Transient thermal stresses and displacements in a transversely isotropic semi-infinite circular cylinder subjected to an arbitrary surface heat generation and a convective heat loss

The transient thermal stresses and displacements in a traction-free, transversely isotropic, semi-infinite circular cylinder subjected to a convective heat loss on the end surface is analyzed by means of a set of stress functions and a generalized Fourier-integral representation for exponential function.The purpose of this paper is to present the results of theoretical analysis which considers the effects of the thermal and elastic anisotropes of the material properties on the thermal stresses and displacements in the transversely isotropic, semi-infinite circular cylinder due to an arbitrary cylindrical surface heat generation.A detailed numerical result of the thermal stresses and displacements is given for various values of the ratios of the thermal conductivity coefficients, Young's moduli and linear thermal expansion coefficients in the axial and radial directions and it is demonstrated that the sensivity of the maximum stress and displacement to the anisotropy of the thermal conductivity coefficient is significant.     

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。     

Conjugate heat transfer and thermoelastic analysis of heat-generating rods

The exact mathematical formulation and solution of a number of problems dealing with temperatures and stress fields in canned cylinders with arbitrary internal power distribution and fully developed axial turbulent flow are discussed. The problem areas are: (1) the exact linear, thermal, thermoelastic and mechanical stress analysis for a rod in symmetric flow with general integral end force/displacement relations; (2) heat transfer from a clad cylinder with a symmetric heating and temperature dependent conductivity; and (3) asymptotic linear thermal analysis of a rod and associated sub-channel from a square or triangular array.     

Thermal properties of redeposition layers in the JT-60U divertor region

Thermal properties of the redeposition layer on the inner plate of the W-shaped divertor of JT-60U have been measured with laser flash method so as to estimate transient heat loads onto the divertor. Morphology analysis of the redeposition layer was conducted with a scanning electron microscope. Measurement of a redeposition layer sample of more than 200 μm thick, which had been produced near the most frequent striking point, showed following results: (1) the bulk density of the redeposition layer is about half of that of carbon fiber composite material; (2) the specific heat of the layer is roughly equal to that of the isotropic graphite; (3) the thermal conductivity of the redeposition layer is two orders of magnitude smaller than that of the carbon fiber composite. This low thermal conductivity of the redeposition layer is considered to be caused by a low graphitization degree of the redeposition layer. The difference between the divertor heat loads and the loss of the plasma stored energy becomes smaller taking account of thermal properties of the redeposition layer on the inner divertor, whereas estimated heat loads due to the ELMs is still larger than the loss. This is probably caused by the poloidal distribution of the thermal properties.     

Thermoelastic stresses in pellet fragments and conditions for fragments formation

A finite element code CHICAM in which thermal and elastic effects are analysed together has been used to calculate the thermoelastic stress fields in whole and hollow pellets in a radial temperature field of approximately parabolic shape. In addition, the residual thermoelastic stresses were calculated for whole and hollow pellets containing a crack and for various geometries of pellet fragments. The residual stress fields in the pellet fragments differ very considerably from the axial symmetrical stress fields of the whole and hollow pellets. The stress intensity factor K_I is given as a function of the crack length for the whole pellet and for the half pellet showing that during the first rise to power a fuel pellet will fracture into several pie-shaped fragments not necessarily of the same angular width. The calculations show that the residual stress fields in fractured pellets cannot be simulated by a stress and strain analysis with axial symmetry.     

Thermal shock in nuclear reactors

Most types of liquid cooled reactors undergo rapid changes in coolant temperature during abnormal operating conditions, which cause components adjacent to the coolant to undergo rapid surface temperature changes. The differential expansion between surface and bulk material caused by repeated thermal shocks can induce fatigue or cyclic creep damage. In thick walled components individual thermal shocks may cause rapid fracture from pre-existing defects. This paper reviews the reported experimental and analytical work defining the temperature, stress and resultant damage in reactor structural components subject to thermal shock.     

Background to the elastic creep-fatigue rules of the ASME B&PV Code Case 1592

Recent modifications associated primarily with elastic analysis have been made to the creep-fatigue rules of the ASME B&PV Code for use in design of elevated temperature nuclear components. The changes involve: (1) a modified equation for predicting the total strain range per cycle based on a more accurate approximation to the Neuber equation; (2) a variable Poisson's ratio in calculating stresses and strain using elastic equations to account for local thermal strain concentrations; (3) a modified approach for determining the equivalent strain range and; (4) an adjusted design fatigue curve. The modified Neuber approach accounts for the beneficial effects of a residual stress related to the relaxation strength of the material and includes a strain concentration factor greater than the elastic stress concentration factor for geometrical notches for mechanical loads. Notch concentration factors are taken equal to the elastic stress concentration factors for peak thermal and creep strains. The background and intent for these rules are discussed. Correlations of the strain range predictions to those obtained using more rigorous detailed inelastic analysis and test data are presented.     

膨润土-砂混合物缓冲层轴向热传导性研究

膨润土-砂混合物作为高放废物处置库缓冲材料,在高放废物衰变释热作用下,其物理力学性能对处置库的稳定和安全性具有重要影响。本研究采用自行设计的装置,对按比例缩小后的不同干密度、含水率、掺砂率试样进行热传导模拟试验,并对缓冲层热-力耦合过程进行数值模拟分析,得到了缓冲层温度、应力和应变的变化及分布情况,重点分析了温度的影响。结果表明,增大试样干密度、含水率和掺砂率均可提高其导热性,应变也随之增大,应力受温度影响较早达到平衡;缓冲层靠近热源的位置温度、应力和应变最大,沿轴向方向递减,初始时刻变化明显。     

工艺管振动应变状态的测试

针对某核反应堆工艺管断裂破坏的原因,进行了堆内动态应力振动形态测试.介绍了堆内应变计的粘贴与防护问题.测试结果表明,工艺管各截面应力随管位和流量的改变而变化.根据同一截面应力与位移的对比关系可推知,动态位移很大,运动轨迹为弓形回旋运动,致使工艺管产生很大的动态变形.这是导致工艺管破坏的主要原因.     

蒸汽发生器管子—管板接头设计的计算方法

本文主要介绍液压胀接管子-管板接头设计的一种理论计算方法。该方法是:将接头的整个胀接过程分为四个阶段,以此为数学模形,用弹性理论和塑性理论分阶段对管子或管子和管板的应力、应变和位移进行分析,从而估算胀管压力、变形、残余应力和拔脱力等。该方法可用于设计蒸汽发生器和钢制管壳式换热器。文中指出,对核蒸汽发生器的管子-管板接头来说,采取液压胀后再局部滚胀比只进行液压胀更好。     

Transient thermal stresses and displacements in a transversely isotropic, heat-generating circular cylinder

The transient thermal stress field in a traction-free, transversely isotropic, heat-generating circular cylinder is studied by means of a set of stress functions developed by Singh. The purpose of this paper is to present the results of theoretical analysis which consider the effects of the thermo-elastic anisotropies of the material on the thermal stress field in the transversely isotropic, circular cylinder due to an abrupt axial change in the internal heat-generation rate. A detailed qualitative picture of the thermal stress state for two values of the Biot number and various values of the ratio of the thermal conductivity coefficients in the axial and radial directions is given and it is demonstrated that the magnitude of the maximum thermal stress is significantly sensitive to the anisotropies of the thermal conductivity coefficient and the Young's modulus.     

Numerical study of self-heating effects of small-size MOSFETs fabricated on silicon-on-aluminum nitride substrate

1 Introduction Silicon-on-insulator (SOI) technology possesses many advantages over bulk silicon technology such as the reduction of parasitic capacitance, excellent sub-threshold slope, elimination of latch up, and resis-tance to radiation.[1] Hence, it is preferred for high-speed, high-temperature, and low-power micro-electronic devices. SOI MOS devices employ a thin layer of insulating material, usually made of silicon dioxide, to electrically insulate the device from the bulk of the sem…     

Finite element elastic-plastic analysis of LMFBR components

The present effort involves the development of computationally efficient finite element methods for accurately predicting the isothermal elastic-plastic three-dimensional responses of thick and thin shell structures subjected to mechanical and thermal loads. This work will be used as the basis for further development of analytical tools to be used to verify the structural integrity of liquid metal fast breeder reactor (LMFBR) components. The methods presented here have been implemented into the three-dimensional solid element module (HEX) of the Grumman PLANS finite element program. These methods include the use of optimal stress points as well as a variable number of stress points within an element. This allows monitoring the stress history at many points within an element and hence provides an accurate representation of the elastic-plastic boundary using a minimum number of degree of freedom. Also included is an improved thermal stress analysis capability in which the temperature variation and corresponding thermal strain variation are represented by the same functional form as the displacement variation. Various problems are used to demonstrate these improved capabilities.     

Effect of Porosity on the Thermal Conductivity of ThO2

The eflect of porosity on the thermal conductivity of sintered ThO₂ of densities ranging from 90 to 95% of theoretical density was investigated at 20°C. The thermal conductivity was determined by means of laser pulse. Discrepancies were observed between the experimental results and the relation derived by Loeb. This discrepancy was explained by applying a modified Maxwell formula derived by Brailsford et al.

The samples were also examined metallographically. It is concluded that the existence of fluid continuous regions contribute significantly to the thermal resistance of such materials as ThO₂.     

The effect of stress levels on the coefficient of thermal expansion of a fine-grained isotropic nuclear graphite

Due to the fluctuation and non-uniform distribution of temperature within the core structure of high-temperature gas-cooled reactors (HTGRs), the thermal expansion behavior of graphite materials plays an important role in the design of graphite components, especially of large-scale components. In the present paper, in order to investigate the influence of stress levels on the coefficient of thermal expansion (CTE) of IG-110 graphite, the strain gauge method was used to measure the CTE on the cylindrical specimens under a series of loads applied using a universal tensile testing machine. In addition, a more precise measurement using a thermal dilatometer was employed to validate the tests using the strain gauge method. A good agreement has been obtained between the experimental results using these two methods. The results show that when the specimens were under compressive loads, the CTE along the loading direction of the specimens increased and that along the perpendicular direction decreased, with more changes in the former. The absolute changes of the CTE in the two directions increased with increasing applied load. When graphite specimens were subjected to a compressive load of 40 MPa, the axial CTE of specimens sectioned along the radial direction of the graphite brick as it is installed in the core structure increased from 4.13 × 10⁻⁶ to 5.35 × 10⁻⁶ K⁻¹, while the axial CTE of specimens sectioned along the vertical direction increased from 3.97 × 10⁻⁶ to 5.58 × 10⁻⁶ K⁻¹. Moreover, the residual change of the CTE, which was caused by the permanent residual strain after unloading, was observed. The change of the CTE with stress levels should be considered in the stress analysis and life prediction of the nuclear graphite components.     

热管堆固态堆芯典型栅元设计优化

热管堆固态堆芯设计是影响堆芯传热性能和结构完整性的关键问题。为避免固态堆芯设计中间隙热阻导致的温度和应力过大,本文建立了四种堆芯典型栅元的三维热力学模型,对不同填充物下间隙尺寸和栅元截面尺寸等关键参数进行了优化分析。结果表明,尽管高热导率的液态钠填充装配间隙能够有效降低燃料包壳和芯块温度,但热应力反而可能增大;圆管插入液态钠方案的热力学性能最优;固态堆芯方案中,六角管拼接氦气填充方案的热力学性能最优。      

Effective thermal conductivity of MOX raw powder

The radial temperature distribution of plutonium and uranium mixed oxide powder loaded into a cylindrical vessel was measured in air and argon gas, and the effective thermal conductivity was calculated from the measured temperature distribution and the decay heat. The effective thermal conductivities were small values of 0.061-0.13 W m^-1 K^-1 at about 318 K, and changed significantly with O/M, bulk density and atmospheric gas. The results in this work were analyzed by the model of Hamilton and Crosser and a new model for the effective thermal conductivity of the powder was derived as functions of powder properties and thermal conductivity of atmospheric gas.     

多轴疲劳模型对比研究及其在核电结构热疲劳寿命预测中的应用

压水堆核电站余热排出系统冷热水混合区管道发现的热疲劳问题影响核反应堆的安全。本文通过一种采用单轴疲劳试验数据拟合疲劳寿命曲线，进而用于预测多轴疲劳寿命的分析方法，基于文献中的疲劳试验数据，对Dang Van模型、Matake模型和Fatemi-Socie模型进行了余热排出系统冷热水混合区管道材料304L不锈钢疲劳寿命预测结果的对比研究。基于余热排出系统冷热水混合区管道的三维简化有限元模型，分别应用Dang Van模型、Matake模型和Fatemi-Socie模型对管道热疲劳寿命进行了预测，并与试验结果进行了对比验证。研究结果表明，基于应变（含平均应力修正）的Fatemi-Socie模型比较适用于304L不锈钢的疲劳分析，其热疲劳寿命预测结果相对Dang Van模型、Matake模型较合理。     

On the thermal conductivity of UO2 nuclear fuel at a high burn-up of around 100 MWd/kgHM

A study of the thermal conductivity of a commercial PWR fuel with an average pellet burn-up of 102 MWd/kgHM is described. The thermal conductivity data reported were derived from the thermal diffusivity measured by the laser flash method. The factors determining the fuel thermal conductivity at high burn-up were elucidated by investigating the recovery that occurred during thermal annealing. It was found that the thermal conductivity in the outer region of the fuel was much higher than it would have been if the high burn-up structure were not present. The increase in thermal conductivity is a consequence of the removal of fission products and radiation defects from the fuel lattice during recrystallisation of the fuel grains (an integral part of the formation process of the high burn-up structure). The gas porosity in the high burn-up structure lowers the increase in thermal conductivity caused by recrystallisation.     

中子衍射残余应力分析技术及其应用

中子衍射方法可以用来测量材料内部的三维残余应力分布,是一种重要的无损检测分析手段.本文概述了中子衍射残余应力分析的基本原理、特点和发展现状,并简要介绍了其在工程和材料科学领域的应用情况与前景.