PEGASOS Hazard Reduction

Have the PEGASOS results lost weight in the meantime?

Following completion of the PEGASOS project in 2004, the results were the subject of lengthy discussion in the technical community. Given the data available in Switzerland, the methods used reached their limits and the results are marked by a large range of uncertainties and numerous open questions. The result was that the seismic hazard was assessed as being too high.

As a result, in 2005 ENSI took the first step in tightening the seismic hazard assumptions for probabilistic safety assessments. The new earthquake curves corresponded to the hazards resulting from PEGASOS with the acceleration values at the power plant sites reduced by 20% - conservatively to be seen as a small reduction compared with the experience and knowledge from both Switzerland and abroad available at the time. Although the PEGASOS results were reduced by 20%, they were still considerably higher than the previous estimates by earthquake experts. The result was that upgrades and improvements were carried out at the power plants.

To improve the quality of the seismic hazard analysis, in 2008 the power plant operators requested the independent experts from the PEGASOS project to carry out the PEGASOS Refinement Project (PRP). Exploratory boreholes were drilled beneath the power plant sites to provide a more solid database.

Following completion and review of the PRP, ENSI will redefine the requirements for probabilistic seismic hazard analyses for all the power plant sites. The power plant operators will then have to update the current safety demonstrations.