스펙트럼 맞춤형 비상관성 입력운동을 적용한 원자력발전 시설물의 개선된 지진해석

Abstract

Seismic response analysis of nuclear power plant facilities can be generally accomplished through the procedures of defining the seismic input motion, modeling of structure or system, conducting seismic analysis of the structure, and evaluating seismic response of secondary systems. However, such conventional seismic analysis procedures applied to nuclear power plant facilities often result in over-conservative seismic design loads or physically incompatible seismic responses because of inherent analysis assumptions and limitations. Alternative methodologies for demonstrating adequate seismic margins are frequently required when nuclear power plant facilities have to be evaluated for increased earthquake loads. The purpose of this study is to reduce unnecessary conservatisms inherent in the conventional seismic analysis procedures by applying specific enhanced seismic analysis techniques. Such enhanced techniques focus on three technical areas: (1) generation of earthquake input motions, (2) seismic analysis of secondary systems, and (3) soil-structure interaction analysis incorporating spatially incoherent seismic ground motion input. For technical area (1), two iterative methods of developing time histories compatible with multi-damping design response spectra are presented. The common method of forcing agreement between the design and calculated response spectral values at several frequencies and multiple damping values often give poor or even meaningless results. The two simple iterative techniques developed in the research presented here use acceleration impulse functions for correcting the time histories for spectrum-compatibility. In the first method, the correction is calculated separately for each frequency and damping value and the maximum coefficient values required for the correction is used to correct the time history for each iteration. In the second method, the solution is further improved by introducing a convergence-facilitating scale factor in each iteration. The effectiveness of the proposed methods is illustrated by comparison of a set of six multi-damping design spectra with the response spectral values computed for a time history generated using the improved methods. For technical area (2), floor response spectra for dynamic responses of subsystems such as equipment and piping are conventionally generated from structural response motions without considering dynamic interaction between the main structure and the subsystems. An enhanced, rigorous analytical method (the Tseng method) is described whereby equipment response spectra can be obtained through dynamic analysis incorporating dynamic equipment-structure interaction effect. The validity of the method is demonstrated by comparing the floor response spectrum generated by applying this method with the floor response spectrum generated from rigorous structure-equipment-coupled analysis at a prototypical containment building location. In order to investigate equipment-structure interaction effect on the response of equipment to be negligible due to decoupling by regulation, response values generated from the Tseng method are compared with the ones generated from the conventional approach without considering equipment-structure interaction. Floor response spectra from the Tseng method for varying equipment mass and damping ratio show lower spectral amplitudes and are seen to be also more reasonable than the conventional floor response spectra. In technical area (3), seismic responses of nuclear power plant structures with incoherent seismic input ground motions characterized using the new hard-rock coherency functions proposed by Abrahamson are investigated. The characteristics of the new hard-rock coherency functions are described and the application of these functions for soil-structure interaction analysis is implemented into the computer code SASSI-INCOH. Seismic response results for a practical nuclear power plant structure obtained from analyses using the hard-rock coherency functions are compared with the conventional seismic response analysis results without using the hard-rock coherency functions. Approximate response spectrum reduction ratio in the high frequency range derived from the analysis results is also compared to the corresponding reduction ratio obtained from other references. Further efforts have also been tried to reduce the structural response spectral amplitudes using incoherent seismic motion input and modeling technique. Numerical examples are provided to demonstrate reduction of seismic responses for a structure with a relatively small-size basemat, which is modeled together with the adjacent other structures. This study shows that reduction in high frequency response motion of the structure due to presence of adjacent structures can be achieved. Enhanced techniques developed and applied in this thesis can lead to increase in calculated seismic margins as compared to the margins that would be obtained using the conventional seismic analysis procedures. Consequently, they lead to more economic designs of plants by reducing the unnecessary over-conservatism included in the conventional techniques and enhance seismic resistance reliability of nuclear power plant structures.