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International audience; Nanomaterials have attracted considerable interest in recent interdisciplinary research for their technological applications related to the nanometric size of the building blocks composing the solids (ex crystallite or atomic and molecular groups).[1,2] Nanostructured materials can be defined as solid samples exhibiting a microstructure the characteristic length scale of which is roughly ranging between 1 and 10 nm.[1] The controlled microstructure of materials at the atomic level offers new physical and chemical properties in comparison to similar bulk materials already applied, for instance, in catalysis, synthesis of luminescent materials, preparation of cosmetic and solar creams, preparation of solar cells, etc.[3,4] Such paradigm has been attributed to the increased surface-to-volume ratio of the shrinking particle size which increase the number of surface and interface atoms generating stress, stain, and structural perturbations.[2] In actinide chemistry, the synthesis and characterization of nanomaterials is very scarce but is of growing interest due to the contribution of actinide nanomaterials in environment (ex migration of actinides) and industry (ex high burn up structures). Recently, we observed the nanostructuration of PuO2 and noticed its outstanding reactivity under ultrasound irradiation which stirred up our curiosity concerning the local environment of this oxide at the nanoscale.[5] In this work, we investigate the synthesis and relevant characterization of nanostructured PuO2 and ThO2. Th can be considered as a good surrogate for Pu because both elements exhibit close ionic radii, their oxide crystallize in the fluorite Fm-3m structure, and they both exist at the (+IV) oxidation state. More precisely, Th only exists at the +IV oxidation state thus avoiding misinterpretations related to the potential contribution of other oxidation states in the crystalline structure. The synthesis studies allowed us to select the nanostructuration conditions for the various oxides and a careful characterization and correlation with AFM, HR-TEM, Raman spectroscopy, XRD and XAS techniques allowed us to probe the local disorder for the various oxides as a function of the particle size. Particularly, EXAFS investigations clearly show a linear decrease of the coordination number for An-O and An-An spheres with the shrinking particle sizes. The crystalline nature of the particles (HR-TEM, XRD) suggest that these observation are correlated to the increasing surface contribution of these particles. These new results raise the question of the physico-chemical properties of oxide nanomaterials crystallizing in the fluorite structure which are materials of paramount importance for engineering applications such as nuclear energy, solid oxide fuel cells, catalysis, or sensors.

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International audience; Plutonium dioxide PuO2 is one of the main end-product in nuclear spent fuel reprocessing. This compound is classically stored in sealed containers under more or less moisty atmosphere before its reuse to manufacture new mixed-oxide (MOx) fuel. Plutonium dioxide is a very hygroscopic material and then the water molecules present in the moisty atmosphere are quickly adsorbed on plutonium dioxide surface [1-3]. This sorbed water then undergoes an a-radiolysis due to the plutonium radioactive decay. It has been showed that the decomposition of sorbed water on plutonium dioxide leads to the release of dihydrogen H2 [4-7]. Obviously, the kinetics of hydrogen evolution in the atmosphere depends on several physico-chemical parameters such as the specific surface area and isotopic composition of plutonium dioxide and the relative humidity of the atmosphere.However, the influence of the atmosphere above plutonium dioxide in the containers on dihydrogen generation has not been examined to a large extend. Most of the published data have been obtained under air or inert (Ar) atmosphere. Comparison of the data is not obvious and the effect of O2 and N2 on H2 formation is quite difficult to discern. In our experiments an increase in dihydrogen formation has sometimes been observed under an atmosphere of air compared to an Ar atmosphere, but the opposite effect, a decrease in hydrogen production, has happened too and unfortunately no clear conclusion can be drawn from these observations.Since carbon dioxide CO2 seems to be quickly and widely adsorbed on plutonium dioxide surface [8], it has then been decided to look at the effect of this gaseous compound on the kinetics of dihydrogen release in moisty atmospheres. The analysis has been extended to carbon monoxide and mixture of CO and CO2. The formation of dihydrogen as well as the consumption of carbon oxides has been followed using gas chromatography. The carbon oxides were introduced in the experiment cells at a level amount of 5-10 vol. percent as a mixture with Argon and the relative humidity is kept constant in the cells. It has been observed that the amount of produced dihydrogen slightly increases in the presence of CO2 in comparison of what have been measured under pure argon. In the same time, no measurable consumption of CO2 has been detected. Dihydrogen formation is significantly enhanced in the presence of CO, this latter being oxidized as CO2. The formation of H2 under an atmosphere containing both CO and CO2 is particular: in a first step, the formation of H2 is lowered and in a second step it looks like the one observed in the presence of CO only. Mechanisms have been postulated to account for these phenomena.

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International audience; Up to now, plutonium dioxide is the major end product of nuclear fuel reprocessing. Due to the radioactive properties of plutonium, handling and management both of plutonium dioxide and of contaminated wastes involve to take into account the radiolysis phenomena plutonium can induce. For instance, plutonium is known to be a highly hygroscopic compound. Kept under a humid atmosphere, up to 3% wt. water can be sorbed on the surface of plutonium dioxide. The sorbed water then is going to undergo a radiolytic decomposition and this latter will lead to hydrogen evolution. It has been shown through our studies and other reported results that hydrogen formation kinetic depends on the specific surface area of plutonium dioxide, the isotopic composition of plutonium and the relative humidity in the atmosphere. Unfortunately no clear dependences between the rate of hydrogen formation and these parameters can be established. However, it has been shown that in many cases hydrogen amount in the atmosphere reaches a steady-state. Such behaviour is probably linked to surface evolution of plutonium dioxide in presence of moist air and under radiolytic decomposition of water. Studies are still in progress to identify surface changes due to these phenomena. Hydrogen can be produced under radiolysis of some plastic waste contaminated by plutonium dioxide too. Whereas $\gamma$-radiolysis of plastic materials is quite well described, $\alpha$-radiolysis of such compounds, especially the ones in close contact with actinides, is very poorly known. It can be demonstrated that only a small part of the whole energy emitted by plutonium dioxide is going to be deposited in the material and leads to a radiolytic decomposition with hydrogen and others hazardous gaseous products (CH$_4$ for instance) formation. Taking into account the amount of energy transferred to the plastic material, $\alpha$-radiolytic yield of hydrogen can be calculated and then used for waste repository scale-up. It should be noticed that plutonium dioxide can exhibit specific behaviour towards some radiolytic products. For instance, it has been shown that chlorine arising from PVC radiolytic decomposition is going to be retained mainly on plutonium dioxide surface, whereas without plutonium dioxide chlorine is evolved as gaseous HCl. In the same way, $\alpha$-radiolysis induced by plutonium dioxide on several other materials is now studied, such as hydrogen, carbon oxides and short-length alkane production from some lubricating compounds used in fuel pellet manufacturing or from specific waste embedding materials (bitumen, mineral polymers). As the present trend is still to increase fuel burn-up in one hand, and, in the other hand to manufacture new fuel with increasing amount of plutonium, it appears necessary to look closer at the radiolytic phenomena induced by actinide oxide and especially plutonium dioxide to actualize the safety analysis for handling and management of future nuclear fuels and wastes.

International audience; In France, industrial reprocessing of spent nuclear fuel involves the dissolution of the irradiated fuel in concentrated nitric media in view of the recycling and separation of uranium and plutonium. Among others, the current treatment of spent fuel raises the questions of waste and effluent management. With the aim of preparing future GENIV reactors and answering to the world demand in the development of a more sustainable nuclear energy, new alternatives are nowadays investigated. Sonochemistry, which studies the influence of ultrasonic waves on chemical systems, has been showed to enhance chemical yields and accelerate reaction rates for several homogeneous and heterogeneous systems. The aim of the present study is to evaluate the potential of high-power ultrasound towards the control and stabilization of Pu oxidation states in aqueous nitric solution.