Control Rods

It’s in the name with these, they help control the fission reaction inside the reactor. They do this by absorbing neutrons that are shot out by fission reactions, which if not absorbed would cause more fission reactions.  

In old school reactors like AGRs control rods are raised out of the reactor core to allow more fission to happen, therefore raising the power output of the reactor, or they are lowered deeper to reduce the power output. In modern PWRs, control rods are used for starting and stopping the reaction only; alternative methods, such as boric acid, are used for controlling the power output.

Because they can stop fission reactions happening, control rods form a key part of emergency shutdown systems such as the SCRAM in modern reactors. In an emergency they can be lowered or even dropped fully into the reactor core to totally stop reactions happening inside the reactor core.

In modern PWRs control rods are held above the reactor vessel by electromagnets so if the power goes out they will be dropped into the reactor, stopping the fission process and shutting down the reactor. This is part of ensuring the reactor ‘fails to safety’.

They typically are arranged in assemblies as shown in the image here. 

Control rods are made of materials that are good at absorbing neutrons. This is typically Boron, Cadmium or Hafnium.