[en] An epithermal neutron beam design for an accelerator-based neutron source for neutron capture therapy (NCT) has been studied. Calculation shows that, of many possible neutron-producing reactions, the Li-7(p,n)Be-7 and H-3(p,n)He-3 reactions are the more promising reactions for producing high neutron yields for NCT. The total neutron yield from the Li-7(p,n)Be-7 and H-3(p,n)He-3 reactions are 1.5 x 10^-4 and 2.5 x 10^-4 neutrons/proton with 2.5 and 1.77 MeV protons, respectively. For a 20 mA proton beam (typical current has been achieved with Radio Frequency Quadrupole accelerators so far), the total neutron yield would be about 1.8 x 10¹³ and 3.2 x 10¹³ neutrons/sec for the Li-7(p,n)Be-7 and H-3(p,n)He-3 reactions, respectively. The reactions must be coupled with a moderator to reduce the energy of the neutrons to an acceptable range. A three-dimensional Monte Carlo code (MCNP) was used to predict neutron flux and neutron and gamma doses. The modified moderator assembly consists of a cylindrical moderator surrounded by a cylindrical reflector. Li-6 with 0.05% atomic concentration was determined to be the concentration in the moderator and reflector, for mitigating the production of gamma rays by removing thermal neutrons. For the Li-7(p,n)Be-7 reaction and a 20 mA proton beam, BeO (20 cm thick, 25 cm diameter cylinder) and MgO (85 cm diameter cylinder) appear to be the optimum selection of moderator and reflector materials. The useful neutron (neutron energy > 1 eV) flux in air, calculated at the irradiation position, is 2.0 x 10⁹ n/cm²/s. For the H-3(p,n)He-3 reaction, Al₂O₃ as moderator (34 cm thick) and also as reflector material gave the best performance. The useful neutron flux, calculated in air at the irradiation position, is 2.4 x 10⁹ n/cm²/s

[en] Highlights: • The nanosized ZnTiNb_2O_8 powders were obtained at 700 °C by the aqueous sol–gel process. • The ixiolite-structure ZnTiNb_2O_8 ceramics were sintered at 1050 °C for 4 h. • The microwave properties were ε_r = 35.3, Q ⋅ f = 66,700 GHz and τ_f = −55.4 ppm/°C. - Abstract: Microwave dielectric ceramics based on ixiolite-structure ZnTiNb_2O_8 were prepared by an aqueous sol–gel process. Highly reactive nanosized ZnTiNb_2O_8 powders with particle sizes of 30–50 nm were successfully obtained at 700 °C as precursors. Sintering characteristics and microwave dielectric properties of ZnTiNb_2O_8 ceramics were studied depending on sintering temperatures ranging from 950 °C to 1100 °C. The dielectric properties were strongly dependent on the compositions, the densifications and the microstructures of the specimens. With the increase of sintering temperature, density, ε_r and Q ⋅ f values were increased and saturated at 1050 °C with excellent microwave properties with an ε_r of 35.3, an high Q ⋅ f of 66,700 GHz, and a τ_f of −55.4 ppm/°C. The correlations of microstructure and dielectric properties for ZnTiNb_2O_8 ceramics were also investigated. The relatively low sintering temperature and high dielectric properties in microwave range would make these ceramics promising for application in electronics

[en] The MnZrNb₂O₈ ceramics with a monoclinic wolframite crystal structure were prepared through the solid-state method and the microwave dielectric properties were also investigated systematically as a function of sintering temperatures. The trend of ε_r was similar to those of apparent density and shrinking ratio. When the sintering temperature was below 1250 °C, the Q·f value mainly depended on the density. However, when the sintering temperature was higher than 1250 °C, the Q·f value mainly depended on the crystal structure. All the τ_f values were fluctuated around −36.88 ppm/°C, ranging from −21.82 to −52.11 ppm/°C. Moreover, the theory of the bond energy was firstly used to predict the variation of the τ_f values. The ceramic sintered at 1250 °C for 4 h exhibited the excellent dielectric properties of ε_r = 24.62, Q·f = 27,936 GHz and τ_f = −52.11 ppm/°C. The relatively low sintering temperature and high dielectric properties would make these ceramics promising for applications in electronics. - Highlights: • New MnZrNb₂O₈ ceramics were prepared by the solid state method. • The dielectric properties were ε_r = 24.62, Q·f = 27,936 GHz and τ_f = −52.11 ppm/°C. • Rietveld refinement was used to analyze the structure of the crystalline phase. • The chemical bond energy was calculated for the analysis of microwave properties.

[en] The effects of H_3BO_3 additions on sintering characteristic, phase composition, microstructure and microwave dielectric properties of MnZrNb_2O_8 ceramics were investigated. The MnZrNb_2O_8 ceramics were prepared by the solid-state method. The phase composition, microstructure and elemental composition of the ceramics were studied using X-Ray Diffraction, Scanning Electron Microscopy and Energy Dispersive Analysis. Only a single-phase MnZrNb_2O_8 was formed in MnZrNb_2O_8 ceramics with H_3BO_3 addition. A small amount of H_3BO_3 successfully reduced the sintering temperature of MnZrNb_2O_8 ceramics from 1250 to 1200 °C without much degradation of the microwave dielectric properties. In addition, the τ_f values were shifted to positive direction with the increase of H_3BO_3 contents. Typically, the MnZrNb_2O_8 ceramic with 1 wt% H_3BO_3 sintered at 1250 °C for 4 h exhibited excellent microwave dielectric properties with ε_r = 25.80, Q·f = 28,419 GHz and τ_f = −8.4 ppm/°C. - Highlights: • Addition of H_3BO_3 reduced the sintering temperature of MnZrNb_2O_8 ceramics. • The τ_f values were shifted to positive direction with the H_3BO_3 addition. • The MnZrNb_2O_8 ceramic with 1 wt% H_3BO_3 was obtained sintered at 1250 °C for 4 h. • The microwave properties were ε_r = 25.80, Q·f = 28,419 GHz and τ_f = −8.4 ppm/°C.

[en] The Takahashi-Tachiki proximity-effect theory is applied to the Nb/Ta multilayer system. The diffusion coefficients of the two metals and the critical temperature of Nb are used as free parameters in fitting experimental phase diagrams. Magnetic-coherence-length scaling is used in order to obtain phase diagrams that best reproduce the measured data. Several parameter sets can compete in fitting an experimental curve. It is not always possible to decide which set gives the most realistic result. (author)

[en] A novel Ba₂BiSbO₆ microwave dielectric ceramic was prepared via the traditional solid-state method, and the microwave dielectric properties of Ba₂BiSbO₆ were investigated for the first time. The analysis of X-ray diffraction patterns indicated that the pure monoclinic Ba₂BiSbO₆ phase could be formed in the temperature range of 950–1150 °C. Rietveld refinement results further confirmed that Ba₂BiSbO₆ belonged to a monoclinic system with space group I2/m (12). Dense and homogeneous microstructures of Ba₂BiSbO₆ ceramics sintered at 1100–1150 °C could be revealed from the scanning electric microscope. The bond ionicity, lattice energy and coefficient of the thermal expansion were initially calculated based on the complex chemical bond theory. The complex dielectric constant and loss were gained from the fitting data of far infrared spectra, which were in good agreement with the measured values. The optimum microwave dielectric properties of ε_r = 26.61, Q·f = 13,764 GHz (at 5.95 GHz) and τ_f = −26.6 ppm/°C were achieved in Ba₂BiSbO₆ ceramic sintered at 1100 °C.

[en] Highlights: • Microwave properties of ε_r = 14.70, Q·f = 200,600 GHz and τ_f = −11.83 ppm/°C were obtained. • A small amount of (Mg_1/3Ta_2/3)⁴⁺ can obviously improve properties. • Rietveld refinement and spectra were used to analyze properties. • Relationships between intrinsic parameters and properties were established. - Abstract: Li₂Mg₃Ti_1-X(Mg_1/3Ta_2/3)_XO₆ (x = 0.00, 0.05, 0.10, 0.15, 0.20) solid solution ceramics were prepared by the conventional solid-state reaction method. Rietveld refinements, Raman spectra, infrared spectra, polarizability and bond energy were used to research the changes of microwave dielectric properties of these solid solutions. With the increase of x values from 0 to 0.2, dielectric constants (ε_r) gradually increased, which could be explained by the variations of the polarizability, infrared resonant modes and Raman vibration modes. The variations of the quality factor (Q·f) could be explained by the changes of the bond valences. The temperature coefficient of resonant frequency (τ_ƒ) correlated with the bond energy of TiO bonds. A small quantity of (Mg_1/3Ta_2/3)⁴⁺ can obviously improve the microwave dielectric properties. Excellent microwave dielectric properties of ε_r = 14.70, Q·f = 200,600 GHz and τ_f = −11.83 ppm/°C were obtained for Li₂Mg₃Ti_0.95(Mg_1/3Ta_2/3)_0.05O₆ ceramic sintered at 1550 °C.

[en] A concept design of natural circulation steam generator for the national 1000MWe PWR of the Chinese National Nuclear Program has been proposed and a relevant research program to validate the efficiency and/or effectiveness of some new design assemblies and/or components has been completed. There are three salient features in the steam generator design. Firstly, steam separation equipment was improved and carryover moisture was further reduced to below 0.1%. Secondly, the water level at the secondary side being elevated, secondary side water volume was expanded to satisfy the EPRI-URD requirements of LFB20. Finally, an inactive device, sludge collector was incorporated to enhance the secondary sludge control. The validation research program consists of two parts; cold state screening test, hot state validation test and corresponding computational analysis, and cold state test and corresponding computational analysis of sludge collector design. The validation tests were completed in 2001. Three sets of steam separator were selected from cold screening tests for hot validation. The hot test showed the most important parameters, outlet carryover, of all three sets were under 0.1%. The best case was only 0.0018%. The sludge collector showed a collection efficiency of over 50%

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