Fracture mechanical analysis of tungsten armor failure of a water-cooled divertor target

The inherent brittleness of tungsten at low temperature and the embrittlement by neutron irradiation are its most critical weaknesses for fusion applications. In the current design of the ITER and DEMO divertor, the high heat flux loads during the operation impose a strong constraint on the structure–mechanical performance of the divertor. Thus, the combination of brittleness and the thermally induced stress fields due to the high heat flux loads raises a serious reliability issue in terms of the structural integrity of tungsten armor. In this study, quantitative estimates of the vulnerability of the tungsten monoblock armor cracking under stationary high heat flux loads are presented. A comparative fracture mechanical investigation has been carried out by means of two different types of computational approaches, namely, the extended finite element method (XFEM) and the finite element method (FEM)-based virtual crack tip extension (VCE) method. The fracture analysis indicates that the most probable pattern of crack formation is radial cracking in the tungsten armor starting from the interface to tube and the most probable site of cracking is the upper interfacial region of the tungsten armor adjacent to the top position of the copper interlayer. The strength threshold for crack initiation and the high heat flux load threshold for crack propagation are evaluated based on XFEM simulations and computations of stress intensity factors and J-integrals.     

Design and cooling of the edge segments of the DEMO divertor target plates

In support of shadowing of the divertor target plate edges in toroidal direction against damage caused by the incident particles, the fingers at the boundary of the target plate should ideally form a flat surface. The reference cooling fingers are of hexagonal shape and when assembled together, their edge boundary cannot be flat. Therefore, the boundary segments need to be designed in a different way. Three possible designs are investigated: non-symmetric pentagonal fingers and two square-shaped fingers of different sizes, all cooled by the same type of concentric cartridge as in the reference design. Their heat transfer performance is analyzed from the point of view of maximum allowable temperature of the thimble structure. The computational fluid dynamics (CFD) analysis is performed to obtain the minimum mass flow rate of the coolant which is necessary to keep the structure's temperature below the permissible limit at an acceptable pressure loss.     

Comparison between FEM and high heat flux thermal fatigue testing results of ITER divertor plasma facing mock-ups

The divertor is one of the most challenging components of “DEMO” the next step ITER machine, so many tasks regarding modeling and experiments have been made in the past years to assess manufacturing processes, materials and thus the life-time of the components. In this context the finite element analysis (FEA) allows designers to explore multiple design options, to reduce physical prototypes and to optimize design performance.The comparison between the hydraulic thermal-mechanical analysis performed by ANSYS WORKBENCH 14.5 and the test results [1] on small-scale mock-ups manufactured with the Hot Radial Pressing (HRP) [2] technology is presented in this paper.During the thermal fatigue testing in the Efremov TSEFEY facility to assess the heat flux load-carrying capability of the mock-ups, only the surface temperature was measured, so the FEA was important because it allowed to know any other information (temperature inside the materials, local water temperature, local stress, etc.). FEA was performed coupling the thermal-hydraulic analysis, that calculated the temperature distributions on the components and the heat transfer coefficient (HTC) between water and heat sink tube, with the mechanical analysis.The comparison between analysis and testing results was based on the temperature maps of the loaded surface and on number of the cycles supported during the testing and those predicted by the mechanical analysis using the experimental fatigue curves for CuCrZr-IG, that is the structural material in the component. Also the behavior for Cu-OFHC interlayer material based on the experimental fatigue curves was considered and the ultimate tensile strength for W, because their failure affects the heat removal capability of the component.The good correlation found between FEA results and testing campaign validated again the use of FEA itself for future design improved concepts.     

Potential and limits of water cooled divertor concepts based on monoblock design as possible candidates for a DEMO reactor

In this paper water-cooled divertor concepts based on tungsten monoblock design identified in previous studies as candidate for fusion power plant have been reviewed to assess their potential and limits as possible candidates for a DEMO concept deliverable in a short to medium term (“conservative baseline design”). The rationale and technology development assumptions that have led to their selection are revisited taking into account present factual information on reactor parameters, materials properties and manufacturing technologies.For that purpose, main parameters impacting the divertor design are identified and their relevance discussed. The state of the art knowledge on materials and relevant manufacturing techniques is reviewed. Particular attention is paid to material properties change after irradiation; phenomenon thresholds (if any) and possible operating ranges are identified (in terms of temperature and damage dose). The suitability of various proposed heat sink/structural and sacrificial layer materials, as proposed in the past, are re-assessed (e.g. with regard to the possibility of reducing peak heat flux and/or neutron radiation damages). As a result, potential and limits of various proposed concepts are highlighted, ranges in which they could operate (if any) defined and possible improvements are proposed.Identified missing point in materials database and/or manufacturing techniques knowledge that should be uppermost investigated in future R&D activities are reported.This work has been carried out in the frame of EFDA PPPT Work Programme activities.     

Results and consequences of high heat flux testing as quality assessment of the Wendelstein 7-X divertor

The series manufacturing of the first 282 Wendelstein 7-X divertor elements was concluded in 2011. The divertor is designed to remove a steady-state heat load of 10 MW/m². 940 target elements of five different types made of CuCrZr heat sinks and covered with 16,000 CFC NB31 flat-tiles have to be produced. Additional to quality assessment during the manufacturing process, a final assessment of the delivered elements with operational heat load is indispensable to ensure a constant high thermal performance of the installed divertor.Based on the results of the pre-series testing a statistical quality assessment method has been developed for the series production. The application of this method to the series elements ensures their thermal performance with reasonable high heat flux test effort.     

Experimental study on heat transfer and pressure drop in mercury flow system for spallation neutron source

In the design of MW-class spallation target system, using mercury to produce practical neutron applications, keeping the highest level of safety is vitally important. To establish the safety of spallation target system, it is essential to understand the thermal hydraulic properties of mercury. Through thermal hydraulic experiments using a mercury experimental loop, which flows at the rate of 1.2 m³/hr maximum, the following facts were experimentally confirmed. The wall friction factor was relatively larger than the Blasius correlation due to the effects of wall roughness. The heat transfer coefficients agreed well with the Subbotin correlation. Furthermore, for validation of the design analysis code, thermal hydraulic analyses were conducted by using the STAR-CD code under the same conditions as the experiments. Analytical results showed good agreement with the experimental results, using optimized turbulent Prandtl number and mesh size.     

Conceptual design and analysis of a passive residual heat removal system for a 10 MW molten salt reactor experiment

A conceptual design of a passive residual heat removal system was developed for a 10 MW molten salt reactor experiment (MSRE) designed by Oak Ridge National Laboratory (ORNL). The principle, main components and design parameters of the system were presented, and thermal-hydraulic behaviors, such as natural circulation and heat removal ability, were numerically analyzed in the code of C++, especially for the bayonet cooling thimbles. The results show that the system can effectively remove decay heat in the molten salt in an MSRE and has a heat removal rate that approximates to the decay heat generation rate, thus causing the temperature of the molten salt to decrease steadily. The width of the gas gap in the bayonet cooling thimbles has little effect on either the heat exchange or the natural circulation inside the thimbles, while the width of the steam riser, in spite of its slight effect on the heat transfer of the system, greatly influences the natural circulation. With the width of the steam riser increase from 3.6 to 5.1 mm, the mass flow rate increases from 1.9 kg/s to 4.79 kg/s. Finally, three operational schemes were proposed for the passive residual heat removal system, among which that of reducing the bayonet cooling thimbles by three-quarters had the best comprehensive performance.     

快堆主容器内氩气空间传热实验原理及分析

本文研究了池式快堆主容器内氩气空间模拟实验的模拟准则及原理.根据模拟准则和对流换热方程组,研究了模拟实验装置需满足的物理相似和边界相似的条件,并对氩气空间的换热状况进行了计算,确定了实验装置的设计参数范围,从理论上解决了池式实验快堆钠池覆盖气体--氩气空间模拟实验装置的模拟问题.     

Modelling of heat and mass transfer processes during core melt discharge from a reactor pressure vessel

The objective of this paper is to study the heat and mass trasnfer processes related to core melt discharge from a reactor vessel in a light water reactor severe accident. The phenomenology modelled includes the convection in, and heat transfer from, the melt pool in contact with the vessel lower head wall, the fluid dynamics and heat transfer of the melt flow in the growing discharge hole and multi-dimensional heat conduction in the ablating lower head wall. A research programme is underway at the Royal Institute of Technology (Kungliga Tekniska Högskolan, KTH) to (1) identify the dominant heat and mass transfer processes determining the characteristics of the lower head ablation process: (2) develop and validate efficient analytical/computational models for these processes; (3) apply models to assess the character of the melt discharge process in a reactor-scale situation; (4) determine the sensitivity of the melt discharge to structural differences and variations in the in-vessel melt progression scenarios. The paper also presents a comparison with recent results of vessel hole ablation experiments conducted at KTH with a melt simulant.     

高温液滴流辐射换热及蒸发特性

本文针对可应用于空间堆的矩形液滴辐射器（LDR）,研究其液滴流的辐射换热及蒸发特性。在传统的高温液滴流辐射换热模型的基础上,添加了液滴流蒸发模型,并将辐射换热模型与蒸发模型进行耦合,在该模型的基础上开发了高温液滴流辐射换热-蒸发特性分析程序LDFAC。将该程序的液滴层温度分布计算结果进行校核,其相对误差不超过1.9%。使用该程序对装载DC705硅油下不同光学厚度及长度的液滴层辐射换热蒸发特性进行了分析。结果表明：在液滴层的光学厚度较大的情况下,液滴层内部的温度分布非常不均匀,液滴层中心的温度没有显著降低,而液滴层接近外表面部分的温度下降较为明显;温度对LDR的系统寿命有着较大影响,温度每降低10 K,系统寿命可提高约450%,同时,液滴层光学厚度越大,系统寿命也越长。     

Numerical simulations on gaseous flow and heat transfer in multiple narrow channels of IFMIF High Flux Test Module

The International Fusion Materials Irradiation Facility (IFMIF) is designated to generate a materials irradiation database for the future fusion reactors. In the High Flux Test Module (HFTM) the test specimens will undergo a severe structural damage caused by neutron fluxes. The HFTM will be with helium gas. This paper presents the comprehensive thermo-hydraulic simulations of the HFTM as a part of the design activities. The turbulence models were assessed by comparing the simulations with in-house annular channel experiments. Since the required coolant flow rates are different for different compartments, multiple fluid domains were employed and simulated with appropriate turbulence (laminar) models individually. The flow distributions and heat transfer characteristics among various HFTM sub-channels will be discussed. Sensitivity study was carried out to assess the impacts of several factors on the simulation results.     

Experimental and numerical study on transient heat transfer for a twisted plate with different length in helium gas at various velocities

This study is aimed to investigate the transient heat transfer process between the solid surface and the coolant (helium gas) in very high temperature reactor or intermediate heat exchanger. Transient heat transfer from a twisted plate with different length in helium gas was experimentally and theoretically studied. The heat generation rate was increased with an exponential function, Q = Q₀exp(t/τ), where t is time and τ is period. Experiment was carried out at various periods ranged from 35 ms to 14 s. Platinum plates were twisted with the same helical pitch of 20 mm, and the effective lengths are 26.8, 67.8 and 106.4 mm (pitch numbers of 1, 3 and 5), respectively. It was clarified that the average heat transfer coefficient approaches quasi-steady-state value when the period τ is larger than about 1 s, and it becomes higher when τ is shorter than about 1 s. The heat transfer coefficient decreases with the increase of plate length. An empirical correlation for forced convection heat transfer for a twisted plate with various lengths was obtained based on the experimental data. Moreover, numerical simulation results were obtained for average surface temperature difference, heat flux and heat transfer coefficient of the twisted plates with different length and showed reasonable agreement with experimental data. Through the numerical simulation, distribution of heat transfer coefficient on heater surface, temperature distribution and velocity distribution were clarified.     

Measurement of heat transfer coefficients for steam condensation on a vertical 21.5-mm-O.D. tube in the presence of air

ABSTRACT

Governing the rate of heat transport by condenser tubes in the passive containment cooling system (PCCS), the steam condensation over a vertical cylinder in the presence of air was investigated experimentally. The main objective of this study was to explore if the condensation heat transfer coefficient relies on the tube dimension, which has been a variable missed in most condensation models or has been embraced without experimental demonstration under phase change environments. The mean heat transfer coefficient was measured in the condensation test facility named JERICHO (JNU Experimental Rig for Investigation of Condensation Heat transfer On tube). The outer diameter of the condenser tube used in this study was set to 21.5 mm. The measured heat transfer coefficients were compared to those obtained from the 40-mm-O.D. tube, and a multiplier to correct the variation of the heat transfer coefficient with the tube diameter was proposed for its application to Lee correlation. The proposed correlation was further validated against another set of experimental data obtained from a separate test facility housing the 31.8-mm-O.D. tube.     

Optimization of zigzag flow channels of a printed circuit heat exchanger for nuclear power plant application

Printed circuit heat exchanger (PCHE) is recently considered as a recuperator for the high-temperature gas cooled reactor. In this study, shape optimization of zigzag flow channels in a PCHE has been performed to enhance heat transfer performance and reduce the friction loss based on three-dimensional Reynolds-averaged Navier–Stokes analysis with the Shear Stress Transport Turbulence model. A multi-objective genetic algorithm is used for the multi-objective optimization. Two non-dimensional objective functions related to heat transfer performance and friction loss are employed. The shape of a flow channel is defined by two geometric design variables, viz. the cold channel angle and the ellipse aspect ratio of the cold channel. The experimental points within the design space are selected using Latin hypercube sampling as the design of the experiment. The response surface approximation model is used to approximate the Pareto-optimal front. Five optimal designs on the Pareto-optimal front have been selected using k-means clustering. The flow and heat transfer characteristics, as well as the objective function values, of these designs have been compared with those of the reference design.     

CFD analysis of flow and heat transfer of turbulent pulsating flow in a tube in rolling motion

The investigation of flow and heat transfer of turbulent pulsating flow is of vital importance to the nuclear reactor thermal hydraulic analysis in ocean environment. In this paper, the flow and heat transfer of turbulent pulsating flow is analyzed. The calculation results are firstly verified with experimental data. The agreement between them is satisfactory. The effect of spanwise and wall-normal additional forces is significant in small Reynolds number, and decreases with Reynolds number increasing. The rolling axis and rolling radius contribute slight to the flow and heat transfer. The effect of velocity oscillation period on the heat transfer is limited than that of Reynolds number and oscillating velocity Reynolds number. The traditional empirical correlations could not predict the flow and heat transfer of turbulent pulsating flow in rolling motion.     

CEFR热传输及发电能力分析

针对中国实验快堆（CEFR）发电能力问题,对CEFR 3条回路的输热能力及热电转换能力进行建模并分析计算,通过40%额定功率发电工况下的试验数据验证了所建模型的正确性。依据CEFR的实际情况,提出优化运行工况的改进措施,其效果得到了汽轮发电机组厂家数据的验证,并结合输热能力验证模型模拟CEFR满功率下的热传输及发电能力。结果表明：基于优化后的运行工况,CEFR热传输系统和热电转换系统可达到设计要求。     

高放废物处置库近场裂隙水流-传热-处置室间距相互作用的三维数值分析

为研究高放废物地质处置库近场裂隙水流-传热-处置室间距的相互作用机理,采用3DEC软件计算裂隙水流-传热-处置室间距相互作用对处置库近场温度分布影响。结果表明:(1)在处置室间距相同条件下,流动的裂隙水显著改变了处置库近场温度场,使岩体温度降低,缩短模型达到稳态所需要的时间。(2)处置室间距增大,温度叠加效应减弱,处置库近场温度越低,并且废物罐表面膨润土温度越低,裂隙出水口水温越低,模型达到稳态所需要的时间越短。(3)水平和垂直裂隙水流共同传热使处置库近场裂隙水流下游区域温度显著高于裂隙水流上游区域。(4)处置室间距为6 m和8 m时,水平裂隙出水口水温高于垂直裂隙,处置室间距为10 m时,水平裂隙出水口水温低于垂直裂隙。     

中国ITER固态实验包层模块热工水力优化设计和分析

在中国氦冷固态增殖剂实验包层模块(CH HCSB TBM)热工水力优化设计的基础上,利用有限元程序ANSYS和计算流体力学程序FLUENT对实验包层模块进行了相应的分析.分析结果表明热工水力优化是合理的,是可以接受的.     

小型模块式反应堆螺旋管蒸汽发生器设计和热工水力分析

近年来,国际上一体化小型模块式反应堆发展飞速,我国也正在加速研制一体化小型模块式反应堆。本文针对15 MW的一体化小型模块式反应堆,设计一种螺旋管式蒸汽发生器,共12个蒸汽发生器组件均匀分布在反应堆堆芯围板外侧和压力容器内侧壁的环形空间中,每个组件含5层、25根螺旋管,整个蒸汽发生器共300根螺旋管。给出了蒸汽发生器的具体参数,分析了蒸汽发生器组件中换热系数、温度、温差和热流密度等沿管长的变化,并给出了螺旋管内流体的动力特性曲线。