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Featured Application: The results can be applicable to the plant equipment design for seismic enhancement. The purpose of this paper is to analyze the seismic response effect caused by the mass eccentricity of individual equipment when conducting base isolation for the improvement of the seismic performance of a nuclear power plant. Recent research has interpreted and confirmed through analysis and testing that base isolation for safety-related equipment in nuclear power plants is an efficient alternative to designing for excessive seismic loads. Depending on the equipment, unavoidable mass eccentricity can occur, which necessitates verification of the response impact caused by eccentricity. In this paper, we analyze the seismic response impact of equipment with mass eccentricity using small base isolators. To do so, sensitivity analysis of the seismic response due to mass eccentricity is conducted for a base-isolated concentrated mass model. Furthermore, three efficient mass eccentricity models suitable for testing are designed and manufactured. Simulation analyses using the finite element method (FEM) models are performed, followed by three-axis shake table tests to validate the seismic response impact of mass eccentricity. In conclusion, it is confirmed that applying small base isolators to equipment with mass eccentricity can affect seismic response impact to some extent when compared for a beyond-design-basis earthquake (BDBE). [ABSTRACT FROM AUTHOR]

The purpose of this study was to develop a vertical seismic isolation device essential for the three-dimensional seismic isolation design of nuclear power plant equipment. The vertical seismic isolation device in this study has a concept that can be integrally combined with a conventional laminated rubber bearing, a horizontal seismic isolator with a design vertical load of 10 kN. To develop the vertical seismic isolation device, the vertical spring and the seismic energy dissipation device capable of limiting the vertical displacement of the spring were designed and their performances were verified through actual tests. In this study, the target elevation of the floor is 136 ft, where safety-related nuclear equipment, such as cabinet and remote shutdown console, etc., is installed. The sensitivity studies were carried out to investigate the optimal design vertical isolation frequencies for the target building elevation. Based on the results of the sensitivity study, a disc spring and a helical coil spring were selected for the vertical stiffness design, and the steel damper was selected for the seismic energy dissipation, and their performance characteristics were tested to confirm the design performance. For the steel damper, three types were designed and their energy dissipation characteristics by hysteretic behavior were confirmed by the inelastic finite element analyses and the tests in static fully reversed cyclic conditions. Through the study of the vertical seismic isolation device, it was found that 2.5 Hz~3.0 Hz is appropriate for the optimal design vertical isolation. With results of the vertical seismic isolation performance analysis, the appropriate number of steel dampers are proposed to limit the vertical seismic displacement of the spring within the static displacement range by the design vertical load. [ABSTRACT FROM AUTHOR]

Response characteristics of small-sized laminated rubber bearings (LRBs) with partial damage and total failure were investigated. For nuclear component seismic isolation, ultimate response characteristics are mainly reviewed using a beyond design basis earthquake (BDBE). Static tests, 3D shaking table tests, and verification analyses were performed using optional LRB design prototypes. During the static test, the hysteresis curve behavior from buckling to potential damage was observed by applying excessive shear deformation. The damaged rubber surface of the laminated section inside the LRB was checked through water jet cutting. A stress review by response spectrum analysis was performed to simulate the dynamic tests and predict seismic inputs' intensity level that triggers LRB damage. Shaking table tests were executed to determine seismic response characteristics with partial damage and to confirm the stability of the superstructure when the supporting LRBs completely fail. Shear buckling in LRBs by high levels of BDBE may be quickly initiated via partial damage or total failure by the addition of torsional or rotational behavior caused by a change in the dynamic characteristics. Furthermore, the maximum seismic displacement can be limited within the range of the design interface due to the successive slip behavior, even during total LRB failure. [ABSTRACT FROM AUTHOR]