Dry recovery of ceramic uranium dioxide waste material

Three dry recovery routes of ceramic uranium dioxide waste material were studied: (a) calcination of waste material to U₃O₈ and blending with the original UO₂ powder; (b) calcination to U₃O₈, then reduction to UO₂ and blending with the original UO₂ powder; (c) grinding the waste material to a fine powder and blending with the original UO₂ powder. The first route resulted in an increased open porosity and decreased closed porosity of the sintered UO₂ pellets. The second recovery route had almost no effect on the quality of the sintered pellets, while the third route caused an increase in open porosity up to about 15–18 wt% of the added scrap material and increased closed porosity for higher concentrations of the added recovery material. Effects of sieving the recovered scrap powders were also investigated. No effects were observed for the first and second recovery routes, while for the third route more pronounced effects were obtained for coarsely ground powders.     

Preparation,characterisation and dissolution of a CeO2 analogue for UO2 nuclear fuel

The behaviour of spent nuclear fuel under geological conditions is a major issue underpinning the safety case for final disposal. This work describes the preparation and characterisation of a non-radioactive UO₂ fuel analogue, CeO₂, to be used to investigate nuclear fuel dissolution under realistic repository conditions as part of a developing EU research programme. The densification behaviour of several cerium dioxide powders, derived from cerium oxalate, were investigated to aid the selection of a suitable powder for fabrication of fuel analogues for powder dissolution tests. CeO₂ powders prepared by calcination of cerium oxalate at 800 °C and sintering at 1700 °C gave samples with similar microstructure to UO₂ fuel and SIMFUEL. The suitability of the optimised synthesis route for dissolution was tested in a dissolution experiment conducted at 90 °C in 0.01 M HNO₃.     

Pyrochemical reduction of uranium dioxide and plutonium dioxide by lithium metal

The lithium reduction process has been developed to apply a pyrochemical recycle process for oxide fuels. This process uses lithium metal as a reductant to convert oxides of actinide elements to metal. Lithium oxide generated in the reduction would be dissolved in a molten lithium chloride bath to enhance reduction. In this work, the solubility of Li₂O in LiCl was measured to be 8.8 wt% at 650 °C. Uranium dioxide was reduced by Li with no intermediate products and formed porous metal. Plutonium dioxide including 3% of americium dioxide was also reduced and formed molten metal. Reduction of PuO₂ to metal also occurred even when the concentration of lithium oxide was just under saturation. This result indicates that the reduction proceeds more easily than the prediction based on the Gibbs free energy of formation. Americium dioxide was also reduced at 1.8 wt% lithium oxide, but was hardly reduced at 8.8 wt%.     

Study of the properties of ammonium polyuranate precipitates and UO<Subscript>2</Subscript> powders and pellets obtained with different technologies

The results of a differential-thermal analysis are used to compare the properties of ammonia polyuranate precipitates, UO₂ powders and pellets, obtained by different methods as well as metallic uranium. It is found that the phase NH₃·3UO₃·5H₂O forms in regular precipitation of ammonium polyuranate. When using nanotechnology, the phases NH₃·2UO₃·3H₂O and 4NH₃·6UO₃·8H₂O are also present in the precipitate. UO₂ powder prepared from such precipitate has high activity, since all phase transformations in it occur at a lower temperature. Modified fuel pellets of uranium dioxide, which are obtained by means of nanotechnology or mechanical addition of ammonia-containing reagents to powder, differ from the standard powders by a lower rate and more complex mechanism of oxidation, similar to metallic uranium. Modified UO₂ fuel pellets fabricated at the Physics and Power-Engineering Institute, are now undergoing tests in the BOR-60 reactor. After tests on the irradiated new modified fuel have been completed, it will be possible to judge its reliability.     

Preparation and characterization of lithium-titanate pebbles by solid-state reaction extrusion and spherodization techniques for fusion reactor

For the development of TBM for fusion reactors, lithium containing ceramics as against the metal are preferred as tritium breeding material. Lithium titanate (Li₂TiO₃) is one such chosen ceramic tritium breeder. Li₂TiO₃ pebbles are conventionally prepared by sol-gel process and wet process. Solid state reaction of lithium carbonate with titanium dioxide is preferred route for the bulk production of Li₂TiO_3. Thermo-gravimetric and differential thermal analysis (TG-DTA) techniques have been used in the present study to understand the solid state reaction of intimate mixture of lithium carbonate and titanium dioxide. It was found out that single phase lithium titanate (Li₂TiO₃) is produced at 750 °C and the reaction is completed in 6 h. Fine powders of lithium titanate obtained after milling and classification were mixed with aqueous solution of PVA to prepare green pebbles of desired size and shape. The pebbles were subsequently sintered at 900 °C and the effect of sintering time on the properties of sintered pebbles was studied. The reaction mechanisms and the product qualities obtained by the solid state reaction, extrusion and spherodization techniques are discussed in this paper.     

Treatment of UO2 pellets used for preparing fuel elements of HTR-10 followed by supercritical fluid extraction

International interest in high temperature gas-cooled reactor (HTGR) has been increasing in recent years. It is important to study on reprocessing of spent nuclear fuel from HTGR for recovery of nuclear resource and reduction of nuclear waste. Treatment of UO₂ pellets used for preparing fuel elements of the 10 MW high temperature gas-cooled reactor (HTR-10) followed by supercritical fluid extraction was investigated. When UO₂ pellets were dissolved and extracted with tri-n-butyl phosphate (TBP)–HNO₃ complex in supercritical CO₂ (SC-CO₂), the extraction efficiency was less than 7% under experimental conditions. After UO₂ pellets were ground into UO₂ fine powders, the extraction efficiency of the UO₂ fine powders with TBP–HNO₃ complex in SC-CO₂ could reach 92%. After UO₂ pellets broke spontaneously into U₃O₈ powders under O₂ flow and 600 °C, the extraction efficiency of the U₃O₈ powder with TBP–HNO₃ complex in SC-CO₂ could reach more than 98%.     

Pyrochemical method of salvaging weapons plutonium in oxide for fabricating mixed fuel for fast reactors

Experimental results of investigations of pyrochemical conversion of weapons plutonium into plutonium oxide for fabricating fast-reactor fuel are presented. Weapons plutonium was hydrogenized by hydrogen at 220°C, after which the plutonium hydride obtained was acidified at 550–560°C with the formation of PuO₂. To increase fire and explosion safety of the process, a mixture of oxygen with nitrogen, helium, or argon was used or nitriding with nitrogen and oxidation of plutonium mononitride were introduced. The particle size of the plutonium oxide powders obtained was less than 15 μm. The powders showed poor flowability, but after granulation they were suitable for fabricating kernels with mixed fuel. The gallium was removed by reduction of Ga₂O₃ by hydrogen to Ga₂O, which was sublimated. The mixed-fuel kernels sintered at 1600–1700°C in a hydrogen atmosphere contained <0.001 wt.% gallium, and their density was 94–97% of the theoretical value.     

Improvement of UO2 Pellet Properties by Controlling the Powder Morphology of Recycled U3O8 Powder

Powder morphology evolution of recycled U₃O₈ according to the thermal treatments has been studied. The defective UO₂ pellets are oxidized to U₃O₈ powders at a conventional temperature of 350 or 450°C in air. Those powders are pressed into green pellets and then sintered at 1,500 and 1,730°C in H₂ gas flow. Final reoxidized U₃O₈ powers are obtained by reoxidizing those sintered pellets at 450°C in air. This paper shows that the reoxidized U₃O₈ powder morphology and the BET surface areas are greatly dependent on the density of sintered UO₂ pellets before reoxidation. Reoxidized U₃O₈ powders are added to virgin UO₂ powders to fabricate UO₂ pellets and the effect of such addition on the UO₂ pellet properties is investigated. The reoxidized U₃O₈ powders having a certain range of BET surface area significantly promote the grain growth of UO₂ pellets.     

Combustion synthesis of 5 and 10 mol% YO1.5 doped ThO2 powders

Nanocrystalline 5 and 10 mol% YO_1.5 doped ThO₂ powders were prepared by the combustion technique using citric acid as a fuel and nitrates as oxidants. The auto-ignition of the fuel-deficient precursors (prepared by thermal dehydration of the aqueous solutions containing metal nitrates and citric acid in required molar ratio) directly resulted in the well crystalline powders of the desired solid solutions along with traces of carbonaceous material. The as-prepared and calcined powders were characterized by X-ray diffraction (XRD), high-temperature XRD and by their sinterability. The YO_1.5 doped ThO₂ powders when cold-pressed and sintered at 1300 °C for 2 h resulted in ?95% of their theoretical densities with nanograin microstructure.     

Investigation of ThO2 and (U, Th)O2

The effect of the properties of ThO₂ and (U, Th)O₂ powders, prepared with different technological regimes, on the properties of the finished items is investigated. The work includes detailed investigations of ThO₂ and (U, Th)O₂ powders (x-ray phase analysis, electron-microscope investigation) and sintered fuel pellets (determination of density, study of microstructure, thermophysical investigations). The temperature dependences of the crystal lattice parameters and the sizes of the crystallites in ThO₂ and (U, Th)O₂ powders with different UO₂:ThO₂ ratio are obtained. The temperature dependences of the thermal conductivity of sintered ThO₂ and (U, Th)O₂ pellets with different UO₂:ThO₂ ratio are studied.     

Microstructure and properties of tungsten–samarium oxide composite prepared by a novel wet chemical method and spark plasma sintering

W–1 wt% Sm₂O₃ powders doped with highly uniform Sm₂O₃ were successfully synthesized by a novel wet chemical method followed by hydrogen reduction. The powders were consolidated by spark plasma sintering (SPS) at 1800 °C to suppress grain growth during sintering. The FE-SEM and HRTEM analysis, tensile test and thermal conductivity measurements were used to characterize these samples. The grain size, relative density of the bulk samples fabricated by SPS sintering were 4 μm and 97.8%, respectively. The tensile strength values of Sm₂O₃/W samples were higher than those of pure W samples. As the temperature rises from 25 to 800 °C, the thermal conductivity of pure W and W–1 wt% Sm₂O₃ composites decreased with the same trend and the thermal conductivity of both samples was above 160 W/m K at room temperature.     

Synthesis and characterization of Li4SiO4 nano-powders by a water-based sol-gel process

The water-based sol-gel process for the synthesis of Li₄SiO₄ nano-powders was reported for the first time. LiOH·H₂O and aerosil SiO₂ were used as the starting materials with citric acid (C₆H₈O₇·H₂O) as the chelating agent. Li₄SiO₄ powders with particle size as small as 100 nm were successfully synthesized at the temperature as low as 675 °C. Phase analysis, morphology, sintering behavior of the powders and ionic conductivity of the sintered bodies were investigated systematically. The experimental results showed that the powders obtained by the water-based sol-gel process (SG) possessed excellent sinterability, exhibiting a linear shrinkage of 5.2% while sintered to 900 °C, more than 3 times that of the powders obtained by solid state reaction (SSR). The bulk conductivity of the SG sintered bodies was much higher than that of the SSR samples at the same testing temperature.     

Densification of nano-CeO2 ceramics as nuclear oxide surrogate by spark plasma sintering

The sintering and resulting microstructure of nano-grained CeO₂ ceramics were investigated as functions of the spark plasma sintering (SPS) parameters. Ceria powders could be sintered to a relative density over 97% with a grain size of about 30 nm. The applied uniaxial pressure during sintering had a significant effect on densification. The combination of high pressure and fast heating rate produces a marked reduction in the sintering temperature to densify CeO₂ with very limited grain growth. Heating rate and holding time, however, had insignificant effect on density but a measurable effect on grain size.     

Etude cinetique de la carboreduction du dioxyde d'uranium

The reduction by graphite, in a carbon monoxide atmosphere, of uranium dioxide, UO₂ + 4C ái UC₂ + 2CO has been studied thermogravimetrically in a pressure range of 60–510 torr from 1709 to 1818°C. The kinetic law is parabolic; solid-state diffusion in the compact dicarbide layer is probably rate-limiting. The rate constant is a linear decreasing function of pressure; this permits an estimate of the equilibrium pressure by extrapolation to zero rate.     

Comparative study of structural and morphological properties of CuIn3S5 and CuIn7S11 materials

CuIn₃S₅ and CuIn₇S₁₁ powders were prepared by solid-state reaction method using high-purity elemental copper, indium and sulphur. The films prepared from CuIn₃S₅ and CuIn₇S₁₁ powders were grown by thermal evaporation under vacuum (10^?6 Torr) on glass substrates at different substrate temperature Ts varying from room temperature to 200 °C. The powders and thin films were characterized for their structural properties by using X-ray diffraction (XRD) and energy dispersive X-ray (EDX). Both powders were polycrystalline with chalcopyrite and spinel structure, respectively. From the XRD data, we calculated the lattice parameters of the structure for the compounds. For CuIn₃S₅ powder, we also calculated the cation–anion bond lengths. The effect of substrate temperature Ts on the structural properties of the films, such as crystal phase, preferred orientation and crystallinity was investigated. Indeed, X-ray diffraction analysis revealed that the films deposited at a room temperature (30 °C) are amorphous in nature while those deposited on heated were polycrystalline with a preferred orientation along (1 1 2) of the chalcopyrite phase and (3 1 1) of the spinel phase for CuIn₃S₅ and CuIn₇S₁₁ films prepared from powders, respectively. The morphology of the films was determined by atomic force microscopy AFM. The surface roughness and the grain size of the films increase on increasing the substrate temperature.     

Characterization of ThO2-UO2 pellets made by co-precipitation process

The co-precipitation technique renders an excellent route to obtain a homogeneous mixture of ThO₂ and UO₂ powders. In this process, after the nitrate solutions of Th and U are mixed in the intended ratio, oxalic acid is added for co-precipitation. The precipitate is then dried and calcined to get a solid solution of ThO₂ and UO₂. In this study, ThO₂-30%UO₂ and ThO₂-50%UO₂ (% in weight) powders were characterized in terms of particle size, particle shape, surface area, phase content, O/M ratio etc. The pellets obtained by sintering these powders were characterized with the help of optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The XRD data for ThO₂-30%UO₂ and ThO₂-50%UO₂ pellets showed the presence of a small amount of U₃O₈ phase besides fluorite phase. The grain size of ThO₂-30%UO₂ and ThO₂-50%UO₂ was found to be 5.7 and 4.5 μm, respectively.     

Compactibility and sintering behavior of nanocrystalline (Th1−xCex)O2 powders synthesized by co-precipitation process

Thoria (ThO₂) based ceramic material is a versatile and very important matrix for immobilization of plutonium and other tetravalent actinides either as a burning or a deposition material for final disposal. The aim of this study was to investigate the influence of the actinide concentration (simulated with cerium), the fabrication conditions and the properties of the produced powders on the compactibility and sinterability of the final products. The (Th_1−xCe_x)O₂ powders with ceria concentration varying from 5 to 50 mol% were synthesized by co-precipitation method. The pellets were then compacted from calcined and ground powders at pressures varying from 250 to 750 MPa. The produced pellets had a homogenous grain size and sintered densities of 0.88% to 0.95% TD, respectively.     

Thermal decomposition of tetravalent and trivalent plutonium oxalates

Using a Kurnakov registering pyrometer, we investigated the thermal decomposition of tetra- and trivalent plutonium oxalates. The composition of the intermediate products was determined with a Berg gas burette, by potentiometric titration and by Penfild's method. It was established that freshly precipitated Pu (IV) oxalate lost three molecules of water at 110 ° C. After standing for 3–4 days at 110 ° C the oxalate also gave 1.5–2.7% of CO+CO₂ due to decomposition induced by the -radiation of the plutonium. At the same time partial reduction to trivalent plutonium occurred. In the temperature range 170–200 ° C two more molecules of water and 13% CO+CO₂ were liberated and the plutonium was reduced to the trivalent state to form mainly Pu₂(C₂O₄)₃H₂O; at 380 ° the oxalate was converted to plutonium dioxide. At 140 ° Pu (III) oxalate was completely dehydrated and at 270 ° C in air it was converted to plutonium dioxide. In an inert medium at 330 ° C the oxalate decomposed to give oxalate-carbonate. At 460 ° C the oxalate-carbonate decomposed and the trivalent plutonium was oxidized to the tetravalent state with the formation of the dioxide.     

Recent activities in the field of radiation measurement and nuclear data

The characteristics, that is, morphology, size distribution, alloy phase and microstructure of U₃Si and U₃Si₂ powders, solidified rapidly by a centrifugal atomization, were investigated. The atomized powders consist of spherical particles with a relatively narrow size distribution independent of the alloy composition. The particle size distribution can be controlled by adjusting the atomization parameters such as feeding rates of the melt and revolution speeds of the disk. The major phases of atomized U₃Si and U₃Si₂ powders are α-U and U₃Si₂ and U₃Si₂, respectively. The atomized U₃Si powder has a dendritic structure of very fine and non-faceted U₃Si₂ precipitates with less fibric and eutectic U₃Si₂ structure. The microstructure of U₃Si₂ powder shows a cellular structure with fine U₃Si₂ grains and finely dispersed silicon-rich precipitates. The time for complete peritectoid reaction of the atomized U₃Si particles and the resulting grain size are greatly reduced, due to the refinement of primary U₃Si₂ precipitates.     

Transmission electron microscopy of irradiated boron carbide

Boron carbide powders and a section of a high density pellet were irradiated at 350 to 400 °C to a burnup of about 8% of the ¹⁰B (16 × 10²⁰ captures/cm³). The resulting damage was studied by transmission electron microscopy. Techniques of sample preparation are described. A very high density uniform black spot damage was observed. The damage appeared to be similar to that observed in metals due to lattice strain after very high fast fluences. There was no evidence of movement or agglomeration of the reaction products.