共查询到20条相似文献,搜索用时 15 毫秒
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《Journal of Nuclear Science and Technology》2013,50(6):467-474
The oxidation-reduction properties and the phase relations of mixed oxides in the system La-U-O were examined by means of thermogravimetry and X-ray diffraction method. The O/M ratios of the mixed oxides LayU1-y O2±x, prepared at 1,650°C in vacuo by the reaction of La2O3 and UO2, were expressed as 2–0.5 y except 0.2≤y≤0.4. The oxides were the single fluorite phase at y0.7. These oxides of 0.2≤y≤0.6 were oxidation-susceptible in air even at room temperature. For the oxides heated or prepared in air at 1,000°C, the O/M ratio vs. y relationship was composed of three line with the turning points, 2.26 at y = 0.27 and 1.92 at y = 0.70. The fluorite phase was observed in the regions of 0.3≤y≤0.45 and 0.7≤y≤0.9, and the rhombohedral phase was in ?0.55≤y-0.7. The O/M ratios of the oxides reduced with hydrogen were nearly 2 at y ≤0.2 and decreased with increasing y at y >0.2. The oxide of y =0.5 showed the curious property: The O/M ratio had a minimum when the oxide reduced with hydrogen was oxidized in air. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(12):973-982
Actinide oxides used for nuclear reactor fuels have fluorite-type cubic structure. In this crystal structure, the self-diffusion coefficients of the constituent ions are closely related to its sublattice structure. Characteristics of the actinide oxides are reviewed with respect to self-diffusion and chemical diffusion. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(4):229-242
A new four-factor formula is proposed for deriving a “finite multiplication factor” k*—the ratio between neutron production and absorption rates in a finite medium—from experimentally determined lattice parameters in a slightly-enriched Pu-U lattice. The “two-group multiplication factor” k* ∞2—related to τ+ L2 and equal to (1 τ B2 C). (1 +L2B2 G at criticality—is derived from k*. The experimental lattice parameters are corrected to account for neutron leakage, from which the “infinite multiplication factor” K∞ is derived. There are found differences between k*, k*∞2 and k ∞ amounting to about 1–2% in the Advanced Thermal Reactor (ATR)-type heavy-water lattice, despite the fact that all these three quantities are often indiscriminately called “infinite multiplication factor.” The proposed four-factor formula for deriving k* was applied to the Deuterium Critical Assembly (DCA) of 1.2%-enriched UO2 lattices of 28-pin clusters in square arrays spaced at 22.5 cm. The values of k* calculated with a lattice design code METHUSELAHH II were in fairly good agreement wTith those determined by experiment. The calculation tended to somewhat overestimate k*, particularly in lattices of highly voided coolant. A similar tendency was observed in the case of 1.5%-enriched UO2 lattices of 22.5 cm pitch in the ATR Sumitomo-Two-Region Critical Facility. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(9):841-847
Irradiation-induced change of the lattice parameter in ThO2 doped with small amounts of CaO, Y2O3 or Nb2O5 was investigated in the fission dose to 2.5×1023 fissions-m?3. The fission dose dependence of the lattice growth was well approximated by a simple exponential model, suggesting that irradiation-induced interstitials exist stably at low fission doses. On the other hand, the lattice growth was found to be enhanced by doping, though no clear difference between dopants was observed. A mutual relationship between irradiation-induced lattice growth and lattice defects generated by irradiation and by doping was examined. Also, a simple model was proposed to explain the effect of grain size on the irradiation-induced lattice growth. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(6):539-545
Highly-dense spherical particles of thorium-based oxides, ThO2 and (Th, U)O2, prepared by the sol-gel method were subjected to dissolution with nitric acid containing 0–0.05 mol/l NaF at high temperatures above 120°C. The dissolution rate depended upon temperature, fluoride concentration and UO2 content. High-temperature in the range of 120–200°C enhanced the dissolution of the ThO2-based fuels. At low temperatures and/or low U02 concentrations, insoluble tetrafluoride precipitates were formed on the particle surfaces and they resulted in the decrease of the dissolution rates. In the present study, the apparent activation energies for the high-temperature dissolution were obtained. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(5):587-595
By means of a linear sweep voltammetry, a cyclic voltammetry and a chronopotentiometry, the electrolytic reduction of uranium oxide has been studied to establish the reduction mechanisms, which are based on a simultaneous uranium oxide reduction and a Li2O electrowinning, and the formation and electrolysis of lithium uranate. From the voltammograms, the reduction potentials of the uranium oxide and Li2O were obtained. From the chronopotentiometries based on the results of the voltammograms, the uranium oxide was reduced to uranium metal through the reduction mechanisms showing a more than 99% conversion. For a verification of the reduction mechanisms feasibility, basic data on the electrolytic reduction of the uranium oxide was obtained from the experiments and the characteristics of the closed recycle of Li2O were discussed. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(4):269-274
The self-diffusion coefficients of the oxygen ion in polycrystalline Li2O were determined in the range of 874~1,129°C by means of the gas-solid isotope exchange technique where the diffusion coefficient was calculated from the concentration variation of the tracer 18O in the diffusion atmosphere with assumption of grain-boundary enhanced diffusion. The determined oxygen self-diffusion coefficient was smaller than the previously reported lithium self-diffusion coefficient. The relative diffusivities of the cation and the anion were discussed with respect to the sublattice structure of the antifluorite cubic Li2O in comparison with the fluorite cubic crystals. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(7):594-601
Effects of irradiation on the dimension and microstructure in (Th,U)O2 pellets were examined by measurements of lattice parameter and bulk density changes, and observations of pore structures. The concentrations of fission-induced defects and the damage volume were estimated by a simple model. Both macroscopic and microscopic dimensional changes were found to increase initially with fission dose and then fall off. The difference between macroscopic and microscopic ingrowths increased with dose, suggesting that fission-induced interstitials would cluster or go to sinks and the concentration of vacancies would be in excess of that of interstititials. The damage volume for vacancies was estimated to be about 1x10?22m3·fiss.?1, and almost agreed with that for fission Xe release. Observations of the pore structure indicated that the volume fraction of pores smaller than 2–3 μm decreases with irradiation and the distribution of pore size shifts toward the larger side. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(11):629-641
Lattice parameters φ28, φρ25, ρ28 and C* were measured on cluster-type fuel lattices of the ATR (Advanced Thermal Reactor) by using a two region critical facility (D20-cluster test and H2O-rod driver regions). Their dependence on lattice pitch, coolant-void ratio and fuel composition (whether UO2 or PuO2-UO2) have been made clear by this experiment. A foil handling technique has been developed for determining the lattice parameters of the Pu02- UO2 fuel pins, and the resulting measurement errors are almost as small as those obtained on the U02 fuel pins. The effects of the Cd-filter and of the presence of UO2 buttons in the measurement of ρ28 and χ25 were studied experimentally and correction factors have been determined. A method of observing the spatial distribution of the γ-ray source in an activated foil has been developed, and the relation between the spatial distribution and the coincidence counting efficiency of the foil has been examined. 相似文献
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《Journal of Nuclear Science and Technology》2013,50(4):382-385
Semiconductor bulk crystals and multilayer structures with controlled isotopic composition have attracted much scientific and technical interest in the past few years. Isotopic composition affects a large number of physical properties, including phonon energies and lifetimes, bandgaps, the thermal conductivity and expansion coefficient and spin-related effects. Isotope superlattices are ideal media for self-diffusion studies. In combination with neutron transmutation doping, isotope control offers a novel approach to metal-insulator transition studies. Spintronics, quantum computing and nanoparticle science are emerging fields using isotope control. 相似文献
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采用循环伏安法和计时电位法研究了Li2O在KCl-LiCl熔盐中的电化学行为,并利用卷积伏安法计算了923 K下O2-在KCl-LiCl熔盐中的扩散系数(D),得到D=0.5×10-5 cm2/s。以Gd2O3、Nd2O3、Sm2O3和Dy2O3为阴极,在KCl-LiCl-Li2O(w=1%)熔盐中进行电解(恒电压3.40 V、电解温度923 K、电解时间25 h)。通过X射线衍射分析(XRD),证实稀土氧化物被部分还原为金属,并分析了电解过程中可能发生的反应。同时利用PRS模型(该模型可将固态阴极内离子的极限扩散速率与固态氧化物孔隙P、金属/氧化物摩尔体积R、阴极还原后的体积收缩率S等参数关联)分析了这些稀土氧化物的电解还原模型,得到Gd2O3、Nd2O3、Sm2O3和Dy2O3的最优孔隙率分别为18.7%、24.2%、30.6%、16.7%,最短电解时间分别为133、157、143、119 h,将这些结果与电解实验结果进行对比,发现阴极的孔隙率和电解时间均不满足金属氧化物完全被还原的要求,并给出了相应的解释。 相似文献
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《Journal of Nuclear Science and Technology》2013,50(7):679-686
Formation process of the pellet-cladding bonding layer was studied by EPMA, XRD, and SEM/TEM for the oxide layer on a cladding inner surface and the bonding layer in irradiated fuel rods. Specimens were prepared from fuels which had been irradiated to the pellet average burnups of 15, 27 and 42 GWd/t in BWRs. In the lower burnup specimens of 15 and 27GWd/t, no bonding layer was found, while the higher burnup specimens of 42 and previously reported 49 GWd/t had a typical bonding layer. A bonding layer which consisted of two regions was found in the latter fuels. One region of the inner surface of the cladding was made up mainly of ZrO2. The structure of this ZrO2 consisted of cubic phase, while no monoclinic crystals were found. The other region, near the pellet surface, had both a cubic solid solution of (U, Zr)O2 and amorphous phase. Even in the lower burnup specimens having no bonding layer, cubic ZrO2 phase was identified in the cladding inner oxide layer. The formation process of the bonding layer were discussed in connection with phase transformation by irradiation damage of fission products and conditions for contact of pellet and cladding. 相似文献