Study on Application of Hollow Springs in Seismic Isolation in Nuclear Power Plants

Earthquake is always an important consideration when constructing nuclear power plants. There are many studies on how to resist or avoid earthquake damage. And recently more and more seismic isolation technology has been applied to nuclear structure in nuclear power plants using isolators mainly made of rubber which can not only bear the vertical load, but also reduce the response of seismic on the upper structure due to their low lateral stiffness so as to improve the safety of the nuclear structure in the earthquake. Compared with traditional anti-seismic technology, seismic isolation can make the design of the main structure of nuclear power plants achieve standardization unrestricted by the magnitude of design ground motion. The two main categories of seismic isolators are elastomeric bearings and spherical sliding bearings. And low damping bearings (LDR), lead rubber bearings (LR) and spherical sliding bearings (FP) are currently considered sufficiently well characterized for use in nuclear facilities. However, the rubber which is the main material of elastomeric bearings cannot withstand high temperature, so the properties of the bearings might be affected and even lost under the complex and severe environment of nuclear power plants, which could compromise the safety of nuclear reactor. In addition, both rubber bearings and spherical sliding bearings can only apply to horizontal seismic isolation. The metal springs have more stable performance than rubber under high temperature condition and also has better isolation effect. Springs are usually made of solid spring wire by metal forming. In production practice, springs are used for vertical isolation because of their low vertical stiffness, and calculations show that they also work in the lateral direction. Springs have been widely used for vibration isolation in many light load and high requirements situations like precision instruments at this stage, but need more research to be applied in nuclear structure of nuclear facilities. Compared with air spring, under the same load-bearing capacity, the stiffness of solid spring is higher, which leads to the higher natural frequency of the isolation system, so it is not suitable for the situation that requires low frequency and heavy load. This paper tries to use springs made of hollow wire to solve the problem. In this paper, some research results of hollow springs are given, including frequency and damping property when filled with damping materials. In addition, a calculation formula of hollow springs’ mechanical property, including the vertical and lateral stiffness and carrying capacity, is derived, and the finite element calculation are used to validate the formula. And the paper also gives a finite element calculation case on how the hollow spring reduce the natural frequency of an isolation system.