Taking Nuclear Power Plants out of Service

The Federal Council and the Swiss Parliament have decided to abandon nuclear energy. Existing nuclear power plants are not to be replaced by new ones at the end of their operating lifetimes. However, they can still be operated as long as their safety is guaranteed. The plants will therefore be taken out of service in an orderly manner, with safety aspects always in mind. ENSI is prepared for this process.

Swiss legislation on nuclear energy makes no provision for restrictions on the periods for which nuclear power plants can be operated. However, an operating licence for an unlimited period does not mean operation for an unlimited period. A nuclear power plant may be operated for as long as it meets the statutory safety requirements. According to the Nuclear Energy Act (NEA), the operator is responsible for the safety of its plant. ENSI verifies whether the operator fulfils this responsibility, and it forms an independent picture from its own analyses, inspections – over 300 per year – and supervisory discussions. ENSI also operates online monitoring of selected operating parameters that are important for safety purposes, and of radioactivity emissions and immissions.

Provisional taking out of service of nuclear power plants

A nuclear power plant operator is obliged by law to review the design of its plant if it has to assume that cooling of the reactor is no longer ensured in case of incidents, or that the integrity of the reactor circuit and the safety barriers around the reactor is no longer guaranteed. After every significant event at a foreign nuclear power plant, the operator must also examine whether its own plant would be equipped to deal with an event of the same sort. The operator must demonstrate to ENSI that the safety design of the nuclear power plant prevents any impermissible release of radioactivity. If this is not the case, the operator must provisionally take its nuclear power plant out of service without delay and – if it intends to continue producing power – must implement safety-related back-fitting measures.

A review of the design of a nuclear power plant calls for time-consuming model calculations in order to map the physical and chemical behaviour of the plant as correctly as possible in case of incidents, and to obtain reliable results. The nuclear power plant may continue to operate until the results are available. There is no legal basis for a precautionary shutdown unless there are specific reasons to fear the occurrence of an incident with potential consequences for people and the environment in the near future, i.e. if there is an immediate threat of danger. According to the Nuclear Energy Act, ENSI may order the shutdown of a nuclear power plant in such cases. If an event occurs at another nuclear power plant elsewhere in the world – such as the event at Fukushima, Japan – this does not as yet mean that there is an immediate danger as regards the Swiss nuclear power plants. Likewise, a ruling that requires the implementation of back-fitting measures does not mean that there is an immediate threat of danger, but that potential for improvement has been identified.

Following the accident at Fukushima, ENSI issued a series of rulings which require nuclear power plant operators to review the design of their plants in the light of the first findings from the accident, and to implement back-fitting measures. The schedule for these activities is strict, both for the work to be carried out by the operators and for its assessment by ENSI. The assessments are to be published.

Another criterion for provisionally taking a plant out of service is ageing damage in the reactor circuit and on the safety barriers around the reactor (steel pressure shell and concrete shell). These components must be examined regularly to determine ageing damage such as material embrittlement, fissures and decreases in wall thickness. If measured values for these variables reach specified limits, the power plant must be provisionally taken out of service until the permissible condition is restored.

Long-term operation and decommissioning (definitive shutdown)

Three of the five Swiss nuclear power plants are about 40 years old. Nuclear energy legislation aims to guarantee the protection of people and the environment by constantly improving safety. Learning from other events is therefore enshrined in the nuclear energy legislation, as is the operators’ duty to keep track of the latest developments in science and technology and to implement ongoing back-fitting measures in their plants. This is the basis for safe operation in the long term.

In addition to ongoing supervisory activities, the integral technical safety assessment of a nuclear power plant that must be carried out every 10 years (the “Periodic Safety Review” or PSR) constitutes a key supervisory instrument as regards long-term operation. For this purpose − on the one hand − the specific operating experience of the power plant over the last 10 years must be evaluated and compared with the operating experience of other nuclear power plants. On the other hand, the current condition of the nuclear power plant must be compared with the latest developments in science and technology. Since the early 1990s, the nuclear power plants have also been obliged to implement a systematic ageing monitoring programme in the areas of mechanical, electrical and construction engineering. Prior to the 40th year of operation, a report on the long-term operation of the power plant must be submitted to ENSI in connection with the Periodic Safety Review. This report includes the results from the ageing monitoring programme, an analysis showing that none of the decommissioning or shutdown criteria are attained in case of continued operation as planned, updated safety analyses and an evaluation of completed and planned back-fitting projects.

Long-term operation and reviews of compliance with criteria for the decommissioning/taking out of service of nuclear power plants are included among ENSI’s normal supervisory activities. The organisation is therefore well prepared for the decision to abandon nuclear power by the Federal Council and the National Council, entailing operation of the existing nuclear power plants without replacements as long as they are safe. As the ageing mechanisms of nuclear power plant components are well-researched and extensive international operating experience is available, good forecasts of the the future progression of ageing are possible. Everything in a nuclear power plant can be replaced, with the exception of a few large components such as the reactor pressure vessel, the steel pressure shell and the reactor buildings. The period for which a nuclear power plant can be operated depends ultimately on the condition of these major components and the operator’s readiness to implement back-fitting measures that can be expected in the future. Consequently, definitive shutdown (decommissioning) will not usually be ordered by the authority due to safety defects; rather, the operator will take its plant out of service in an orderly manner when it decides to do so for technical and economic reasons.

Decommissioning and dismantling

The decommissioning and dismantling of a nuclear power plant is a labour-intensive and time-consuming undertaking. However, there are already numerous international reference projects, so relevant practical experience is available for all the planned work. In this context, as during the actual operation of power plants, it is important for the work to be carried out diligently and in compliance with the radiation protection requirements.

A safe technical post operational phase must be established following the final cease of operation. This process requires compliance with the same safety measures which were already in place during operation.  The plant is then definitively shut down when all fuel elements have been transferred to the spent fuel pool and the technical and organisational measures required for establishing the technical post operational phase have been implemented.

The establishment of the technical post operational phase must be followed by further cooling of the fuel elements in the spent fuel pool. The technical post operational phase ends at the point when all fuel elements have been removed from the nuclear power plant.

The dismantling work, which is ordered in the decommissioning ruling can be roughly broken down into the following steps:

  • Dismantling of installations in the controlled zone
  • Dismantling of core internals
  • Dismantling of the reactor pressure vessel
  • Dismantling of the biological shield
  • Remaining disassembly of the installations in the controlled zones
  • Decontamination and clearance of the building
  • Material handling and waste disposal
  • Dismantling of installations in the conventional area and demolition of the buildings in the conventional manner (if planned)

Insofar as possible, radioactively contaminated components and buildings are decontaminated in order to minimise the volume of radioactive waste. Following proof of absence from contamination (clearance), the components, the buildings or the demolition waste can be released from the supervision required by nuclear energy legislation.

The decommissioning and dismantling of the Swiss nuclear installations will be subject to supervision by ENSI. The costs of decommissioning – and also of radioactive waste management – is pre-financed by the nuclear power plant operators on the basis of contribuitions paid into the decommissioning and waste management funds. ENSI is already conducting regular reviews of the existing decommissioning plans for the nuclear power plants, which specify the planned dismantling work, the time requirements, a radiation protection plan, and information on the nature and quantity of radioactive waste expected for each individual power plant. These plans also justify the amount of relevant contributions to the funds. A decommissioning guideline which stipulates further details of decommissioning projects in depth was issued by ENSI to implement the international recommendations of IAEA and WENRA.


(This post has been updated on 7. December 2015.)