Abstract
Actinides, and mainly plutonium, are the main contributors to the long-term radiotoxicity of spent nuclear fuel. In conditions representative for geological repository, the interactions between such radioelements and silicate species could influence the mobility of the actinides in the environment and thus, could affect the safety of the storage facilities. Especially, the formation of actinide silicate, AnSiO4, has to be considered carefully, since thorium and uranium silicate are ubiquitous in environmental silicate rich media and under reductive conditions. Because the chemistry of AnSiO4 is poorly known, their preparation is a prerequisite in order to determine thermodynamic data associated to their formation and to evaluate their stability under geological repository conditions.The first part of this work consisted in the preparation, under hydrothermal conditions, of ThSiO4, USiO4 and CeSiO4 as surrogates of PuSiO4. This study allowed us to determine the preponderant parameters which impact the formation of silicate phases, to establish optimized conditions for their synthesis as pure phases and then to propose potential mechanisms of formation. For all of these syntheses, the competition between the complexation of metal cations by silicate ions and their hydrolysis played a predominant role. Three different and efficient strategies have been identified for the formation of actinide silicates. The first one was characterized by the use of weakly acid reactive media to limit the hydrolysis of actinide in solution. The second involved the use of strong ligands to maintain the actinide in solution in weakly basic media. The third was developed with element oxidation states that also protected it against hydrolysis. The transposition of these strategies to the preparation of PuSiO4 allowed to determine the suitable synthesis conditions to form PuSiO4 and then to identify the differences in terms of reactivity between Pu(IV) and surrogate elements.