堆内辐照氚增殖剂球床有效热导率分析

氘-氚聚变反应堆中,固态氚增殖剂包层能不断为聚变反应提供氚核素,是实现聚变反应堆商用的关键技术之一。由锂陶瓷小球堆积形成的球床形式的固态氚增殖剂包层具有比表面积大、产氚效率高等优点,是我国重点发展的氚增殖剂包层形式。氚增殖剂球床须能支撑在堆内辐照时的高温环境,这就要求氚增殖剂球床有较好的导热特性。球床的有效热导率在球床设计和辐照过程中的安全分析十分重要,因此在中国先进研究堆（CARR）开展了氚增殖剂球床在堆内辐照环境下的有效热导率测量实验。根据MCNP计算得出的球床发热功率,结合实验测量的球床温度分布反推得到氚增殖剂球床的有效热导率,并与广泛应用于球床有效热导率计算的改进型ZBS模型计算结果以及堆外实验结果进行对比分析,理论值与实验值能较好吻合。     

Effective Thermal Property Estimation of Unitary Pebble Beds Based on a CFD-DEM Coupled Method for a Fusion Blanket

Lithium ceramic pebble beds have been considered in the solid blanket design for fusion reactors.To characterize the fusion solid blanket thermal performance,studies of the effective thermal properties,i.e.the effective thermal conductivity and heat transfer coefficient,of the pebble beds are necessary.In this paper,a 3D computational fluid dynamics discrete element method(CFD-DEM) coupled numerical model was proposed to simulate heat transfer and thereby estimate the effective thermal properties.The DEM was applied to produce a geometric topology of a prototypical blanket pebble bed by directly simulating the contact state of each individual particle using basic interaction laws.Based on this geometric topology,a CFD model was built to analyze the temperature distribution and obtain the effective thermal properties.The current numerical model was shown to be in good agreement with the existing experimental data for effective thermal conductivity available in the literature.     

A Measurement Platform Scheme and Data Post-processing Method for Thermal Conductivity of Li<Subscript>4</Subscript>SiO<Subscript>4</Subscript> Pebble Bed

The effective thermal conductivity of tritium breeder pebble bed is an important thermal parameter and must be known for the thermo-mechanical design of solid tritium breeder blankets. In order to obtain the parameter, experimental measurement is an effective method. A measurement platform was designed by University of Science and Technology of China for CFETR solid blanket scheme to measure the immediate thermal conductivity data and study the effect of pebble bed temperature, the purge gas pressure and pebble deformation on the thermal conductivity of pebble bed. Measurements were performed based on about 1 mm diameter Li₄SiO₄ pebbles in the temperature range between 100 and 800 °C, with purge gas pressure ranging from 0.1 to 0.3 MPa. This paper described a measurement platform scheme by thermal probe method. On the other hand, for the sake of increasing the precision of thermal conductivity data transformed from temperature data, some improvements for the data post-processing using Monte Carlo inversion method were made in this paper too.     

氚增殖剂球床组件堆内辐照物理热工计算分析

为验证在中国先进研究堆(CARR)内进行国际热核聚变实验堆(ITER)氚增殖包层模块(TBM)辐照实验的可行性和安全性,进行了氚增殖剂球床组件堆内辐照物理及热工计算分析。氚增殖剂包层模块主要是固态氚增殖剂陶瓷球床。本文采用Monte Carlo粒子输运模拟程序对氚增殖剂球床进行堆内建模,计算球床的中子注量率、能量沉积和产额,得到不同功率下球床的中子注量率、发热功率和产氚速率以及球床组件引入反应堆的反应性。根据物理计算得到的组件各部件发热情况建立热工计算一维模型,通过更改反应堆功率得到满足实验要求的工况并采用三维程序进行验证。物理与热工计算分析的结果表明,在反应堆运行功率为20 MW的工况下球床组件各部件的温度均不超过限值。     

硅酸锂球床传热传质特性与吹扫气体流动的DEM-CFD耦合模拟

氦冷固态增殖包层是中国聚变工程实验堆（CFETR）的3种候选包层概念之一,氚增殖球床是包层的核心部件,采用硅酸锂颗粒作为氚增殖材料。球床结构对氚在球床内的输运行为及流动和传热均有重要影响。本文基于离散单元法（DEM）生成了满足氚增殖球床填充率要求的随机堆积结构,通过CFD计算获取了球床结构下氚在吹扫气体内的等效扩散系数及吹扫气体的流动特性,包括速度分布、压力分布及进出口压降;开展了外加热流及有内热源两种工况下球床等效导热系数的模拟。计算结果表明,球床结构下氚在吹扫气体内的等效扩散系数为二元气体扩散系数的40%;受球床结构影响,球床内存在流动迟滞区,壁面出现流动加速;拟合得到Ergun方程的黏性阻力系数C1=87;有内热源工况下的球床等效导热系数低于外加热流工况下的球床等效导热系数。     

Status and progress of Indian LLCB test blanket systems for ITER

The lead–lithium ceramic breeder (LLCB) TBM and its auxiliary systems are being developed by India for testing in ITER machine. The LLCB TBM consists of lithium titanate as ceramic breeder (CB) material in the form of packed pebble beds. The FW structural material is ferritic martensitic steel cooled by high-pressure helium gas and lead–lithium eutectic (Pb–Li) flowing separately around the ceramic breeder pebble bed to extract the nuclear heat from the CB zones. Low-pressure helium is purged inside the CB zone for in situ extraction of bred tritium. Currently the LLCB blanket design optimization is under progress. The performance of tritium breeding and high-grade heat extraction is being evaluated by neutronic analysis and thermal–hydraulic calculations for different LLCB cooling configurations and geometrical design variants. The LLCB TBM auxiliary systems such as, helium cooling system (HCS), lead–lithium cooling system (LLCS), tritium extraction system (TES) process design are under progress. Safety analysis of the LLCB test blanket system (TBS) is under progress for the contribution to preliminary safety report of ITER-TBMs. This paper will present the status of the LLCB TBM design, process integration design (PID) of the auxiliary systems and preliminary safety analysis results.     

Comparative studies for two different orientations of pebble bed in an HCCB blanket

The Indian Test Blanket Module(TBM) program in ITER is one of the major steps in its fusion reactor program towards DEMO and the future fusion power reactor vision. Research and development(RD) is focused on two types of breeding blanket concepts: lead–lithium ceramic breeder(LLCB) and helium-cooled ceramic breeder(HCCB) blanket systems for the DEMO reactor. As part of the ITER-TBM program, the LLCB concept will be tested in one-half of ITER port no. 2, whose materials and technologies will be tested during ITER operation. The HCCB concept is a variant of the solid breeder blanket, which is presently part of our domestic RD program for DEMO relevant technology development. In the HCCB concept Li_2TiO_3 and beryllium are used as the tritium breeder and neutron multiplier, respectively, in the form of a packed bed having edge-on configuration with reduced activation ferritic martensitic steel as the structural material. In this paper two design schemes, mainly two different orientations of pebble beds, are discussed. In the current concept(case-1), the ceramic breeder beds are kept horizontal in the toroidal–radial direction. Due to gravity, the pebbles may settle down at the bottom and create a finite gap between the pebbles and the top cooling plate, which will affect the heat transfer between them. In the alternate design concept(case-2), the pebble bed is vertically(poloidal–radial) orientated where the side plates act as cooling plates instead of top and bottom plates. These two design variants are analyzed analytically and 2 D thermal-hydraulic simulation studies are carried out with ANSYS, using the heat loads obtained from neutronic calculations.Based on the analysis the performance is compared and details of the thermal and radiative heat transfer studies are also discussed in this paper.     

高温球床壁面区域有效导热系数模型优化

有效导热系数用来表征高温气冷球床堆堆芯综合传热能力,提高球床有效导热系数的预测精度对于高温气冷球床堆的热工设计和安全分析十分重要。为了优化球床壁面区域有效导热系数模型,本文针对无序石墨球床有效导热系数开展数值研究,分析了无序堆积球床主体区域、近壁面区域以及壁面区域有效导热系数的分布特性。结果表明：壁面区域有效导热系数相对于主体区域和近壁面区域显著降低,其平均降幅约为22%。因此引入了修正系数C_w对ZBS模型在壁面区域进行优化,对于球床主体区域及近壁面区域修正系数C_w=1,对于壁面区域,修正系数C_w=0.78。通过与前期无序球床实验数据和南非HTTU实验数据的对比,验证了优化后的ZBS模型能较好地预测球床壁面区域有效导热系数。     

Thermal transient and the temperature profile in a HELICA mock-up simulated by a new finite element homogenous model

This paper deals with a numerical approach for simulating the thermal and mechanical behaviour of pebble beds used as breeder and neutron multiplier in breeding blanket of nuclear fusion reactor. The model of the pebble beds is based on the results of a theoretical and experimental research activity performed by the Authors on ceramic pebble beds (lithium ortosilicate and lithium metatitanate). The results of this activity permitted to determine the effective thermal conductivity of the beds, versus the temperature and the axial pressure and to implement a homogenous model of pebble bed in a FEM code.This paper illustrates an application of the implemented model, considering pebble beds under several cycles of heating and cooling. The examined geometry corresponds to the HELICA mock-up tested by ENEA in the research centre Brasimone. The experimental tests performed on HELICA have been used as a benchmark problem in order to assess the different approaches for simulating pebble beds. In this paper, the simulations performed with two-dimensional models are illustrated. Moreover the numerical results are compared with the experimental ones. Finally, a discussion on results obtained by other authors involved in the benchmark is reported.     

Solid breeder thermal transient under purge-line-break accidents

The breeder thermal performances under a purge line break have been analyzed for two blanket design options: a blanket design using a packed breeder bed and a blanket design using a sintered breeder product. Under a purge line break open to a vacuum environment, the packed bed breeder temperature exceeds its operating temperature limit at a faster rate than that of the sintered breeder blanket design for the same breeder temperature gradient. Depending on the breeder material and nominal operating conditions, the breeder reaches its maximum operating temperature in time ranging from 32 seconds to 125 seconds for a break area of 10 cm² in packed bed designs. However for the sintered product design, the consequence of this transient might not result in the breeder exceeding its maximum operating temperature if a reasonable contact pressure could be established at the interface. To reduce the safety hazards, the tritium concentration build up in the vacuum vessel in conjunction with the purge gas pressure inside the blanket module should be used as a measure for initiating the reactor shutdown for this type of accident. The consequence of the purge line break outside the vacuum vessel on the breeder transient thermal performance is less significant because of a longer transient time involved.     

高温气冷球床堆有效导热系数模型的改进

有效导热系数是高温气冷球床堆热工设计和安全分析程序中的基本参数,ZBS模型广泛应用于球床结构有效导热系数的预测。本文针对ZBS模型中的关键经验型参数——接触面积系数φ进行了分析,通过对不同堆积结构球床有效导热系数的数值分析,获得了12组接触直径比和配位数及其对应的φ值,然后通过多元线性分析获得φ的计算公式。与德国SANA实验结果进行比较,发现改进后的ZBS模型预测能力优于其他模型。改进后的ZBS模型的计算结果与先前实验测量的球床主体区域的有效导热系数吻合也很好。本文研究结果可为高温气冷球床堆的设计和安全分析提供理论支持。     

聚变堆混和球床包层中子学和热工水力特性研究

在聚变堆初步概念设计的基础上，针对固态包层设计路线，提出了一个先进的氦冷固态包层概念。设计采用Be₁₂Ti和Li₂TiO₃陶瓷小球混和球床，物理和化学相容性好；采用SiC作为结构材料，提高耐高温性能及氦气出口温度。计算结果表明：选择Be₁₂Ti和Li^₂TiO₃球体积比在2和4之间较合理；在Be₁₂Ti和Li₂TiO₃球体积比为3时，⁶Li富集度取30％~80％较适宜；球床的最高温度低于材料的温度限值，温度分布合理均匀。该方案可较大程度提高热效率和改善中子学以及氚增殖性能。     

Design optimization of first wall and breeder unit module size for the Indian HCCB blanket module

The Indian test blanket module(TBM) program in ITER is one of the major steps in the Indian fusion reactor program for carrying out the RD activities in the critical areas like design of tritium breeding blankets relevant to future Indian fusion devices(ITER relevant and DEMO).The Indian Lead–Lithium Cooled Ceramic Breeder(LLCB) blanket concept is one of the Indian DEMO relevant TBM,to be tested in ITER as a part of the TBM program.Helium-Cooled Ceramic Breeder(HCCB) is an alternative blanket concept that consists of lithium titanate(Li_2TiO_3) as ceramic breeder(CB) material in the form of packed pebble beds and beryllium as the neutron multiplier.Specifically,attentions are given to the optimization of first wall coolant channel design and size of breeder unit module considering coolant pressure and thermal loads for the proposed Indian HCCB blanket based on ITER relevant TBM and loading conditions.These analyses will help proceeding further in designing blankets for loads relevant to the future fusion device.     

Void fraction and effective thermal conductivity of binary particulate bed

In many industrial processes, solid particles of different sizes are mixed in different volume or mass fractions for various applications, primarily to reduce void volume or to increase the density of the mixture. A few of these processes include; production of high density ceramics, mortar, concrete, graphite, bricks and carbon blocks. Experiments were carried out with alumina and lithium titanate pebbles (size ≥ 1 mm) and particles (size < 1 mm) of different sizes and volume fractions in cylindrical and rectangular vessels to study the variation of void fraction with volume fraction of component pebbles and with large pebble to small pebble or pebble to particle size ratio. It was observed that the variation of void fraction with volume fraction of smaller pebbles or particles is ‘V-shaped’. Thus there exists a minimum void fraction and two different volume fractions of smaller pebbles or particles which can give same void fraction. The effect of void fraction on the effective thermal conductivity of binary bed of lithium titanate and alumina pebbles and particles of different sizes and volume fractions was investigated. From the experimental results it was found that the binary particulate bed has higher effective thermal conductivity that that of a unary particulate bed. Effective thermal conductivity of binary particulate bed is the maximum when its void fraction is the minimum. The binary particulate bed with less volume fraction of small particles has higher conductivity than that of the binary bed of higher volume fraction of small particles and having same void fraction. This is due to the fact that with increase in volume fraction of small pebbles or particles, the number of small-to-small and large-to-small pebbles and or particles contact point increases resulting in higher resistance to heat transfers. The experimental details and results are discussed in this paper.     

Design and validation of breeder unit of helium-cooled pebble bed blanket for demo fusion reactor

In the framework of European helium-cooled pebble bed (HCPB) blanket development, an HCPB breeder unit based on the design of pebble beds between flat cooling plates is proposed for a DEMO fusion reactor. The performances of the designed breeder units are validated by supporting analyses. By applying the thermal boundary conditions obtained by neutronics simulations for the DEMO reactor, results of finite element calculations of the breeder unit are analyzed in views of thermal-hydraulics and thermal stress to identify the adherence to maximum temperatures in structural and functional materials and the abidance by the stress criterion imposed by the structural material. The layout of the internal meandering channels in the cooling plates is optimized by using numerical methods. Finally, possible improvements of the new designed breeder unit are proposed.     

球床包层聚变—裂变混合堆热工安全分析

球床包层混合堆与板状元件包层混合堆相比较，前者在核燃料生产和安全方面可能具有更多的优越性。本应用ＴＨＥＲＭＩＸ程序和辅助程序对我国开发的托卡马克堆芯氮气冷却球床包层聚变－裂变合堆的包层进行了热工计算。计算中考虑了不同的燃料球材料及稳态，卸压和断流事故工况。计算结果表明，只要选用合适的燃料球材料和设置适当的控制保护系统，具有快速卸料罐的托卡马克堆芯氦气包层聚变－裂变混合堆的概念设计在安全上的可行的。     

Construction of PREMUX and preliminary experimental results,as preparation for the HCPB breeder unit mock-up testing

One of the European blanket designs for ITER is the Helium Cooled Pebble Bed (HCPB) blanket. The core of the HCPB-TBM consists of so-called breeder units (BUs), which encloses beryllium as neutron multiplier and lithium orthosilicate (Li₄SiO₄) as tritium breeder in form of pebble beds. After the design phase of the HCPB-BU, a non-nuclear thermal and thermo-mechanical qualification program for this device is running at the Karlsruhe Institute of Technology.Before the complex full scale BU testing, a pre-test mock-up experiment (PREMUX) has been constructed, which consists of a slice of the BU containing the Li₄SiO₄ pebble bed. PREMUX is going to be operated under highly ITER-relevant conditions and has the following goals: (1) as a testing rig of new heater concept based on a matrix of wire heaters, (2) as benchmark for the existing finite element method (FEM) codes used for the thermo-mechanical assessment of the Li₄SiO₄ pebble bed, and (3) in situ measurement of thermal conductivity of the Li₄SiO₄ pebble bed during the tests.This paper describes the construction of PREMUX, its rationale and the experimental campaign planned with the device. Preliminary results testing the algorithm used for the temperature reconstruction of the pebble bed are reported and compared qualitatively with first analyses completed with the FEM codes.     

Status of ceramic breeder pebble bed thermo-mechanics R&D and impact on breeder material mechanical strength

Among the international fusion solid breeder blanket community, there exists steady progress on the experimental, phenomenological, and numerical characterizations of the pebble bed effective thermo physical and mechanical properties, and of thermomechanic state of the bed under prototypical operating conditions. This paper summarizes recent achievements in pebble bed thermomechanics that were carried out by members of the IEA Fusion Nuclear Technology Subtask I Solid Breeding Blanket. A major goal is on developing predictive capability while identifying a pre-conditioned equilibrium stress state that would warrant pebble bed integrity during operations. The paper reviews and synthesizes existing computational modeling approaches for pebble bed thermomechanics prediction, and differentiating points of convergence/divergence among existing approaches. The progress toward modeling benchmark is also discussed. These advancements have led to a framework to help navigate future research.     

高氚增殖比包层的设计及热工水力分析

本文设计了一种高氚增殖比包层(HBRB),该包层采用多孔U-10Zr合金作为中子倍增剂,Li4SiO4球床作为增殖剂,低活化马氏体(RAFM)钢作为结构材料。在详细研究包层加工工艺、流量分配、中子性能等问题的基础上,完成了包层内部详细结构设计。利用中子学软件分析计算了包层的氚增殖比(TBR)和热沉积分布,并根据计算结果对包层进行热力耦合分析。结果表明:包层TBR较高,且核性能稳定;冷却剂的流量分配情况和压降合理;包层内各组件冷却充分,温度和结构材料热应力不超过限值。     

高温气冷堆热工分析模型改进与堆芯温度场分析

更准确地模拟球床式高温气冷堆堆芯温度分布,是反应堆安全分析尤其是超高温运行研究中的关键问题之一。由于堆芯球流运动具有不确定性,石墨块和碳砖等结构材料采用散体布置,堆内冷却剂流道复杂,对热工水力准确模拟造成困难,可进一步优化。本文结合HTR 10的结构特点和流道特征,简要分析了堆芯传热过程,说明了在热工模拟中准确划分结构和流道对获取更精确的堆芯温度分布的重要意义。详细梳理了冷却剂流动路径,改进了在THERMIX程序下建立的HTR 10原有热工分析模型,更合理地模拟了堆芯冷却剂漏流行为,使得模型对堆芯冷却剂流动和传热过程的描述更准确。与试验数据对比,改进后的模型对堆芯外围系统的温度分布模拟准确性显著提升。计算结果表明,反应堆在额定设计工况下满功率稳态运行时,燃料和反射层最高温度均未超过材料的耐热限值。