Synthesis and characterization of homogeneous (U,Am)O₂ and (U,Pu,Am)O₂ nanopowders

This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were $U_{0.75}Pu_{0.20}Am_{0.05}O_{2}$, $U_{0.90}Am_{0.10}O_{2}$ and $U_{0.80}Am_{0.20}O_{2}$. After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered $U_{0.80}Am_{0.20}O_{2}$ materials was investigated applying $U M_{4}$ and $Am M_{5}$ edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are $U_{(IV)}/U_{(V)}$ and $Am_{(III)}/Am_{(IV)}$. The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for $PuO_{2}$, $AmO_{2}$, $(U,Pu)O_{2}$ or $(U,Am)O_{2}$

Nanocomposites of silver nanoparticles embedded in glass nanofibres obtained by laser spinning

Nanocomposites made of non-woven glass fibres with diameters ranging from tens of nanometers up to several micrometers, containing silver nanoparticles, were successfully fabricated by the laser spinning technique. Pellets of a soda-lime silicate glass containing silver nanoparticles with varying concentrations (5 and 10 wt%) were used as a precursor. The process followed to obtain the silver nanofibres did not agglomerate significantly the metallic nanoparticles, and the average particle size is still lower than 50 nm. This is the first time that glass nanofibres containing silver nanoparticles have been obtained following a process different from electrospinning of a sol–gel, thus avoiding the limitations of this method and opening a new route to composite nanomaterials. Antibacterial efficiency of the nanosilver glass fibres, tested against one of the most common Gram negative bacteria, was greater than 99.99% compared to the glass fibres free of silver. The silver nanoparticles are well-dispersed not only on the surface but are also embedded into the uniform nanofibres, which leads to a long lasting durable antimicrobial effect. All these novel characteristics will potentially open up a whole new range of applications.Xunta de Galicia | Ref. 10DPI303014PRMinisterio de Ciencia e Innovación | Ref. MAT2009-14542-C02-0

Produción de nanofibras cerámicas mediante a técnica de fibrado láser

Trátase dunha novidosa técnica para a obtención de nanofibras chamada Laser Spinning. Esta técnica permite obter micro- e nanofribras de materiais que non son fitrábeis por outras técnicas. As fibras producidas son amorfas, de composición homoxénea, e con unha xeometría uniforme, con diámetros que van dende unhas dúcias de nanómetros ate unhas poucas micras. Fronte a outras técnicas existentes, o Laser Spinning permite producir nanofibras a velocidades realmente altas, a partir de material en bruto (sen preparación previa do mesmo) e baixo condicións atmosféricas (sen necesidade de atmósfera controlada). Dado que o Laser Spinning é unha técnica con pouco tempo de existencia, a influencia que as condicións de traballo teñen sobre a xeometría das fibras obtidas segue sen estar totalmente clara. Por iso, vai ser abordado o estudo da influencia dos parámetros de procesamento nas características físico-químicas das nanofibras obtidas. Unha vez estudado o proceso en profundidade, pasaremos a funcionalizar as fibras producidas. Segundo a composición das mesmas, pdemos traballar en diferentes liñas de investigación: * Silicatos de Litio. Trátase de un material apto para a captura e secuestro de CO2 (c.c.s. polas súas siglas en ingles). Pretendemos producir fibras de este material para poder utilizalas a modo de filtro en industrias emisoras de alta cantidad de CO2 (especialmente centrais térmicas). * Aluminosilicatos con alto contido en alúmina. Poderían ser utilizadas como soporte para certos catalizadores que precisan de un material base con alto contenido en Alúmina. Outra aplicación seria a de utilizar as fibras para fabricar tecidos ignífugos, dada a alta temperatura de fusión de estas cerámicas

Hydrothermal decomposition of actinide(IV) oxalates: a new aqueous route towards reactive actinide oxide nanocrystals

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

TEM analysis of irradiation-induced interaction layers in coated UMo/X/Al trilayer systems (X= Ti, Nb, Zr, and Mo)

Uranium-molybdenum (UMo) alloy powder embedded in an Al matrix is considered as a promising candidate for fuel conversion of research reactors. A modified system with a diffusion barrier X as coating, UMo/X/Al trilayer (X = Ti, Zr, Nb, and Mo), has been investigated to suppress interdiffusion between UMo and the Al matrix. The trilayer systems were tested by swift heavy ion irradiation, the thereby created interaction zone has been analyzed by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX). Detailed structural characterization are presented and compared to earlier µ-xrd analysis.JRC.G.I.3-Nuclear Fuel Safet

Radiation effects in alpha-doped UO2

UO2 samples doped with the strong α-emitter 238Pu were periodically characterised to study the potential effects of α-self-irradiation on spent nuclear fuel during storage. The degradation of the thermal properties and microstructure of the samples were monitored using the Laser Flash technique (LAF), X-Ray Diffraction (XRD), and Scanning and Transmission Electron Microscopy (SEM, TEM). The thermal conductivity decreased with a non-linear behaviour as a function of the α-dose. Three clearly different stages in the thermal conductivity degradation could be detected, and saturation was reached at relatively low doses (0.035 dpa). In combination with analyses of the microstrain measured by XRD, and TEM images, these three phases could be attributed to three stages of radiation-induced defect generation and clustering. SEM inspection showed the thermal conductivity degradation cannot be due to loss of integrity or grain boundary opening. The radiation damage produced resulted in the formation of dislocation loops as observed by TEM.JRC.G.I.3-Nuclear Fuel Safet

Low temperature decomposition of U(IV) and Th(IV) oxalates to nanograined oxide powders

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

Plutonium and americium aluminate perovskites

Both AmAlO3 and PuAlO3 perovskites have been synthesised and characterised using powder X Ray Diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FT IR) and 27Al Magic Angle Spinning - Nuclear Magnetic Resonance spectroscopy (MAS-NMR). AmAlO3 perovskite showed a rhombohedral configuration (Space group R-3c) in agreement with previous studies. The effect of americium alpha-decay on this material has been followed by XRD and 27Al MAS-NMR analyses. In a first step, a progressive increase of disorder in the crystalline phase was detected, associated with a significant crystallographic swelling of the material. In a second step the crystalline AmAlO3 perovskite was progressively converted into amorphous AmAlO3, with a total amorphisation occurring after 8 months and 2 x 1018 alpha-decay/g. For the first time, PuAlO3 perovskite was synthesised with an orthorhombic configuration (Space group Imma), showing an interesting parallel to CeAlO3 and PrAlO3 lanthanide analogues. High-temperature XRD was performed and showed a phase transition Imma→R-3c occurring between 473 K and 573 K. The thermal behaviour of R-3c PuAlO3 was followed from 573 K to 1273 K, and extrapolation of the data suggests that cubic plutonium perovskite should become stable at around 1850 K (R-3c→Pm-3m transition).JRC.G.I.3-Nuclear Fuel Safet

TEM-EELS analyses of protactinium

A fragment of metallic protactinium (Pa) has been studied using a transmission electron microscope (TEM) equipped with an electron energy loss spectroscopy (EELS) detector. Bright and dark field TEM images have been acquired, and selective area electron diffraction (SAED) has been used to study the crystal structures. The results showed the presence of domains both of metallic and of oxidized material that likely occurred during sample preparation. EELS edges have been collected for the first time for the oxide compounds and compared with those computed using many-electron atomic spectral methods.JRC.G.I.3-Nuclear Fuel Safet