Shigeki Okamura

Seismic Isolation Design for JSFR

This paper describes the seismic design of Japan Sodium-Cooled Fast Reactor (JSFR), which includes the seismic condition, the seismic isolation system, and the seismic evaluation of the primary components. Since the design seismic loading is set out severely than ever since The Niigata-ken Chuetsu-oki Earthquake in 2007, an advanced seismic isolation system is aimed to reduce the seismic force loaded on the primary components of JSFR to be less than that of the previous seismic isolation system. The advanced seismic isolation system is developed by optimizing the performance based on the previous seismic isolation system considering the natural frequency of the primary components. The laminated rubber bearings thicker than the previous ones and oil dampers are adopted for the advanced seismic isolation system of SFR. The seismic evaluation of nuclear reactor components applying the advanced seismic isolation system is performed and its feasibility is confirmed.

Parametric Design Study About Seismic Isolation System for Fast Reactor JSFR

Japanese seismic conditions are getting severer and natural frequencies of components are getting lower due to the enlargements of components’ size, therefore response accelerations and buckling margins of reactor vessels were parametrically surveyed with attention to thicknesses, diameters, and isolation frequencies for reviewing necessary isolation specification.For the first, Japanese seismic condition and present specification of JSFR isolation system are introduced in this paper. RV installed floor responses were calculated based on this seismic condition and the relationship of natural frequencies, initiated stresses, and buckling margins against vessel thicknesses and diameters were shown with trend.Expansion characteristic of isolation system was evaluated by parametric acceleration response analyses. Comparing the response of isolation system with 8Hz vertical natural frequency with other natural frequency’s isolation, response ratios against natural frequencies were calculated.Japanese seismic design condition may become severer than present one, and a natural frequency of main component may decrease. However based on the buckling margin with present plant specifications and the expansion characteristic of isolation system, the advanced isolation system with 8Hz vertical natural frequency was selected as the isolation system of JSFR at still present occasion.Copyright

Motion Analysis of Pendulum-Type Isolation Systems During Earthquakes: Dynamic Test and Response Analysis on a Three Story Steel Frame Model Supported by Four Friction Pendulum Bearings

After the Hanshin-Awaji Earthquake Disaster, the number of earthquake isolated buildings is increasing. Most of the base isolated buildings or structures are built on laminated rubber bearings in order to give them certain natural periods. This situation, however, also encourages structural engineers to research and develop nonrubber-type isolation systems such as linear motion bearing isolators and friction pendulum systems. It is considered that the nonrubber-type isolation systems can be applied to important industrial facilities, such as LNG tanks, boiler facilities and so on to refine their seismic reliabilities. In the device of the nonrubber-type isolation systems, the device which applied the sliding is especially noticed. However, when using nonrubber-type isolation systems with sliding in the open air circumstances, long term durability of the systems must be taken into account and it may be very difficult to maintain the friction coefficient of the system. In this study, the dynamic motion analysis and the experimental tests on the isolated structure mounted on four Friction Pendulum Bearing (FPB) Systems were carried out to investigate the performance of isolation due to the rotational motion which might be induced by the friction force difference in FPB system.

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

In Japan, mechanical structures installed in nuclear power plants, such as piping and equipment, are usually designed statically in an elastic region. Although these mechanical structures have sufficient seismic safety margin, understanding the ultimate fatigue endurance is very important in order to improve the seismic safety reliability for unexpected severe earthquakes. Moreover, clarifying a margin of seismic resistance of mechanical structures that suffered a severe earthquake is being required. In this study, the energy balance equation that is one of valid methods for structural calculation is applied to the above-mentioned issues. The main feature of the energy balance equation is that it explains accumulated information of motion. Therefore the energy balance is adequate for the investigation of the influence of cumulative load such as seismic response. The investigation is implemented by forced vibration experiments. The experiment models are simple single-degree-of-freedom models that are made of stainless steel and carbon steel. In the experiment, random waves having predominant frequency similar to natural frequency of the experimental model are input in order to obtain adequate response not only in the elastic region but also in the plastic region. As a result, experimental models vibrate under resonance condition, so response acceleration is approximately seven times as big as the input. The excitation is continued until the experimental models fracture, and is carried out with various input levels. In the experiment, models suffered cracks at the bottom end, and fractured finally. As a result, input energy for failure increases as time for failure. In other words, more input energy for failure is needed in case of small input. Moreover the correlation between increment in input energy and input energy for failure is investigated. It was confirmed that input energy for failure is inversely proportional to increment in input energy per unit time. Additionally energy for failure of stainless steel is about twice as big as carbon steel. The correlation between fatigue failure and energy is confirmed from the vibration experiment. Therefore it is expected that time for fatigue failure can be evaluated by the energy balance equation.

Development on Rubber Bearings for Sodium-Cooled Fast Reactor: Part 1 — Examination Plan

This paper describes the past preliminary examination results and the new characteristic examination plans of the thick laminated rubber bearing for the application to Sodium-cooled Fast Reactor (SFR).The preliminary examination focusing on mechanical characteristics such as shear modulus with a 1/8-scale model (ϕ= 200 mm) and creep characteristics with a 1/13-scale model (ϕ= 120 mm) were carried out [1]. With the basic mechanical characteristic examination, the basic design formulas were temporarily confirmed. Furthermore, to establish the creep prediction formula, the creep characteristic examination has been carried out and the creep amount during the plant life has been approximately predicted.To obtain the mechanical properties of thick laminated rubber bearing with the scale-effect, the basic mechanical characteristic examination, including the ultimate behavior test, with the 1/2-scale model (ϕ= 800 mm) will be planned to confirm design margin with clarifying the ultimate limit curve under the bi-axial loading such as shear strain and the normal stress. Moreover, the deterioration promoted examination will be planned. The object of this examination is intended to grasp the degree of aging that may be caused by environmental effects during the plant life. By the accelerated deterioration examination results, the influence by aging on the mechanical characteristic and ultimate behavior of the laminated rubber bearing could be evaluated.Copyright

Development on Rubber Bearings for Sodium-Cooled Fast Reactor: Part 2 — Fundamental Characteristics of Half-Scale Rubber Bearings Based on Static Test

This paper described the results of the static loading tests using a half-scale thick rubber bearing to investigate the fundamental characteristics such as horizontal and vertical restoring force of a rubber bearing applied to a Sodium-cooled-Fast-Reactor (SFR). Since the SFR has thin-walled component structures, a seismic isolation system is employed to mitigate the seismic force. A rubber bearing with thick rubber layers is used for the seismic isolation system applied to the SFR, it was developed aiming for isolation of not only horizontal response acceleration, but also vertical response acceleration. The thick rubber bearing of 1600 mm in diameter full-scale was designed to provide about a 10000 kN rated load with a horizontal natural period of 3.4 s and a vertical one of 0.125 s. Moreover, a linear strain limit of the thick rubber bearing was designed to accept a horizontal displacement of 700 mm or more in order to ensure a double safety margin for response displacements against a design basis ground motion. The static loading tests were performed using a half-scale thick rubber bearing with a diameter of 800 mm to investigate the horizontal/vertical stiffness, damping ratio, a linear strain limit in horizontal direction and a tensile yield stress in the vertical direction. The fundamental characteristic of rubber bearings employed to the SFR and the validity of a design formula became clear through the static loading tests.Copyright

Motion analysis of pendulum type isolation systems during earthquakes (Probabilistic study of isolation performance of base isolated structure considering characteristic dispersion of pendulum type isolation systems)

Most of the base isolated buildings or structures are built on laminated rubber bearings in order to give them certain natural periods. This situation, however, also encourages structural engineers to research and develop nonrubber-type isolation systems such as linear motion bearing isolators and friction pendulum systems. It is considered that the nonrubber-type isolation systems can be applied to important industrial facilities, such as LNG tanks, boiler facilities, and so on, to refine their seismic reliabilities. This device of a nonrubber-type isolation system uses the energy loss associated with sliding to reduce the deleterious effects of earthquakes. However, when using nonrubber-type isolation systems with sliding in the atmosphere, long term durability of the systems must be taken into account. It may be difficult to maintain the friction coefficient of the system. In this paper, a stochastic study of the effect on rotational motion and isolation performance of two structures subjected to an earthquake with a friction pendulum bearing is analyzed with a Monte Carlo method.

Fatigue Failure Evaluation Method and Fragility Curve Using Energy

In this paper, a fatigue failure evaluation method and a fragility curve using energy are proposed. The energy is one of damage indicating parameters and it can evaluate cumulative damage such as fatigue failure. The authors have already reported qualitative relationships between the energy and fatigue failure in previous papers. Great East Japan Earthquake had some features that are different from others, because it was the largest earthquake in Japanese history. For example, the earthquake had long duration time, and many aftershocks. Therefore structures were damaged by cumulative damage. On the other hand, seismic design and evaluation of mechanical structures are generally based on static force in the elastic region, so cumulative damage is not considered. Therefore damage indicating parameters that can evaluate fatigue failure have been required, and the energy is suitable for the evaluation of fatigue failure. In addition, the seismic probabilistic risk assessment (PRA) has attracted attention in recent years, so this paper deals with the fragility curve based on energy as well.Copyright

Study on Motion Analysis of Friction Pendulum Isolation Systems during Earthquake (5th Report, The Effect of the Rotary Motion for the Number of the Isolation System)

Most of non-rubber type isolation systems apply frictional force. One of the famous non-rubber type isolation systems is Friction Pendulum Bearing (FPB) system. When using friction pendulum system in the open air circumstances, some have pointed out that long term durability of the systems must have been taken into account and it might have been very difficult to maintain the friction coefficient. In this study, the behavior of the structure and the isolation systems in considering the dispersion of the friction coefficient of the each device and vertical load on the each device is examined during earthquake. In this report, the analytical models used the square structure, and this model are considered as rigid body models. The dispersion of the friction coefficient and vertical load is made to occur from the nomal random number. The center of frictional force in the isolation systems was calculated using a Monte Carlo method. The relation between the number of supported isolation device and the center of frictional force was investigated.