Carrying out the PEGASOS Project

How was the PEGASOS project carried out?

The PEGASOS project, which was carried out between 2000 and 2004, is a probabilistic seismic hazard assessment (PSHA). The aim is to estimate the likelihood with which a specific ground vibration at a power plant site will not be exceeded within a specified time interval (for example 10,000 years) and to give the uncertainty contained in this prediction.

PEGASOS comprised four subprojects that addressed the following issues: 

  • SP1 - Characterisation of seismic sources: where and at what depth are earthquakes to be expected, with what intensity and how frequently?
  • SP2 - Attenuation modelling: how do the vibrations propagate from the earthquake hypocentre through the surrounding rock? How strongly are they attenuated by the time they reach their place of impact?
  • SP3 - Site effects: How does the local building ground at the power plant site behave as a result of the vibrations? What accelerations ultimately have an effect on the facility?
  • SP4 - Calculation of the site-specific hazard based on the results and expert models from 1 to 3 above.

Relying on the judgment of independent experts

By its very nature, such a complex analysis cannot be based on exact data as these are either unavailable or cannot be measured sufficiently accurately. Evaluations and opinions of experts therefore have to be used, with the aim of taking into account all the aspects and knowledge to arrive at the best possible result. If necessary, missing data can be provided by expert opinions and interpretations. The aim is not to identify the individual opinion of the “best” expert, but to find the representative opinion of the technical community that reflects a technically defensible interpretation.

The Swiss Federal Nuclear Safety Inspectorate (then HSK, now ENSI) called for a study to be carried out following the American SSHAC Level 4 procedure. Before PEGASOS, a study at this level had been carried out only once before worldwide, for the planned Yucca Mountain repository in the USA.

The team for the PEGASOS project was made up of around 25 recognised, independent experts from seven European countries and the USA. A further 30 Swiss and foreign specialists made their data and expertise available to the project.