Although nuclear power plants are some of the most earthquake-resistant buildings in Switzerland, earthquakes are a major part of the risk of nuclear power plants. “This is why it is so important to ensure that nuclear plant operators regularly update their comprehensive seismic safety cases and that these comply with the latest scientific evidence – especially if this evidence suggests that stricter requirements are now necessary,” explains Hans Wanner, director general of ENSI. This procedure complies with a legal provision and is safety oriented.

Operators presented a new report in 2013

The umbrella organisation for Swiss nuclear power plant operators, swissnuclear, submitted a new report to ENSI at the end of 2013. Experts from Switzerland and other countries had reassessed seismic hazards for Swiss nuclear power plant sites as part of a complex process culminating in this report. The aim of the new PEGASOS Refinement Project (PRP) was to refine the analyses conducted in the PEGASOS Project which was completed back in 2004. New scientific findings and extensive new data were considered as part of this process.

ENSI has reservations about a sub-project forming part of the study

It took ENSI considerably longer than originally anticipated to examine the report and formulate definitive seismic hazard assumptions. The refinements incorporated in the very core of the project, sub-project 2, on attenuation models, and sub-project 3, examining site influences, were acknowledged by ENSI as being technically correct. “However, sub-project 1, which examined seismic sources, was not covered in sufficient depth in the eyes of our independent review team,” explains Ralph Schulz, Head of the Safety Analysis Division, explaining the delay in completing the project.

ENSI has therefore decided to replace the rejected PRP models with data and models from the Swiss Seismological Service (SED2015). The necessary work for the elaboration of the so-called ENSI hybrid model was carried out by experts commissioned by ENSI.

New hazard assumptions higher than PRP and SED2015

Incorporating the SED data in the PRP led to results which were in most cases higher than the results obtained by either the PRP or the SED2015. “Our new hazard assumptions are more stringent as a result,” explains Ralph Schulz. In some cases they even exceed the hazard assumptions used for seismic safety cases immediately after the Fukushima reactor accident. An interim assessment from the PRP study was used at the time.

Seismic safety case to be staggered

In 2011, in response to Fukushima, ENSI called for a rapid review of seismic safety at all nuclear power plants in Switzerland. The results were to be submitted within a year and concentrated on the most important aspects. They showed that all Swiss nuclear power plants are sufficiently protected against earthquakes and flooding potentially caused by a breach in a dam following a seismic event. Further investigations also revealed that all nuclear power plants in Switzerland have a safety margin.

“However, the seismic safety case that the plants now need to provide is considerably more extensive than the document that the nuclear power plants had to provide after Fukushima,” Ralph Schulz points out. One significant additional requirement concerns the method used to assess the seismic resistance of the major components needed to keep the incident under control. Due to the complexity of the new safety case procedure, ENSI will allow operators to stagger the process in three stages.

• Safety case stages

  • The safety case drawn up in 2012 after the Fukushima reactor accident must be updated by the end of 2018. It must demonstrate that the plants are able to withstand a 10,000-year earthquake whilst not exceeding a dose limit of 100 millisieverts.
  • The probabilistic safety analysis must be updated by mid-2019, permitting a quantitative assessment of the risk of beyond-design-basis incidents.
  • Finally, an extended seismic safety case must be submitted during autumn 2020. This will cover not only a 10,000-year earthquake (incident category 3), but also a 1,000-year earthquake (incident category 2) with a dose limit of 1 millisievert. The methods used to establish the seismic resistance of major components will also need to be more detailed in this safety case. ENSI set out the requirements for this extended safety case in a memo as early as 2014.

ENSI will then examine the safety cases submitted by the operators and comment accordingly.

Mühleberg Nuclear Power Plant has to provide a seismic safety case in spite of its scheduled shutdown in 2019

Although it will stop generating electricity by the end of 2019, Mühleberg Nuclear Power Plant will stay a hazardous nuclear installation for several years. Different parts of the plant are still important after stopping power operation. Therefore, BKW has provide a seismic safety case. The operator must also submit a completely revised seismic safety case to replace the document submitted after Fukushima. However, the information to be submitted after 2019 may be restricted to those systems required in the post-operating phase after power generation comes to an end in late 2019.