Variable friction pendulum system for seismic isolation of liquid storage tanks

Earthquake response of liquid storage steel tanks isolated with variable friction pendulum system (VFPS) is investigated under normal component of six recorded near-fault ground motions. The continuous liquid mass of the tank is modeled as lumped masses known as sloshing mass, impulsive mass and rigid mass. The corresponding stiffness constants associated with these lumped masses are worked out depending upon the properties of the tank wall and liquid mass. The governing equations of motion of the tanks isolated with variable friction pendulum system are derived and solved by Newmark's step-by-step method assuming linear variation of acceleration over small time interval. In order to verify the effectiveness of the VFPS in tanks, the seismic response of tanks isolated with VFPS is compared with that of the same tanks isolated using the conventional friction pendulum system (FPS). Furthermore, a parametric study is also carried out to critically examine the behaviour of tanks isolated with VFPS. The various important parameters considered are the tank aspect ratio, the isolation period and initial time period of the VFPS. In addition, the seismic response of tanks isolated with VFPS under trigonometric cycloidal pulses is also investigated. From these investigations, it is concluded that with the installation of VFPS in tanks, the seismic response of tanks during near-fault ground motions can be controlled within a desirable range. Finally, it is also observed that the response of tanks isolated with VFPS under the near-fault ground motions and trigonometric cycloidal pulses matches well only when the isolation period reaches high values.     

Non-linear seismic response of base-isolated liquid storage tanks to bi-directional excitation

Seismic response of the liquid storage tanks isolated by lead-rubber bearings is investigated for bi-directional earthquake excitation (i.e. two horizontal components). The biaxial force-deformation behaviour of the bearings is considered as bi-linear modelled by coupled non-linear differential equations. The continuous liquid mass of the tank is modelled as lumped masses known as convective mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses has been worked out depending upon the properties of the tank wall and liquid mass. Since the force-deformation behaviour of the bearings is non-linear, as a result, the seismic response is obtained by the Newmark's step-by-step method. The seismic responses of two types of the isolated tanks (i.e. slender and broad) are investigated under several recorded earthquake ground to study the effects of bi-directional interaction. Further, a parametric study is also carried out to study the effects of important system parameters on the effectiveness of seismic isolation for liquid storage tanks. The various important parameters considered are: (i) the period of isolation, (ii) the damping of isolation bearings and (iii) the yield strength level of the bearings. It has been observed that the seismic response of isolated tank is found to be insensitive to interaction effect of the bearing forces. Further, there exists an optimum value of isolation damping for which the base shear in the tank attains the minimum value. Therefore, increasing the bearing damping beyond a certain value may decrease the bearing and sloshing displacements but it may increase the base shear.     

Multi-story base-isolated buildings under a harmonic ground motion — Part II: Sensitivity analysis

A series of sensitivity analyses for multi-story base-isolated structures subject to a harmonic ground excitation is carried out. Several base isolation and energy dissipation devices are considered. The effects of variations in properties of structures and base-isolation systems on the peak responses are studied. For a range of natural frequency, friction coefficient, and damping of the isolator, the peak responses are evaluated. The sensitivity of the base-isolated structure to variations in the amplitude and the frequency of ground excitation is also studied. Particular attention is given to the effects of long-period excitations. The study shows that the peak structural responses for base isolators with a frictional element are less sensitive to variations in intensity and frequency of ground excitation when compared with non-frictional systems. Furthermore, the performances of frictional isolators are insensitive to small variations in friction coefficient and its velocity-dependence.     

CAP1400双乏燃料贮存格架地震条件下流固耦合试验研究

CAP1400是中国自主研发的新型反应堆，其乏燃料贮存格架自由地放置在乏燃料水池内，水可在格架内部通道中流通，同时水也存在于格架与格架、格架与池壁之间的间隙中。在地震情况下，格架与流体之间的流固耦合特性非线性度非常高，很难用软件准确地模拟计算。为准确地探究CAP1400乏燃料贮存格架的流固耦合特性，建立了乏燃料贮存格架和水箱缩比试验模型，开展了振动台试验。在扫频试验中，测量了格架支脚与水箱底板间的摩擦系数；同时研究了正弦波、地震波工况下，格架与格架间的间隙对格架滑移现象、流体压力的影响。通过分析试验结果，得到了格架支脚与水池间的摩擦系数；且格架间隙越大，流体压力越小，但格架间隙对滑移现象影响不明显。本试验的结果可为CAP1400乏燃料贮存格架的流固耦合参数选取提供依据。     

高温气冷堆用石墨摩擦性能研究

采用标准摩擦试验机研究了3种石墨(兰州石墨、上海三高石墨和IG-11)的摩擦性能,并用电子扫描显微镜分析了摩擦表面。试验的环境气氛为氦气和空气。研究结果表明:在相同载荷作用下,3种石墨在不同环境气氛中表现出不同的摩擦性能。当滑动速度较大时,在空气环境中的摩擦系数高低依次为兰州石墨>上海三高石墨>IG-11;在氦气环境中的摩擦系数则为上海三高石墨>IG-11>兰州石墨。     

Response of piping system on friction support to bi-directional excitation

Installation of friction devices between a piping system and its supporting medium is an effective way of energy dissipation in the piping systems. In this paper, seismic effectiveness of friction type support for a piping system subjected to two horizontal components of earthquake motion is investigated. The interaction between the mobilized restoring forces of the friction support is duly considered. The non-linear behavior of the restoring forces of the support is modeled as an elastic-perfectly plastic system with a very high value of initial stiffness. Such an idealization avoids keeping track of transitional rules (as required in conventional modeling of friction systems) under arbitrary dynamic loading. The frictional forces mobilized at the friction support are assumed to be dependent on the sliding velocity and instantaneous normal force acting on the support. A detailed systematic procedure for analysis of piping systems supported on friction support considering the effects of bi-directional interaction of the frictional forces is presented. The proposed procedure is validated by comparing the analytical seismic responses of a spatial piping system supported on a friction support with the corresponding experimental results. The responses of the piping system and the frictional forces of the support are observed to be in close agreement with the experimental results validating the proposed analysis procedure. It was also observed that the friction supports are very effective in reducing the seismic response of piping systems. In order to investigate the effects of bi-directional interaction of the frictional forces, the seismic responses of the piping system are compared by considering and ignoring the interaction under few narrow-band and broad-band (real earthquake) ground motions. The bi-directional interaction of the frictional forces has significant effects on the response of piping system and should be included in the analysis of piping systems supported on friction supports. Further, it was also observed that the velocity dependence of the friction coefficient does not have noticeable effects on the peak responses of the piping system.     

The influence of roughness on tribological properties of nuclear grade graphite

The influence of surface roughness on tribological properties of graphite IG-11 was investigated on a standard SRV tester. The experimental condition was selected as: 30 N normal load, room temperature and a 10 Hz frequency with different strokes. The experiments environments included helium and air. Five types of roughness were studied in the experiments. The experiments revealed that the surface roughness greatly affected the graphite friction behavior. When the friction surface was smooth, the friction coefficient was high because of intensive adhesion accompanied by many pits at the friction surface. When the friction surface was rough, the adhesion was very poor, but the wear was excessive and generated many graphite particles at the friction surface. These particles can separate the friction surfaces, which reduced the friction action between them. For very rough specimens, the friction coefficient decreased with sliding velocity at about 0.004 m/s and then increases gradually.     

Aseismic design of structure–equipment systems using variable frequency pendulum isolator

Sliding isolation systems have emerged as very useful vibration control technique that incorporate isolation, energy dissipation and restoring mechanism in one unit. However, most currently available systems such as friction pendulum system (FPS) and pure friction (PF) system have practical limitations and are of limited effectiveness when input excitation level is significantly different from its design level. To overcome these limitations while preserving the advantages, a new system called the variable frequency pendulum isolator (VFPI) has been developed by the authors. The isolation period of VFPI continuously decreases with increase in horizontal sliding displacement which results in a more robust isolation system. The VFPI also limits the maximum force transmitted to the structure by providing a restoring force-softening mechanism. In this paper, it has been shown that isolating a structure using VFPI is very effective for vibration control of structure–equipment and other primary–secondary systems. An example five-storey structure with equipment mounted at its top has been analysed to demonstrate the effectiveness of VFPI. It is shown that VFPI provides better vibration control properties compared to other friction isolation systems, and excellent response reduction is observed for a wide range of equipment properties and excitation characteristics. The performance of the example structure with VFPI has been compared with the other frictional systems such as FPS and PF system.     

基于抽样的敏感性分析方法在LBLOCA质能释放PIRT评级中的应用

基于随机抽样的非参量敏感性统计分析方法是一种有效的敏感性分析方法,通过计算热工水力分析程序多个抽样输入参数与输出参数之间的相关系数来评价各输入参数对输出参数影响的重要程度。通过耦合DAKOTA和WCOBRA/TRAC程序,开发了基于抽样的适用于非能动核电厂大破口失水事故质能释放的敏感性分析方法,该方法可全面定量评估各敏感性参数对计算结果的影响。计算结果表明：堆芯初始功率、燃耗、衰变热、安注箱初始水温、初始水体积、安注箱管道阻力系数、堆芯补水箱初始水温、喷放系数及破口阻力系数对破口质能释放具有显著影响。该分析结果可为大破口失水事故质能释放分析现象识别和重要度排序表评级提供定量依据。     

Experimental studies on rewetting of hot vertical annular channel

Studies on the rewetting behaviour of hot vertical annular channels are of interest in the context of emergency core cooling in nuclear reactors following LOCA. Experimental studies were carried out to study the rewetting behaviour of a hot vertical annular channel, with hot inner tube, for bottom flooding and top flow rewetting conditions. The length of the inner tube of the test section was 3030 mm for bottom flooding rewetting experiments and 2630 mm for top flow rewetting experiments. The tube was made of stainless steel. Experiments were conducted for water flow rates in the annulus upto 7 lpm (11.7×10⁻⁵ m³ s⁻¹). The initial surface temperature of the inner tube was varied from 200 to 500°C. The experimental studies show that for a given initial surface temperature of the tube, the rewetting velocity increases with an increase in flow rate of water and it decreases with an increase in the initial surface temperature for a given water flow rate. For a given water flow rate and initial surface temperature, the rewetting velocity is higher in the case of rewetting under bottom flooding conditions as compared to that in the case of rewetting under top flow conditions. These conclusions agree with the conclusions reported in the earlier literature. Using the experimental data of the present work, correlations for bottom flooding and top flow rewetting velocities are developed.     

A trilinear base-isolator concept for nuclear power plants

A friction limited base-isolation concept for moderate seismicity areas is presented. The idea of the concept is to combine frictional damping with base-isolation. The aim of the design effort is to develop a system whose stiffness and damping characteristics are selected to optimize the operational properties of the isolator device for the given site conditions, so that most favourable combination of acceleration and displacement responses of the system is achieved.     

锆合金包壳和GH4169镍基合金的微动摩擦磨损性能研究

燃料棒在冷却剂流过时易受到扰动而发生微振动，导致在格架弹簧与包壳管接触点附近产生微动摩擦磨损，严重时会导致燃料棒破损，放射性产物泄漏，从而影响核电厂安全运行，因而需要对燃料包壳的微动摩擦磨损性能进行充分研究。本研究旨在比较分析2种牌号、2种状态的锆合金（Zr-4）和N36与格架材料GH4169镍基合金在不同环境条件下的微动摩擦磨损性能，分析载荷、循环次数、环境条件对其摩擦磨损性能的影响，并结合磨损表面的形貌、成分分析结果，揭示其微动摩擦磨损机理。研究结果表明，微动摩擦磨损时摩擦系数随载荷的增加呈线性增加趋势；相同条件下，Zr-4/Zr-4摩擦副组合的微动摩擦系数最大，GH4169/N36摩擦副组合的微动摩擦系数最小；预氧化对材料的微动摩擦系数影响显著，预氧化态样品的摩擦系数均高于非预氧化态的样品。      

Cost-effectiveness analysis of seismically isolated pool structures for the storage of nuclear spent-fuel assemblies

This study introduces a method for evaluating cost-effectiveness of seismically isolated pool structures, taking account of fluid–structure interaction effects. As a measure of cost-effectiveness, the procedure estimates minimum life-cycle cost, sum of initial construction cost and excepted damage cist over life-cycle of the structure. The expected damage cost is the function of failure probabilities, which are computed by frequency domain analysis for convenience. Input ground motions are represented by a power spectral density function compatible with site-dependent response spectrums. The interaction effects between flexible walls and contained fluid are considered in the form of the added mass matrix. The thickness of wall and the stiffness of isolator are considered two key design variables in achieving a design with minimum life-cycle cost. The numerical examples show that seismically isolated pool tanks are highly cost-effective in low-to-moderate seismic regions.     

华龙一号非能动安全壳冷却系统循环水箱的热分层现象数值研究

华龙一号核电技术采用了非能动安全壳冷却系统的先进设计。作为一种自然循环系统，系统的冷却能力与其循环水箱的水温直接相关，循环水箱中的热分层现象研究对循环系统冷却能力的准确评估以及工程设计优化均有重要的现实意义。本文基于计算流体力学（CFD）技术对循环水箱升温过程进行了三维流动传热的数值模拟。研究表明，循环水箱中存在较为明显的热分层现象，总体上呈现水池顶部温度波动大，而底部等温层较为平缓的特点，系统循环功率和循环流量均会对水箱的升温过程产生影响：功率增大、流量减小均会促使水箱内产生较明显的热分层现象，同时也会使水箱平均温度偏高，出口水温也相应较高。2列循环系统出现循环功率或流量不均衡对水箱平均温度以及出口温度的升高过程基本无明显影响，因此非能动安全壳冷却系统水箱对系统循环能起到一定的自稳定的效果。     

聚变堆用难变形CLAM钢三通成形模拟及试验研究

利用有限元软件对聚变堆用CLAM钢三通管件的内高压整体成形工艺进行了数值模拟,研究了压力加载路径及摩擦条件关键工艺参数对支管高度以及壁厚分布的影响,从而确定了CLAM钢三通管件内高压成形的优化工艺参数组合。使用满足优化摩擦系数的表面涂层对CLAM钢管坯进行润滑处理,并根据优化结果对CLAM钢管材进行了内高压成形试验。实际成形的三通管件在几何尺寸及厚度分布方面,实验结果和模拟结果吻合。研究结果表明,通过有限元数值模拟可准确预测难变形材料CLAM钢三通的复杂冷成形,并确定工艺参数;通过确定的工艺参数可制备出几何尺寸及厚度分布均满足要求的高质量聚变堆用三通管件。     

基于Dragon与Donjon程序的液态熔盐实验堆临界计算与分析

基于组件输运程序Dragon与堆芯节块法程序Donjon，对包含有上下熔盐腔室、控制棒、实验孔道与中子源孔道的液态熔盐实验堆堆芯进行了计算与分析，给出了液态熔盐实验堆不同组件的等效均匀化模型。根据液态熔盐实验堆特性将中子能群划分为5种少群能群结构，基于所划分的每一种少群能群结构，对单根控制棒与不同控制棒组插入堆芯后的有效增殖因数和控制棒价值进行了计算分析。结果表明，7群能群结构具有更好的计算结果。基于7群能群结构开展了堆芯径向与纵向功率分布，以及控制棒拔出后堆芯的温度反应性系数计算分析，其计算结果与MCNP5计算结果相近，证明了模型等效的合理性以及Dragon和Donjon程序对液态熔盐实验堆的适用性。     

Effects of random surface roughness in pixe analysis of thick targets

The ratio of proton-induced X-ray yields from thick targets with perfectly smooth and randomly rough surfaces is computed as a function of proton energy, target-detector geometry and X-ray attenuation coefficient. The rough surfaces are obtained by a Monte Carlo technique. Attenuation coefficient is shown to be the dominant parameter governing the effect of roughness.     

Theoretical magnitude of the stresses produced in fuel cladding by the expansion of cracked pellets: Effect of interfacial dislocations (interfaceons) and pellet relaxation

Because the stresses to which metals are subjected in service are a much smaller proportion of the elastic modulus than are the stresses commonly imposed on rubbery polymers, the coefficient of friction when interfacial dislocations (interfaceons) are present is much lower for metals than for rubber. It is also much lower than the coefficient of friction for ordinary sliding (in the absence of interfaceons) and so the stress concentration predicted to be produced by pellet cracks is negligible if there are interfaceons present. If they are absent then the stress concentration is significant. In either case a low rate of power increase causes the lowest stresses and stress concentrations in the clad. This is shown to be due to relaxation creep in the pellet.     

基于扩散界面法的较大气泡上升过程数值模拟

基于扩散界面法和有限元法,对较大气泡在上升阶段的形态和速度进行了模拟,结果与实验吻合较好,说明该方法能准确地模拟气泡的运动特性。利用该模型,对初始直径不同的较大气泡上升过程中的形态、速度和振荡随时间变化的规律进行了分析。并分析了14mm直径的气泡在不同尺寸通道中上升过程的形态、速度的变化规律。结果表明:气泡的稳定形态随着气泡初始直径的增大由椭球形变为球帽形,且达到稳定形状的时间更长。气泡初始直径越大,气泡的顶端速度越快,并稍有波动。而气泡的底端速度开始快速增大使气泡向内凹陷,随后回落并在气泡顶端速度上下振荡。气泡上升通道越窄,气泡达到稳定形态的时间越长,顶端速度越小,气泡的高宽比越大。