In a nuclear reactor, a type of nuclear reaction called nuclear fission occurs. Nuclear fission is a process where the nucleus of an atom is split into two or more smaller nuclei, accompanied by the release of a great amount of energy. This energy is what is harnessed in nuclear reactors to generate electricity.

In most commercial nuclear reactors, the nuclear fission reaction involves the splitting of uranium-235 atoms. Neutrons are used to bombard uranium-235 atoms, causing them to split into smaller atoms, such as krypton and barium, along with the release of additional neutrons and a significant amount of energy in the form of heat.

The released neutrons from the fission process can then go on to split other uranium-235 atoms, creating a chain reaction. To control this chain reaction and maintain a steady rate of fission, control rods made of neutron-absorbing materials are used to absorb excess neutrons and regulate the reactor's power output.

This carefully controlled nuclear fission process allows nuclear reactors to generate large amounts of heat energy, which is then used to produce steam and drive turbines for electricity generation.