In 2016, the Swiss Federal Nuclear Safety Inspectorate (ENSI) set new requirements for the seismic hazard of Swiss nuclear power plants (NPP). These new guidelines take into account the latest scientific findings. In 2016, ENSI decreed that NPP operators must demonstrate in three steps that their plants can withstand extremely infrequent severe earthquakes:

• Step 1: Update earthquake safety proof by the end of 2018
• Step 2: Update probabilistic safety analysis by the middle of 2019
• Step 3: Submit extended deterministic safety proof by September 2020

• Stages of verification

  Step 1: NPP operators were obliged to update their existing deterministic proofs of earthquake safety by the end of 2018. The operators had drawn up such proofs in 2012 following the reactor accident in Fukushima. Among other things, operators of NPPs had to demonstrate that their plants are capable of withstanding a 10,000-year earthquake without any released radiation exceeding a dose limit of 100 millisievert.

  Step 2: The operators were obliged to update their probabilistic safety analyses (PSA) by the middle of 2019. A PSA is used to quantify the risk of beyond design basis accidents (in German). For example, the frequencies of nuclear damage or releases needed to be demonstrated.

  Step 3: The operators were obliged to submit an extended deterministic earthquake proof to ENSI by autumn 2020. In addition to the 10,000-year earthquake (accident category 3), this also includes the 1,000-year earthquake (accident category 2) with a dose limit of one millisievert be complied with. In addition, more detailed methods for determining the earthquake capacities of important components must be used for this proof. ENSI had already defined the requirements for the extended proof in a note in 2014.

Final step for the Gösgen nuclear power plant partially confirmed

ENSI examined the submitted and updated documents in detail. In 2021, ENSI approved step 1 (deterministic proof of earthquake safety in accident category 3) for all NPPs. In 2022, ENSI approved step 2 (probabilistic safety analysis) for all NPPs.

In 2023 and 2024, ENSI approved the extended deterministic safety cases (Step 3) for the Mühleberg, Leibstadt and Beznau nuclear power plants. ENSI now partially approves the extended deterministic safety case for the Gösgen nuclear power plant as well.

The review of the documents submitted by the Gösgen nuclear power plant has shown that both the core cooling and the cooling of the spent fuel ponds at the Gösgen nuclear power plant are ensured in the event of a very severe category 3 earthquake, which is expected to occur every 10,000 years. This also applies to a combination of an earthquake and earthquake-related flooding. The maximum permissible dose of 100 millisievert would not be exceeded in the event of such an accident. In its statement, ENSI makes demands regarding the selective need for improvement of the evidence for accident category 3, but these do not fundamentally call into question the overall results of the review and will be followed up as part of the ongoing supervision.

ENSI also concluded that the technical acceptance criteria are met for the 1,000-year earthquake in accident category 2. However, ENSI did not conclusively accept the radiological proof of compliance with the dose value of 1 mSv for earthquakes in accident category 2 in its current form. ENSI therefore requested that the Gösgen nuclear power plant revise and resubmit this proof by mid-2026.

• What exactly did ENSI check as part of step number 3?

  In the third and final step of the verification process, NPP operators had to provide very detailed and methodologically demanding deterministic earthquake safety proofs for accident categories 2 and 3. These included technical and radiological analyses. As a minimum, the following earthquake effects had to be taken into account: ground vibration, ground settling, landslides, destruction of nearby facilities that could endanger the safety of the nuclear installation, loss of non-earthquake-resistant systems, fire and flooding.

• Continuous improvement of earthquake safety over the last 20 years

  During the years 2001 to 2004, the NPP operating companies investigated the seismic hazard at the NPP sites in greater depth as part of the PEGASOS project (probabilistic seismic hazard analysis for the NPP sites in Switzerland). The procedure was carried out in accordance with SSHAC Level 4, which is the highest and thus most demanding stage of an internationally recognised procedure.

  Following the completion of the PEGASOS project, significant seismic data was collected nationally and worldwide, and seismological models were developed. With the aim of reducing the scatter of hazard results by taking into account new data and models, the nuclear power plant operators launched the “PEGASOS Refinement Project” (PRP) in 2008. The main focus of the refinements was on new release reductions laws and site-specific foundation investigations.

  In May 2016, ENSI implemented the results referred to as “Seismic Hazard Assumptions ENSI-2015”. At the same time, ENSI asked NPP operators to assess the impact on the safety of the plant and, in particular, on the risk. For this purpose, in respect of earthquakes, the safety proofs required by ENSI after the Fukushima reactor disaster in March 2011 were required to be updated by the end of 2018, and the probabilistic safety analysis was required to be submitted by the middle of 2019, while the extended deterministic accident analysis was required to be submitted by the end of September 2020. ENSI has now completed its comments on this.

  Based on the findings from the safety proofs of the last two decades, improvements have been continuously derived that have increased the robustness of the NPPs against the impacts of earthquakes.

More Information

• Updated hazard assumptions for earthquakes require new safety proof from the Swiss nuclear power plants
• Leibstadt and Beznau nuclear power plants can withstand infrequent severe earthquakes
• Topic Earthquakes
• PEGASOS: New seismic hazard assumptions and proofs for NPPs (video interview in German)
• PEGASOS: Updated seismic hazard assumptions for Swiss NPPs
• ENSI final report on the PEGASOS Refinement Project (PRP) and PRP Summary Report”
• PEGASOS: SSHAC-Verfahren für genaue Erdbebenwerte