Abstract
Due to their interesting properties, several phosphate-based ceramics have been considered as potential matrices for the long-term conditioning of radionuclides, including actinides, in deep geological storage. Among these ceramics, phases derived from monazite (Ln,AnIII)PO4 have been widely studied due to their high structural flexibility and remarkable chemical durability. However, although the direct incorporation of An(III) has already been reported through wet and dry synthesis methods, the incorporation of An(IV) into ceramics has mainly been achieved via dry synthesis methods, due to issues related to the precipitation of initial precursors via wet methods. Nevertheless, wet methods can offer significant improvements in terms of chemical homogeneity and sinterability, which can enhance the chemical durability of materials during leaching tests.This work focused on the development of synthetic monazites doped with tetravalent actinides (An = U, Th) bycoupled substitution to prevent the formation of cationic vacancies. This mechanism involves the substitution of two Ln(III) cations by one An(IV) cation and one M(II) cation. The incorporation of An(IV) into the monazite structure was first carried out using rhabdophane as the initial precursor of Nd1-2xCaxAnxPO4 (x≤0.1). Hydrothermal conditions were optimized by adjusting the Ca/An stoichiometric ratio to prepare single-phase samples. The thermal conversion of rhabdophane into monazite-cheralite was studied under inert (argon) and oxidizing (air) atmospheres, followed by direct sintering under air at different temperatures and durations of isothermal holding. This one-step approach enabled the direct preparation of dense monazite-cheralite pellets. Sintering maps were developed to control the final microstructure of the ceramics. Finally, dissolution experiments were conducted at different temperatures and leaching media to obtain multiparametric dissolutionrates. The different experiments revealed a chemical durability as high as that obtained for undoped monazites.Finally, since thorium-containing phases are known to be refractory to dissolution, solid solutions of monazite-cheralite containing both uranium and thorium were manufactured. Similar to compounds containing only uranium, synthesis, sintering, and long-term behavior were examined. This research thus optimized the synthesis process of wetmethod-incorporated tetravalent actinide monazite-cheralites and deepened understanding of their long-term behavior.