Application of base isolation to nuclear equipment

The paper presents a base isolation concept and discusses its potential application to nuclear equipment subjected to high frequency (20-80 Hz) loads which can result from Loss-of-Coolant Accident (LOCA) and/or Safety/Relief Valve (S/RV) discharge events. The high frequency content of these loads has little effect on the structural integrity of most types of equipment. However, serious problems can be encountered in the operability of some electromechanical devices such as relays. Typical of this situation is control panels. Response spectra computed at component locations can exhibit high acceleration peaks in the high frequency range - often considerably above the levels for which the components can be qualified. When equipment is subjected to high accelerations in the low frequency range, the equipment is usually designed in such a way that its fundamental frequency is well above the frequencies of the applied loads so that little or no amplification occurs in the equipment. This approach is typically used for equipment excited by seismic loads whose dominant frequencies occur in the 1 to 15 Hz range. When the fundamental frequency of the equipment is raised to 33 Hz or higher, it is considered rigid for seismic loads and no amplification is assumed to occur. This approach may be used to reduce high accelerations occurring in equipment resulting from high frequency loads. However problems arise with local modes of vibration in the 20-100 Hz range. It may be neither practical nor economical to qualify such equipment by stiffening because of the difficulty in raising the frequencies of the local modes out of the frequency range of the applied loads. (orig./RW)