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Shutdown of a nuclear power plant

During the power operation of a nuclear power plant, a self-sustaining chain reaction occurs in the reactor core. This chain reaction is maintained by the fact that more neutrons are produced in the fission process than are used in the absorption process. To shut down a nuclear power plant, the reactor must be brought into a permanently uncritical state (subcriticality) and the heat that continuous to generate must be discharged safely.

Shutdown technique

Subcriticality is achieved by lowering the neutron-absorbing control rods between the fuel elements in the reactor core. The control rods catch the neutrons generated in the reactor and thus end the nuclear chain reaction. In the case of a pressurised water reactor, water with added boron is additionally fed into the reactor for permanent subcriticality.

Residual heat removal

The fission products generating inside the fuel elements are radioactive and generate large amounts of heat, even after the reactor has been shut down. If the heat would not be removed, this so-called residual heat would increase the temperature far beyond the melting point of the fuel elements. Therefore the spent fuel elements are initially stored in a water-filled pool inside the nuclear power plant (cooling pond). The water largely shields the radiation and at the same time absorbs the generated residual heat.

Within one year after having been unloaded from the reactor, the activity contained in the irradiated fuel decreases to about 1/100 of the original level and slowly decreases further in the following years.

State of 2017.03.09

© Federal Office for the Safety of Nuclear Waste Management