Sonohydrothermal conversion of uranium(VI) compounds into UO2+x

The hydrothermal conversion of actinide carboxylates is a promising route for the synthesis of oxide powders intended for the manufacture of nuclear fuels. This method relies on the precipitation of a precursor at room temperature, followed by its conversion to an oxide under so-called “mild” hydrothermal conditions (T = 250°C, P < 100 bars). Simple An(IV)- oxalate systems and, more recently, U(VI)-oxalic acid mixtures have been studied for about a decade, particularly at the ICSM, and have shown that hydrothermal conversion allows for the quantitative precipitation of actinides from solution and yields a nanostructured powder suitable for densification. Nevertheless, the processes involved are generally characterized by slow kinetics and produce powders that are not well-suited for shaping and sintering.

The use of high-power ultrasound for the synthesis of materials is a rapidly growing field that has the potential to meet these requirements, particularly for hydrothermal syntheses—which are known to be time-consuming and energy-intensive but remain indispensable even on an industrial scale. The combination of ultrasound and hydrothermal conditions is a particularly innovative technique that we have been developing at the ICSM for the synthesis of catalytic materials for several years. In fact, our recent studies have shown that the synthesis of LTA-type zeolites is accelerated by an order of magnitude compared to conventional syntheses under silent hydrothermal conditions, while simultaneously reducing the particle size and polydispersity of the material.

This thesis topic aims to study the effect of sonohydrothermal treatment on the reductive conversion of uranium(VI) compounds into oxides, focusing on the mechanisms involved, their kinetics, and the characteristics of the powders obtained using various analytical techniques. It will also draw on the study of the mechanisms and kinetics of the phenomena observed under the innovative conditions employed—which generate both chemical and physical effects—in order to refine our understanding of the processes leading to the precipitation of the final oxide and to enable their future application to other systems of interest.