Keywords: seismic isolation, anti-seismic engineering, nuclear reactors.

Today, over 10,000 buildings are seismically isolated all over the world; most of them are provided with high damping rubber bearings (HDRBs). Seismic isolation is also widely used for bridges and viaducts and for industrial plants, and is considered the most promising technology to protect nuclear power plants (NPPs) from an earthquake. In spite of this, only two nuclear facilities are currently provided with base isolation: four pressurized water reactors (PWRs) at Cruas (France) isolated with 3600 neoprene pads, and two PWRs at Koeberg (South Africa) isolated with 1830 rubber bearings coupled with friction plates. It is worth noting that these isolation systems were designed in the 1970s and manufactured in the early 1980s and their performance may be non-optimal; in fact, today's seismic isolators have better characteristics and can provide better performance. In addition to these two old applications, the Jules Horowitz Reactor, now under construction at Cadarache (France) with a seismic isolation system made of rubber bearings, must be cited.

The extremely limited number of existing isolated NPPs is probably due to the relatively low seismic input assumed as design for the Generation II reactors, and also because most of them were water reactors, which are characterized by quite stiff structures and rigid components. Among the new designs, only IRIS (International Reactor Innovative and Secure) and 4S (Super Safe, Small and Simple) are provided with base isolation. On the contrary, among the fast reactors, most of the recent designs already include the seismic isolation: ALMR (Advanced Liquid Metal Reactor), S-PRISM (Power Reactor Innovative Small Module), DFBR (Demonstration Fast Breeder Reactor), DFBR (Demonstration Fast Breeder Reactor), STAR-LM (Secure Transportable Autonomous Reactor-Liquid Metal) and EFR (European Fast Breeder Reactor). Unfortunately, no application of these reactors has been done, and there is a dramatic lack of information and experimental results about the behavior of large isolators under severe dynamic conditions.

The paper will present the state-of-the-art of seismically isolated NPPs. Then, it will focus on the main problems encountered in the application of this technology to so specific structures (characterized by huge masses, large sizes and severe safety requirements) such as the need of manufacturing and testing very large isolators, the difficulty to obtain reliable seismic inputs for very long return periods and the lack of standards specifically addressed to isolated NPPs. Reference will be made to the lessons learned during the design of the IRIS reactor.

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