Innovative Synthesis Methods of Mixed Actinides Compounds: Control of the Composition Homogeneity at a Molecular or Nanometric Scale

Abstract

Actinides contained in the used nuclear fuel need to be managed in the future fuel cycles for the sustainability of this source of energy. The major ones such as uranium or plutonium are very valuable for energy production within a new fuel. The minor ones such as neptunium, americium or curium are responsible for the long-term radiotoxicity of the ultimate waste if not separated and transmuted within new fuels or dedicated targets. Whatever the choice of management in the present or future, innovative synthesis methods are studied in many research institutions to elaborate new actinides based materials.

Innovative concepts for future fuels or transmutation targets focus on mixed actinides or mixed actinide-inert element materials. For their synthesis, wet methods fulfill very useful requirements such as flexibility, compatibility with a hydrometallurgical fuel processing, less dissemination of radioactive dusts during processing, and above all a better accessibility to very homogeneous compounds and interesting nanostructures. When dealing with plutonium or minor actinides, this last characteristic is of great importance in order to avoid the so-called “hot spots” and to limit macroscopic defects in the fuel material.

In this communication, experimental results are given to illustrate interesting achievements to control the composition or the structure of mixed actinides compounds at a molecular or at a nanometric scale using co-precipitating techniques or sol-gel methods.

The first illustration describes the flexibility of the oxalate ligand to modulate the nanostructure of actinides-based solid precursors and obtain mixed actinides oxide following a thermal treatment of the oxalate precursor. New mixed oxalate structures which present original features such as accepting in the same crystallographic site either a tetravalent actinide or a trivalent one are noticeably detailed. Monocharged cations equilibrate the charge in the 3D structure depending on the molar ratio of trivalent to tetravalent actinides. These oxalate compounds are particularly suitable precursors of oxide solid solutions for various actinides systems.

The second illustration deals with the control of inorganic condensation reactions of tri- and tetravalent cations in solution by using suitable ligands with a view to obtaining homogeneous oxy-hydroxyde mixtures. The results obtained using Zr(IV), Y(III) and Am(III) or Nd(III) are quite original: a very stable colloidal sol is obtained at pH 5-6 and a nanostructured mixed oxy-hydroxide phase is formed by adapting the sol-gel transition conditions. The initial interactions between the oxy-hydroxide Zr nanoparticles, the ligand and the trivalent cations at a nanometric scale in the sol give access, after gel formation and thermal treatment, to a crystallized phase (Am-bearing cubic Y-stabilized Zirconia) at comparatively low temperatures.

In both cases, the simultaneous co-precipitation or co-gelation of the involved actinides remains a challenge because of the specific properties of each actinide, properties which moreover differ according to various possible oxidation states.