The life cycle of a nuclear power plant extends from the first spade cutting to the moment the site is released for the development of other activities. In the case of Belgian nuclear power plants, it spans a total of 60 to 70 years - depending on the unit.

Besides construction and operation, shutdown and decommissioning are also part of the life cycle of a nuclear power plant. Initial preparations for shutdown and decommissioning start years before the nuclear power plant produces its last megawatt hours of electricity. The objective: safely remove all radioactive materials and clear the site for future new activities.

De post-operational phase

The post-operational phase of the nuclear power plant starts at the moment of final shutdown. The aim of this phase: to prepare the nuclear power plant for the actual decommissioning by already eliminating most of the nuclear risk.

During the first weeks of the post-operational phase, employees perform the activities they are used to from annual maintenance periods. Once the plant has cooled down sufficiently, they disconnect all cables from the reactor, unscrew the reactor cover bolts and open the reactor. They fill the reactor dock with water - which blocks radiation from the reactor core very effectively - and remove the upper internal parts of the reactor. Then, using a special discharge machine, they lift the fuel elements out of the reactor piece by piece. The machine places the fuel elements on a transport system - also completely underwater - which transfers them to the cooling docks, located in a bunker adjacent to the reactor building.

In the next phase, activities at the cooling docks will start. The fuel elements that were already removed from the reactor several years earlier - and thus have already had sufficient time to cool down - will be transferred underwater to special storage and transport containers. The filled containers are hoisted out of the water, dried, hermetically sealed and transferred to a specially designed building for the temporary storage of spent fuel on the nuclear power plant site.     

More info on fuel containers and temporary storage buildings

Of course, more than just that happens during the post-operational phase. As long as fuel is still present in the cooling docks, certain systems must remain in service or on standby to ensure the cooling of the fuel under all conditions: control, measurement and control systems, electrical boards and switches, emergency generators, fire-fighting systems, ventilation systems, pumps, piping and storage tanks for cooling water and other liquids, filtration systems, etc. Some of the employees will thus continue to monitor, test and maintain all these systems.

In the course of the shutdown phase, the nuclear risk due to further cooling of the nuclear fuel will be reduced, however, and thus fewer and fewer safety systems will be needed. The systems that are no longer needed will be taken out of service according to careful planning.

After four to five years, when the cooling docks are also completely empty and all the fuel containers have found a place in the temporary storage buildings, staff will proceed to thoroughly clean the cooling docks and the associated cooling systems. Meanwhile, the reactor and the rest of the primary circuit have also been cleared of about 97% of the remaining radioactivity using chemical cleaning. The last waste waters and hazardous substances are still being removed. Everything is now ready for the complete decommissioning of the plant.

The decommissioning phase

Once the activities of the shutdown phase are sufficiently advanced and the licence allows, Electrabel will start decommissioning the facilities - about 5 years after the shutdown of the reactor.

The duration of the decommissioning phase will be determined by the activities in the reactor building. The dismantling of the other installations - such as the machine hall, the buildings containing the electrical installations and control systems, the 2nd-level safety systems bunkers - will be done in parallel.

The dismantling of the reactor building starts with cutting up and removing the internal parts of the reactor. Examples include the gratings that hold the fuel elements in place, the bottom plate on which everything rests and the control rod control systems. These steel parts have become radioactive due to decades of exposure to radiation from the reactor core. Cutting is therefore done underwater, with teleoperated robots. The cut-up parts are then stored in specially designed containers for this purpose.

After the internal parts, the reactor vessel itself is also dismantled. Special sawing machines cut the 20-centimetre-thick steel walls of the vessel into smaller pieces, which, like the internal parts of the reactor, are stored in containers.

The next step is the dismantling of the primary circuit. The steel pipes of the primary cooling circuit are cut with saws made of tungsten or diamond and are detached from the major components, such as the steam generators and the pressure control vessel. These components are removed in their entirety and transferred to an on-site location where they are dismantled. Since these components are considerably less radioactive, the cutting does not have to be done underwater, although even here, tele-operated machines are sometimes used.

Once the reactor vessel is completely gone, workers can tackle the biological shield, a thick concrete wall more than 2 metres thick surrounding the reactor vessel. Due to decades of exposure to high levels of radiation, the inside of this concrete ring around the reactor has become radioactive. The radioactive concrete layer is carefully separated from the rest, treated and transferred to Belgoprocess as radioactive waste.

After removing the biological shield, employees check all remaining concrete structures in the reactor building. Using special measuring equipment, they check every centimetre for the possible presence of radioactivity. Where necessary, they scrape off surfaces to remove all radioactive particles. On completion of these works, the reactor building is ready for conventional demolition.

The degradation phase

At the end of the decommissioning phase, all parts of the nuclear power plant are completely free of radioactivity or hazardous substances. The remaining structures and buildings are completely demolished using conventional demolition methods. Once all units are demolished, the site is prepared to welcome new industrial activities.

An important note, however, is that at that time there will still be nuclear fuel present at both sites, in the temporary storage buildings mentioned earlier. The security perimeter around these storage buildings will possibly be smaller than the current site, but their presence does mean that certain - mainly safety-related - restrictions will apply to the reuse of the site.