SSHAC Process - swissnuclear

Why are guidelines needed for the seismic hazard analysis?

The seismic hazard at a specific site cannot be determined using natural scientific methods alone as not all the data required for this approach are available or can be acquired. A key element is the analysis of existing data and their uncertainties by experienced, independent experts whose opinions cannot be influenced by the contracting body.

At the request of the Swiss Federal Nuclear Safety Inspectorate HSK (now ENSI), the PEGASOS project was carried out in compliance with SSHAC Level 4 procedure. The guidelines (NUREG-CR 6372) for this procedure were developed by the American Senior Seismic Hazard Analysis Committee (SSHAC).

The aim of the procedure is to take into account as many opinions as possible from the qualified technical community and to ensure the independence of the expert judgments to the greatest extent possible. This is a very challenging and demanding process requiring a large investment of time and resources. Level 4 is the highest of four levels and is intended to ensure the maximum independence of the experts and the robustness of the results.

The Swiss PEGASOS study went one step further, setting new international standards in terms of transparent documentation and quality assurance of the seismic hazard assessment; the experience has since been integrated into new guidelines (NUREG 2117).

Level 4 means that the procedure goes beyond collecting expert opinions. The experts themselves are responsible for the technical assessments and the models and discuss their results with one another in order to arrive at the best possible estimation of the hazard. The power plant operators cannot exert any influence of the opinion-forming process of the experts, nor can the regulatory authority ENSI. The experts prepare their results to the best of their knowledge and belief. ENSI checks that the SSHAC rules have been observed.

Systematic approach to ensuring independence

The PEGASOS project team comprised around 25 renowned professors and experts from seven European countries and the USA. Around 30 other Swiss and foreign specialists made their data and expertise available to the project. The study according to SSHAC Level 4 was led by two American experts who were originally involved in developing the methodology in the 1990s.

A Level 4 procedure has been carried out only twice to date: for the deep geological repository at Yucca Mountain in the USA and PEGASOS. Switzerland thus ranks at the forefront in terms of the quality of estimating the seismic hazard of its power plants.