Etude thermodynamique de la complexation des ions actinide(III) et lanthanide(III) par des ligands polyazotés en milieu homogène

Les recherches menées au cours de cette thèse s'inscrivent dans une démarche de compréhension des mécanismes intervenant lors de la complexation des ions f trivalents en milieu homogène par un ligand polyazoté tridente, l'Adptz. L 'approche thermodynamique des interactions a été utilisée afin d'évaluer, d'un point de vue énergétique, l'influence des diverses contributions sur la réaction et d'acquérir une base de données thermodynamiques la plus complète possible sur cette réaction. Dans un premier temps, l'influence de la nature du cation sur les valeurs des grandeurs thermodynamiques a été considérée. Les paramètres thermodynamiques (enthalpie libre, enthalpie et entropie) des complexes 1/1 formés ont été déterminées par la méthode de van't Hoff et/ou par microcalorimétrie dans le solvant mixte méthanol/eau (en proportions volumiques 75/25%) pour tous les ions lanthanide(III), pour l'yttrium(III) et pour trois cations actinide(III) : le plutonium, l'américium et le curium. La comparaison des valeurs pour les deux familles d'éléments f met en évidence l'augmentation de la stabilité des complexes d'un rapport 20 en faveur des actinides. Ce gain de stabilité s'explique par une réaction plus exothermique, et a été associé, dans l'interprétation des données à un effet covalent plus marqué lors de la formation de la liaison actinide(III)-azote. L'influence du changement de composition du solvant d'étude sur la thermodynamique de complexation a ensuite été abordée dans le cas de la réaction entre l'europium(III) et l'Adptz. Lorsque la fraction de méthanol augmente dans le mélange, le complexe formé devient plus stable grâce à une augmentation du terme entropique, mettant en valeur l'importance des phénomènes de réorganisation du solvant au cours de la complexation sur le bilan énergétique de la réaction dans ces systèmes.The aim of the researches carried out during this thesis was to better understand the chemical mechanisms of the complexation of trivalent f-elements by Adptz, a tridentate N-donor ligand, in homogeneous conditions. A thermodynamic approach was retained in order to estimate, from an energetical point of view, the influence of the different contributions to the reaction, and to acquire a complete set of thermodynamic data on this reaction. First, the influence of the nature of the cation on the thermodynamics was considered. The thermodynamic parameters (free energy, enthalpy and entropy) of the 1/1 complexes were systematically determined by van't Hoff method and/or microcalorimetry in a mixed solvent methanol/water (in volumic proportions 75/25%) for every lanthanide(III) ion, yttrium(III) and for three actinide(III) cations: plutonium, americium and curium. The comparison of the values obtained between the two series of f-element highlights the increase of stability of the complexes by a factor of 20 in favour of the actinide cations. This gap is explained by a more exothermic reaction and is associated, in the data interpretation, to a higher covalency of the actinide(III)-nitrogen bond. Then, the influence of the change of solvent composition on the thermodynamic of complexation was studied for the reaction between europium(III) and Adptz. The stability of the complex formed increases with the percentage of methanol in the mixed solvent, owing to an increase of the entropic contribution. The solvent reorganization upon complexation seems to be the prominent process from the energetical point of view.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

New Insights into the Recovery of Strategic and Critical Metals by Solvent Extraction

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Optimization of new extractant molecules for U(VI)/Pu(IV) separation

International audienceThe standard industrial solvent extraction process PUREX involves tri-n-butyl phosphate (TBP) as extractant to separate uranium(VI) and plutonium(IV) from fission products. TBP extractant has been used for decades but has some limitations such as its non-incinerable nature and the formation of some troublesome degradation products by radiolysis. The partition between uranium and plutonium requires the reduction of Pu(IV) to Pu(III) by introduction of reducing and stabilizing agents such as uranium(IV) and hydrazinium nitrate. New monoamide extractants showing higher selectivity are studied in order to separate U(VI) from Pu(IV) in one single cycle without reducing plutonium. This new extraction system would allow the reprocessing of MOX fuels with a higher Pu content. Amongst potential candidates, unsymmetrical N-methyl-N-alkyl monoamides are so far the most promising molecules to achieve U(VI) and Pu(IV) separation without redox chemistry. The effect of structural modifications (alkyl chains length and ramifications) on extraction performances (U and Pu distribution ratios, U/Pu selectivity) and physico-chemical properties (viscosity and loading capacity) was evaluated. In particular, the length of alkyl chains (total number of carbon atoms on the molecule) was adjusted to give enough lipophilicity preventing from third phase formations, but in the same time maintain a reasonable viscosity (compatible with a good hydrodynamic behavior)

Séparation de l'américium : Développement d'un nouveau procédé d'extraction liquide-liquide

International audienceThe separation of minor actinides from spent nuclear fuels would allow tosignificantly reduce the long-term radiotoxicity and heat power of vitrified high activitywaste. One strategy would consider the recycling of americium alone, leaving curium in finalwaste with fission products. In this framework, a liquid-liquid extraction process calledEXAm (Extraction of Americium) was developed to separate americium alone from curium and other fission products (especially lanthanides). This separation is challenging consideringvery similar physico-chemical properties of americium and curium. In EXAm the selectivityis reached by introducing the TEDGA complexing agent in the high acidity feed solution ofthe process. Another process was also developed in the European SACSESS project usingTPAEN ligand as a selective americium stripping agent in low acidity solution.La séparation-transmutation des actinides mineurs des combustibles nucléaires irradiés permettrait de réduire significativement la radiotoxicité et la puissance thermique à long terme des déchets de haute activité. Une des stratégies considérée consisterait à recycler l'américium tout en laissant le curium avec les produits de fission vitrifiés. Dans ce contexte, le procédé d'extraction liquide-liquide EXAm a été développé pour séparer l'américium seul du curium et des produits de fission (dont les lanthanides). Cette séparation est difficile car ces deux éléments ont des propriétés physico-chimiques très similaires

New Bitopic Ligands for the Group Actinide Separation by Solvent Extraction

The synthesis and evaluation of solvent extraction performance of N,N,N',N'-tetraalkyl-6,6''-(2,2':6',2''-terpyridine)diamides and N,N'-diethyl-N,N'-diphenyl-6,6''-(2,2':6',2''-terpyridine)diamide are reported here. These new bitopic ligands were found to extract actinides in different oxidation states (U(VI), Np(V and VI), Pu(IV), Am(III), and Cm(III)) from 3 M nitric acid. The presence of three soft nitrogen donors led to the selective extraction of actinides(III) over lanthanides(III) (Ce, Eu) and the presence of two amide functional groups grafted to the terpyridine unit allowed the extraction to occur from a highly acidic medium by minimizing the basicity of the ligand. Ligands bearing long alkyl chains (C4 and C8) or phenyl groups showed increased performances in a polar diluent like nitrobenzene

Modelling of Americium Stripping in the EXAm Process

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Uranium (III) precipitation in molten chloride by wet argon sparging

International audienceIn the context of pyrochemical processes for nuclear fuel treatment, the precipitation of uranium (III) in molten salt LiCl-CaCl$_2$ (30–70 mo per cent) at 705 C is studied. First, this molten chloride is characterized with the determination of the water dissociation constant. With a value of 10$^{−4.0}$, the salt has oxoacid properties. Then, the uranium (III) precipitation using wet argon sparging is studied. The salt is prepared using UCl3 precursor. At the end of the precipitation, the salt is totally free of solubilized uranium. The main part is converted into UO2 powder but some uranium is lost during the process due to the volatility of uranium chloride. The main impurity of the resulting powder is calcium. The consequences of oxidative and reductive conditions on precipitation are studied. Finally, coprecipitation of uranium (III) and neodymium (III) is studied, showing a higher sensitivity of uranium (III) than neodymium (III) to precipitation

Uranium (III)-Plutonium (III) co-precipitation in molten chloride

International audienceCo-management of the actinides in an integrated closed fuel cycle by a pyrochemical process is studied at the laboratory scale in France in the CEA-ATALANTE facility. In this context the co-precipitation of U(III) and Pu(III) by wet argon sparging in LiCl-CaCl2 (30–70 mol%) molten salt at 705 °C is studied. Pu(III) is prepared in situ in the molten salt by carbochlorination of PuO2 and U(III) is then introduced as UCl3 after chlorine purge by argon to avoid any oxidation of uranium up to U(VI) by Cl2. The oxide conversion yield through wet argon sparging is quantitative. However, the preferential oxidation of U(III) in comparison to Pu(III) is responsible for a successive conversion of the two actinides, giving a mixture of UO2 and PuO2 oxides. Surprisingly, the conversion of sole Pu(III) in the same conditions leads to a mixture of PuO2 and PuOCl, characteristic of a partial oxidation of Pu(III) to Pu(IV). This is in contrast with coconversion of U(III)-Pu(III) mixtures but in agreement with the conversion of Ce(III)