The nuclear fuel cycle is the series of industrial processes which involve the production of electricity from uranium in nuclear power reactors. Uranium is a relatively common element that is found throughout the world.
Uranium is a very heavy metal which can be used as an abundant source of concentrated energy. Uranium occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum.
Neutrons in motion are the starting point for everything that happens in a nuclear reactor. When a neutron passes near to a heavy nucleus, for example uranium-235, the neutron may be captured by the nucleus and this may or may not be followed by fission.