PEGASOS Refinement Project PRP

Why is PEGASOS being refined?

The PEGASOS seismic hazard analysis commissioned by the power plant operators and carried out between 2000 and 2004 by an international team of experts led to the conclusion that there were still significant uncertainties in the reassessment of the actual seismic hazard at the Swiss power plant sites.

To address the open questions, and with the approval of ENSI, the power plant operators initiated a refinement study in September 2008. The objective of this PEGASOS Refinement Project (PRP) is to carry out measurements rather than making estimates wherever possible, to integrate the most recent scientific developments and to quantify the remaining uncertainties more accurately in order to arrive at as realistic as possible an estimate of the seismic hazard.

As for the preceding PEGASOS project, the PRP was carried out in line with the guidelines of the American Senior Seismic Hazard Analysis Committee (SSHAC) Level 4. The same method and independent experts were used as for PEGASOS.

New in the PRP is that there is a fifth subproject that defines earthquake scenarios for the nuclear power plants in such a way that all the operators can use the results equally and directly.

As was the case for PEGASOS, the power plant operators and the safety authority ENSI have no influence on the judgment of the experts. The responsibility for defining and evaluating the models used lies with the independent experts themselves.

Tailor-made for Switzerland

The following aspects of PEGASOS were refined as part of the PRP:

Consideration of the updated earthquake catalogue of the Swiss Seismological Service at the ETH Zürich. The strong earthquakes (magnitude and depth) documented for Central Europe are very significant for evaluating the seismic hazard in Switzerland.
In the last ten years, the models of the propagation and attenuation of the vibrations from the earthquake source used at an international level have been improved and refined thanks to more dense measurement networks in areas with high seismic hazard. The Swiss Seismological Service has prepared an attenuation model for Switzerland based on the new extensive database for earthquakes in Switzerland. Both of these factors make it easier to transfer the experience from actual strong earthquakes abroad to the geological conditions in Switzerland.
As part of the PRP, comprehensive ground investigations (boreholes and laboratory investigations) were carried out at all the power plants sites. These provided actual site-specific data to be used in evaluating the effects of earthquake vibrations on the local ground conditions.
Thanks to the experience gained by the experts in the PEGASOS project and better definition of the subprojects, it was possible to avoid double-counting of uncertainties. This means that the remaining uncertainties can be defined more accurately and the seismic hazard can be estimated more realistically.
In an additional fifth subproject, the analysis has been extended in such a way that the results can be input directly by the engineers into the calculation of the building models.

The work of the experts within the framework of the PRP project will be presented to ENSI for review. The results will then be published.

Interim analysis following the Fukushima accident

Following the accident in the Fukushima-Daiichi power plant, ENSI called for an immediate review of the safety of the Swiss nuclear power plants in the event of earthquakes and floods.

The PEGASOS Refinement Project was not sufficiently far advanced at that point to allow the hazard assumptions to be taken over directly from the project. In May 2011, swissnuclear therefore called for an Intermediate Seismic Hazard analysis based on the available quality-assured results of the PRP to date. This interim analysis is based on significantly more extensive data than the PEGASOS project (including new measurements) and state-of-the-art understanding, but does not represent the definitive result of the PRP.