热金属水分解动力学及固定床反应器研究

【作者】 姚勇；

【导师】 罗德礼；

【作者基本信息】 中国工程物理研究院 ， 核燃料循环与材料， 2013， 博士

【摘要】 ITER是由欧盟、中国、日本等七方共同参与建造的磁约束核聚变实验装置。其氚增殖实验包层模块(TBM)的主要目的是验证未来核聚变反应堆氚自持的可行性和可靠性,是ITER的核心技术之一。中国设计采用氦冷陶瓷增殖剂实验包层(HCCB),采用含0.1%H2/He作为吹洗气经同位素交换提取模块中的氚,通过氚提取系统(TES)分析评估实验模块的总产氚量和元素氚与氚化水的份额。为了实现上述目的,需要将以氚化水形态存在的氚转化为元素态氚以便有效回收氚和分析不同形态氚的量。热金属法是一种可靠的氚水分解方法,其原理是用金属(或合金)将氚化水还原成元素态的氚。该方法不需要引入复杂的机械体系,具有体积小、成本低和安全可靠等优点,欧盟和中国的HCCB-TBM-TES设计中都采用了该方法。但目前还没有针对TBM特殊要求的相关实验研究报导。因此有必要对水分解材料和水分解反应器进行相应研究,为TBM-TES氚化水处理床的设计提供技术支撑。本文面向TBM-TES氚化水处理需求,根据气固反应器研究的一般方法开展了下述两方面工作：①材料水分解动力学实验,考察四种侯选材料的水分解动力学性能,建立水分解动力学方程；②固定床反应器研究,考察反应器的质量、能量传递过程,并与动力学方程耦合,建立了相应的反应器模型方程。具体内容如下：(1)基于热金属床填充材料氚滞留量低的要求,在分析文献的基础上,优选了ZrMnFe(ST909)和ZrNi5两类商用锆合金；并采用溶液燃烧-钙还原法制备了超细ZrMnFe合金微粉。考察了这三种锆合金的高温吸氢性能和水分解动力学性能。试验结果表明1000Pa氢压下吸氢质量比均<0.04‰,能够满足热金属床低氚滞留量的要求。在673K-823K范围内考察了三种材料在不同温度和水浓度条件下的水分解反应动力学性能,结果表明ST909的水分解反应可用Ginstring-Brounshtein模型方程描述,反应活化能为118kJ·mol-1;化学合成ZrMnFe可用Jander模型方程描述,反应活化能是47kJ·mol-1,活化能较ST909显著降低；ZrNi5可用均相反应模型方程描述,反应活化能是55.8kJ·mol-1。上述各反应级数均为1级。(2)针对热金属材料能够可逆使用的要求,考察了Fe/Fe304体系的水分解反应动力学。采用溶液燃烧-氢还原法合成纳米铁粉,平均反应速率较微米级铁粉提升10倍以上。纳米铁粉中掺加Zr能显著提升抗烧结性能,优化添加量是l0at.%；添加Rh能显著提升反应动力学性能,优化添加量是1at.%。锆铑共掺杂的优化体系是Zr10%-Rhl%-Fe,循环20次后其动力学性能仍无明显下降,水分解反应级数为1级,反应过程可用均相反应方程描述,反应活化能是30.8kJ·mol-1。(3)进行了微型反应器的实验和模型验证。一维拟均相恒温模型的计算结果与恒温微型反应器的实验结果一致性良好,表明该动力学模型能够较好地描述微型反应器的实际情况。通过对质量衡算方程的变形处理,建立了归一化传质带的计算方法；对比表明ST909的归一化传质带长度是其余三种材料的10倍以上,不适合作为TBM-TES水分解反应器的填充材料。建立了由归一化传质带结合反应器操作参数的穿透时间预测方法,实验结果表明该方法可较准确预测穿透点,可作为反应器设计的基本方法。(4)考察了原料气中加氢对穿透曲线的影响,结果表明氢对锆系合金穿透曲线没有明显影响,TBM-TES工况下不需要考虑氢对锆合金水分解性能的影响。但氢会严重降低铁的水分解效率,铁不适合作为TBM-TES在线氚化水分解材料。考察了同等条件下D2O替换H2O和He替换Ar的穿透曲线,结果表明水质量数改变及载气质量数改变对穿透曲线没有明显影响；TBM-TES工况下不需要考虑HTO与H2O质量数差异对反应速度的影响。(5)针对TBM-TES中试规模反应器,分别考察了反应器的动量传递(床层压降)、轴向扩散和能量传递过程。首先以二氧化锆球形颗粒作为模拟填料,考察了颗粒直径、气体流速、温度对压降的影响规律,压降可采用McDonald方程进行描述。试验结果表明粉体材料装填反应器压降大,必须对材料进行造粒处理才能满足TBM-TES低压降要求,适宜的造粒颗粒直径为2mm。针对惰性核滚动造粒颗粒,建立了由粉体动力学方程和孔扩散两项组成的修正动力学方程,获得了造粒颗粒的传质带修正方程。考察了轴向扩散对质量传递过程的影响,结果表明轴向扩散系数增大会导致传质带增长,传质带利用率下降；采用数值方法建立了轴向扩散系数对传质带的修正方程。按一维非均相绝热模型计算了反应器的热点温度,热点温度受水浓度影响显著,而与流速基本无关；TBM-TES工况由于水浓度为ppm量级,可视为等温反应。基于修正传质带的穿透点预测方法可以较准确预测中试反应器的穿透点。(6)进行了手套箱惰性气氛氚化水热金属床分解实验,针对放射性源项为0.1Ci/L氚化水的手套箱惰性气氛,手套箱氚化水浓度随循环时间的下降而快速下降,流出热金属床的氚为元素态,实验结果验证了轻水分解模型,表明该模型可用于TBM-TES氚化水分解反应器设计。(7)建立了基于传质带方法的TBM-TES氚化水分解反应器的设计程序；拟定10ppm和100ppm两种载气水含量,根据反应器压降5000Pa的设计标准,分别对装填ZrNi5、化学合成ZrMnFe和Zr10%-Rhl%-Fe的反应器进行了设计,获得相关的反应器尺寸、利用率等参数。更多还原

【Abstract】 ITER is an experimental fusion reactor based on magnetic confinement fusion, with the cooperation of seven countries including EU, China, Japan, et al. The fundamental purpose of TBM (Test Blanket Module) is to verify the feasibility and reliability of tritium self-sufficiency in fusion reactor. China has decided to develop and test the helium-cooled ceramic breeder (HCCB) test blanket module. The tritium in breeder materials will be extracted by0.1%H2/He, hydrogen is used to enhance the performance of isotope exchanging. There will be some amount of tritiated water in the purge gas. In order to evaluate the total amount of tritium and recovey tritium from tritiated water, tritiated water should be transfered to elemental tritium effectively.Hot metal bed is a convenient and reliable method to decompose tritiated water, the principle is reduction of water by metal/alloy to produce hydrogen. This method is simple, without mechanics, compact, cost-effective and safe. Hot metal bed has been designed to split HTO in HCCB-TBM TES both by EU and China, but nowadays the is still no corresponding research report, so it is necessary to carry out experimental research to provide project for water decomposition in TBM TES. The general research method for gas-solid reaction was used in this study, and the detailed work is as following:(1) Based on the demand of low tritium inventory in material, two kinds of commercial zirconium alloy was selected:one is ST909(ZrMnFe), another is ZrNi5. In addition, ultrafine ZrMnFe was synthesized by solution-combustion and calcium reduction method aimed to improving dynamics property. The hydrogen absorption properties was investigated in the temperature range673-773K and with hydrogen pressure below1000Pa, the hydrogen absorption mass ration is less than0.04%o for all the three alloys, they can meet the demand of low tritium inventory in TBM TES. The kinetics of reaction between the three alloy and water was studied, the results showed that the process of ST909could be described by Ginstrign-Brounshtoerin model with the activation energy118kJ· mol-1, the synthesized ZrMnFe could be described by Jander model with the activation energy47kJ· mol-1, and ZrNi5could be described by homogeneous reaction model with the activation energy55.8kJ· mol-1;the reaction are all one order for water.(2) To meet the demand of renewablity of material, the system of Fe/Fe3O4was selected. Nanometer Fe powder was synthesized by solution combustion-hydrogen reduction, and the reaction rate increased more than10times than micrometer Fe powder. The recyclability was promote by addition of zirconium and the optimal proportion is10at%; the addition of Rh significantly improve dynamics performance and the optimal proportion is lat%. Zr10%-Rhl%-Fe was selected as the optimal system, cycle experimental result showed that the dynamics performance didn’t drop even after20cycles, the kinetics experiment indicated that the reaction is first order for water and the process could be described by homogeneous reaction model with the activation energy30.8kJ· mol-1.(3) The water decomposition experiment in micro-fixed bed reactor was carried out for above four materials and the breakthrough performance was investigated at different conditions. One-dimensional isothermal plug-flow reactor model was established to predict the breakthrough curves. The outcomes of modeling agree well with the experimental results, it showed that the reaction model is correct and is suitable to describe the reaction process. The normalized mass transfer zone was calculated by the modified mass balance equation, the results showed that the length of the mass transfer zone of ST909is about ten times larger than the other three materials, so ST909is not suitable to be used as packing material in TBM-TES water handling. A method for predicting the breakthrough point was established based on the normalized mass transfer zone, the breakthrough time could be accurately predicted by this method.(4) The breakthrough performance with hydrogen added in the reactant gas was investigated; the result showed that hydrogen adding has no significant effect on the breakthrough curve of zirconium alloys; but hydrogen has significant negative effect on Fe, so Fe is not suitable for online water decomposition in TBM-TES purge gas. The breakthrough curve of D2O instead of H2O and He instead of Ar was also investigated with the same condition, the result showed that the difference in mass of water or purge gas didn’t affect the reaction rate.(5) In order to design the larger scale reactor, momentum transfer mass transfer and energy transfer process were investigated. Firstly, the pressure drop in fixed-bed reactor packed with three size of zirconia pebbles was studied at different temperature and flow rate, pressure drop was calculated by McDonald equation and Ergun equation, the result showed that McDonald equation can accurately describe the pressure drop; the powder of metal/alloy must be made into pellet, and the pellet with the diameter no less than2mm will be needed in TBM-TES HTO decomposition reactor. Secondly, the kinetics of pellet was investigated and the kinetic equation was described by the grain equation adding the diffusion equation of water into interval in particle; the length of the mass transfer zone of pellet is larger than powders. Thirdly, the axial dispersion coefficient was investigated, the mass transfer zone will prolong with the increasing axial dispersion coefficience and the utilization rate of mass transfer zone will decrease. Fourthly, hot point temperature in fixed-bed was investigated, the result showed that hot point temperature depends directly on water concentration in reactant gas but didn’t affected obviously by the flow rate. The water concentration in TBM-TES is ppm grade, the water decomposition reactor can be treated as an isothermal.(6) The experiment of practical tritiated water decomposition was carried out in glove-box closed system, the result indicated that tritiated water could be splited completely by the ZrNis bed; the conversion efficiencies of H2O and HTO were measured independently and were observed to be similar, so the isotope effect in the reaction needn’t be considered.(7) The water decomposition reactor for TBM-TES was designed based on the mass transfer zone theory. The programs were established for ZrNi5, ultrafine ZrMnFe and Zr10%-Rhl%-Fe, the parameters can be obtained in these programs, such as dimension of reactor、the loading mass of alloy or metal、the rate of utilization、et al.更多还原