Following the final shutdown of a nuclear power plant, a post-operational phase may take place during which preparatory measures for decommissioning are implemented.

Measures in the post-operational phase

Measures in the post-operational phase are carried out under the operating licence held by the nuclear power plant operator:

• Typically, fuel elements are unloaded from the reactor and initially stored in the spent fuel pool.
• Once activity has sufficiently decreased, they are placed in storage casks and transferred to the on-site interim storage facility.
• Recoverable radioactive materials are reused, and radioactive waste from the operating phase is conditioned and disposed of.
• The plant and its systems are decontaminated, generally including complete or partial primary circuit decontamination to remove activated oxide layers formed during operation, thereby reducing radiation exposure for staff during the post-operational phase and subsequent dismantling. Systems no longer required are placed in a drained, depressurised, and cold condition before permanent decommissioning; once permanently decommissioned, systems can be dismantled in a systematic manner.
• Samples from systems and components are analysed to provide an overview of the facility’s condition at the end of the operating phase; the results are used to define the dismantling concept and planning and to prepare the decommissioning licence application.

Information on decontamination

Taking into account the respective nuclear hazard potential, the operator may request further modifications during the post-operational phase from the competent nuclear licensing and supervisory authority.

Safety in the post-operational phase

During the post-operational phase, precautionary measures must be taken in line with the state of the art in science and technology to prevent damage. As in the operational phase, radioactive materials must be safely contained and radiation exposure limited.

As long as fuel elements remain in the plant, measures for cooling and reactivity control must be maintained. Reactivity control must ensure

• that during operation, on average, exactly one neutron produced by fission induces another fission (critical state) and
• that no unintended criticality occurs in the shut-down reactor.

Precautionary measures during the post-operational phase are governed by the plant’s operating licence, which remains in force.

After the post-operational phase

The decommissioning of a nuclear power plant after the post-operational phase requires a comprehensive licensing procedure, addressing in particular radiation protection and potential environmental impacts. Any work that significantly modifies systems or components and ultimately leads to dismantling may only begin once a decommissioning licence has been granted.