Oxidation-Reduction Properties of Mixed Oxides in Lanthanum-Uranium-Oxygen System

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 La_yU_1-y O_2±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.     

Diffusion Characteristics of Actinide Oxides

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.     

Determination of Multiplication Factors Using Experimental Lattice Parameters of Cluster-Type Fuel Lattices

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.     

稠密栅元内湍流流体的数值模拟

采用CFD软件Fluent对37棒束内的湍流流体进行了分析。利用实验数据对计算结果进行了验证,分析了棒 棒间隙的减小对稠密栅元内局部流动、传热和相干结构的影响。稠密栅元的临界P/D（棒间距/棒直径）约为1.03。随着P/D减小,相干结构和流体交混先增加然后迅速衰减。当通道间隙非常小时,相干结构运动非常弱以至于可将其忽略。其流速、壁面剪应力和壁面温度的波动也非常小,但其参数的空间分布的差异非常明显。     

Effect of Additives on Irradiation-Induced Change of Lattice Parameter in ThO2

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.     

Dissolution of ThO2Based Oxides in Nitric Acid Solutions at Elevated Temperatures

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.     

新型栅元湍流流动及涡结构数值模拟

本文利用非稳态雷诺平均模拟(URANS)对非均匀壁厚新型栅元中的准周期性大尺度涡结构和湍流流动特性进行了模拟和分析。结果表明:新型栅元的对流换热能力优于传统栅元的;增加周向角可强化涡结构强度;随周向角的增大,新型栅元摩擦阻力系数呈现先减小、后趋于恒定的变化趋势,而传热系数则会在达到极小值后略有上升。     

Electrochemical Study on the Reduction Mechanism of Uranium Oxide in a LiCl-Li2O Molten Salt

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.     

Determination of Oxygen Self-Diffusion Coefficient in Polycrystalline Li2O by Gas-Phase Analysis

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.     

Dimensional Changes in Irradiated (Th,U)O2

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.     

内蒙古二连盆地腾格尔坳陷南缘巴彦塔拉矿床成矿地球化学研究

在多次野外踏勘取样和大量地质资料研讨的基础上研究了潜水氧化带型铀矿床的成矿特征,并按照铀元素成矿过程“源、运、积”的演化轨迹,对矿床成矿机理进行深入探讨,并按照不同成矿阶段,对成矿作用进行定量判别;对该区二次还原成矿作用以及潜水转层间的特殊成矿模式进行归纳和总结;对蚀源区花岗岩体开展古铀场和铀差场研究,取得了新的认识和成果。开展的高精度磁法测量对查明支谷底部基岩起伏变化和埋深,以及划分支谷边界、探测深部构造均提供了重要依据。     

Neutron Behavior in Cluster-Type Fuel Lattices, (I)

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.     

Characterization of Uranium Oxide Thin Film Prepared by Vacuum Deposition Method

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.     

稀土氧化物Gd2O3、Nd2O3、Sm2O3和Dy2O3在KCl-LiCl-Li2O熔盐中的电解

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。     

两相自然循环系统的静态漂移特性及输热能力限分析

基于分岔理论及其ＤＥＲＰＡＲ数值方法,运用最简单的均相模型计算出典型两相自然循环系统的静态分岔解图,详细讨论了由浮力和阻力随加热功率（含汽率）的非线性变化特性引起的静态分岔机理;导出对应于强迫循环系统的Ｌｅｄｉｎｅｇｇ不稳定性现象及其判断准则;定义稳定性裕度、自然循环系统输热能力限、静态分岔迟滞现象;讨论了系统压力、欠热度、阻力、几何构型等参数对运行稳定性及输热能力限的影响;强调指出了简单理论预测     

Formation of Pellet-Cladding Bonding Layer in High Burnup BWR Fuels

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.