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The molecular characterization based on multi-technique approach has led to major highlights on revealing the coordination environment of americium (Am) surrounded by citric acid (H3CitH). The structure of the different complexes at pH 1 and 3 are described. These characterizations are made possible by the comparison of the americium-citric acid system with the americium-tricarballylic acid (one analogue of the citric acid without the alpha-hydroxo group). The structural analyses (Vis spectrophotometry, NMR, EXAFS, TRLFS and capillary electrophoresis) were carried out after the establishment of the speciation distribution diagrams so that the complex percentages in solution are known, allowing to take into account the species repartition for structural analysis data treatment. With this combination of means, it was proved for the 1:1 complex that the hydroxo group is counter intuitively deprotonated and coordinated to the Am(III) at pH 1 as well as two carboxylate functions, whereas at pH 3 the hydroxo is not coordinated and stays protonated allowing the three carboxylate functions to coordinate the metallic cation. Therefore, the hydroxo group affects the Am complexation differently depending on the pHs: the complexation is favored by inductive effect at pH 3, and by direct coordination at pH 1.

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International audience; In the present work, we have combined spectroscopic and computational methods to elucidate the structures of plutonium(IV) ions with dialkyl monoamide and tetraalkyl-carbamide ligands in the solid state and in solution.

International audience; L’étude de la spéciation des actinides en présence d’acides alpha-hydroxycarboxylate est cruciale aussi bien pour le retraitement du combustible que pour le stockage des déchets nucléaires : d’une part, cette famille de molécule est envisagée dans les procédés de séparation des actinides mineurs en tant qu’agent complexant ou agent tampon en phase aqueuse (notamment pour la séparation de l’américium seul), et d’autre part, elle constitue une partie importante des produits de dégradation de la cellulose. Dans ce double contexte, le comportement et la complexation de l’américium avec l’acide citrique, un des représentants de cette famille de ligand, a été étudié. Certaines études se sont déjà intéressées au système Am(III)-acide citrique, mais ces dernières sont bien souvent incomplètes du fait de la complexité du système, avec une coordination possible par trois fonctions acide carboxyliques et une fonction hydroxyl. Les études thermodynamiques, d’une part, ont permis uniquement de déterminer la stœchiométrie des complexes sans qu’aucune information sur le mode de coordination ne soit apportée (nombre de groupements carboxylate complexant, état de protonation du ligand, présence ou non de la fonction hydroxo dans la sphère de coordination…). D’autre part, les rares études structurales ont concerné des espèces polynucléaires1 et polymériques2-5, différentes des complexes obtenus en phase aqueuse lors des études thermodynamiques précédentes.Dans la présente étude, les données thermodynamiques ont été associées aux données structurales acquises dans les mêmes conditions chimiques afin de parfaitement décrire la spéciation du système Am-acide citrique. Une attention particulière a été portée sur le rôle de la fonction hydroxo : coordonne-t-elle ? Est-elle protonée ? Pour mieux comprendre son rôle, le système Am-citrate a été comparé au système Am-tricarballylate (analogue de l’acide citrique sans fonction hydroxyde). Les constantes de complexation ont donc été déterminées et les complexes ont été identifiés et caractérisés par spectroscopies UV-visible, d’absorption X), RMN, SLRT et par électrophorèse capillaire.Références[1]. Fedosseev, A.M.; Grigoriev, M.S.; Budantseva, N.A.; Guillaumont, D.; Le Naour, C.; Simoni, E.; Den Auwer, C.; Moisy, P.; Comptes Rendus Chimie, 2010, 13, 839-848.[2]. Baggio, R.; Perec, M. Inorg. Chem. 2004, 43, 6965.[3]. Yuan, Y.-Q.; Xu, Y. Q.; Wu, M.-Y.; Hong, M. C. Acta Crystallogr. 2005, E61, m108.[4]. Liu, S.-G.; Zuo, W. J.-L.; Li, Y.-Z.; You, X.-Z. Inorg. Chem. Commun. 2005, 8, 328.[5]. Baggio, R.; Calvo, R.; Garland, M. T.; Pena, O.; Perec,M. Rizzi, A.; Inorg. Chem., 2005, 44, 8979-8987.

International audience; Alpha-hydroxycarboxylic acids are important molecules in nuclear field. They can be used as complexing agent or buffer in aqueous phase in actinide selective separation processes. These molecules are also present in the environment as a degradation product of cellulose. Therefore the alpha hydroxyl-carboxylate ligands are likely to influence the speciation in the fuel cycle processes and in the migration mechanisms. In this double context, the behavior and complexation of americium with this type of ligand was studied, taking citric acid as a representative of this family. Some studies have already been published on actinide(III)-citric acid complexation, but most of them are incomplete because the citric acid is also one of the most complicated compounds of the family due to its three carboxylic acids able to be deprotonated and to coordinate the metallic cation. On one side, the thermodynamic studies provide only the stoichiometry of the different complexes without any information on the coordination spheres (the number of complexing carboxylates, the presence or not of the hydroxo function as a coordinating group and the ligand protonation state). On the other side, the rare structural studies relate to polynuclear or polymeric species , , , , unlike the complexes identified in solution in the previous thermodynamic studies. In this study, thermodynamics and structural data were assessed in solution for the same chemical conditions to finally describe the speciation of the Am(III)-citrate system. A focus was done on the role played by the hydroxyl group does it coordinate the actinide? Is it protonated? To better understand its role, the Am-citrate system was compared to the Am-tricarballylate system (analogue to citric acid without the hydroxyl group). The two systems were studied at pH = 1 and 3. The complexation constants have been determined and, different complexes were characterized using X-ray absorption, NMR, TRLFS and capillary electrophoresis.

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International audience; Alpha-hydroxycarboxylic acids are important molecules that can be used as complexing agent or buffer in aqueous phase in actinide selective separation processes. They are also present in the environment as a degradation product of cellulose. Therefore the alpha these molecules are likely to influence the speciation in the fuel cycle processes as well as in the migration mechanisms. In this double context, the behavior and complexation of americium with this type of ligand was studied, taking citric acid as a representative of this family. Actinide(III)-citric acid system was already examined, but most of the studies are incomplete because the citric acid is one of the most complicated compounds of the family due to its three carboxylic acids able to be deprotonated and to coordinate the metallic cation. On one side, the thermodynamic studies provide only complexation constants and the stoichiometry [ , ] of the different complexes without any information on the coordination spheres (the number of complexing carboxylate groups, the presence or not of the hydroxo function as a coordinating group and the ligand protonation state). On the other side, the rare structural studies relate to polynuclear [ ] or polymeric species [ , , , ] unlike the complexes identified in solution in the previous thermodynamic studies. In this study, thermodynamics and structural data were assessed in solution for the same chemical conditions to finally describe the speciation of the Am(III)-citrate system. A focus was done on the role played by the hydroxyl group does it coordinate the actinide? Is it protonated? To better understand its role, the Am-citrate system was compared to the Am-tricarballylate system (analogue to citric acid without the hydroxyl group). The complexation constants have been determined and the complexes were characterized using X-ray absorption, NMR, TRLFS and capillary electrophoresis.

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Three tetravalent actinide (An(IV)) hexanuclear clusters with the octahedral core [An6(OH)4O4]12+ and (An(IV) = U(IV), Np(IV), Pu(IV)) were structurally characterized in solid state and in aqueous solution using single crystal X-ray diffraction, X-ray absorption, IR, Raman and UV-Visible spectroscopy. The observed structure, [An6(OH)4O4(H2O)8(HDOTA)4].HNO3.nH2O (An = U (I), Np (II), Pu (III)), consists of a An(IV) hexanuclear pseudo- octahedral cluster stabilized by (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) (DOTA) ligands. The six actinide atoms are connected through alternative µ3-O2- and µ3-OH- groups. EXAFS investigations combined with UV-vis spectroscopy evidence the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physical chemical properties (pH range stability, solubility and protonation constant) of the cluster were determined. The results underline the importance (i) to consider such polynuclear species in thermodynamic models and (ii) importance of competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) leads to the formation of a dimer, Th2(H2O)10(H2DOTA)2.4NO3.xH2O (IV), which in contrast to the structure of the other An(IV) hexamers.

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A singular Pu(IV) hexanuclear cluster [Pu6(OH)4O4]12+ stabilized by DOTA ligands has been structurally characterized for the first time both in the solid state and in water solution using X-ray diffraction, Vis-NIR and X-ray absorption spectroscopies. The cluster solubility in water and its high stability in a relatively large pH range are of the upmost importance for plutonium environmental speciation.

International audience; Americium oxalate single crystals, Am2(C2O4)3(H2O)6·4H2O, were prepared by in situ oxalic acid generation by slow hydrolysis of the diester. Their structure was determined by single-crystal X-ray diffraction and was solved by the direct methods and Fourier difference techniques. The structure (space group P21/c, a = 11.184(4) Å, b = 9.489(4) Å, c = 10.234(4) Å, β = 114.308(8)°, Z = 2) consists of layers formed by six-membered rings of actinide metals connected through oxalate ions. The americium atoms are nine-coordinated by six oxygen atoms from three bidentate oxalate ligands and three water molecules. The distances within the coordination sphere as well as infrared and Raman spectra of several isostructural lanthanide (Ce(III), Pr(III), Nd(III), Sm(III), Gd(III)) and actinide (Pu(III), Am(III)) oxalates were compared to evaluate the similarities and the differences between the two series.

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National audience; Les techniques spectroscopiques basées sur les processus d’absorption des rayons X ont progressées au cours des dernières années au point de devenir des outils de choix dans le domaine de la radiochimie. Ainsi, les méthodes d’analyses telles que l’EXAFS (Extended X-ray Absorption Fine Structure) et le XANES (X-ray Absorption Near Edge Structure) s’appuyant sur le rayonnement synchrotron sont aujourd’hui fréquemment employées pour caractériser l’environnement des actinides à l’échelle atomique et leurs degrés d’oxydation. Ces techniques ont été employées afin de sonder la structure fine des complexes formés lors de l’extraction du Pu(IV), de l’U(VI) et du Tc(VII) par les N,N-dialkylamides candidats potentiels pour remplacer le Tributylphosphate (TBP) - actuellement utilisé dans le procédé PUREX. Les données expérimentales ont été couplées à des calculs de chimie quantique notamment pour déterminer l’influence des chaines alkyles portées par le groupe fonctionnel C=O sur les propriétés extractantes observées. S’appuyant sur un modèle cristallographique, les analyses EXAFS montrent que la coordination du Pu(IV) avec les amides à chaine linéaire et de l’U(VI) avec les amides à chaine linéaire et ramifiées est identique en solution et en phase solide (i.e. complexe de stoechiometrie An:L 1 :2). Pour l’amide à chaine ramifiée la structure de coordination du plutonium est plus difficile à établir sans ambiguïté. Des calculs de DFT ont été réalisés sur une série de structures de coordination potentielles et les paramètres de structure et facteurs de Debye-Waller dérivés ont été utilisés pour simuler les spectres EXAFS ab initio. La comparaison des spectres expérimentaux et théoriques a permis de déterminer la structure de coordination des complexes Pu-DEHiBA et ainsi de mettre en évidence la formation de complexes de sphère externe avec un ligand protoné. Une méthodologie similaire, qui consiste à interpréter des spectres EXAFS et FTIR sur la base de résultats de chimie quantique a également été employée pour identifier le complexe (DEHiBA)2(UO2)(NO3)(TcO4) impliqué dans la co-extraction du technétium et l’uranium par le DEHiBA

The neutron population in a nuclear reactor is subject to fluctuations in time and in space due to the competition of diffusion by scattering, births by fission events, and deaths by absorptions. As such, fission chains provide a prototype model for the study of spatial clustering phenomena. In order for the reactor to be operated in stationary conditions at the critical point, the population of prompt neutrons instantaneously emitted at fission must be in equilibrium with the much smaller population of delayed neutrons, emitted after a Poissonian time by nuclear decay of the fissioned nuclei. In this work, we will show that the delayed neutrons, although representing a tiny fraction of the total number of neutrons in the reactor, have actually a key impact on the fluctuations, and their contribution is very effective in quenching the spatial clustering., 14 pages, 8 figures

International audience; Crystallographic studies play a key role in coordination chemistry. It provides information on the coordination mode, the geometry of the complex, the interaction strength between the ligand and the cation. As such, our group studies the crystallization of different actinides with monoamides considered as good models for extraction of U(VI) and Pu(IV) candidates [1,2]. Previous researches reveal that the monoamide structure has a huge impact on the uranium(VI)-plutonium(IV) selectivity and the extraction properties [3]. The single crystal syntheses and structural characterizations of actinides (U(VI), Pu(IV)) or lanthanides (Ce(IV) surrogates of Pu(IV)) complexes with four different monoamides are reported herein. To the best of our knowledge it is the first structures of tetravalent cation (Ce(IV) or Pu(IV)) with monoamide ligand. To favor crystallization, the studied monoamides are short chain ligands considered as surrogates of the real extractant molecules. The comparison of the complexes allows analyzing the impact of the nitrogen atom dissymmetry and the ramification of the carbonyl chain. For all the synthesized complexes, the structure is solved by single-crystal X-ray diffraction and the compounds are fully characterized by IR spectroscopy and EXAFS as a solid, or dissolved in an organic phase. The comparison of the results between the different solid complexes as well as the comparison between solid and liquid phases (structures, IR frequencies, and distances form EXAFS and XRD) give some hints to better understand the Actinide-monoamide systems in extraction processes.

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The oxalic conversion of plutonium into oxide is one of the most important steps of the present fuel cycle dealing with the recycling of plutonium contained in the used fuel. This operation, which occurs at the end of the PUREX process, includes two steps, oxalic precipitation followed by calcination in air, and permits to elaborate one of the main starting materials for the MOX fuel fabrication. Coordination chemistry of plutonium with oxalic acid is rich and several plutonium oxalate hydrates, the quite well-known PuIV(C2O4)2.6H2O1 and PuIII2(C2O4)3.10H2O2 and the other novel structures described here, can lead to PuO2. Considering the morphology of the plutonium oxalate particles, it is interesting to notice that there is a strong correlation between the shape of these particles and the crystal structure of the plutonium oxalate.