Abstract
The chemistry of actinides (An) is extremely rich and complex, and generally well referenced in the literature. However, there is only few data on actinide colloids, in particular on those resulting from the hydrolysis properties of An cations (intrinsic colloids) and on the related formation mechanisms. The physico-chemical properties of these species in relation with their structure (size, morphology, local structure, etc.) are of fundamental interest, as well as technological purpose for the preparation of advanced nuclear fuel. Particular attention is also given to the observation of colloidal An nanoparticles in the environment and their potential role in the migration of radioactivity. The aim of this work is to improve the knowledge of intrinsic uranium and plutonium colloids, in particular by providing a better understanding of their formation mechanisms and their structural properties. This information could provide a better fundamental knowledge of these species and contribute to a better prediction of their behaviour in environmental or industrial conditions but also to a possible control their formation in the context of potential industrial applications. In a first step, the formation mechanisms of intrinsic Pu(IV) colloids were studied, by carrying out kinetic studies in different media (H2O, D2O or electrolytes). A kinetic isotopic effect was observed and attributed to the difference in zero-point energy of the OH and OD bonds. This work also allowed us to observe, for the first time, the participation of a reaction intermediate (oxo-hydroxo cluster of Pu(IV)) in the formation mechanism of intrinsic Pu(IV) colloids. On the other hand, colloidal nanoparticles of uranium (IV) and uranium (VI) were prepared under ultrasound irradiation. This work showed that sonochemistry offers an alternative to existing preparation methods. The uranium and plutonium colloidal species were characterised using state-of-the-art techniques (synchrotron and laboratory techniques including SAXS, XAS, TEM, etc.) in order to propose a formation mechanism and provide information on their structures and morphologies.