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.     

Chemically produced nanostructured ODS-lanthanum oxide-tungsten composites sintered by spark plasma

High purity W and W-0.9La₂O₃ (wt.%) nanopowders were produced by a wet chemical route. The precursor was prepared by the reaction of ammonium paratungstate (APT) with lanthanum salt in aqueous solutions. High resolution electron microscopy investigations revealed that the tungstate particles were coated with oxide precipitates. The precursor powder was reduced to tungsten metal with dispersed lanthanum oxide. Powders were consolidated by spark plasma sintering (SPS) at 1300 and 1400 °C to suppress grain growth during sintering. The final grain size relates to the SPS conditions, i.e. temperature and heating rate, regardless of the starting powder particle size. Scanning electron microscopy revealed that oxide phases were mainly accumulated at grain boundaries while the tungsten matrix constituted of nanosized sub-grains. The transmission electron microscopy revealed that the tungsten grains consist of micron-scale grains and finer sub-grains. EDX analysis confirmed the presence of W in dispersed oxide phases with varying chemical composition, which evidenced the presence of complex oxide phases (W-O-La) in the sintered metals.     

Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization

Nano-crystalline W-1%Y₂O₃ (wt.%) powder was produced by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. The precursor powder was found to consist of particles of bimodal morphology i.e. large APT-like particles up to 20 μm and rectangular yttrium containing ultrafine plates. After thermal processing tungsten crystals were evolved from W-O-Y plate like particles. spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200 °C for different holding times in order to optimize the sintering conditions to yield high density but with reduced grain growth. Dispersion of yttrium oxide enhanced the sinterability of W powder with respect to lanthanum oxide. W-1%Y₂O₃ composites with sub-micron grain size showed improved density and mechanical properties as compared to W-La₂O₃ composites. Sample sintered in two steps showed improved density, due to longer holding time at lower temperature (900 °C) and less grain growth due to shorter holding time at higher temperature i.e. 1 min at 1100 °C.     

Thermal shock properties of 2D-SiCf/SiC composites

This paper dealt with the thermal shock properties of SiC_f/SiC composites reinforced with two dimensional SiC fabrics. SiC_f/SiC composites were fabricated by a liquid phase sintering process, using a commercial nano-size SiC powder and oxide additive materials. An Al₂O₃–Y₂O₃–SiO₂ powder mixture was used as a sintering additive for the consolidation of SiC matrix region. In this composite system, Tyranno SA SiC fabrics were also utilized as a reinforcing material. The thermal shock test for SiC_f/SiC composites was carried out at the elevated temperature. Both mechanical strength and microstructure of SiC_f/SiC composites were investigated by means of optical microscopy, SEM and three point bending test. SiC_f/SiC composites represented a dense morphology with a porosity of about 8.2% and a flexural strength of about 160 MPs. The characterization of SiC_f/SiC composites was greatly affected by the history of cyclic thermal shock. Especially, SiC_f/SiC composites represented a reduction of flexural strength at the thermal shock temperature difference higher than 800 °C.     

Formation of oxides particles in ferritic steel by using gas-atomized powder

Oxides dispersion strengthened (ODS) ferritic steel was prepared by using gas-atomized pre-alloyed powder, without the conventional mechanical alloying process. By adjusting the volume content of O₂ in the gas atmosphere Ar, the O level in the ferritic powder can be well controlled. The O dissolves uniformly in the ferritic powder, and a very thin layer of oxides forms on the powder surface. After hot deformation, the primary particle boundaries, which retain after sintering, can be disintegrated and near fully dense materials can be obtained. The oxide layer on the powder surface has a significant effect on the microstructural evolution. It may prevent the diffusion in between the primary particles during sintering, and may dissolve and/or induce the nucleation of new oxides in the ferritic matrix during recrystallization. Two kinds of oxide particles are found in the ferritic steel: large (∼100 nm) Ti-rich and fine (10-20 nm) Y-Ti-rich oxides. The hardness of the ferritic steel increases with increasing annealing temperatures, however, decreases at 1400 °C, due to the coarsening of precipitates and the recrystallization microstructure.     

Thermophysical properties of several nitrides prepared by spark plasma sintering

Relatively high-density nitride pellets of TiN, ZrN, DyN, UN and their composites have been prepared by spark plasma sintering (SPS) technique without any milling process and sintering additive. The sintering process finished within 30 min for all the samples. The short sintering time and moderate sintering temperature strongly prohibited the grain growth in the sintering process. The SPS-prepared sample showed high Vickers hardness due to the small grain size. Despite of the grain size, the thermal conductivity remains the high value. The result indicates that the impurity layer on particle surface was removed in SPS process. The SPS pelletizing permits easy densification of nitrides without any deterioration of thermal properties, considered to be suitable as a preparation method of nitride fuels.     

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.     

Microstructure and oxidation properties of 16Cr–5Al–ODS steel prepared by sol–gel and spark plasma sintering methods

The 16Cr–5Al oxide dispersion strengthened (ODS) ferritic steel was fabricated by sol–gel method in combination with hydrogen reduction, mechanical alloying (MA) and spark plasma sintering (SPS) techniques. The phase characterization, microstructure and oxidation resistance of the 16Cr–5Al–ODS steel were investigated in comparison with the Al free 16Cr–ODS steel. X-ray diffraction (XRD) patterns showed that the Al free and Al added 16Cr–ODS steels exhibited typical ferritic characteristic structure. The microstructure analysis investigated by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) revealed that Y–Ti–O complexes with particle size of 10–30 nm were formed in the Al free matrix and Y–Al–O complexes with particle size of 20–100 nm were formed in the Al contained high-Cr ODS steel matrix. These complexes are homogeneously distributed in the matrices. The fabricated 16Cr–5Al–ODS steel exhibited superior oxidation resistance compared with the Al free 16Cr–ODS steel and the commercial 304 stainless steel owing to the formation of continuous and dense Al₂O₃ film on the surface.     

Oxidation of accident tolerant fuel candidates

In this study, the oxidation of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the reference standard – UO₂. The candidates considered in this study were UN, U₃Si₂, U₃Si₅, and a composite material composed of UN–U₃Si₂. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-density composite of UN–U₃Si₂?(10%). Using thermogravimetry in air, the oxidation behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidation can be improved compared to U₃Si₅ and UO₂, and compete favorably with the principal ATF candidates, U₃Si₂, which shows a particularly violent reaction under the conditions of this study, and the UN–U₃Si₂?(10%) composite.     

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.     

Fabrication of particle dispersed inert matrix fuel based on liquid phase sintered SiC

In the present work, liquid phase sintered SiC (LPS-SiC) was proposed as an inert matrix for the particle dispersed inert matrix fuel (IMF). The fuel particles containing plutonium and minor actinides were substituted with pure yttria stabilized zirconia beads. The LPS-SiC matrix was produced from the initial mixtures prepared using submicron sized α-SiC powder and oxide additives Al₂O₃, Y₂O₃ in the amount of 10 wt.% with the molar ratio 1Y₂O₃/1Al₂O₃. Powder mixtures were sintered using two sintering methods; namely conventional high temperature sintering and novel spark plasma sintering at different temperatures depending on the method applied in order to obtain dense samples. The phase reaction products were identified using X-ray diffraction (XRD) and microstructures were investigated using light microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) techniques. The influence of powder mixing methods, sintering temperatures, pressures applied and holding time on the density of the obtained pellets was investigated. The samples sintered by slow conventional sintering show lower relative density and more pronounced interaction between the fuel particles and matrix in comparison with those obtained with the fast spark plasma sintering method.     

含碘废物固化基材及固化方法综述

核燃料后处理厂是目前环境中¹²⁹I的主要来源,其产生的二次废物分为碘洗脱液和固体吸附剂两类,通常含碘废物的固化方法有三种,即水泥固化、玻璃固化和陶瓷固化等。本文综述了各方法的研究进展和应用现状,并对我国后处理厂产生的含碘废物未来处理路线提出了建议。     

Microwave sintering of 8 mol% yttria-zirconia (8YZ): An inert matrix material for nuclear fuel applications

This study focused on reducing overall processing time and temperature for fully stabilized zirconia, an inert matrix material candidate, to minimize the loss of actinides (that will be incorporated into the matrix material), while maintaining at least 90% theoretical density (TD). The effects of different processing routes on bulk density and microstructure were evaluated. The results obtained by adopting microwave sintering for 8 mol% Y₂O₃-ZrO₂ were compared to conventional sintering. A 20 min soak time at 1300 °C resulted in pellets with 90% TD for microwave-processed samples, compared to 77% TD for pellets processed conventionally. A similar density was obtained at lower temperature (1200 °C) by increasing the soak time to 100 min in microwave processing. This time and temperature resulted in 60% TD conventionally processed pellets. Compressive strength values obtained for a 1300 °C (20 min soak time) microwave-processed sample were higher (1600 MPa) as compared to a conventionally processed sample (1300 MPa).     

Reformation of hazardous wastes into useful supporting materials for fast reactor fuels

Novel concepts for effective utilization of molybdenum (Mo) from nuclear waste and magnesium silicates from hazardous asbestos wastes are proposed. A fast reactor cycle scheme that incorporates each material is described in the present paper. Results of basic studies on several fundamental technologies for the present cycle are given. Basic separation features of Mo by using the LIX63 microcapsule and tertiary pyridine resin were investigated. A simple chemical synthesis route for Mo precursor powder from Mo-containing HNO₃ solution was demonstrated. Effects of impurities in recovered Mo on sintering behavior of Mo-pellets were experimentally investigated.     

Synthesis and characterization of zirconia-magnesia inert matrix fuel: Ce homolog studies

X-ray powder diffraction, microscopy, thermal analysis and electron probe microanalysis were used to characterize a ZrO₂-MgO inert matrix containing CeO₂ as a homolog for PuO₂ and Er₂O₃ as a burnable poison. The synthesis was carried out using a precipitation method. A large composition range of MgO to ZrO₂ was evaluated to determine phases present, phase mixing, phase composition, microstructure and thermal properties. It was found that most compositions of the material consist of two phases: MgO (periclase) and ZrO₂ (cubic zirconia). The zirconia phase incorporates 5% (wt/wt) MgO and up to 14% and 12% (wt/wt) CeO₂ and Er₂O₃, respectively. The MgO phase remains pure, which will enable it to retain its heat transfer and solubility properties and will improve the overall thermal conductivity and reprocessing component of the inert matrix fuel. The results with Ce will be used as the basis of future studies with actinides.     

Thermophysical properties of Th1−xUxO2 pellets prepared by spark plasma sintering technique

Th_1?x U _x O₂ solid solutions were synthesized by solid-state reaction and pelletized using the spark plasma sintering (SPS) technique. Pellets with >90% theoretical density were easily obtained within 40 min of sintering without any additive. The thermal conductivity, Young's modulus, Debye temperature, Vickers hardness, and heat capacity were systematically investigated, and the values for ThO₂ agree with the literature data. The thermal conductivity of Th_1?x U _x O₂ decreased with increasing U content up to x =～0.5. This tendency corresponds to phonon-point defect scattering theory. The Young's modulus and Debye temperature linearly decreased with increasing U content. The Debye temperature and standard molar entropy derived from the low-temperature heat capacity agree with the reported values.     

SiC添加量对真空烧结SiC/Zr复合材料显微组织及性能的影响

以SiC粉和ZrH₂粉为原料,用真空烧结法制备SiC/Zr复合材料。研究了SiC添加量对SiC/Zr复合材料显微组织及性能的影响。结果表明:未添加SiC时,材料为致密的Zr金属烧结体,但材料硬度和耐腐蚀性能相对较差;添加SiC后,SiC与Zr发生界面反应生成ZrC和Zr₂Si,SiC和基体Zr通过界面层紧密结合,与未添加SiC的试样相比,材料的致密度有所下降,但硬度和耐腐蚀性能均得到提升;随着SiC添加量的增加,试样的生坯致密度和烧结致密度降低,而组织中的第二相增多,烧结试样的硬度值先增大后减小,其中含15%SiC的烧结试样硬度值最大,耐腐蚀性能随SiC添加量的增加而提高。      

Fabrication and improvement of the density of Li2TiO3 pebbles by the optimization of a sol-gel method

Li₂TiO₃ is regarded as a promising candidate breeder in solid blanket concepts. Pebble configuration has been a preferred option due to its potential advantages in blanket design. Li₂TiO₃ pebbles were successfully fabricated by a water-based sol-gel method previously. However, the sintered density of the pebbles was very low (less than 70% theoretical density). The process parameters were optimized and the sintered density of the pebbles was improved significantly in this work. Li₂TiO₃ pebbles with density as high as 85% were obtained at a relatively lower sintering temperature (1100 °C) and shorter sintering time (4 h). The experimental results showed that the viscosity of the sol was influential to the sphericity of the gel-spheres and thus the sintered pebbles. The variety of lithium source, the pH value of the solution and the sintering conditions demonstrated significant influences on the microstructure and density of the sintered Li₂TiO₃ pebbles.     

Local Gamma-Ray Counting Efficiency of Halogen Counter with Thick Anode

A substance for solidifying waste containing ¹²⁹I is sought that effectively sorbs iodine to inhibit its release from repository into the environment. Three candidate media—commercial alumina cement mixed with calcium sulfate and/or calcium hydroxide—were investigated. The criterion applied for evaluating iodine sorption performance was the distribution coefficient Kd of iodine between solid and solution in an emulsion of sodium iodide solution and powder prepared from the alumina cement/calcium compound mixture, which was molded, cured and ground. Batch sorption experiments were performed on different combinations of the above-mentioned calcium compounds added to alumina cement. The solidified substance was also examined for mechanical strength. The highest iodine sorption performance was obtained with calcium sulfate added to alumina cement to a SC₄/Ca mole ratio of 0.16, which ensured a Kd value raised to a level of not lower than 0.2 m³/kg from the (1.8–3.2)x10^-3 m³/kg obtainable with alumina cement before calcium compound addition. The enhancement of iodine sorption with addition of calcium sulfate is attributed to formation in the substance of monosulfate (3CaO·Al₂O₃·CaSO₄12H₂O) or tetracalcium aluminate hydrate (4CaO·Al₂O3·XH₂O (X=13 to 19)).