in a number of actinide systems.

Speaker: Dr Sergei Butorin (Department of Physics and Astronomy, Uppsala University)

09:15 Advanced X-ray spectroscopy of actinides

This contribution will provide a brief overview of recent progress in X-ray spectroscopy of actinides in the hard and tender X-ray range. We will focus on the high-energy-resolution X-ray fluorescence detected (HERFD) absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) at the U L3 and M4,5 edges of different uranium systems. In connection with latest results, the capabilities and limitations of the experimental techniques and theoretical methods for data analysis will be discussed in details.

Speaker: Dr Kristina Kvashnina (European Synchrotron Radiation Facility)

09:50 → 10:20 Coffee

10:20 → 12:20 Actinides in Environmental and Life Sciences

Convener: Dr David Shuh (LBNL)

10:20 Cs sorption-desorption mechanisms on clay minerals by XAFS, STXM, DFT calculation methods

Three years have passed since Fukushima Dai-ichi Nuclear Power Plant (FDNP) Accident; a serious contamination problem still remains. Therefore, Japanese and local governments promotes the decontamination projects in which contaminated surface soil is removed and then, is moved it to interim storage area. Since soil waste is extremely large and space of an interim storage is limited, a suitable waste reduction method would be required, also in anticipation of final disposal of the waste after about 30 years. On these backgrounds, we have performed elucidation of the sorption-desorption behaviors of Cs on clay minerals by XAFS: EXAFS/XANES, DXAFS, STXM, TEM/SEM-EDS, and theoretical calculation methods, aiming at soil waste volume reduction in Fukushima. In this presentation, we plan to touch uranium behaviors in this area. In this area, species of clay minerals on which Cs adsorbed were unexpectedly few according to the radio autography and SEM and TEM-EDS analyses; mainly were a several micaceous minerals and its weathered minerals. Therefore, illite and vermiculite were used to elucidate sorption behavior on clay minerals in Fukushima. The Fourier-transformed EXAFS oscillation for the clay mineral samples prepared under laboratory scales concentration indicated that cesium existed in the interlayer for vermiculite and mainly on the surface for illite. The DXAFS experiment supported this result of the Cs sorption behavior in vermiculite in addition to detection of the detailed sorption dynamics. Furthermore, the chemical bond of Cs with the clay minerals revealed two types of futures by EXAFS, i.e., Cs showed heavy elements feature like actinides in chemical bond. It might be related with selectivity on sorption on clay minerals in environment and it’s stability in clay minerals.

Speaker: Dr Tsuyoshi Yaita (Japan Atomic Energy Agency)

10:55 Speciation of reduced U(IV) in contaminated sediments and laboratory reactors

In situ groundwater remediation or waste storage strategies often have the goal of limiting the mobility of a contaminant by decreasing its solubility in the passing aqueous phase. The chemical or enzymatic reduction of U(VI) to U(IV) presents an approach to achieve this goal by exploiting the lower solubility of uraninite (UO2) relative to that of U(VI) species. This property of U has led to extensive research aimed at understanding the mechanism of U(VI) reduction and the stability of uraninite. However, recent evidence from U LIII-edge x-ray absorption spectroscopy suggests that uraninite may not be the predominant form of U(IV) in reduced sediments on the time scale of remediation activities. We will present an example from contaminated sediments at the Oak Ridge Integrated Field Research Center,1 followed up by several laboratory studies where we find that factors such as the mechanism of enzymatic U(VI) reduction,2 groundwater components such as phosphate or carbonate,2 and the ratio of U to specific mineral surface sites,3 can prevent the precipitation of uraninite and result in the formation of complexed or adsorbed U(IV) species. The spectroscopic features allowing the identification of these species from XANES and EXAFS measurements will be highlighted. The predominance of non-uraninite U(IV) species in reduced sediments presents new analytical and transport modelling challenges that will need to be addressed in future research. 1.“In situ Bioremediation of Uranium with Emulsified Vegetable Oil as the Electron Donor”, D. B. Watson, W-M. Wu, T. Mehlhorn, G. Tang, J. Earles, K. Lowe, T. M. Gihring, G. Zhang, F. Zhang, J. Phillips, M. Boyanov, B. Spalding, C. Schadt, K. M. Kemner, C. S. Criddle, P. M. Jardine, S. C. Brooks, Environ. Sci. Technol., Environ. Sci. Technol., 47 (12), 6440–6448 (2013) 2.“Solution and Microbial Controls on the Formation of Reduced U(IV) Species”, M. Boyanov, K. Fletcher, M.-J. Kwon, X. Rui, E. O’Loughlin, F. Löffler., K. Kemner. Environ. Sci. Technol. 45, 8336-8344 (2011) 3.“Stable U(IV) complexes form at high-affinity mineral surface sites”, D. E. Latta, B. Mishra, R.E. Cook, K. M. Kemner, M. I. Boyanov, Environ.Sci.Tech., 48 (3), 1683–1691 (2014)

Speaker: Dr Maxim Boyanov (Argonne National Laboratory)

11:20 U(IV) release from a mining impacted wetland

U contamination as a result of U mining and processing is ubiquitous. In the environment, uranium is typically present in either its oxidized, soluble hexavalent state [U(VI)] or its reduced, insoluble tetravalent state [U(IV)]. The concentration of U was observed to increase as stream flowed through a wetland contaminated with U. This was unexpected as U is usually found in its sparingly soluble tetravalent state in the reducing environment of wetlands and, as a result, release would be expected to be minimal. This investigation aimed at characterizing the speciation of U to attempt to explain this phenomenon. U was present in the top 40 cm of the soil and was identified as almost exclusively a non-crystalline U(IV) species using X-ray absorption spectroscopy. Additionally, electron microscopy revealed that U was associated primarily with amorphous Fe-P-Al-Si assemblages. Surprisingly, little U was associated with the clay phases (e.g., muscovite, smectite) that dominate the mineralogy of this soil. In contrast, a relatively high concentration of porewater U was distributed uniformly throughout the soil depth in the porewater. Further characterization of the speciation of U revealed the presence of U associated with colloids of Fe and organic matter. This finding suggests that U is labile and mobile in the soil and is released into the porewater and eventually into the stream due to the formation of U-bearing colloids. Using laser fluorescence spectroscopy, these colloids were shown to be associated with tetravalent uranium. This study highlights the mobility of U from a wetland even though U is present in a reduced valence. This is the first time that such mobility is demonstrated in the environment and it brings into question the use of constructed wetlands for the treatment of U-impacted waters.

Speaker: Prof. Rizlan Bernier-Latmani (Ecole Polytechnique Federale de Lausanne (EPFL))

11:55 Interaction of plutonium and neptunium with magnetite under anoxic conditions: Reduction, surface complexation, and structural incorporation

For the redox-reactive fission products and actinides Se, Tc, U, and Np, it is assumed that the strongly reducing conditions in deep underground, anoxic nuclear waste repositories will reduce their mobility, since their lower-oxidation states commonly form solids of very low solubility. This is not necessarily the case for Pu, where the hexa- and pentavalent aquo-complexes prevalent at higher pe are replaced at lower pe by a tetravalent solid of low solubility, PuO2, but also by a trivalent aquocomplex at lower pH. FeII-bearing mineral phases, especially those with low bandgap, play an important role in this process, since they are able to catalyse redox reactions at their surface. Magnetite, FeIII2FeIIO4, is an important mineral in this context, since it forms by steel corrosion under anoxic conditions and is also prevalent as geogenic mineral. Therefore, we investigated the reaction of PuV, and in comparison also of NpV, with magnetite in sorption and coprecipiation experiments with X-ray absorption spectroscopy. We found that PuV is indeed reduced to the trivalent oxidation state in the presence of magnetite. The PuIII aquo-complexes are, however, strongly sorbed by forming tridentate surface complexes [1]. PuIII can also be partly incorporated by the structure of magnetite by rapid coprecipitation. During aging, however, it is expelled from the structure and readsorbed at the magnetite surface. This behaviour of Pu is then compared to that of Np.

Speaker: Dr Andreas Scheinost (HZDR Institute of Resource Ecology)

12:20 → 13:20 Lunch

13:20 → 15:30 Actinides in Environmental and Life Sciences

Convener: Dr Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source)

13:20 Interfacial reactivity of Pu and Th at the muscovite (001) basal plane

The geochemistry of the actinides is of utmost importance in understanding and predicting their behavior in contaminated legacy sites as well as nuclear waste storage facilities. The unique chemistry of this group of elements including strong hydrolysis, complex redox chemistry, and the potential for polymerization reactions in combination with the actinides’ inherent radioactivity and toxicity makes studies challenging. However, especially for artificial elements like Pu and other transuranics, no natural analogues are available and homologues frequently fall short in accurately reproducing the actinides’ behavior. We will present and discuss recent results from in situ resonant anomalous x-ray reflectivity (RAXR) and crystal truncation rod (CTR) experiments, shedding light on the inter-action of Th(IV) as well as Pu(III) and Pu(IV) with the negatively charged muscovite (001) basal plane. The example of Th(IV) demonstrates how the strong hydration of the highly charged cations prevents a close approach to the surface, instead favoring adsorption as a highly hydrated extended outer sphere complex. Subsequently, it will be shown how similar adsorption behavior in combination with the complex redox chemistry of plutonium, leads to a surface-enhanced formation of nanoparticles. Results from surface x-ray scattering will be supplemented by ex situ alpha-spectrometry quantification and atomic force microscopy (AFM), to yield a more complete understanding of the interfacial structure.

Speaker: Dr Moritz Schmidt (HZDR)

13:55 Actinide environmental speciation: Source-dependent vs. source-independent species

Design of the nuclear waste repositories in geological conditions, development of effective remedial actions at radionuclide contaminated sites and evaluation of radioecological impact on the environment requires the molecular-level speciation of radionuclides. In this content the long-lived actinides are of the primary importance since their high radiotoxicity. Recently it have been recognized that environmental behavior of actinides are controlled either by the local geochemical conditions (source independent speciation) or by the source or origin of the contamination (source dependent speciation). In the last case depending on the release scenarios the highly inert “hot-particles” could be formed that demonstrate very high kinetic stability independent on the geochemical conditions. Such “hot-particles” demonstrate very slow solubility and leaching rates. At the same time it has been shown that sorption and redox reactions of actinides at the mineral-water interface also may result in the formation of very low soluble surface precipitates that demonstrate very low leaching rates. To study actinide speciation and to define the conditions that favor the formation of low-soluble inert source dependent and source independent actinide containing particles various spectroscopic (XAFS, XPS, XRF, etc.) and microscopic techniques (radiography, SEM-EDX, HR-TEM) have been applied. In model experiments under well-defined laboratory conditions the interaction of Pu(IV,V,VI) with minerals (hematite and anatase) at various total concentrations, pH and time intervals have been studied. It was shown that redox transformation of oxidized plutonium, i.e. Pu(V) and Pu(VI), occurs which kinetics is highly dependent on the total concentration of Pu. Redox speciation is done using the kinetics of sorption and leaching, solvent extraction and XAFS. Using Pu LIII XAFS and HR-TEM it was shown that at micromolar concentrations plutonium forms PuO2+x•nH2O nanoparticles with sizes of 1.5-2 nm that are aggregating on the hematite surface. At very low concentration, i.e. <10-14 M, it is chemosorbed on the surface of hematite. For anatase only the chemosorbed species are identified at various concentrations of plutonium. The experimental data on plutonium sorption and surface precipitation at various concentrations is interpreted as a competition between chemosorption reaction: =Xe-OH + Pu → =Xe-O-Pu + H (X= Fe or Ti) and surface precipitation: =Pu-OH + Pu → =Pu-O-Pu + H. The precipitation of PuO2+x•nH2O nanoparticles result in the formation of highly inert and kinetically stable forms. The work was supported by RFBR (project 14-03-00821).

Speaker: Prof. Stepan Kalmykov (Lomonosov Moscow State University)

14:30 Microscale analysis of U(VI) uptake by argillaceous rocks using synchrotron radiation X-ray methods

Argillaceous rock formations, due to their high clay content and low permeability, have significant radionuclide retention/retardation capacities, and are therefore in the focus for high level and long lived nuclear waste (HLW) repositories in several countries. In Hungary two geological sites (Gorica Block and W-Mecsek Anticline Block in the perianticlinal structure of W-Mecsek Mountains) of the Boda Claystone Formation (BCF) have been selected for the study of potential host rocks for HLW. The aim of the measurements was to obtain information on the U(VI) uptake mechanism on the micrometer scale, in order to complete molecular scale sorption results and understand the behaviour of uranium dissolving from HLW and to identify the possible differences in uptake capacity between the two possible sites. Synchrotron radiation microscopic X-ray fluorescence (SR µ-XRF) has sufficient sensitivity to study the metal uptake on the microscale [1]. SR µ-XRF measurements were performed on thin sections subjected to uptake experiments involving U(VI) using 5 µm spatial resolution. The thin sections were prepared on high-purity silicon wafers from geochemically characterized cores of both sites of BCF. Correlation analysis of µ-XRF elemental maps indicated that the U enrichment was not only correlated to the argillaceous matrix, but also the cavity filling minerals played an important role in the uptake. By using positive matrix factorization as a new multivariate approach the factors with higher uptake capacity could be identified. The uptake capacity of the different mineral phases could be quantified with additional mineralogical information [2]. The results were compared with cluster analysis when the regions dominated by different mineral phases were segmented. The multivariate approach based on µ-XRF to identify the minerals was finally validated using microscopic X-ray diffraction. Our results revealed that in the sample, taken from W-Mecsek Anticline Block, where dolomites have ankerite rims and U-bearing rings, newly formed FeOOH precipitations were observed, which partly replaced the ankerite at near neutral pH (6.8). In this sample the dissolution of ankerite was followed by FeOOH formation, which easily can bind U(VI) due to the enhanced specific surface area and high adsorption capacity. The oxidation of Fe(II) required to the formation of FeOOH is caused by partial reduction of U(VI) to U(IV). Eighty percent of uranium was taken up by clay minerals and 20% by FeOOH although the ankerite concentration is as low as 6% in W-Mecsek Anticline Block of BCF. The study demonstrated that the different mineralogy of Gorica Block and W-Mecsek Anticline Block significantly influences the U retention capacity of the Boda Claystone Formation [3]. [1] C. Walther, M.A. Denecke. Chemical Reviews 113 (2013) 995-1015. [2] J. Osán, A. Kéri, D. Breitner, M. Fábián, R. Dähn, R. Simon, S. Török. Spectrochimica Acta Part B 91 (2014) 12–23. [3] D. Breitner, J. Osán, M. Fábián, P. Zagyvai, C. Szabó, R. Dähn, M. Marques Fernandes, I.E. Sajó, Z. Máthé, S. Török. Submitted to Environmental Earth Sciences

Speaker: Dr Janos Osan (Hungarian Academy of Sciences Centre for Energy Research)

14:55 Synchrotron radiation study of actinide sorption and diffusion in natural clay

In several European countries, e.g., France, Germany and Switzerland, argillaceous rocks are considered as a potential host rock for the construction of nuclear waste repositories. Detailed information on the interaction between the clay and the actinides, which are major contributors to the radiotoxicity of spent nuclear fuel after storage times of more than 1,000 years, is required for the safety assessment of future nuclear waste repositories. The interaction between redox-sensitive actinides (Np and Pu) and natural clay has been investigated in batch and diffusion experiments and by X-ray techniques. Opalinus Clay (OPA, Mont Terri Rock Laboratory, Switzerland), which was used as a reference for natural clay, consists of different clay minerals (66%), quartz (14%), calcite (13%), iron(II) minerals (4%) and traces of other minerals and organics. Due to this mineralogical heterogeneity, a combination of different synchrotron based techniques with micrometer resolution was used to study the sorption and diffusion of Np(V) and Pu(V), which have a high solubility at near neutral pH conditions. Different thin sections of OPA were contacted with 8 µM Np(V) or 20 µM Pu(V) solutions at pH 7.6 under aerobic conditions for several days. In the diffusion experiments, neptunium and plutonium were allowed to diffuse from a reservoir with 8 µM Np(V) or 20 µM Pu(V) into intact OPA bore cores during several weeks before they were removed from the diffusion cells. The thin sections and small pieces of the bore core were investigated at the microXAS beamline at the Swiss Light Source using monochromatic synchrotron radiation with a beam size of typically 3 x 1.5 µm2. µ-XRF mappings of both sorption and diffusion samples showed heterogeneous distributions of neptunium and plutonium and other heavy elements (Ca, Mn, Fe, Sr) contained in OPA. Regions with high actinide concentrations, which frequently occurred in close vicinity to areas enriched in iron, were investigated by µ-XANES spectroscopy. By comparing these Np and Pu LIII-edge XANES spectra to those of reference spectra of neptunium and plutonium in different oxidation states, it was shown that the highly soluble pentavalent actinide species were reduced to the less soluble tetravalent oxidation state in all investigated samples. In some areas also Pu(III) was identified, although Pu(IV) remained the dominating oxidation state. Further information about the minerals present in areas enriched in neptunium or plutonium was obtained by µ-XRD. In addition to the clay mineral illite, siderite (FeCO3) and pyrite (FeS) could be identified near actinide spots in several thin section samples, indicating that these Fe(II) minerals act as a reducing agents and causes the immobilization of neptunium and plutonium in OPA. The examples given in this presentation illustrate that µ-XRF, µ-XANES, and µ-XRD are a powerful combination of techniques for studying the speciation and migration behaviour of actinides in heterogeneous natural systems like argillaceous rocks with high spatial resolution.

Speaker: Prof. Tobias Reich (Johannes Gutenberg-Universität Mainz)

15:30 → 16:00 Coffee

16:00 → 17:40 Actinides in Environmental and Life Sciences

Convener: Dr Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source)

16:00 Competitive uptake behaviour of radionuclides in cementitious material

Cement is an important constituent of the engineered barrier systems in underground repositories for low and intermediate level radioactive waste. Cement is used to solidify and stabilize the waste materials and, furthermore, it is used in huge amounts for the construction and backfilling of the cavern. The release of radionuclides from the cementitious near field into the underground is retarded due to their strong interaction with cement mineral phases. In hydrated cement, calcium silicate hydrates (C-S-H), (CaO)x(SiO2)y(H2O)z) are quantitatively the most abundant and most important phases. They are chemically the most stable minerals in a cementitious environment and exhibit a wide diversity of structural sites exposed for cation binding. Three different modes of heavy metal interaction with C-S-H phases can be envisaged: 1) surface complexation, 2) uptake in the interlayer, and 3) incorporation in the octahedral Ca sheets. In this study, 11 Å tobermorite, a crystalline C-S-H phase, was doped with both Nd and Zn in order to investigate whether or not the competition between the two metals has an influence on the binding mechanism and the coordination environment of these metals in the C-S-H structure. On the basis of previous EXAFS studies a structural model for the site occupation of Zn in C-S-H phases was proposed. Zn(O,OH)4 tetrahedra substitute for silicate bridging tetrahedral and/or they are bound at terminal silicate chain sites, thus suggesting Zn binding in layer positions of C-S-H phases rather than the interlayer. In contrast, spectroscopic studies on lanthanide binding to crystalline C-S-H phases indicate that Eu (and Nd) forms inner sphere surface complexes with the C-S-H surface in the early phase of the sorption process. However, with time, Eu and Nd binding into the structure of crystalline C-S-H phases occurs, occupying positions in the interlayer and the Ca layer of the C-S-H structure. Therefore, Nd and Zn are expected to be taken up into the structure of C-S-H phases whereby they occupy neighbouring crystallographic positions. In this study Zn K-edge and Nd LIII-edge EXAFS spectroscopy was used to discern the possibility of backscattering contributions from neighbouring Zn and Nd atoms on the absorption spectra. The Zn and Nd doped 11 Å tobermorite samples had metal concentrations of 50000 ppm, while the reaction times varied from one to six months. X-ray absorption spectroscopic data suggest that Nd has an influence on the incorporation of Zn in the tobermorite structure. The Zn K-edge EXAFS data exhibit distinct differences in the presence and absence of Nd as neighbouring element. Zn has a tetrahedral coordination environment with Zn-O distances of 1.96 Å, whereas Nd is octahedral coordinated with Nd-O distances of 2.45 Å. These findings indicate that competitive uptake of metal cations with similar sorption behaviour by C-S-H phases can take place, which deserves further attention in future assessments of the safe disposal of radioactive wastes in cement-based repositories. Currently molecular modelling calculations are ongoing to better constrain the uptake mechanism involved.

Speaker: Dr Marika Vespa (KIT-INE)

16:25 XAS studies of radionuclide incorporation into mineral phases

Many countries have a legacy of radioactively contaminated land and nuclear wastes. Current policy is to dispose of waste in a geological disposal facility, and decommission legacy nuclear sites over the next decades. These processes could lead to the release of radionuclides (e.g. 137Cs and Tc). Radionuclide mobility in the environment is often controlled by adsorption onto, and incorporation into mineral phases, which can significantly limit their transport and bioavailability. Key to understanding the long-term behaviour of radionuclides within environmental systems is a molecular-scale understanding of the mechanisms by which they become incorporated into minerals. In this study the incorporation of U, Tc and Cs into iron oxide (e.g. hematite and magnetite) or aluminosilicate clay (e.g. illite) minerals have been characterised at the atomic scale using X-ray Absorption Spectroscopy (XAS) in conjunction with aberration corrected (scanning) transmission electron microscopy ((S)TEM), and density functional theory modelling. The results of this work provide a new mechanistic understanding of U and Tc incorporation into hematite and magnetite, respectively. We show that U(VI) and Tc(IV) substitute directly for Fe within the iron oxides. Firstly, U(VI) was incorporated into a distorted, octahedrally coordinated site replacing Fe(III) in hematite. XAS showed the uranyl bonds lengthened from 1.81 to 1.87 Å, in contrast to previous studies. Secondly, Tc(IV) was incorporated into octahedrally coordinated sites replacing Fe(III) in magnetite, with the majority of the Tc located near the particle surface. In addition, XAS and STEM analysis showed that following initial sorption onto the frayed edges, Cs migrates into the illite interlayer becoming incorporated. This process is irreversible as Cs is held in the collapsed interlayers and is not exchangeable. During longer term experiments Cs did not remain at the edge of the illite particle, but migrated into the collapsed interlayer through exchange with K.

Speaker: Dr Sam Shaw (University of Manchester)

16:50 Monazite as promising candidates for nuclear waste management: Structural characterization by X-ray Absorption Spectroscopy

Minor actinides such as Np, Am, and Cm contribute a major part of the heat load and radiotoxicity of spent nuclear fuel. Their separation and incorporation into stable ceramic phases may provide a route for their safe storage in deep-geological repositories or for their transmutation to short-lived nuclides. Natural monazites, early-lanthanide phosphate minerals of old age, contain up to 27 wt % UO2 + ThO2, demonstrating thereby a high incorporation capacity, high chemical durability and high resistance to radiation damage. Therefore, monazite-type ceramics are promising candidates for the long-term incorporation of minor actinides. The aim of this work is to reveal the structural changes of LaPO4 monazite induced by the incorporation of europium, an analogue for trivalent actinides. La1-xEuxPO4 monazite solid-solutions with x from 0.2 to 1.0 were synthesized and characterized by Eu L3-edge and La L1-edge, extended X-ray absorption fine-structure (EXAFS) spectroscopy at the Rossendorf Beamline (ESRF, France). Structural refinement of X-ray diffraction data showed a Vegard-like behavior of metal-oxygen and metal-metal distances, i.e. a linear decrease of these distances with increasing Eu content. In contrast, EXAFS analysis revealed that only the La-O distances in the first coordination shell and the first metal-metal distances decrease according to Vegard’s law, while the Eu-O local coordination remains unchanged. These new EXAFS results provide important insight into the structural basis of the ideality and therefore stability of monazite solid-solutions; they will be used in the future to develop the thermodynamic constants needed for long-term stability predictions.

Speaker: Dr M. Janeth Lozano-Rodriguez (1) HZDR, Institute of Resource Ecology, 01314 Dresden, Germany; (2) The Rossendorf Beamline at ESRF, CS 40220, 38043, Grenoble, CEDEX 9, France)

17:15 Bioaccumulation and speciation of europium in sponge A. Cavernicola

The fate of radionuclides in the marine environment remains a major concern in our modern societies[1], specially after the recent event of Fukushima in 2011. Among the environmental compartments, the hydrosphere is ubiquitous and can transport compounds or elements over long distances. Among the radionuclides of concern, actinides are the heaviest elements involved in nuclear activities. Surprisingly, very little is known about the speciation of actinides in sea water[2] and their accumulation in marine organisms. Improving knowledge on the interaction between actinides, sea water and marine organisms is therefore essential to better understand the transfer mechanisms from the hydrosphere to the biosphere and to evaluate their global impact on the environment. Marine sponges have already been identified as hyper-accumulators of several trace elements and are proposed in this case as model biomonitor organisms[3]. To address this question, we have focused the present study on americium (III), an actinide with a relatively simple RedOx chemistry compared to plutonium or uranium. Yet because of the relatively high specific radioactivity of americium, europium (a stable lanthanide with chemical properties very close to that of americium) was also used as a chemical surrogate. The use of speciation calculations conforted by the X-ray Absorption Spectroscopy (XAS) allowed to study the speciation of europium and americium (III) in the sea water. The europium is assumed to interact with carbonates and humic substances. A similar behaviour seems to be observed for americium. In a second step, the accumulation of europium (III) was investigated in the Mediterranean sponge A. Cavernicola. This sponge is commonly found in the caves off the coasts of the Northwestern Mediterranean and previous studies have shown that it has a substantial capacity for metal accumulation[3]. The amount of europium integrated in sponges exposed to the radiotracer 152Eu was monitored using a high-purity Ge gamma spectrometer. Despite the relatively low levels of accumulation, the X-ray Absorption Spectroscopy (XAS) spectra at the europium LII edge showed a specific phosphate interatomic distance. This phenomena could be explain by an interaction with the bacterial membrane. Furthermore, the use of STXM (Scanning Transmission X-ray Micrsoscopy) showed that the europium in the sponge is located where the bacteria are. [1] Maher, K., J.R. Bargar, and G.E. Brown, Environmental Speciation of Actinides. Inorganic Chemistry, 2013. 52(7): p. 3510-3532 [2] Choppin, G.R., Soluble rare earth and actinide species in seawater. Marine Chemistry, 1989. 28(1-3): p. 19-26 [3] Genta-Jouve, G., et al., Comparative bioaccumulation kinetics of trace elements in Mediterranean marine sponges. Chemosphere, 2012. 89: p. 340-349

Speaker: Ms Melody Maloubier (Institut de Chimie de Nice)

18:00 → 19:00 Apero

Convener: Dr Rainer Dähn (Paul Scherrer Institut)

19:00 → 22:00 Dinner

Thursday 22 May

08:40 → 09:50 Actinides in Environmental and Life Sciences

Convener: Dr Rainer Dähn (Paul Scherrer Institut)

08:40 The role of phosphorous biochemistry in actinide human contamination

In case of accidental exposure to radioelements, internal actinide toxicity is related to both emitted radiation and to the in-vivo circulation scheme. Blocking the biological pathways of the actinides in the human (or more generally mammalian) systems and/or increasing their elimination rate would considerably decrease the toxicity of these elements. Overall the need for a better understanding of the actinide pathways in biological systems is of fundamental importance with regards to the assessment of nuclear risk. The use of bio-mimicking molecules or molecular building blocks (like simple aminoacids with important chelating groups, small peptides etc.) is one of the methods to better understand these chemical pathways that drive actinide incorporation into cells and organs. Our present strategy has focused on the cation coordination itself in a so-called bioactinidic approach corresponding to actinide chelation by important biological actors or building blocks of biological molecules that may be considered as simplified mimicking actors. Note that actinide chemistry is complicated by complex RedOx behavior and large ionic size radius that induces significant changes in ligand conformation with respect to the essential biological cations such as iron. These complications are why a fundamental and simplified approach to the question of actinide transfer in biological systems is essential. The phosphate chemical function is ubiquitous in biological systems. The phosphorylations of proteins are transient phenomena, which play a key role in the signalization cascades, and actinide bound to the phosphorylated groups might disturb some biochemical pathways. While it is involved in phosphorylated proteins phosphates are also the major functions of the nucleotides. Consequently phosphorylated amino acid building blocks as well as phosphorylated proteins may be considered as possible targets for actinide complexation in the various compartments of the biological machinery. This was evidenced in previous studies aiming at identifying proteins able to bind uranyl starting from cell extracts. This presentation will give a background of actinide bioinorganic chemistry in the framework of nuclear toxicology. It will in particular browse examples of actinide coordination mechanisms with two distinct biological systems that involve phosphorylated biomolecules. The first one is the nucleotide family, which is involved in many enzymatic reactions but also as essential building blocks in the nucleic acids. The second one is a targeted protein involved in the skeleton turnover, namely osetopontin (OPN). We have in parallel investigated an hexapeptide that is representative of the active site of osteopontin on the bond surface. Three actinide cations will be discussed: thorium(IV) as a representative of actinide(IV), uranyl(VI) as the most ubiquitous example of actinyl oxocations and americium(III) as a representative of oxidation state +III and middle actinides. In addition, lutetium(III) will be also discussed as a chemical analogue of the middle actinides.

Speaker: Prof. Christophe DEN AUWER (University Nice Sophia Antipolis, Nice Chemistry Institute)

09:15 Using XAS to probe radionuclide biogeochemistry in complex environmental systems

Understanding the behaviour of radionuclides in natural and engineered environments is key to the management and control of radioactively contaminated materials. Recent work has demonstrated that biogeochemical cycling of radionuclides is critical in controlling radionuclide behaviour in the shallow sub-surface. More recently, we have been exploring the role of microbial processes in controlling radionuclide behaviour under conditions relevant to geological disposal of radioactive waste and to some radioactively contaminated land scenarios. Specifically we have explored both high pH conditions and the indirect effects of microbial reduction on silicate minerals and the resultant reactivity of the bioreduced mineral phases on radionuclide speciation. For the high pH geomicrobiology work, we took alkaline sediments from an old lime workings, and incubated the sediments with an electron donor and uranium-(VI), technetium-(VII) and neptunium-(V). In addition, we also added excess Fe(III) as ferrihydrite into an additional set of experiments to probe the impact of microbial Fe(III)-reduction on radionuclide behaviour. As the systems became reducing, we tracked the biogeochemical behaviour of the stable elements and tracked the solubility and speciation of U, Tc and Np using geochemical and spectroscopic techniques (XANES and EXAFS). For the experiments exploring the indirect impacts of microbially induced Fe(III)-reduction on radionuclide speciation, we exposed chlorite and biotite to the model Fe(III)-reducing microorganism Geobacter sulfurreducens in the presence of an electron donor. The resultant bioreduced minerals contained elevated levels of Fe(II) compared to the unreacted biotite and chlorite. After washing to remove cells, the bioreduced minerals were then spiked with radionuclides to explore the reactivity of the bioreduced minerals compared to the unreacted materials. U, Tc and Np solubility and speciation were again tracked using geochemical and, at selected time points, spectroscopic techniques (XANES and EXAFS). Here, we present the results of this work and highlight the significance of biogeochemistry in underpinning the understanding of radionuclide speciation and fate in the environment.

Speaker: Prof. Katherine Morris (The University of Manchester)

09:50 → 10:20 Coffee

10:20 → 12:20 Theoretical and Modeling Tools

Convener: Prof. Christophe DEN AUWER (University Nice Sophia Antipolis, Nice Chemistry Institute)

10:20 Coordination of actinyl ions to N-heterocyclic ligands: A joint theoretical and experimental study

Nuclear energy represents a critical tool available to meet the demand of increasing energy supply while limiting green house gas emissions. To reduce the need for long-term nuclear waste storage, it is important to develop efficient strategies for selective separations. A better molecular-level understanding of the coordination modes and affinities of ligands with multiple binding sites to actinyl ions can pave the way to designing new ligands with improved extraction efficiency and selectivity. We will discuss the coordination chemistry of actinyl ions with ligands composed of multiple competitive binding sites, including sulfur, nitrogen and oxygen chelating groups. We will present the interactions between actinide centers and selected nitrogenous heterocyclic ligands using first-principle methods that include relativistic effects and electron correlation. The theoretical results will be further validated by gas phase and solution spectroscopic characterizations.

Speaker: Dr Ping YANG (Pacific Northwest National Laboratory)

10:55 Theoretical studies of orbital mixing in actinide-ligand bonds

For the past few years we have put a significant amount of effort into the development of reliable computational methodologies for quantifying the amount of covalent interactions in actinide-ligand bonds. These studies aided the interpretation of ligand core-level spectroscopy measurements by some members of the team and the two efforts together yielded solid evidence of covalent bonding and 5f participation in that interaction. On a more applied side, our studies have been instrumental in elucidating fundamental differences in electronic structure that lead to stronger bonds and selectivity in chemical separations of actinides. In this lecture we will discuss the computational methodologies and the application at prediction of XAS spectra in actinide species. As a corollary we will also discuss the connection between the covalent nature of the bonds and the strength of the bond, which is a concept that has confused the community for a long time. An implicit misunderstanding has been the connection between this evidence of covalent interaction and the relative strength of these bonds. In this presentation we address that issue with quantum mechanical methods to try to clarify and separate the two concepts. These developments are illustrated in a series of molecular complexes where the bonds are clearly defined and described in “simple” language.

Speaker: Dr Enrique Batista (Los Alamos National Laboratory)

11:30 Mechanism of Zn adsorption on montmorillonite inferred from atomistic simulations, P-EXAFS-spectroscopy and sorption experiments

The uptake of heavy metals by clay minerals such as montmorillonite and illite is an important process controling the migration of contaminants in the geosphere. The 2-site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model [1] has been used over the past decade or so to quantitatively describe sorption of metals with valences from II to VI on montmorillonite [2]. One of the main assumptions in this model is that there are two broad categories of edge sorption sites; the so called strong (>SSOH) and weak (>SW1OH) sites. Because of their different sorption characteristics, it was expected that the coordination environments of the surface complexes on the two site types should be different. The aim of this study is to identify the nature of (>SSOH) and (>SW1OH) sites at an atomistic level combining sorption measurements, P-EXAFS spectroscopy and atomistic simulations. Two different montmorillonites were chosen for the experimental part of this study: Milos (Island of Milos, Greece) and STx-1 (Gonzales County, Texas, USA). As a reference for Zn substituted for Al in the clay octahedral sheet a MILOS sample was prepared without adding any Zn. Milos was chosen because it contains 1.8 [mmol/kg] Zn incorporated into the structure, compared to only 0.3 [mmol/kg] for STx-1. Compared to the foreseen added Zn loadings of ~2 mmol/kg and ~30 mmol/kg, respectively, the quantity of incorporated Zn in the STx-1 is quite low and thus does not impair the EXAFS signal form the adsorbed species. In a first step Zn isotherm data on two montmorillonites, Milos and STx-1, were measured and modelled using the 2SPNE SC/CE sorption model. The results were used to define the most favourable experimental conditions under which Zn sorption was either dominated by the strong (>SSOH, ~2 mmol kg-1) or by the weak sites (>SW1OH, ~40 mmol kg-1). Highly oriented self-supporting films of montmorillonite were prepared to measure P-EXAFS spectra of Zn complexes adsorbed at low (~2 mmol kg-1), and medium Zn loadings (~30 mmol kg-1) as well as Zn incorporated in the montmorillonite structure. The molecular structure of potential Zn complexes on the (010), (110), (130) and (100) edges of montmorillonite were derived based on ab initio molecular dynamics and geometry optimization runs. These optimised molecular structure were used to calculate theoretical EXAFS spectra. The molecular mechanism of Zn uptake on montmorillonite was then derived comparing the measured and calculated EXAFS spectra. The analysis of the measured EXAFS spectra [3] and simulation results [4] suggests that Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading Zn forms mono- and bi-dentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling. [1] M.H. Bradbury, B. Baeyens, J.Cont. Hydrol, 27, 223-248 (1997). [2] M.H. Bradbury, B. Baeyens, Geochim. Cosmochim. Acta, 69, 875-892 (2005). [3] Dähn R., Baeyens B., and Bradbury M. H. (2011) Investigation of the different binding edge sites for Zn on montmorillonite using P-EXAFS – the strong/weak site concept in the 2SPNE SC/CE sorption model. Geochim. Cosmochim. Acta 75(18), 5154-5168. [4] Churakov S.V., Daehn R. (2012) Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy. Environental Science and Technology. 46, 5713-5719

Speaker: Sergey Churakov (Paul Scherrer Institut)

11:55 Analysis of EXAFS spectra with the aid of neural networks: Aqueous U(VI) complexes with aliphatic (hydroxy-) carboxylic acides

Sixty U LIII-edge EXAFS spectra from 13 structurally different aliphatic ((di-)hydroxy-) carboxylic acids (acetic, succinic, tartaric, lactic, 3-hydroxybutyric, citric, formic, malic, maleic, malonic, oxalic, propionic, and tricarballylic acid) were measured at different pH, uranium and ligand concentrations. Each of the ligands can form several metal complexes, which may coexist as mixtures depending on the physicochemical parameters (pH, concentration).. The ligands were chosen in such a way that they would allow a structural analysis of the complexes solely by using the exclusion principle, i.e. by comparing the spectra with respect to pH, concentration, presence/absence, number, position and the type of the functional group/s. Due to the high number of different constellations in this highly correlated system, simple inspection by eye and other conventional tools will possibly lead to different solutions and is therefore prone to misinterpretation. Artificial neural networks, such as self-organizing maps (SOM), are expected to be better adapted and more specialized for dealing with such highly complex systems. We show that the inclusion of the Beer-Lambert law, in the training period of SOM, leads to a new kind of supervised learning algorithm [1] which enables the determination of the spectra and fractions of the different U(VI) complexes. Moreover, we show that the new SOM algorithm allows the inclusion of available information such as the ligand structures and the physicochemical parameters so that latent relationships between them and the spectra of the complexes are revealed. [1] Domaschke, K. et al. (2014) Proceedings of ESANN.

Speaker: Dr André Rossberg (Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, 01314, Dresden, Germany)

12:20 → 13:20 Lunch

13:20 → 14:40 Facility Reports and Upcoming Techniques

Convener: Prof. Tobias Reich (Johannes Gutenberg-Universität Mainz)

13:20 Radioactive matter investigations at the MARS beamline of synchrotron SOLEIL

The Multi-Analyses on Radioactive Samples (MARS) beamline at the French synchrotron SOLEIL is opened to the international community since 2010. This beamline is fully devoted to advanced structural and chemical characterizations of radioactive matter (solid or liquid), coupling analytical tools such as X-ray absorption spectroscopy, X-ray diffraction and X-ray fluorescence. Since September 2013, analyses on radioactive samples at ambient temperature and pressure with activities up to 20 000 times above the French exemption limit are now allowed (thus for actinides up to activities of 200 MBq). Yet, the final aim is to get an extension of the possibilities of analyses onto a larger variety of experiments and to activities up to 18.5 GBq. Currently, six different types of experiments are available: standard X-ray absorption spectroscopy (XAS), High-Resolution X-ray absorption spectroscopy (HRXAS), Transmission X-ray diffraction (TXRD), High-Resolution X-ray diffraction (HRXRD), Wide-Angle X-ray Scattering (WAXS) and associated X-ray microbeam techniques (µXRF/µXRD/µXAS). In this contribution we will describe the progresses that have been made on the beamline in the last years and we will give a brief overview of experiments on a selection of topics related to the nuclear or radiochemical field.

Speaker: Dr Pier Lorenzo Solari (Synchrotron Soleil)

13:40 Scientific capabilities of the Advanced Light Source for radioactive materials investigations

The Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory (LBNL) is a third-generation synchrotron radiation (SR) light source that began operations more than twenty years ago. The ALS is a U.S. Department of Energy (DOE) national user facility generating intense light for scientific and technological research. As one of the world's brightest sources of ultraviolet and soft X-ray beams and the world's first third-generation synchrotron light source in its energy range, the ALS makes previously impossible studies possible. Access to the resources of the ALS is through a competitive proposal mechanism within the general user program. Multiple ALS beamlines (BLs) are currently being employed for a range of radioactive materials investigations. These experiments are reviewed individually on a graded hazard approach implemented by the ALS in conjunction with the LBNL Environmental, Health, and Safety (EH&S) Radiation Protection Program that conform to DOE regulations. The ALS provides radiological work authorization (RWA), radiological control technician support, and general assistance for accepted user experimental programs for work with radioactive materials. There are three basic categories of experiments with radioactive materials at the ALS that serve to define the general safety measures that must be present to perform experiments. There are no fully-dedicated BL facilities for studying radioactive materials at the ALS, that have been primarily actinides but have included technetium plus others. The SR techniques that are or have been utilized at ALS BLs with a diverse set of materials containing actinides include photoelectron spectroscopy (PES), angle-resolved PES (ARPES), near-edge X-ray absorption fine structure (NEXAFS), soft X-ray emission spectroscopy (XES), resonant inelastic X-ray scattering (RIXS), spectromicroscopy with scanning transmission X-ray microscopes (STXM), micro-diffraction, and of course, the suite of X-ray absorption spectroscopy (XAS) methods. Several ALS BLs are used on a regular basis for radioactive materials investigations with part of the core complement consisting of soft X-ray BLs 4.0.2-3, 7.0.1 (now temporarily unavailable) 5.3.1-2, 8.0.1, 10.0.1, and 11.0.2. Another group of useful BLs is composed of the higher energy BLs 9.3.1, 10.3.2, small molecule crystallography at BL 11.3.1, and superbend BLs located at 12.2.2. and 12.3.2. Recent and past results highlights from the ALS with these aforementioned techniques and BLs will be briefly described to illustrate the capabilities of the ALS for the investigation of radioactive materials. As the ALS is recently undergoing a great deal of change resulting from BL upgrades and from recent DOE evaluations, particular attention will be given to the newest developments and to the long-range ALS scientific priorities. A full description of experimental capabilities, access, and safety policies of ALS BLs can be found on the ALS website (http://www-als.lbl.gov).

Speaker: Dr David Shuh (Lawrence Berkeley National Laboratory)

14:00 Actinide and radionuclide speciation by X-ray absorption spectroscopy methods at ANKA – A facility report

X-ray Absorption Spectroscopy (XAS) and related techniques have become essential ingredients for many speciation studies on radionuclides in various technical, (geo-)chemical, biological or environmental systems over the past two decades. The high applicability of XAS in the field of radionuclide research is mainly attributed to its ability to provide direct speciation information on a selected element without (or with just a minimum of) sample pre-treatment or target element isolation. At the same time adequate sample containment is easily provided, due to the penetration strength of medium to hard X-rays. XAS has attracted an increasing number of researchers in nuclear sciences with a focus on the nuclear waste repository safety case, but also covering fundamental actinide studies. However, this trend is counteracted by the limited access to the few, strongly overbooked synchrotron X-ray facilities in the world equipped with technical infrastructure, radiation safety expertise and sample handling know-how required to perform XAS measurements on radioactive materials with activities beyond the specific exemption limits. KIT at Karlsruhe, Germany, provides dedicated synchrotron beamline facilities for the investigation of radionuclide containing samples and controlled area labs in close proximity on the same research campus. Research efforts at the INE-Beamline [1] at the KIT synchrotron light source ANKA [2] are driven by the Karlsruhe based institutes KIT-INE and JRC-ITU and various national and international user groups - many of them applying for beamtime in the frame of joint research projects funded through the EURATOM FP7 access initiative ACTINET-i3 or its follow-up TALISMAN [3]. Operational since 2005, the INE-Beamline offers sufficient flexibility to adapt sample environments and detection systems to many specific scientific questions, including recent implementation of high resolution X-ray emission spectroscopy (HRXES). CAT-ACT, a new X-ray beamline at ANKA for CATalysis and ACTinide research, is currently being constructed and will be jointly operated by KIT-ITCP (Institute for Technical and Polymer Chemistry) for in-operando catalysis experiments and KIT-INE for radionuclide research. CAT-ACT will help serve growing demand to establish additional capabilities for high flux / high energy spectroscopy measurements at ANKA, complementing the existing, strongly overbooked XAS, SUL-X and INE endstations. The new beamline comprises two alternately operated experimental stations at a superconducting high-field wiggler in a 1m straight ANKA ring section next to the INE-Beamline. The CAT-ACT wiggler source provides a spectrum extending into the hard X-ray regime beyond 50 keV, thus giving access to actinide L1- and lanthanide K-edges to Gd. At the same time, the photon flux increase by about two orders of magnitude compared to ANKA bending magnet Radiation at, e.g., the Pu L3-edge will improve HRXES studies requiring high photon flux, as well as increase the spectroscopic sensitivity for dilute sample systems, e.g., for far field studies or radionuclides in environmental samples. The CAT-ACT beamline at ANKA is scheduled to start regular user operation at the beginning of 2016. [1] J. Rothe, S. Butorin, K. Dardenne, M. A. Denecke, B. Kienzler, M. Löble, V. Metz, A. Seibert, M. Steppert, T. Vitova, C. Walther, H. Geckeis (2012). The INE-Beamline for actinide science at ANKA. Rev. Sci. Instrum., 83, 043105 [2] www.anka.kit.edu [3] www.actinet-i3.eu

Speaker: Dr Joerg Rothe (Karlsruhe Institute of Technology - Institute for Nuclear Waste Dispoasal)

14:20 MicroXAS beamline project: The radioactive microprobe facility at the Swiss Light Source

Speaker: Dr Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source)

14:40 → 14:50 Concluding Remarks

Conveners: Dr Daniel Grolimund (Paul Scherrer Institute, Swiss Light Source), Dr Rainer Dähn (Paul Scherrer Institut)

14:50 → 15:10 Coffee