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Thorium carbide to be tested as target material for the production of 225 Ac with the ISOL method, was produced via carbothermal reduction of ThO 2 nanoparticles by graphite and graphene oxide, respectively. The use of graphene oxide (GO) as carbon source resulted in a reduced reactivity compared to graphite, confirmed by the presence of unreacted ThO 2 mainly in the core of the samples. The reacted ThO 2 or ThC 2 -GO showed a faster reactivity in air, mainly observed as ThC 2 amorphization. The specific surface area of the ThC 2 -GO samples was almost doubled compared to ThC 2 -graphite samples. The effect of these microstructural features was analysed in terms of thermal diffusivity and calculated thermal conductivity that were both reduced in ThC 2 -GO samples, however the difference with ThC 2 -graphite samples decreased at increasing temperature. The present study shows that the use of unreduced GO inhibits the solid-state reaction between ThO 2 and C; on the other hand, the high reactivity of the ThC 2 so produced is expected to be beneficial for the 225 Ac production with the ISOL method, affording a high release efficiency. It is expected that the use of reduced GO could represent a good solution for highly efficient ThC 2 targets.

Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO 2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO 2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO 2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu L III -edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO 2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO 2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO 2 cores and hydrolyzed Pu(IV) moieties at the surface shell.