【作者】 张浩；

【导师】 李正宏； 叶民友；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2017， 博士

【摘要】 作为聚变堆的核心功能部件,聚变包层承担着实现高效核-热转换及氚自持的重任。按照氚增殖剂形态,聚变堆氚增殖包层可分为液态包层和固态包层两种形式。固态包层由于其较好的材料相容性、热稳定性以及无磁流体动力学(MHD)效应等优点被认为是聚变堆包层的重要候选结构设计之一。由于面临较高的破碎风险及难以预测的界面接触,固态包层增殖剂结构设计从早期烧结的块状或者柱状结构形式逐渐演变为增殖剂球床结构形式。我国ITER-TBM氚增殖区采用氦气冷却的正硅酸锂(Li4SiC4)球床填充结构设计,Li4SiO4球床安全、稳定地运行将直接影响该包层核热转换及氚增殖比。Li4SiO4球床在聚变中子辐照下产生的热量由高压冷却氦气载出发电,同时增殖的氚由低压氦气载出以维持聚变堆氚自持。由于聚变堆内严苛的运行环境及球床特殊的离散堆积结构,Li4SiO4球床内载气流动及热-力行为异常复杂。从20世纪90年代开始,国际上针对陶瓷增殖剂球床内载气流动特性及热力性能开展了广泛的实验及理论研究,但目前还未能透彻认识上述行为特性及机理。为了进一步认识Li4SiO4球床内气体流动及热-力行为特性,本文利用实验与模拟相结合的研究思路,对影响上述行为特性的关键影响因素进行了系统的研究与分析。模拟部分,采用离散单元法(DEM)实现球床堆积结构及循环单向压缩力学性能测试;使用离散元结合计算流体力学(CFD)的方法实现球床内载气流动以及组分混合输运过程模拟。实验部分,通过自研陶瓷增殖剂球床热力耦合性能测试平台,实现陶瓷增殖剂球床等效导热系数及在单向压缩力学实验下的整体应力-应变行为响应特性测量;同时采用显微电镜对实验前后的Li4SiO4颗粒微观形貌特征进行观测。基于DEM-CFD数值模拟方法,随机填充球床结构由DEM生成并采用径向孔隙率分布验证其模型有效性,划分网格后采用CFD计算分析得到球床内颗粒间隙间气体流动特性。计算获得球床内吹扫气体径向流速分布与孔隙率震荡衰减波动规律一致,不同入口流速下(0.05～0.2 m/s)流速归一化分布相同。近壁面区域存在流速极大值,当球床无量纲尺寸小于20时,壁面漏流现象严重。吹扫气体在流向方向上的压降可由Blake-Kozeny关系式较好的预测。球床内增殖的氚能够迅速的被吹扫氦气载出,近壁面吹扫效率更高;在球床内颗粒无破损情况下,球床内不存在氚滞留现象。为了描述球床内部详细的结构及接触力学行为特征,本文基于伺服控制机理,建立了 L4SiO4球床循环单向压缩试验离散元模拟方法,循环加载目标压力为2 MPa至6 MPa。模拟得到球床内颗粒接触应力在不同的外力约束下存在类似的归一化概率密度分布,平均接触力随外力加载提高而增大。在循环加载-卸载实验中,球床填充密实度提高,内部颗粒接触应力有所缓解,球床应力-应变响应高度非线性。Li4SiO4颗粒属性对球床填充及接触力学影响明显,颗粒尺寸分布有利于球床填充密实度并缓解球床内应力;而较低的球形度和较高的表面摩擦系数对球床填充密实度及内应力缓解均存在不利的影响。通过颗粒缩径及分割的方法进行球床内颗粒粉化及破碎模拟,计算得到较低的颗粒粉化率及破碎率即可造成球床填充结构及颗粒力学行为的明显变化。根据上述模拟结果,本文设计搭建了一套氚增殖剂球床热力耦合实验装置。通过结合稳态法导热系数测量与单向压缩力学测试方法,针对Li4SiO4球床密实填充、热力蠕变响应、等效导热系数等特性进行了系列实验,球床实验温度为20～700℃,单向压缩力学强度为1～5 MPa。在常温循环加载实验下,Li4SiO4球床在各自压力下存在有限的密实化行为;而当球床处于550 °C平均温度时,随外力加载循环次数增加,Li4SiO4球床发生的不可逆蠕变形变较难停止。随着加载压力提升,Li4SiO4球床整体塑性形变增加。热力耦合实验后,Li4SiO4球床尤其是在靠近加载压头的区域的填充因子获得很大提高,Li4SiO4球床等效导热系数随球床填充因子及其平均温度提高而增大。相比于实验前规则、光滑的Li4SiO4颗粒形貌,热力耦合实验后的Li4SiO4颗粒烧结、粉化现象突出。某些颗粒表面覆盖熔融析出物阻碍颗粒表面释氚孔隙,并且颗粒间接触区域存在粘连以及塑性变形现象。大量颗粒表面出现了不同程度的裂纹,球床内出现了许多Li4SiO4碎片及粉末。本文建立了 Li4SiO4球床内气体流动及热力耦合行为研究的基本实验及模拟方法,获得了球床内详细的气体流动、混合输运及流量分配特征,进一步深刻认识了球床填充因子、材料属性、温度梯度以及加载方式等因素对球床热-力行为的影响规律。本文不仅为后续Li4SiO4球床性能研究奠定了基础,同时可作为氚增殖包层结构及热工水力设计的重要参考依据。更多还原

【Abstract】 As the primarily functional part of fusion reactor,the fusion blankets are responsible for the high-efficient generation of nuclear heat and self-sufficiency of tritium.Based on the morphology of tritium breeder,the fusion blankets can be classified into solid breeder blankets and liquid breeder blankets.Owing to the advantages of good compatibility between materials,thermostabilization and without magnetohydrodynamic(MHD)effect,the solid breeder blankets are considered as one of the most important candidate for the design of fusion blankets.Because of the high tendency of breeder crush and the unpredictable contacts between the interfaces,the sintered breeder blocks or pins for the early design of the fusion blankets gradually developed as the configuration of pebble bed.The helium cooled lithium orthosilicate(Li4SiO4)pebble bed is chosen as the breeder zone of the China ITER-TBM,safe and stable operation of the Li4SiO4 pebble bed can directly affect the nuclear heat transformation and tritium breeding ratio of the blanket.The nuclear power is generated in the Li4SiO4 pebbles under the irradiation of fusion neutrons with heating the high-pressure helium for electricity generation,while the breeding tritium released from the Li4SiO4 pebbles are purged out by low-preesure helium for tritium self-sufficiency.Due to the rigorous operating condition in the fusion reactor and the discrete packing structures of the pebble bed,the gas flow and thermo-mechnical behaviors are particularly complex in the Li4Si04 pebble bed.Since the 1990s,the gas flow and thermo-mechanical behaviors of the Li4SiO4 pebble beds had beed widely studied by experimernts and simulation throughout the world,while a thoroughly understanding and mechanism of the above behaviors have not achieved yet.To get a deeper understanding of the properties of the gas flow and thermodynamics in Li4SiO4 pebble beds,the influencing factors for the properties were systematically examined by experiments and simulations in this thesis.In the simulations,the discrete element method(DEM)was adopted for the pebbles packing and cyclic loading tests,while the purge gas flow and species transport process were computed by computational fluid dynamics(CFD)method.In the experiments,the effectivel thermal conductivities and stress and strain responses were measured by an in-house experimental apparatus for thermal-mechanical tests of tritium breeder pebble bed.Besides,the microscopic morphologies of the beeder pebbles were observed with the electron microscope.Based on the DEM-CFD simulating method,the randomly packed pebble bed was generated by DEM and verified with the radial porosity distribution,after gridding division,the flow characteristics are computed and analysed by CFD method.The results showed that the radial velocity distribution of the flow was the same pattern with porosity distribution in the pebble bed,and the distributions of the normalized flow velocities under different inlet velocities were nearly the same.The gas flow was accelerated to a maximum in the near wall region of the pebble bed,and the wall channeling is serious when the non-dimensional size of the pebble bed is smaller than 20.Furthermore,the pressure drop of the gas in the flow direction can be well predicted by Blake-Kozeny experssion.Moreover,the breeding tritium can be efficiently purged out by helium and especially in the near wall region,and there is no tritium retention in the pebble bed without pebble crush.To demonstrating the detailed packing structure and the internal contact mechanics of the pebble bed,the cyclically uniaxial compression tests of Li4SiO4 pebble beds were developed by DEM method with servo-control mechanism,the target loading pressure ranged from 2 MPa to 6 MPa.From the simulating results,the normalized contact forces between the pebbles of the pebble beds under different loading pressures had the same probability distributions,and the average contact forces increased with increasing loads.During the cyclic loading and unloading tests,the packing factors of the pebble beds increased while the contact forces between the pebbles allevieated,the stress-strain responses of the pebble beds were highly nonlinear.The packing behaviors and contact mechanics of the pebble bed were obviouly affected by the paroperties of the Li4SiO4 pebbles,the size distribution was advantageous for the densly packing of the pebble bed and alleviated the contact forces between pebbles,while the lower sphericity and higher surface friction of the pebbles were disadvantageous for the densely packing and mechanics of pebble beds.Furthermore,the pulverization and fragmentation phenomena of the pebbles were simulated with the shrinkage and segmentation of the pebbles.The simulation showed that a relatively lower ratio of pulverized and cracked pebbles can resulted with evident changes in the pebble bed packing structure and contact mechanics of the pebbles.According to the simulating results,an experimental apparatus for measuring the themo-mechnical properties of the Li4SiO4 pebble beds were constructed.By coupling the method of effective thermal conductivity with steady heat transfer and the uniaxial compression test,the densly packing behaviors,thermal creep responses and effective thermal conductivities of the Li4SiO4 pebble beds were systematically researched by this apparatus,the average temperature of the pebble beds ranged from 20℃ to 700℃ with the loading pressure ranged from 1 MPa to 5MPa.During the cyclic loading tests at room temperature,the packing factor of the pebble bed had limited increase under each loading pressure,while the irreversible thermal creep of the pebble bed increased continuously at temperature of 550℃.With the increasing loading pressure,the plastic deformations of the pebble bed increased.After the coupled thermal-mechnical experiments,the packing factor of the Li4SiO4 pebble bed,especially in the region near the loading plate,had increased much,and the effective thermal conductivities increased with the increasing packing factor and the average temperature of the Li4SiO4 pebble bed.Compared with the regular and smooth shapes of the Li4SiO4 pebbles before experiments,the sintering and pulverization of the Li4SiO4 pebbles were widespread after thermo-mechanical experiments.On the surfaces of some Li4SiO4 pebbles,the tritium release pores were covered with molten compound,and the contact regions between the pebbles were overlapped with sintered bonds or plastic deformations.Besides,considerable cracks were existed on the surfaces of many Li4SiO4 pebbles,and there were crushed Li4SiO4 pebbles in the pebble bed.In this thesis,basic experimental and simulated methods had been established for the research of gas flow and thermo-mechanical behaviors in the Li4SiO4 pebble bed,the gas flow characteristics,species transport and flow distribution were obtained,and further deeply understanding of the packing factor,material peoperties,temperature gradient and loading patterns on the thermal-mechanical behaviors in the Li4SiO4 pebble bed was achevied.This dissertation is not only the foundation for further studies of Li4Si04 pebble bed,but also can be a significant reference for the structural and thermo-hydraulic design of the tritium breeding blankets.更多还原