Control rods control the nuclear fission reaction by absorbing neutrons in the reactor core. Usually, they are stainless steel tubes filled with a material that is good at absorbing neutrons, like boron.
The further they are lowered into the reactor core, the more neutrons they absorb and the fewer nuclei can be split, reducing the heat produced. If the rods are lowered all the way, the reaction stops completely.
Did you know?
Safety first
Powerful electromagnets hold the control rods above the reactor core, so that in the event of a power cut the control rods would automatically fall into it and stop the nuclear reaction within 4 seconds. This safe shutdown is known as a scram.
