MUÑOZ Maëva: Synthesis and characterisation of actinide polynuclear complexes

Thesis summary: The speciation of actinides is an essential fundamental data to take into account for better understand the behavior of these radioelements in the condensed phase. Recently, it has been accepted that the competition between hydrolysis and complexation can lead to the formation of polynuclear species (or clusters) stabilized at the surface by ligands present in the medium and in which the cations actinides are connected via oxo (O), hydroxo (OH), aquo (H2O), or even peroxo (O2) bridges. These species can appear for low acidities and therefore intervene in several stages of the fuel cycle nuclear or during geological storage. The stabilizing ligands of these species are often ligands with donor oxygen atoms such as formic acid, glycine or DOTA (1,4,7,10-acid tetraazacyclododecane-1,4,7,10-tetraacetic), although some clusters were stabilized by ligands inorganic chlorides or nitrates. Different stoichiometries are possible depending on the conditions ranging from compounds with 2 metal centers to extreme cases involving 38 actinide cations.

Although the existence of these species no longer needs to be demonstrated, they are however not taken into account in the main thermodynamic models of speciation and their mechanism of formation has never been formally studied. Moreover, recent studies suggest the possible involvement of these species in the mechanism of formation of colloidal phases composed of actinide oxide nanoparticles. The formation of colloids is an irreversible process which can lead to industrial problems during the hydro metallurgical cycles involved in the reprocessing of spent nuclear fuel. Thus, understanding the mechanisms of formation of polynuclear species as well as the study of their stability represent an important challenge which could make it possible to better anticipate their behavior on an industrial scale.

The objective of this thesis work, of a fundamental nature, consists in acquiring experimental data to improve (i) the description of polynuclear actinide species (molecular structures, chemical bonds) and (ii) the understanding of their formation mechanisms as well as their evolution in colloids. The methodology is based on a multi-technical experimental approach coupled with calculations of quantum chemistry. Polynuclear complexes with stabilized oxo, hydroxo or peroxo bridges by donor oxygen ligands (e.g. carboxylic acid, amino acid, etc.) will be synthesized and characterized by X-ray diffraction (single crystal and powder), ESI-MS mass spectrometry, and different spectroscopic tools (UV-vis, vibrational-IR-Raman, X-ray absorption)... The evolution of the behavior of these species along the actinide series (between thorium and plutonium) as well as the influence of the degree of oxidation of actinides will be studied to meet the various objectives set. Previous studies have shown the importance of the contribution of quantum chemistry for the interpretation of experimental data as well as for the description of the interaction between actinides and the different ligands present: oxo/hydroxo-type core ligands on the one hand and acid-type surface ligands carboxylic on the other hand. The coupling of experience and theory therefore appears essential.

PhD student: MUÑOZ Maëva - LinkedIn

PhD supervisers : GUILLAUMONT Dominique (CEA Marcoule) - VALLET Valérie

PhD co-supervisors : VIROT Matthieu - TAMAIN Christelle