Oxalate precipitation is a powerful technique for actinide oxide preparation at either laboratory or industrial scales. In this study we focused on the low temperature decomposition of Th(C2O4)2•2H2O and (N2H5)2U2(C2O4)5•nH2O into nanograined ThO2 and UO2 powders, which will be used later as precursors for the generation of materials emulating the nuclear fuel high burnup structure (HBS). The evolution with temperature of the nanoparticles properties has been investigated using several solid state analytical techniques (transmission and scanning electron microscopy, room and high temperature powder X-ray diffraction, Raman spectroscopy, thermogravimetry). Oxide powders with a high degree of crystallinity and grain size from ~10 nm were prepared. Their purity was examined with special focus on the presence of carbonates. It was found that carbonate content in the final powder did not exceed 3.4 wt.%, for the powder treated at 600°C for 1 hour.JRC.E.4-Nuclear Fuel Safet

Production of actinide oxide powder via dry thermal decomposition of corresponding oxalates is currently carried out on the industrial scale at temperatures exceeding 500 °C. Although it is simple, this method presents some disadvantages such as high decomposition temperature with a direct effect on the surface area, pre-organised morphology of the nanoparticles affecting the sintering behaviour, etc. We have recently proposed the decomposition of An^IV-oxalates under hot compressed water conditions as a straightforward way to produce reactive actinide oxide nanocrystals. This method could be easily applied at low temperatures (95–250 °C) in order to generate highly crystalline nano-AnO₂. We present here the formation conditions of AnO₂ (An = Th, U, Np, and Pu) and some associated solid solutions, their stability, and grain growth during thermal treatment. The involvement of water molecules in the mechanism of the oxalate decomposition under the hot compressed water conditions has been demonstrated by an isotopic exchange reaction during the thermal treatment of the hydrated oxalate in H₂[¹⁷O] through MAS-NMR and Raman techniques.

The hydrothermal decomposition of actinide(IV) oxalates (An= Th, U, Pu) at temperatures between 95 and 250 °C is shown to lead to the production of highly crystalline, reactive actinide oxide nanocrystals (NCs). This aqueous process proved to be quantitative, reproducible and fast (depending on temperature). The NCs obtained were characterised by X-ray diffraction and TEM showing their size to be smaller than 15 nm. Attempts to extend this general approach towards transition metal or lanthanide oxalates failed in the 95–250 °C temperature range. The hydrothermal decomposition of actinide oxalates is therefore a clean, flexible and powerful approach towards NCs of AnO₂ with possible scale-up potential.

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 PuO₂ 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 PuO₂ 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 PuO₂ 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 PuO₂ nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO₂ nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO₂ cores and hydrolyzed Pu(IV) moieties at the surface shell.

XRD, EXAFS and HR-TEM study reveals a strong correlation between AnO ₂ (An = Th and Pu) nanoparticle size and An( iv ) local structure.