The development of new technologies of nuclear reactors implies to consider innovative methods for manufacturing (U,Pu)O2 MOx fuels. In this context, recent works focused on the hydrothermal conversion of tetravalent actinide carboxylates, in particular oxalates. This process enables hydrated actinide oxides to be obtained directly by using "mild" conditions. The characteristics of the powders obtained can also be controlled by adjusting the experimental conditions. However, no study involving the uranyl cation UO22+ has been reported, although several studies have highlighted the reduction of U(VI) by organic matter in geological environments.
The aim of this PhD thesis is therefore to address the direct precipitation, under reductive hydrothermal conditions, of UO2+x and associated solid solutions from uranyl-based solutions. The study of simple systems containing only U(VI) will first be undertaken by considering different sources of organic matter. A multiparametric study will specify the experimental conditions for the reduction of U(VI) to U(IV) and the quantitative formation of UO2+x, while the reduction mechanism will be studied using in situ XANES analyses on ESRF FAME and FAME-UHD beamlines. The second part of the work will concern the study of mixed systems initially containing uranium (VI) and a tetravalent cation. The U(VI)-Th and U(VI)-Ce(IV) systems will be studied as a first approach, in order to progressively increase the complexity of the redox behaviour of the samples. Finally, the study will be transposed to the U(VI)-Pu(IV) system at CEA Marcoule's ATALANTE facility, in collaboration with the DES/DMRC/SPTC/LSEM. Regardless of the chemical system studied, a complete physico-chemical characterisation of the solids obtained will be undertaken. The sintering of the powders prepared will also be studied.
The aim of this thesis work is therefore to propose an alternative route for manufacturing/remanufacturing future-generation nuclear fuels, by offering the original possibility of reducing uranium(VI) in situ in the reactor, which constitutes a direct route from ions in solution to the final solid. The successful candidate will have a master's or engineering degree in radiochemistry, separative chemistry or materials chemistry. In the course of his/her work, he/she will be required to master numerous techniques relating to materials chemistry, microscopy and solution chemistry, which will enable him/her to develop skills not only in the nuclear field, but also more broadly in the field of ceramic materials development.