Nouveaux composés d'actinide: des précurseurs au matériau modèle – COMP-ACT

The chemistry of the early actinide elements has attracted a great deal of interest these last two decades with the assessment of nuclear waste disposal and improvement of reprocessing. This interest should increase with the various prospects within the framework 'forum Generation IV' relative to the use of new generation reactors. Besides their practical interest, actinides are also important from a theoretical point of view. Indeed, the 6d and 5f shells participation to the chemical bond confers to them a unique chemistry and crystal chemistry. Three French teams very active in this field and exchanging bilateral collaborations since several years are gathered by the proposed project. The complementarity of these teams should allow the extension of these collaborations and induce a real synergy affirming the role of this group at the national and international levels. This ambitious and innovating project mainly focuses on: i) synthesis using various preparative routes of new actinides bearing inorganic-organic hybrid compounds, precursors of simple or mixed oxides, carbides (or nitrides); ii) their characterization using numerous physico-chemical techniques including structure determinations by X-ray diffraction; iii) their thermal decomposition in various atmospheres in order to understand the global transformations leading to the final products; iv) study of the physico-chemical behaviour of the materials (such as sintering capability, chemical behaviour during dissolution processes (understanding and quantification of the processes concerning dissolution/alteration and associated release/retention/precipitation of radionuclides). This project does not focus on specific steps of the nuclear fuel cycle but is mainly devoted to the elaboration and characterization of new materials prepared through innovative chemical processes. This project is divided into two work packages according to the chemical family of expected materials. WP1: Oxides. This part is supported by the experience acquired by the three teams on the actinides oxalates. Using other organic di-acids such as squaric acids or di-, tri-, tetra-carboxylic acids as linkers, metal-organic porous framework are expected. By adapting the quantity of counter ions (which can also play the role of template) in the liberated spaces, An(III)/An(IV) mixed compounds for example are expected. The microstructure of the mixed oxides obtained by thermal decomposition, and throughout the sintering and dissolution properties, will be influenced by the morphology of the precursors, that depends on various factors such as composition, crystal structure, synthesis route... This very long-term project could find some applications in the management of actinides in the field of future spent fuel cycles. WP2: Carbides (or nitrides). This study is completely innovative. As part of Gen IV program, the use of mixed carbides (nitrides are less envisaged) is considered for its superior thermal properties and greater metal atom density relative to the oxide ones. The reference route to make actinide carbides is the carbothermic reduction of oxide mixtures at high temperature. The major innovation of this project is to prepare carbide compounds at lower temperature. In parallel to the oxides WP1, we plane to prepare actinide carbides by thermal decomposition of carbon enriched (and oxygen depleted) precursors. N-donor ligands will be used, thus nitrides formation can not be excluded in some cases. For the two work packages the same approach will be adopted. In a first step, precursors will be synthesized, through a large choice of chelate molecules and of wet chemistry methods, using lanthanides and uranium or thorium. The second part will be dedicated to the preparation of ceramics through the description of the sintering mechanisms occurring during the densification step. Finally, the chemical behaviour will be examined through the description of the dissolution/alteration process, especially by pointing out the correlation between the microstructure of the material, on the one hand, and the behaviour of the ceramic in use, on the second hand. After realization of all this study, the most interesting compounds will be studied using plutonium and minor actinides in the ATALANTE facility (partner 3).