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Nuclear Facilities - Reportable Events - Decommissioning - Nuclear Accidents

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From power operation to decommissioning

The final shutdown of a nuclear power plant can be followed by a post-operational phase during which measures are carried out to prepare for decommissioning.

Measures in the post-operational phase

Cooling basin in the Brokdorf nuclear power plant Cooling basin in the Brokdorf nuclear power plantSource: BASE

The measures in the post-operational phase are covered by the operating licence held by the nuclear power plant owner. Generally, the following measures are carried out in the post-operational phase:

  • The fuel elements are unloaded from the reactor and initially stored in a water-filled pool inside the nuclear power plant.
  • Once the activity contained in the irradiated fuel has decreased far enough, the fuel elements are transferred to storage casks and stored at the on-site interim storage facilities.
  • Radioactive materials are recycled, and radioactive waste from the operating phase is disposed of.
  • The plant or systems are decontaminated. As a rule, this also includes complete or partial primary circuit decontamination to remove oxide layers with activity, which have formed during operation, and thus to reduce the radiation exposure for staff during the post-operational phase and during subsequent dismantling. Systems that are no longer required are brought into a drained, unpressurised and cold state before being permanently decommissioned. Systems that have been permanently decommissioned can then be dismantled in an organised manner.
  • Samples are taken from systems and components and analysed in order to obtain an overview of the state of the facility at the end of the operating phase. These sample results are used, among other things, to specify the dismantling concept and planning, and serve as input data when preparing application documents for the decommissioning of a nuclear power plant.

Information on decontamination

What is decontamination? Is acid used in the process?show / hide

With light-water reactors, the components (e.g. pipes, pumps) in the primary circuit that come into contact with cooling water will experience a build-up of activity during operation. The dose rate values in the vicinity of the primary circuit will increase as a result. This implies a potential radiation exposure for the personnel working in this area of the plant, which is why the so-called (primary circuit) decontamination has been carried out in reactors worldwide for more than 50 years to reduce this activity.

During this decontamination, parts of the primary circuit are cleaned by means of qualified procedures and special chemicals, such as acid. This can reduce the existing activity by more than 90 per cent. The intensity of decontamination and the associated removal of material can be controlled and depends, among other things, on whether the nuclear power plant will continue its operation or be dismantled afterwards.

Why is decontamination necessary prior to dismantling?show / hide

For one thing, decontamination reduces the radiation exposure for the personnel. Secondly, decontamination reduces the amount of radioactive waste that must be placed in interim and final storage.

Whether the decontamination of the primary circuit is planned in preparation for dismantling remains at the operator's discretion. The implementation must be covered by a valid licence, and is supervised by the competent authority. Decontamination can thus be commissioned by the NPP operator during the post-operational phase of the NPP - within the framework of the operating licence. However, decontamination can also take place during the decommissioning phase of a nuclear power plant - after a corresponding licence has been granted.

Is decontamination also carried out at nuclear power plants abroad?show / hide

(Primary circuit) decontamination is an established procedure that represents the state of the art and has been carried out worldwide for over 50 years. In some NPPs, power operation was subsequently resumed, for example at the Oskarshamn 1 NPP in Sweden or the Loviisa2 NPP in Finland. Decontamination in preparation for decommissioning was carried out, for example, at the US nuclear power plant Haddam Neck and at many German nuclear power plants (e.g. Stade, Obrigheim, Biblis).

Taking into account the respective nuclear hazard potential, the operator can request further adjustments in the post-operational phase from the competent nuclear licensing and supervisory authority.

Timetable decommissioning Timetable decommissioningTimetable for the transition between power operation, post-operational phase and decommissiong of a nuclear power plant until it is released from nuclear regualtory control Source: BASE

Safety in the post-operational phase

The necessary precaution against damages according to the state of the art of science and technology must be taken during the post-operational phase. As in the operational-phase, the radioactive material must be safely enclosed, and radiation exposure must be limited.

As long as there are fuel elements in the nuclear power plant, precautions for cooling the fuel elements and controlling the reactivity must continue. The control of reactivity must ensure,

  • that during operation, on average, exactly one of the neutrons produced by the fission of an atomic nucleus will trigger a new atomic fission (critical state) and
  • that no critical state will occur in the shut-down reactor.

The precautionary measures in the post-operational phase are covered by the nuclear power plant's operating licence, which remains valid.

After the post-operational phase

The decommissioning of a nuclear power plant following the post-operational phase requires a comprehensive licensing procedure comprising, in particular, aspects of radiation protection and possible impacts on the environment. Work that will significantly change the systems and components, and will finally lead to the dismantling of the nuclear power plant, can only be started after a licence for decommissioning has been granted.

State of 2023.12.21

© Federal Office for the Safety of Nuclear Waste Management