Study of the mechanisms of production by hydrothermal process and of alteration in various media of reference actinide oxide particles

I.A.E.A. (International Atomic Energy Agency) ensures the respect of the Nuclear Non-Proliferation Treaty by inspecting the nuclear facilities of signatory countries. The smears collected during these inspections are then sent to the Agency's support laboratories (including that of CEA/DAM) for isotopic analyses aiming to assess the nature of the facility's activities (civilian or military). However, the very small number of particles present on the smears (a few tens) and their size, generally less than one micrometer, represent an analytical challenge for the support laboratories. It is then essential to have reference particles whose chemical nature, isotopy, size and morphology are controlled, and which can be used for inter-comparison tests and for the development of the analytical techniques used, in particular LG-SIMS (Large Geometry Secondary Ion Mass Spectrometry). The work undertaken in the course of this thesis enabled us to consolidate the synthesis protocol for spherical uranium oxide particles of controlled size, based on the literature. Batch of particles with diameters ranging from 320 ± 20 nm to 2450 ± 220 nm were prepared. Several improvements were made to the initial protocol to adapt it to ²³⁵U-enriched isotopies. The synthesis was thus scaled down to reduce the amount of uranium used by a factor of 4. Also, different routes for using hexavalent uranium starting reagents were investigated, in particular the reduction of U(VI) by Ti(III) in the form of TiCl₃. Unfortunately, this method was not found to be robust and needs to be improved. In addition, uranium-thorium mixed oxides microparticles were produced. SEM analysis confirmed the conservation of particle size and spherical morphology. XRD analysis demonstrated that heat treatment in a reducing atmosphere fixed the particles in the (U_1-xTh_x)O_2+δ form. Particles treated under an oxidizing atmosphere were in the U₃O₈:Th form up to a content of 2 mol%, and underwent demixing to form (U₃O₈ + AnO₂) beyond. LG-SIMS analyses also showed a small dispersion in the ²³⁸U/²³²Th molar ratio between the individual particles of one batch. Finally, to envisage the particles storage as suspensions, UO₂ and U₃O₈ particles alteration experiments were carried out in 10^-2 M HNO₃, water and ethanol. Experiments in nitric media showed greater alteration for UO₂ particles than for U₃O₈ particles, with complete disappearance of the spherical morphology. Moreover, the dissolution kinetics of the particles are similar to the dissolution of bulk material. In aqueous media, UO₂ and U₃O₈ particles showed a morphology transformation linked to the formation of schoepite as a secondary phase. In the case of ethanol-altered oxide particles, very little alteration was observed after more than 100 days, particularly for U₃O₈ particles, with preservation of structure, spherical morphology and ²³⁵U/²³⁸U isotopy. In conclusion, a storage in ethanol in the U₃O₈ form and in a dry atmosphere to avoid the formation of schoepite on the surface could be recommended over the long-term. (author)