BackgroundThe Uranium dioxide (UO2) is currently the most widely used nuclear fuel for commercial nuclear reactors. However, the Fukushima Daiichi nuclear disaster revealed the primary safety risks of this fuel in an accident, so various international programs were launched to develop accident tolerant fuel (ATF), a new generation of fuel system developed to enhance the capability of nuclear fuel assemblies in severe accidents.PurposeThis study aims to improve the thermal conductivity of fuel pellets by adding a second material to the UO2 matrix, an important research direction for ATF.MethodsFirst of all, large-grain UO2 particles were used as raw materials, and the high-density large grain UO2-SiC composite fuel pellets were obtained by Spark Plasma Sintering (SPS) sintering process at lower sintering temperature. Then, the properties, such as microstructure and chemical composition, of the composite fuel pellets were characterized by using metallographic microscope (MM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Finally, the high-temperature oxidation resistance in air environment was studied.ResultsThe results show that the UO2-SiC interfacial reaction can be avoided by the SPS sintering at lower temperature, and the density of the prepared pellets is more than 95% theoretical density (TD). Compared with traditional UO2 fuel pellets and SPS sintered UO2-SiC pellets using conventional UO2 powders, the thermal conductivity of large-grain composite fuel pellets is significantly improved. Oxidation tests results indicate that the oxidation weight gain of the composite fuel pellets is significantly weaker than that of traditional pellets when the temperature is lower than 350 ℃. However, when the temperature reaches 350 ℃, the oxidation of UO2 cannot be further prevented by SiC.ConclusionsThis study provides reference for improving the thermal conductivity of UO2 matrix by adding a second phase with high thermal conductivity.

Background The solid fuel thorium element molten salt reactors (MSR) have attracted more attention recent years. A3-3 graphite is chosen as the fuel matrix for MSR, thus its irradiation behavior and mechanical property is very important before the application. Purpose The study aims to observe the irradiation defects and hardness of A3-3 matrix graphite after ion irradiation by slow positron beam and nano-indentation, respectively. Methods The matrix graphite of fuel elements was irradiated with 1 MeV Xe ions to fluence of 5.8×1014 ions·cm-2 and 2.9×1015 ions·cm-2 respectively at room temperature. The slow positron beam and nano-indentation were employed to investigate the effect of Xe ions irradiation on vacancy defects and hardness of matrix graphite. The changes in irradiation induced defects distribution with depth and fluence were analyzed according to the obtained positron annihilation S parameters versus positron incidence energy or depth curves, compared to SRIM (Stopping and Range of Ions in Matter) calculation. Results Results from slow positron beam measurement show that 1 MeV Xe ions irradiation in matrix graphite introduces a damage layer with depth of about 600 nm, and the damage peak locates at about 250~350 nm in depth, consisted with SRIM simulation. The S parameters in irradiation samples increase significantly compared to virgin sample, which suggests that a high concentration of vacancy-type defects appeared within irradiation damage layer. In addition, the S parameters increase with the irradiation fluence, which shows that the concentration or size of vacancy-type defects increases. The nano-indentation results show that the hardness of irradiated graphite matrix is enhanced. Conclusions The enhanced hardness of A3-3 matrix graphite after ion irradiation is ascribed to the pinning of basal plane dislocation by the high concentration of vacancy type defects introduced by irradiation, consisted with the slow positron beam analysis. Slow positron beam is a very sensitive tool to study the irradiation defects.

The effects of electron shuttles (biochar/anthraquinone-2,6-disulphonate (AQDS)) on the process of the Shewanella oneidensis MR-1-induced As(V)-adsorbed ferrihydrite reduction were studied. The results showed that biochar could stimulate Fe(Ⅱ) and As release during the ferrihydrite bioreduction. After the addition of biochar, more dissolved organic matter (DOM) can be consumed as an electron donor to promote the metabolism of microorganisms by the fluorescence excitation-emission matrix spectra analysis. After microbial treatment, cyclic voltammetry (CV) showed that a unique cathodic peak and a distinct anodic peak appeared, which may represent the reduction of Fe(OH)3 to Fe(OH)2 and the complexed oxidation of Fe2+ to Fe3+. No characteristic peak was associated with arsenate reduction or arsenite oxidation. The mineralogical characterization of the final products indicated that AQDS can promote solid-state conversion from ferrihydrite to vivianite (Fe3(PO4)2·8H2O). However, the addition of biochar inhibited solid-state conversion of ferrihydrite. It was shown that after 6 d, the secondary mineral vivianite production in the bacteria alone and AQDS treatments was 8.12% and 15.6% respectively by mössbauer spectroscopy analysis. Moreover, the XPS indicated that As(V) has no species transformation. It provided new data for understanding the iron-reducing bacteria induced mineralization process and related biogeochemical cycles of Fe and As.

Semi-crystalline high density polyethylene (HDPE) samples were irradiated with 1.157GeV 56Fe ion beams to fluences ranging from 1×1011 to 6×1012ions/cm2. The radiation induced changes in nano/microstructure were investigated with small angle X-ray scattering (SAXS) technique. The scattering contributions from HDPE matrix and ion tracks are successfully separated and analyzed through tilted SAXS measurements with respect to the X-ray beam direction. Lorentz correction, one-dimensional correlation function calculation, fractal nature analysis of the isotropic scattering pattern reveal that HDPE long period polymeric structures are damaged and new materials, possibly clusters of carbon-rich materials, are formed inside the ion tracks. Least square curve fitting of the scattering contribution from the ion track reveals that the track is composed of a core of about 5.3nm in radius, characterized by a significant density deficit compared to the virgin HDPE, surrounded by a shell of about 4.3nm in thickness with less density reduction.

Semi-Insulating Gallium Arsenide (SI-GaAs) was implanted with 1.5 MeV He+ ions and THz photoconductive antenna (PCA) was prepared on the implanted SI-GaAs surface. The antenna was applied as the THz wave emission source of a terahertz time domain spectroscopy (THz-TDS) and the THz wave emission ability was studied as a function of the implantation dose. It is found that the THz signal intensity increases with increase of implantation dose, and after reaching to a peak value the THz signal intensity decreases with further implantation. The best THz emission ability was achieved at a dose value between 1 × 1015 and 1 × 1016 ions/cm2. It is believed that the implantation induced defects in the 1 μm-thick surface area are responsible for the enhanced THz emission ability. The work proved that better THz photoconductive antenna than that made by low-temperature-grown GaAs (LT-GaAs) can be produced through He-ion implantation at proper dose.

Polystyrene (PS) films were irradiated with 1.157GeV 56Fe ions at room temperature to fluences of 1×1012ions/cm2 at various electronic energy loss values. Ultraviolet–visible (UV–vis) spectra were measured to investigate the optical properties of the irradiated materials. Optical constants of virgin and irradiated PS were evaluated in the UV–vis frequence range through fitting of UV–vis spectra with the multi-Lorentz model. Furthermore, effective medium theory was applied to evaluate the optical constants of materials in the ion track area. It is found that the refractive index of ion track material decreased significantly in the ultraviolet–visible range whereas its extinction coefficient increased. Gas atom release and carbon atom conglomerating are believed to be the reason. The extracted track radius and its variation with electronic energy loss values are in good agreement with earlier studies.

The article was addressed to the transformation of excluded pyrite during O-2/CO2 combustion of pulverized coal. Raw pyrite mineral was added to a pulverized coal sample, which was density fractionated to remove the excluded minerals, to simulate the excluded pyrite present in coal. The mixed sample was burned in a drop tube furnace in O-2/CO2 and O-2/N-2 conditions to generate the residue ash, which was characterized by Mossbauer spectroscopic and size analyses. It was found that, in comparison with O-2/N-2 combustion at the same oxygen concentration, slightly less iron glass silicate was formed from excluded pyrite and silicates although the transformation of pyrite to oxides was slowed in O-2/CO2 combustion, different from the behaviors of included pyrite in pulverized coal those were observed in previous study. Additionally, less fragmentation of excluded pyrite particles was also observed in O-2/CO2 combustion. (C) 2009 Curtin University of Technology and John Wiley & Sons, Ltd.