EU Stress Test confirms safety of Swiss nuclear power plants

The EU Stress Test provides renewed confirmation that Swiss nuclear power plants maintain a high standard of safety, and that the measures implemented to date on the basis of knowledge gained from Fukushima are correct. Taking account of the submissions from operators, the Swiss Federal Nuclear Safety Inspectorate (ENSI) has, however, identified a further eight “open points” which supplement the 37 checkpoints derived from the Fukushima analysis. ENSI considers that there is a need to clarify the seismic stability of the Wohlensee dam.

Rosa Sardella, Head of the Systems Supervision Area and the team of authors for the Swiss National Report (PDF, 1.5 MB), comments: “The hazard assumptions that were taken as the basis for the stress test in Switzerland are strict compared to those in other countries.” Moreover, Swiss nuclear power plants usually have safety margins that are well in excess of the applicable statutory requirements.

According to the Swiss National Report, the review confirmed “that Switzerland’s nuclear power plants have a very high level of protection against impacts from earthquakes, flooding and other natural hazards, and also against power supply failures and reactor cooling.” Turning to a key problem that contributed to the disaster at Fukushima in March 2011, Rosa Sardella places particular emphasis on one of the results from the EU Stress Test: “The special emergency systems are a major strength of Swiss nuclear power plants – these are separate, well-protected systems that serve the particular purpose of protection against external events of both natural and human origin.”

EU Stress Test confirms measures implemented to date

“The report also confirms the measures initiated by ENSI after the accident in Fukushima, Japan”, according to ENSI Director Hans Wanner. By means of its rulings issued on 18 March, 1 April and 5 May 2011, the Swiss supervisory authority ordered the operators of nuclear power plants to implement immediate measures; and it formulated 37 checkpoints based on an in-depth analysis of the accident in its third Fukushima report at the end of October 2011.

Eight “open points” identified

In addition to these steps, which have already been introduced, ENSI has now identified eight additional “open points” in connection with the EU Stress Test and on the basis of the operators’ submissions; these should result in further improvement measures. Three of these measures relate to protection against earthquakes, two concern emergency management, and and individual points deal with protection in case of flooding, extreme weather events and loss of power supplies.
The “open points” regarding earthquake protection include the question as to whether, in case of an emergency, an automatic emergency shut–down (scram) can be initiated in Swiss nuclear power plants by seismic measuring instruments. Moreover, the containment (as the protective casing that surrounds the nuclear reactor is called) and the primary circuit (in which the coolant comes into direct contact with the nuclear fuel) are to undergo detailed inspection to determine their capacity to resist earthquakes.

In addition, ENSI intends to examine measures at the Gösgen and Leibstadt nuclear power plants that would improve the seismic stability of the containment venting system (the equipment that removes accident-induced overpressure from the containment surrounding the nuclear reactor).

As part of its emergency management, the supervisory authority intends to review the power supply for the venting systems in case of severe accidents, and to examine whether maintenance of the tightness (integrity) of the containment after a power supply failure (Station Blackout) represents a time-critical measure.

Three “open points” concern the following aspects: the potential consequences of debris-induced blockages, i.e. points along rivers where bottlenecks lead to jamming, such as bridges, weir systems and sharp bends; proof of safety in case of extreme weather events; and the deployment of mobile emergency diesel generators if the power supply fails.

These five open points are integrated as checkpoints in the Action Plan based on the Lessons Learned from Fukushima, dated October 2011. The Action Plan for 2012 regarding the checkpoints will be available at the end of January. The operators of the nuclear power plants must implement the measures by 2015.

ENSI issues rulings

The reviews conducted by the operators for the purposes of the EU Stress Test have, however, also highlighted the fact that safety margins are relatively tight in certain areas. This leads to consequences in accordance with Switzerland’s tried-and-tested supervisory practice: “Wherever margins are tight or information is not adequate,” Hans Wanner comments, “it is necessary to clarify the situation.” For this reason, ENSI does not intend to wait until the EU experts have reviewed the Swiss National Report (PDF, 1.5 MB) and have presented their recommendations, probably in June 2012.

ENSI has therefore issued specific rulings to clarify three open points:

1. All Swiss nuclear power plants must carry out a new review of the seismic robustness of the isolation of their reactor containments, and must submit the results to ENSI by 30 September 2012.

2. If necessary, the containment venting systems can be used for filtered reduction of overpressure in the containment. It has become apparent that, in some cases, these systems have lower seismic stability than the relevant containments. ENSI therefore requires the Gösgen and Leibstadt nuclear power plants  to review the seismic stability of their containment venting systems, and to submit the results of this review to ENSI by 30 September 2012. Measures to bring about improvements to the seismic stability of the containment venting system must be proposed by 31 December 2012.

3. Another requirement concerns debris-induced blockages: points along rivers where bottlenecks could lead to jamming, such as bridges or weir systems. By 30 September 2012, the Gösgen and Mühleberg nuclear power plants must identify bottlenecks that could have a relevant influence on the flooding situation for their plants in case of a total blockage. They must assess these bottlenecks in respect of their impact on the safety of the plants.

Additional information on seismic stability

ENSI considers that there is an additional need to clarify the seismic stability of the Wohlensee dam. A ruling was issued on 1 April 2011 requiring the Mühleberg nuclear power plant to submit verification of the seismic stability of the equipment used to bring a 10,000-year earthquake under control; this verification must be submitted by 30 November 2011. This also includes the Wohlensee dam, for which no revised verification of stability has been submitted. ENSI requires submission of this verification by 31 January 2012.

Furthermore, the Mühleberg nuclear power plant must supply ENSI with additional information on the seismic stability of the emergency shut–down (scram) system by 31 January 2012.

National report is reviewed by international experts

The EU Stress Test is the response of those European countries with their own nuclear power plants to the Fukushima reactor accident following the tsunami on 11 March 2011. On 1 June 2011, ENSI issued a ruling which required the operators of Swiss nuclear power plants to take part in the EU Stress Test. The EU Commission adopted a specification for the content and timeframe on 25 May 2011. The following aspects are examined:

  • Earthquakes
  • Flooding
  • Extreme weather conditions
  • Loss of power supply and ultimate heat sink
  • Emergency management

The operators submitted their reports to ENSI within the stipulated period, i.e. by 31 October 2011. ENSI reviewed the reports and processed them to produce the National Report which was delivered to the EU Commission promptly on 31 December 2011.

The final national reports for the EU Stress Test will now be examined by experts in what is known as the peer review process, with the participation of experts from other member states and a representative of the EU Commission. The European Commission will submit the final results to the European Council during its meeting at the end of June 2012.

Reviews continue

The EU Stress Test does not mark the end of the post-processing work in Switzerland on the consequences of Fukushima. By the end of March 2012, the operators must present proof of their ability to control the 10,000-year earthquake and the combination of earthquake and earthquake-induced failure of dam installations whose area of influence includes a nuclear power plant. This safety verification will be based on stricter hazard assumptions than in the case of the EU Stress Test.
ENSI will review the verifications submitted by the plants, and the results can be expected by the end of June. “If the verifications show that the safety of the population is at risk, ENSI will have the nuclear power plant in question taken out of service, unless the operators have already done so,” Hans Wanner states.