二氧化铀核燃料芯块烧结工艺的发展概况

介绍了当今世界上二氧化铀芯块的主要烧结方法,详细讨论了各种工艺方法的特点、优点及不足,提出了综合改进烧结方法的建议.并简要介绍了二氧化铀核燃料芯块的烧结理论.     

晶型控制剂对纳米氧化镁形貌的影响

对晶型控制剂的种类及用量进行对比,通过形貌和物相的表征和分析,研究晶型控制剂对纳米氧化镁形貌及分散作用的影响。结果表明:采用羧基类和羟类两种晶型控制剂,一羧基类和一羟基类晶型控制剂生成纳米氧化镁为球形,团聚严重;而多羧基类和多羟类晶型控制剂均有利于纳米氧化镁颗粒沿片状分布;加入不同用量的晶型控制剂,乙二胺四乙酸用量为6 mg/L时,有利于纳米氧化镁颗粒沿片状分布,且氧化镁晶粒度仅为26.13 nm,聚乙二醇用量为1.2 mg/L时较利于纳米氧化镁生成片状。     

大晶粒UO2燃料芯块性能研究进展

大晶粒UO2芯块相比于传统UO2芯块而言,具有更低的辐照肿胀、更低的裂变气体释放量以及优异的抗芯块-包壳相互作用的能力.大晶粒UO2芯块作为高燃耗、长换料周期新型燃料具有很大的应用潜力,近年来受到越来越多的关注.国内外学者针对大晶粒UO2芯块开展了大量的研究,包括芯块的微观结构、烧结特性以及辐照行为等,结果表明,大晶粒UO2芯块在高燃耗和长换料周期条件下表现出优异的堆内性能,在提高压水堆核电站的经济性的同时,可以有效提高其安全性.近年来,研究人员针对大晶粒UO2芯块在欧洲、美国、俄罗斯和中国等国开展了大量的堆内辐照考验,考验的环境包括压水堆、沸水堆和高温堆等.对大晶粒UO2芯块堆内、外性能的相关研究,为芯块制备、堆内性能预测、反应堆燃料元件设计以及燃料元件工业化生产提供了支撑.本文根据国内外关于大晶粒UO2燃料芯块的研究进展,系统总结了大晶粒UO2芯块微观结构、堆外热-力学性能、大晶粒UO2芯块堆内行为,介绍了不同掺杂类型大晶粒UO2芯块的性能,分析了大晶粒UO2燃料元件行为,并指出了大晶粒UO2芯块应用所面临的问题.     

熟料氧化镁含量与冷却条件对氧化镁形态的影响

为研究熟料中氧化镁膨胀的本质,烧制含氧化镁熟料,其氧化镁含量从1.5%至9%,冷却条件为炉内慢冷、空气快冷和液氮急冷,采用化学方法(硝酸铵提取方镁石)对各熟料方镁石进行定量,通过XRD、SEM/EDS等手段分析熟料矿物组成、形貌及固溶情况,对熟料中氧化镁存在形态及机制进行讨论。结果表明:熟料中氧化镁含量提高,方镁石含量也随之提高,但氧化镁含量较高时,方镁石随氧化镁含量而增加的速率转慢;熟料冷却速度加快,方镁石含量减少,反之则增加。熟料中玻璃相是影响氧化镁形态的重要因素,氧化镁含量的增加以及冷却速度的加快,使熟料中玻璃体的含量增加,分布在玻璃体中的非晶态氧化镁比例增加,从而减少以方镁石存在的氧化镁比例。     

基于振动方式的燃料棒自动装管工艺研究

该文研制了采用振动方式将燃料芯块装入包壳管的燃料棒自动装管装置,探究了装置震动频率和倾斜角度对自动装管效率和装管质量的影响,确定了采用自动装管装置进行燃料芯块装管过程的工艺参数,提升了装管工序的自动化程度及生产效率,提高了燃料芯块的装管质量,有效降低采用推入式方法易产生芯块损坏的风险,增强了芯块装管的可靠性。     

加速器驱动次临界系统用嬗变核燃料研究进展分析

加速器驱动次临界系统（Accelerator driven subcritical system,ADS）是乏燃料安全处理处置关键瓶颈问题的优秀解决方案,而开发适用于该系统的嬗变核燃料正是ADS研发的关键任务之一。然而由于嬗变对象次锕系元素的固有特殊性质、嬗变燃料体系相关机理尚不十分明确、制备技术难度大、嬗变核燃料相关试验数据和运行经验的欠缺等原因,ADS用嬗变核燃料的研发十分复杂且极具挑战。本文系统综述了作为ADS重要候选嬗变燃料的氧化物弥散型燃料CERCER/CERMET、氮化物燃料和金属燃料的研究进展,包括制备工艺、辐照实验和辐照后检验结果、物性参数、主要优缺点等内容,以期为我国ADS用嬗变核燃料的研发提供一定思路和参考。     

氧化镁/PEEK复合材料的制备及其导热与吸声性能

以六水氯化镁、氨水为原料,吐温80为表面活性剂,通过醇水法和前驱体法,制备了粒径分布为50～100 nm的氧化镁导热粉末。将氧化镁与聚醚醚酮粉末混合后,采用模压和烧结工艺,制备了氧化镁/聚醚醚酮(MgO/PEEK)导热复合材料。扫描电子显微照片表明,纳米氧化镁呈球形、分布均匀,在聚醚醚酮中具有较好的分散性。激光闪点法测试了材料的导热性能,当氧化镁添加量为50%(质量分数)时,复合材料的导热率提高到了2.8 W/(m·K),且其具有极好的热稳定性与阻燃性能。将玻璃棉浸泡在熔化的氧化镁/聚醚醚酮复合材料中,测试表明,玻璃棉的导热率由0.056 W/(m·K)提高到了0.24 W/(m·K),解决了吸声材料降噪与散热不能同时兼顾的难题。     

Q245钢表面等离子堆焊Fe-Cr-Ti-C层的组织与耐磨性能

为了研制一种铁基耐磨复合材料,采用等离子堆焊技术制备了3组Fe-Cr-Ti-C系合金堆焊层.在MLS-23型湿砂橡胶轮式磨损试验机上进行磨粒磨损试验,采用JSM-6360LV型扫描电子显微镜(SEM)和布鲁克D8型衍射仪等技术,观察了堆焊层的组织、碳化物形貌及磨损形貌,探讨了磨损机理.结果表明:随着堆焊层中Ti含量的提高,其组织由奥氏体加铁素体向马氏体转变,碳化物TiC及M7C3等硬质相的数量逐渐增多、且分布弥散均匀;当Ti含量为9.67％时,TiC和M7C3硬质相均匀弥散分布在具有较强韧性的板条马氏体中,堆焊层显现出优良的耐磨损性能.     

UO2-ZrO2固溶体的形成机制和致密化规律

采用溶胶-凝胶法制备了UO2-ZrO2芯块,分析了制备过程中芯块的固溶体的形成 、微观组织结构和致密化行为.X射线衍射结果表明低于1600℃时UO2-ZrO2芯块中主要是四方结构的ZrO2固溶体和面心立方结构的UO2固溶体.随着烧结温度的增加,ZrO2固溶体逐渐减少,UO2固溶体不断增加,并伴有2θ的偏移.由于离子扩散速率和晶体结构不同,Zr元素易扩散至UO2中,UO2基体固溶ZrO2同时不断长大,原ZrO2的位置产生缺陷,使得芯块形成新的孔隙,芯块密度下降.烧结温度大于1650℃时,基体组织主要是是面心立方(U,Zr)O2,晶粒尺寸迅速长大.     

UO2+x芯块低温烧结的活化机理

报道了UO2+x芯块低温烧结实验的结果.12组芯块在N2+CO2组成的部分氧化气氛下于立式钼丝炉中低温烧结.UO2芯块要获得密度为10.41g/cm3(94.98％理论密度)需在氢气氛中于2073～2273K下烧结,而UO2+x芯块实现该密度的烧结温度可降低400K以上.建立了超氧化铀缺陷模型来研究低温烧结的活化机理.研究发现铀离子扩散系数与气氛中氧分压或是UO2+x中x成正比.利用铀离子的扩散系数,可预测UO2+x芯块在1073、1273、1473和1673K温度下的烧结密度；还可算出x=0.04时,UO2+x芯块在部分氧化气氛下的理论烧成温度.计算所得烧结密度和烧成温度与实验结果符合得很好.     

Powder metallurgical fabrication of zirconium matrix cermet nuclear fuels

Two powder metallurgical fabrication methods for a zirconium-based cermet nuclear dispersion fuel with oxide microspheres have been demonstrated. A multi-pass, cold-drawing process is shown to have excellent capability to control the final matrix density, though it requires several high-temperature anneals during fabrication to relieve strain hardening and increase matrix–particle bonding. Severe oxide particle damage was observed in the cold-drawn fuel pin and was likely a result of high matrix deformation resistance at room temperature. A single-pass, hot-extrusion process has been demonstrated and was shown to be capable of providing a dense matrix phase with less particle damage. Both processes were shown to be effective fabrication methods for a zirconium-based cermet, and the process variables may be controlled to create the desired fuel properties.     

Nondestructive characterisation of MOX fuel rods using gamma autoradiography (GAR)

During the fabrication of mixed uranium-plutonium oxide (MOX) fuel rods, two important characteristics to be checked in as-fabricated fuel pins are plutonium enrichment and plutonium dioxide agglomerates. The mixed oxide fuel pellets are made via mechanical mixing of uranium dioxide and plutonium dioxide powders by cold compaction and sintering. The chance of loading a wrong Pu enrichment pellet and having pellets with plutonium dioxide agglomerates in a fuel pin cannot be ruled out. A simple nondestructive evaluation technique is felt necessary to ensure at the last stage (in the welded pins) to check these two characteristics. During the fabrication of MOX fuel rods for Boiling Water Reactors at Advanced Fuel Fabrication Facility of BARC at Tarapur, Gamma-auto-radiography was successfully used to evaluate composition of MOX pellets and to detect presence of PuO₂ agglomerates in the peripheral region. The fuel pins are allowed to be in contact with industrial X-ray films loaded in cassettes for a long time and the processed films are carefully evaluated. Experiments were made to standardise the conditions for distinguishing fuel pellets of different composition by gamma-auto-radio-graphy of fuel pins loaded with pellets of different composition. Gamma-auto-radiography of fuel pins containing agglomerates of different sizes was also carried out. This paper describes the experimental details of the technique, results obtained and compare with other nondestructive evaluation techniques available.     

Stable reactor fuel of controlled density using active UO2 powders

This paper describes a method for preparing thermally stable UO₂ fuel pellets from active starting powders with a considerable range of sintered density. This accomplishment is effected by introducing regions of large, stable porosity by variations in the powder preparation procedure. The final density is sensitively dependent on the ratio of the density reached in a prepressing (slugging) operation to the density of the presintered green pellet. As the slug density becomes greater than the green density, the sintered density falls off rapidly.     

Effect of system parameters on size distribution of 304 stainless steel particles produced by electrical discharge mechanism

An electrical discharge machine (EDM) can be used to produce Uranium Carbide (UC) microspheres. This material is useful for dispersed nuclear fuel fabrication. To obtain empirical data about the process parameters for UC modeling, 304 stainless steel (304 SS) was used to produce microspheres. Particle size effects were analyzed via three parameters; arc intensity, dielectric medium, and pulse width. Results indicate that the size and yield of the particles are dependent on all three parameters. The yield of larger particles increased with increasing arc current. Particle size distribution decreased with decreasing pulse width.     

A Factorial Approach to High Dose Product Development by an Extrusion/Spheronization Process

Extrusion/spheronization technology has been used for preparing high drug-loaded pellets. Typical formulations include 40-60% microcrystalline cellulose (MCC) to impart the plastic characteristics required for this process. Studies have suggested that pellets containing greater than 80% drug are difficult to process and require special grades of MCC. Most of these studies focused on either the process or formulation aspects of the product and failed to explore the interactions of process and product. Statistical experimental designs are well suited for exploring both process and product variables and their interactions with each other. This study addresses pelletization of a high dose drug with low density. A Nica® radial (basket-type) extruder was used in extrudate preparation, followed by spheronization on a serrated plate spheronizer. A Plackett-Burman screening design was employed to investigate product and process parameters affecting final pellet drug content, density and roundness. MCC type and concentration, water concentration, spheronizer speed and residence time and extruder screen size were found to be statistically significant in imparting desirable attributes to the final product. Wet mixing time, extruder feed rate and extrusion rate did not significantly affect pellet properties     

Electron microscopy characterization of an as-fabricated research reactor fuel plate comprised of U–7Mo particles dispersed in an Al–2Si alloy matrix

To understand the microstructural development of nuclear fuel plates during irradiation, it is imperative to know the microstructure of a fuel plate after all the fabrication steps have been completed and before it is inserted into the reactor. To this end, a U–7 wt.% Mo alloy research reactor dispersion fuel plate with Al–2 wt.% Si matrix was destructively examined using scanning and transmission electron microscopy to characterize the developed microstructure after fabrication. Of particular interest for this study was how the Si that was added to the fuel matrix partitioned between the various fuel plate phases during fabrication. Si was added to the matrix so that the microstructure that developed during fuel fabrication would exhibit good irradiation behavior. SEM analysis was used to identify the representative microstructure, the compositions of the various phases, and the partitioning behavior of the fuel and matrix constituents. TEM analysis was employed to definitively identify the phases in the U–7Mo alloy and the phases that formed due to diffusional interactions between the fuel particles and matrix during fuel plate fabrication. The TEM results are the first reported for an as-fabricated U–7 wt.% Mo dispersion fuel plate with an Al alloy matrix. SEM results showed that a significant portion of the original γ-(U–Mo) fuel particles had transformed to a lamellar microstructure, comprised of α-U and either γ or γ' phases, and the fuel/matrix interaction layers were enriched in Si. TEM analysis identified an ordered fcc (U–Mo)(Al–Si)₃ type of phase, which formed at the decomposed U–7Mo/matrix interface and extended into the lamellar microstructure. Some regions of the U–7Mo particles retained the single-phase γ-(U–Mo). Small precipitate phases were observed in the fuel meat matrix that contained Fe, Al, and Si. The Si that is added to the matrix of a U–Mo dispersion fuel plate to improve irradiation performance appears to result in the creation of a Si-rich (U–Mo)(Al–Si)₃ type of fuel/matrix interaction layer during fabrication that appears to exhibit favorable behavior during irradiation compared to the behavior of the layers that form in U–Mo dispersion fuel plates without Si in the matrix.     

A mechanistic approach of the sintering of nuclear fuel ceramics

The CEA and the COGEMA, as part of their effort to model the different stages of the MOX fuel fabrication process, have specifically worked, on the sintering stage. A physical mechanistic model of MOX fuel sintering is proposed, as well as the numerical schemes that will lead to the achievement of the corresponding software tool. The model takes into account surface diffusion, grain boundary diffusion, volume diffusion, exchanges between solid and gas, as well as the mechanical strains of grains. The scale at which concentrations and strains are taken into account is smaller than grain size (0.1 μm for green pellets and 10 μm for sintered ones). The numerical resolution schemes of this problem have been conceived, and are also presented. They mainly consist of relaxation procedures to uncouple partial derivative mass conservation equations, flux balances along interfaces, non-linear potential equality conditions at interfaces, and Navier–Lamé equations over each grain. A code (SALAMMBO) is under construction, based on the latter; once validated by experimental and parametric tests, this code will describe Pu distribution, grain and pore size distribution, and local density of the pellets as functions of the sintering conditions, thus enabling further developments of the fabrication process such as the obtention of advanced microstructures of improved MOX fuels.     

Manufacture,Characterization, and Release Profiles of Insulin-Loaded Mesoporous PLGA Microspheres

This paper reports the fabrication of insulin-loaded mesoporous microspheres by a double emulsion-solvent evaporation technique using poly(lactic acid-co-glycolic acid) (PLGA) as carrier materials. PLGA solutions with two different concentrations (4% and 5%) were used as the oil phases to fabricate the mesoporous microspheres. The morphology and the particle size distribution of final microspheres were studied by optical microscope, scanning electronic microscope (SEM), and Malvern 2600 sizer, respectively. The mesoporous microspheres were monodisperse with an average diameter of 7 ± 3.5 µm. Insulin, as a model drug, was encapsulated into the final microspheres. In vitro release studies suggested that insulin was continuously released from the medicated microspheres. Furthermore, the final microspheres obtained from 4% PLGA solution showed a small “burst release” effect for their dense structures, which shortened the lag time to the effective plasma concentration. To summarize, the insulin-loaded PLGA microsphere are very promising for use in pharmaceutical applications.     

Thermal and mechanical properties of uranium nitride prepared by SPS technique

Nitride fuel is a promising nuclear fuel in fast breeder reactor (FBR) or accelerator-driven subcritical reactor (ADSR) system. In this study, high-density UN pellets were prepared by Spark plasma sintering (SPS) technique. The sample density strongly depended on the sintering temperature and pressure, and the pellets with 90% of theoretical density were easily obtained with low sintering temperature and short sintering time without any milling process. The grain size and pore size were much smaller compared with those for samples prepared by conventional sintering process. Despite of the small grain size, the thermal conductivity remains the high value. The SPS process permits easy densification of nitrides without any deterioration of thermal and mechanical properties, considered to be suitable as a preparation method of nitride fuels.     

The influence of drug solubility and particle size on the pellet formulation in a rotoprocessor

Pellets containing drugs of different properties were prepared in a Rotoprocessor in order to study changes in the formulation process and resulting pellet characteristics. Diltiazem hydrochloride, diclofenac sodium, and theophylline were chosen as model drugs. Pellet size distribution, sphericity, density, hardness, friability, and repose angle were determined using standard methods. The amount of water as a wetting agent necessary for successful pellet formulation was observed for each sample and changed depending on drug solubility, concentration, and particle size. The pelletization of freely soluble diltiazem hydrochloride required 24.8-23.1% of the wetting agent and its amount decreased as the drug concentration increased. The demand for water in the formulation of theophylline pellets was 31.0-34.4% and it increased with increasing drug concentration. The pellet samples containing both drugs were easy to prepare. However, the cohesion of micronized diclofenac sodium particles negatively influenced both the pellet size distribution and the formulation process itself. When the drug concentration exceeded 40%, it was not possible to produce pellets of an appropriate size and the process was not reproducible.