ING JIRI PODROUZEK - SKODA PRAHA, a.s., Project Dept. Pilsen, Czech Republic

The original design of the old power plants, fuel or namely nuclear, did not take into account exact external hazards, in particularseismic. Seismic loads were not considered fully explicitly, or fully correctly, in the original design of this plants. Also the level of theoretical knowledges and realistic seismic experiences is rather higher now. For this reason, additional seismic qualifications of the most significant safetyrelated structures and equipment of this units necessary to shutdown its operation and to prevent the population and structures are now urgently required.The seismic safety margin, on the lack of it, depends on the seismic hazard associated with the plant site. Unfortunately, the major problem associated with this power plant are also the lack of realistic seismic hazard information and, consequently, the uncertainty associated with the seismic input.In recent years, increasing knowledge in the geoscience field has led to a better understanding that, although highly unlikely, it is possible for power plants to be subjected to earthquake ground motions greater than those for which they have been designed. The technical community is also well aware of the conservatism that are inherent in current seismic design practice and that provide additional significant margins in these designs. However, there is a concern that an upper boundary of seismic capacity exists, primarily for active equipment, which would render components inoperable if exceeded.