Switzerland is on course with its implementation of recommendations resulting from the EU Stress Test. This emerges from the status report submitted to the European Union (EU) by the Swiss Federal Nuclear Safety Inspectorate (ENSI).
In connection with the peer review for the EU Stress Test, the international experts drew up key recommendations on a number of issues in April 2012:
- natural hazards
- Periodic Safety Review (PSR)
- containment integrity
- beyond-design-basis accidents due to extreme natural events
The peer review Board recommends that WENRA, involving the best available expertise from Europe, develop guidance on natural hazards assessments, including earthquake, flooding and extreme weather conditions, as well as corresponding guidance on the assessment of margins beyond the design basis and cliff-edge effects.
2. Periodic Safety Review
The peer review Board recommends that ENSREG underline the importance of periodic safety review. In particular, ENSREG should highlight the necessity to re-evaluate natural hazards and relevant plant provisions as often as appropriate but at least every 10 years.
3. Containment Integrity
Urgent implementation of the recognised measures to protect containment integrity is a finding of the peer review that national regulators should consider.
The measures to be taken can vary depending on the design of the plants. For water cooled reactors, they include equipment, procedures and accident management guidelines to:
- depressurize the primary circuit in order to prevent high-pressure core melt;
- prevent hydrogen explosions
- prevent containment overpressure.
4. Prevention of accidents resulting from natural hazards and limiting their consequences
Necessary implementation of measures allowing prevention of accidents and limitation of their consequences in case of extreme natural hazards is a finding of the peer review that national regulators should consider.
Improved basis for determining natural hazards
In keeping with the stress test recommendation of the European Nuclear Safety Regulators Group (ENSREG), the Western European Nuclear Safety Regulators Association (WENRA) has revised its safety reference levels, which were published in September 2014. ENSI is an active member of WENRA and its sub-groups that focus on hazard assumptions. The Inspectorate intends to incorporate the revised safety reference levels into its own guidelines.
Work is under way on various aspects related to earthquakes and flooding. At the end of 2013, the operators submitted their final report on the PEGASOS Refinement Project (PRP). ENSI is currently reviewing this report.
For about one year, an interdepartmental working group headed by the Federal Office for the Environment (FOEN) has been developing a joint basis for the assessment of the flooding hazard on the rivers Aare and Rhine, in order to continue improving the hazard assumptions for flooding.
The operators have already submitted several documents regarding extreme weather conditions. These will be followed by more documents which ENSI will review.
The nuclear power plants have already implemented various measures since Fukushima. For example, construction work was carried out to improve the seismic stability of the Wohlensee dam upstream of the Mühleberg nuclear power plant.
Periodic Safety Review: already established practice for many years
Switzerland’s Nuclear Energy Act states that Periodic Safety Reviews (PSR) are mandatory at least once every ten years. This year, ENSI has revised its guideline on this subject: among other changes, it has defined the requirements regarding the proof that must be submitted for long-term operation. ENSI’s definition is based on the requirements in the IAEA Safety Standards.
Analysis of the hydrogen hazard to be completed soon
The formation of hydrogen can pose a particular hazard to the integrity of the protective shield. For this reason, the operators have already undertaken studies on this aspect as part of their probabilistic safety analyses.
After the reactor accident at Fukushima, however, ENSI ordered a new review of this issue, including the dispersion of hydrogen outside of the containment. The operators submitted their reports promptly. As an additional measure, some operators are also planning to install passive hydrogen recombiners to control hydrogen in the containment. ENSI will issue a statement on this aspect early in 2015.
Statement on increasing safety margins to be released shortly
The results of reviews conducted in response to ENSI’s rulings and the EU Stress Test have shown that Switzerland’s nuclear power plants display a high level of protection against the impacts of earthquakes, flooding and other natural hazards as well as power failures and loss of the heat sink. Nevertheless, ENSI has ordered a review of additional potential improvements beyond the design basis in order to increase the existing safety margins.
This year, the operators submitted the reports that were requested on earthquakes and external flooding. ENSI is currently completing its statement on these aspects.
Review of containment integrity during planned maintenance outages
Based on the Swiss status report as at the end of 2012, the reviewers expressed the opinion in April 2013 that there was a need for faster handling of the restoration of containment integrity in the event of a total loss of AC power during planned maintenance outages. This point concerns measures to close larger openings in the containment that are present for the purposes of transporting materials or enabling access by people. It must be expected that the restoration of containment integrity will be made more difficult in the event of an accident during this period which entails a lengthy loss of the power supply (Station Black-Out, or SBO).
After this open point was identified in connection with ENSI’s 2011 National Report on the EU Stress Test, the issue was addressed as part of the 2014 Fukushima Action Plan. The operators submitted the required documents in October 2014, and they are currently under review by ENSI.
Peer review of the status report in April 2015
The extensive status report also includes an account of the measures drawn up on the basis of the recommendations of the second Extraordinary Meeting of the Convention on Nuclear Safety. As was the case with the 2012 status report, provision is again made for interested members of the public to submit questions to ENSREG. In April 2015, a workshop lasting several days will be staged for discussion and peer review of the national reports.