Nuclear reactors work on the principle that nuclear fission releases heat, which can be harnessed and used to heat water into steam to drive turbines.
A typical nuclear reactor uses enriched uranium in the form of fuel 'pellets', each roughly the size of a coin and about an inch long. The pellets are formed into long rods known as bundles, and housed inside a heavily insulated, pressurised chamber.
In many power stations, the bundles are submerged in water to keep them cool. Other types use carbon dioxide or liquid metal to cool the reactor core.
To function in a reactor - ie produce heat through a fissile reaction - the uranium core must be 'critical'. This means that the uranium must be in sufficiently enriched form to allow a self-sustaining chain reaction to occur.
To regulate this process, and allow the nuclear plant to function, control rods are inserted into the reactor chamber. The rods are made of a substance, typically cadmium, which absorbs neutrons inside the reactor.
Fewer neutrons means fewer chain reactions are started, slowing down the fission process. There are more than 400 nuclear power stations across the globe, producing about 17% of the world's electricity. Nuclear reactors are also used to power submarines and naval vessels.