Why has the seismic hazard been reassessed?

Switzerland is located in a zone of low to average seismic activity, in the transition between an area with higher activity to the south (Italy, Greece) and a quiet region in northern Europe.

All the nuclear power plants in Switzerland are constructed in seismically quiet zones and statistics indicate that larger earthquakes are to be expected extremely rarely.

However, the issue was taken into consideration during construction of the current power plants. There have always been stricter requirements in this respect for power plants and dams than for other conventional structures. The former are planned, constructed and continuously upgraded in such a way as to make them resistant to even strong earthquakes. The power plants are thus amongst the most earthquake-resistant structures in Switzerland.

Knowledge is not static and scientific understanding is developing all the time and, since the construction of the Swiss plants, the methods for assessing seismic hazard have evolved further. The safety culture that prevails in Switzerland means that the safety of the power plants has to be reviewed in the light of new methods and understanding and improved if necessary.

In the 1980s, the so-called probabilistic safety assessment (PSA) method was introduced. The spread of the calculations now has to be considered in assessing the seismic risk; this gives indications of the uncertainties in the database and the calculation models.

The PEGASOS project

In view of the advances being made in foreign countries in the area of earthquake research and modelling, the regulatory authority  ENSI - then HSK - called in 1999 for the power plant operators to reassess the seismic hazard using the newest methods available.

As a result, the power plant operators commissioned the PEGASOS project, which was completed in 2004. PEGASOS stands for Probabilistic Seismic Hazard Analysis for the Swiss Nuclear Power Plant Sites. This was breaking new ground for the Swiss operators and, to date, the study is the only one of its kind to be carried out in Europe. In terms of its scope, it is one of only two worldwide. The external costs for PEGASOS amounted to around 10 million Swiss Francs.