The nuclear industry faces important technical challenges for HLW storage and designing new nuclear fuels. Such materials have to perform under severe conditions and accommodate defects safely. Few materials are suitable for those purposes but certain types of ceramics are promising candidates; among them oxide compounds such as pyrochlores and zirconolites. The aim of this PhD project was to synthesise novel actinide oxide particles, in which two different approaches were taken: controlled hydrolysis and solid state chemistry. Some minerals were naturally doped with actinides and rare earth elements and inspired synthetic formulations of new targeted waste forms for High Level Waste (HLW) instead of other disposal routes. The choice of synthetic pyrochlore and zirconolites as actinide hosts was inspired by the existence of naturally occurring actinide rich minerals. Substitutions within the cubic Y2Ti2O7 were investigated via the respective Y2Ti2-2xFexNbxO7 and Y2-xCexTixFexO7 solid solutions.Controlled hydrolysis was based on previous work with Lewis acidic transition metal cations. It was shown that it was possible to control the hydrolysis products in the presence of chelating organic ligands by carefully controlling the experimental conditions of the system. The same principles should apply to the chemistry of the actinide ions as they too are Lewis acidic. Such chemistry has implications for understanding the behaviour of actinides in nuclear processes within aqueous and organic systems. This project was a collaboration between Sheffield University (Immobilisation Science Laboratory, ISL) and The University of Manchester (Centre for Radiochemistry Research, CRR) through the DIAMOND (Decommissioning Immobilisation and Management of Nuclear waste for Disposal) University research consortium.

La zirconolite est une des matrices envisagées pour le confinement des actinides mineurs dans le cadre du stockage des déchets en site géologique profond. Cependant, dans ces conditions, après plusieurs milliers d'années, l'eau peut arriver au contact du colis de déchets et être radiolysée lors de la désintégration des radionucléides. Au cours de cette thèse, nous avons donc étudié les effets de la radiolyse de l'eau, induite par des particules chargées (alphas ou protons), selon deux géométries, sur la livixation d'une zirconoloite synthétique, de formule Ca0,8Nd0,2ZrTi1,8Al0,2O7, où Nd simule la présence des actinides mineurs. Les irradiations externes ont été réalisées sur des zirconolites saines et amorphisées, dans l'eau pure ou en présence d'ions complexants. La dissolution de l'échantillon a été évaluée via le relâchement des cations constitutifs. La production de H2O2 radiolytique a également été mesurée. Le comportement du matériau a été évalué par calculs ab initio.