Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids

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 later 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 LIII-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 hydrolysed Pu(IV) moieties at the surface shell.JRC.G.I.3-Nuclear Fuel Safet

Probing the Local Structure of Nanoscaled Actinide Oxides: A Comparison between PuO2 and ThO2 Nanoparticles Rules out PuO2+x Hypothesis

International audienceActinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO2 and ThO2 nanoparticles displayed as dispersed colloids, nanopowders or nanostructured oxide powders, allow to establish a clear relationship between the size of the nanocrystals composing these oxides and their corresponding An(IV) local structure investigated by EXAFS spectroscopy. Particularly, the probed An(IV) first oxygen shell evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of PuO2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO2+x species

Size vs. Local Structure Relationship for ThO2 and PuO2 Nanoparticles

International audiencePlutonium oxide nanoparticles constitute a hot topic in modern actinide science due to their past release in the environment (e.g. nuclear tests or accident) but also to their potential contribution in industrial processes (e.g. high burn-up structures, fuel synthesis).[1-3] Nanoparticles and related nanomaterials often exhibit interesting or unexpected properties when shrinking in size due to their increasing surface contribution (S/V ratio). Nevertheless, few results have been described in the literature for actinide nanoparticles. A better description of the speciation and formation mechanisms for these nanoscale objects represent a challenging topic that could contribute in the understanding and predictive modelling of their behaviour and reactivity, particularly useful for environmental purposes.[4-6] This presentation describes the synthesis and relevant synchrotron characterization of PuO2 and ThO2 nanoparticles exhibiting different sizes that were prepared using various approaches. A thorough investigation of these nanomaterials with XRD, HR-TEM and EXAFS spectroscopy evidenced strong similarities for both actinide oxides at the nanoscale. A strong correlation between the size of the nanoparticles and their local structure has been established thus eliminating the potential contribution of other oxidation states in the crystalline structure and evidencing a size-dependent local structural disorder driven by a nanoparticle surface effect