Synthesis and characterization of nanocrystalline U1−x_{1-x}Pux_{x}O2(+y)_{2(+y)} mixed oxides

We report here the first synthesis of mixed oxide U$_{1-x}$Pu$_{x}$O$_{2(+y)}$ nanoparticles. The obtained nanopowders were characterized by X-ray diffraction, thermal ionization mass spectrometry, transmission electron microscopy, Raman spectroscopy, and U M$_{4}$ edge high-energy-resolution X-ray absorption near edge structure (HR-XANES). The HR-XANES spectra give evidence for the partial oxidation of U$^{IV}$ to U$^{V}$. This novel route toward the formation of actinide–actinide solid solution opens research opportunities that are not accessible using bulk materials. We give details on the X-ray diffraction study on plutonium oxalate hexahydrate, as a reagent for the synthesis of such nanoparticles

Innovative preparation route for uranium carbide using citric acid as a carbon source

The preparation of uranium carbide (UC) by carbothermal reduction and its sintering into dense pellets by conventional means require high temperatures for long periods. We have developed a preparation route yielding fine UC powder with significantly increased sinterability. At first, a mixture of nanocrystalline UO2 embedded in amorphous carbon (nano-UO2/C) was obtained by thermal decomposition of a gel containing solubilised uranyl nitrate and citric acid. Later, the nano-UO2/C powder was treated in a conventional furnace or in a modified spark plasma sintering facility at elevated temperatures (≥1200°C) in order to obtain uranium carbide powder. The effects of initial composition, temperature, gas/vacuum atmosphere and the overall reaction kinetics are reported

Preparation of a New Water Soluble Polynuclear Peroxide Complex of Pu(IV)

International audienceDue to its acido-basic, complexing and redox properties, hydrogen peroxide has been widely used in nuclear research and industry. Particularly, this salt-free reagent has been highly considered for the stabilization of Pu(IV) in acid conditions and the precipitation of Pu peroxides as precursors for PuO$_2$ preparation. The addition of H$_2$O$_2$ to Pu(VI) acid solutions allows its conversion into Pu(V) which disproportionates and allows the accumulation of Pu(IV).[1] Further addition of H$_2$O$_2$ may lead to Pu(III), or a mixture of Pu(IV) and Pu(III), through the generation of a Pu(IV) peroxo complexe which decompose in acid conditions.[1,2] The continuous addition of H$_2$O$_2$ to Pu(IV) acid solutions leads to the progressive formation of a brown peroxo complex which turns red before insoluble plutonium peroxide precipitate.[2,3] The structure of these complexes which have been postulated in 1949 still remains under debate. Since, only one Pu peroxo structure has been recently proposed and concerned a dimeric molecular compound prepared in alkaline conditions.[4] The current study focuses on the behavior of Pu(IV) aqueous solutions in slightly acid conditions in the presence of hydrogen peroxide. We particularly describe the first observation of a water soluble polynuclear peroxo complex of Pu(IV) which has, up to our knowledge, never been reported in the literature

Evidence for the Formation of a New Water Soluble Pu Peroxo Complex

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Particle Size vs. Local Environment Relationship for ThO2 and PuO

International audienceNanomaterials 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

Sonochemical Preparation and Characterization of Intrinsic Pu Colloids

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Preparation and characterization of intrinsic plutonium colloids by sonolysis of puo2 in water

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