The basic process taking place in nuclear reactors is

A nuclear reactor produces and controls the release of energy from splitting the atoms of uranium, known as nuclear fission. When a fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it undergoes nuclear fission in which the heavy nucleus splits into two or more lighter nuclei. The fission process produces free neutrons and gamma photons, and releases a very large amount of energy.