Keisuke Minagawa

Required Properties of Seismic Isolation System for Nuclear Power Plants

In Japan, applications of seismic isolation systems to new generation nuclear power plants and fast breeder reactors have been expected in order to enhance seismic safety. However there are lots of restrictions for design of isolation systems, such as strong design seismic wave, deformation of piping between an isolated structure and a non-isolated structure, and so on. In addition combination of horizontal and vertical isolation has possibility to cause rocking motion if a three-dimensional isolation system is applied. Therefore isolation systems should be designed properly. Moreover the design of seismic isolation system has to consider influence on inner equipment and piping. This paper describes investigation regarding required properties and performance of seismic isolation system for nuclear power plants. The investigation is carried out by numerical analysis. In the analysis, various isolation devices such as friction pendulum bearings and so on are applied as well as natural rubber bearings.Copyright

Fundamental Study on Shape Dependency of Input Energy for Failure

This paper describes a comparison of input energy for failure between experimental models. Recently, understanding ultimate fatigue endurance in strong earthquakes is strongly required in order to improve seismic safety reliability. We have dealt with this issue from the viewpoint of the energy balance equation, and already confirmed relations between input energy and fatigue failure by a previous paper. However the confirmed relations were qualitative, so that a quantitative investigation is required. Therefore this paper deals with another experimental model that has 4 times as big width as the previous model. Input energy for failure is investigated by vibration experiment using random wave. As a result, same relations as the previous paper are confirmed. Then relations are expressed by power function, and are compared with the previous model. Consequently the rectangle model requires input energy which is about 4 times bigger than the square model.Copyright

Research and Development of Intelligent Seismic Isolation System Using Air Bearing

This study aims at research and development of the intelligent seismic isolation system using air bearings as isolation device and Earthquake Early Warning (EEW) as trigger of isolation system. In October 2007, EEW was started providing to resident of Japan. The EEW system expects earthquake intensity and arrival time at particular place by analysis of seismic wave that was observed near the earthquake center. Therefore social and technical application of the system is strongly expected for suppression of disaster scale. On the other hand, long period seismic waves having predominant period of from a few seconds to a few ten seconds have recently been observed in various earthquakes. Also resonances of high-rise buildings and sloshing of petroleum tanks in consequence of long period seismic waves have been reported. In metropolises of Japan such as Tokyo, Osaka and Nagoya, it is expected that long period seismic waves are excited in large earthquakes because these are located on sedimentary layers. Therefore the isolation system having very long natural period or no natural period is required. In this study, we propose an isolation system having no natural period by using air bearing as isolation device. Air bearing is a bearing that can reduce contact friction between floor and the bearing by thin air film produced by compressed air. In general, the air bearing is used as heavy machinery moving equipment. The approximate friction coefficient is 0.0005 to 0.001, so that the system using air bearing almost isolates seismic wave. In addition, the EEW is applied as trigger of isolation. The EEW is applied for turning gas and electrical heater off, too. P-wave sensor is also equipped and it can operate as trigger in case of near-field earthquake or when EEW system could not work properly. Furthermore, steel plate equipped at the bottom of the air bearing can operate as friction bearing when air bearing does not work. In this paper, we describe results of earthquake response analysis of the intelligent isolation system using air bearing. From results of the analysis, it was confirmed that response acceleration depends on friction coefficient only, and the system has good isolation performance not only against typical seismic wave, such as El Centro wave, but also against long period seismic wave. However residual displacement remains after seismic input stopped. Additionally, experimental test was executed so as to investigate basic performance of isolation. As a result, it was confirmed that the isolation system has good isolation performance.Copyright

Study on the Earthquake Resistant Characteristics of High Natural Frequency Mechanical Structures From a Viewpoint of Energy Balance

This paper describes investigation regarding earthquake-resistant characteristics of mechanical structures that have high natural frequency from a viewpoint of the energy balance. In earthquake-resistant design of a mechanical structure installed in a nuclear power plant, those mechanical structures are basically considered the rigid body and static seismic design is implemented. When vertical component of an seismic wave is considered, floor response acceleration at high natural frequency band is high, and it is expected that excessive inertia force operate against the mechanical structure in design phase. Therefore designing those mechanical structures is so difficult that accurate grasp regarding vibration characteristics, the main cause of fracture and establishment of evaluation technique are required. In this study, earthquake resistant characteristics of mechanical structures that have high natural frequency are investigated from a viewpoint of the energy balance that is one of valid methods for structural calculation. At first, forced vibration experiments were executed in order to confirm differences of input energy between low natural frequency models and high natural frequency models. Then continuous vibration experiments that induce experimental model to fatigue fracture were carried out. Results of these experiments and fracture phenomena were evaluated from the viewpoint of the energy balance.Copyright

Comparison Between Hysteresis Energy and Input Energy for Failure

Many methods for evaluation of seismic resistance have been proposed. Energy balance equation is one of the methods. The main feature of the energy balance equation is that it explains accumulated information of motion. Therefore energy balance is suitable to investigate the influence of cumulative load. We have already conducted some studies that applied the energy balance equation to mechanical structures. In the studies, we confirmed a relationship between input energy and fatigue failure. On the other hand, a relation between fatigue life and hysteresis energy (i.e. area of a hysteresis loop) is well known in the fatigue strength field, and a lot of knowledge has been reported. The input energy of the energy balance equation is essentially equivalent to the hysteresis energy. Therefore it is expected that input energy for failure is equivalent to hysteresis energy for failure. In this paper, two experiments are carried out. One is a fatigue fracture experiment and the other is a vibration experiment that causes fatigue failure to experimental models. Both experiments cause fatigue failure by same bending mode. Finally the input energy and hysteresis energy for failure are compared.Copyright

Dynamic Strength Evaluation of Straight Pipe Using Energy Balance Method

This paper describes a relationship between fatigue failure and the input energy from results of forced vibration experiment using straight pipe. In Japan, mechanical structures installed in nuclear power plants such as piping and equipment are usually designed statically in the elastic region. Although these mechanical structures have a sufficient seismic safety margin, understanding the ultimate strength is very important in order to improve the seismic safety reliability in an unexpected severe earthquake such as the Great Kobe Earthquake (1995) and the Niigataken Chuetsu-oki Earthquake (2007) in Japan. A rational design method is also being required in accordance with a revision of seismic standard in Japan. In this study, the ultimate strength of a mechanical structure is investigated from a viewpoint of the energy balance equation that is one of valid methods for the structural calculation. A main feature of the energy balance equation is that explains accumulated information of motion. Therefore the energy balance is adequate for an investigation into the influence of cumulative load. Authors have already confirmed a relationship between fatigue failure and the input energy from results of experiments using simple single degree of freedom models in a previous study. In this paper, straight pipe models are adopted instead of simple single degree of freedom models, and the relationship between fatigue failure and the input energy is investigated. The straight pipe model is made of stainless steel, and has natural frequency of approximately 21 Hertz. The investigation is implemented by forced vibration experiments that lead the experimental model to fatigue failure. In the experiment, colored random waves whose predominant frequencies are similar to natural frequency of the experimental model are input. The experimental model vibrates in the elasto-plastic region due to the colored random wave input, and cracks finally. As a result of the experiment, it is confirmed that the input energy for failure increase with an increase of time for failure. In other words, much input energy for failure is needed in case of small input level. This tendency is same as the result of the previous study using simple single degree of freedom model. Therefore it is expected that any actual piping satisfy this tendency, and time for failure can be expected by using energy balance.Copyright

Study on Dynamic Strength Evaluation Method of Mechanical Members Based on Energy Balance

This paper describes the dynamic strength evaluation of piping installed in nuclear power plants from a viewpoint of energy balance. Mechanical structures installed in nuclear power plants such as piping and equipment are usually designed statically in elastic region. Although these mechanical structures have sufficient seismic safety margin, comprehending the ultimate strength is very important in order to improve the seismic safety reliability in unexpected severe earthquakes. In this study, ultimate strength of a simple single-degree-of-freedom model is investigated from a viewpoint of energy balance equation that is one of valid methods for structural calculation. The investigation is implemented by forced vibration experiment. In the experiment, colored random wave having predominant frequency that is similar to natural frequency of the experimental model is input. Stainless steel and carbon steel are selected as material of experimental model. Excitation is continued until the experimental model is damaged, and is carried out with various input levels. As a result of the experiment, it is confirmed that input energy for failure increase with an increase of time for failure. Additionally it is confirmed that input energy for failure depend on the material.Copyright

Fundamental Study on the Super-Long-Period Active Isolation System

Background: Since the Hanshin-Awaji Earthquake Disaster, the number of isolated structures has been greatly increased. The natural period of the isolation system is designed around 3 s, because the predominate period of observed seismic waves is usually 0.1 to 1 s. However, relatively long period seismic waves have been observed in various earthquakes, and the resonances of long-period structures, such as high-rise buildings, during earthquakes have been reported at the same time. Therefore the natural period needs to be extended. When extending the natural period of the isolated structure using rubber bearings, its stiffness needs to be reduced. It is more difficult to extend the natural period of the isolation system than the conventional system because of a buckling problem. Therefore we propose a super-long-period active seismic isolation system as a new method for extending the natural period of the isolated structure. This system consists of rubber bearings and hydraulic actuators. Method of approach: In this study, we designed a control system by using the model matching method. This is one of the classical control system design methods. Additionally we applied a genetic algorithm (GA) to select parameters of a transfer function. Results: The system designed by applying the GA could reduce response acceleration sufficiently compared with the input acceleration. Further waveforms of the response acceleration retain almost straight forwardly, so this indicates good performance of isolation. Therefore, application of super-long-period active isolation is an effective technique to improve the performance of isolation. However, the control forces are big, and the system needs 95.5 x 10 6 N for the El Centro NS wave as control force. This force is equivalent to 21 actuators that are used in a large shake table, so there are few possibilities to realize active isolation. Conclusion: The required control force of hydraulic actuators is big, although the super-long-period active isolation system possesses good performance of isolation compared with the conventional isolation system. Therefore it is difficult to apply this isolation system to the real structure. However, the problem regarding requirements of the actuator should be solved because of the realization of an active seismic isolation system. Therefore, we will examine for the parameters of the system and semi-active isolation system.

Vibration Characteristics of High Natural Frequency Mechanical Structures

Recently, it has been reported that a rupture of the piping in an earthquake is produced not by a momentary large load but by cumulative fatigue damage. Then it is expected that the rupture of equipment and piping that have high natural frequency is also caused by cumulative fatigue damage. Therefore it is very important to grasp this effect accurately. In this study, vibration characteristics at high frequency band are investigated. At first numerical analyses are carried out. In the analysis, elasto-plastic model that gives the system elasto-plastic behavior by differential equation is adopted. Then fundamental experiments by using simple single-degree-of-freedom model are carried out to investigate property of experiment model and vibration characteristic. The experiments consist of static experiments, free vibration experiments, and forced vibration experiments.Copyright