Shut down - and then?

The post-operational phase of a nuclear power plant

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Turning off a nuclear power plant is less easy than it may seem. Because with the flipping of a switch, it is still far from done. The nature of the nuclear reaction requires instead a whole series of carefully coordinated measures and a long breath.

To switch off, control rods are inserted between the nuclear fuel rods, here the view into a fuel element. © tanyss / iStock

The problem of residual heat

The first step to shut down is to stop the atomic chain reaction in the reactor core. This is usually done by inserting cadmium- or boron-containing control rods between the rods with the nuclear fuel. They absorb the neutrons released by the decay of uranium and thus prevent further decomposition reactions from being triggered. The reactor is turned off.

The problem, however: Even without a running chain reaction, the fuel rods still produce heat. It is not caused by the decay of the actual nuclear fuel, but by the shorter-lived decay products that have developed during operation. These include above all radioactive isotopes of iodine, cesium, strontium, xenon and barium. Their disintegration immediately after switching off the reactor still generates between five and ten percent decay heat - in a large reactor this may correspond to 20 megawatts of thermal energy after one day, after all three megawatts after three months.

From the reactor core to the cooling pool

This heat development means that the shut-down reactor and even spent fuel rods already removed from the core still need to be actively cooled for some time. If this does not happen, the fuel rods can heat up so much that a meltdown threatens. The consequences of this could be demonstrated by the Fukushima nuclear catastrophe in 2011: Because the earthquake and tsunami caused the cooling circuit to fail, an overheated decay pool of one of the reactors exploded and high-level radioactive gases escaped.

Almost empty fuel storage bin in the sinking basin of the Italian Caorso nuclear power plant. © Simone Ramella / CC-by-sa 2.0

The decay phase lasts between one and five years, depending on the type of reactor and the amount of remaining nuclear fuel. During this time, the reactor core first has to cool down from around 300 degrees to almost room temperature. Then the second step follows: removing the fuel rods from the reactor core. For this purpose, the cover of the reactor core is raised by means of a remote-controlled crane and the core is flooded with water so far that the connection channel to the adjacent cooling basin is under water. display

Cool for five years

Through the so-called storage lock, the fuel rods are now transferred with the crane under water into the sinking pool. This procedure prevents excessive radioactive radiation of the interior of the reactor and at the same time ensures the continuous cooling of the nuclear fuel rods, which are usually still more than 100 degrees centigrade hot. In the decay tank, water serves as a coolant and as a neutron brake. In order to prevent a renewed flare-up of the chain reaction, additional neutron-absorbing substances such as boric acid are added.

Nevertheless, the decay products of the nuclear reaction are still active in the decay pool - as evidenced by a ghostly bluish glow. This Cherenkov light is produced when the energetic electrons released during the decay of radionuclides collide with water particles. The atoms are stimulated by this and return the energy in the form of light when they return to their ground state. Under several meters of water and with constant cooling, the fuel rods remain in the cooling pool until their temperature has dropped to about 40 to 50 degrees.

Then the nuclear fuel rods can be removed from the nuclear power plant. For this purpose, they are transhipped into Castorbeh lter with special tweezers and taken to an interim storage facility. Only with this removal ends the so-called Nachbetriebsphase and the power plant is officially considered shut down.

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- Nadja Podbregar