Nuclear Energy, energy released in large amounts by the splitting or formation of atomic nuclei. The light and heat of the sun and other stars is an example of naturally occurring nuclear energy. Artificially produced nuclear energy can be released either in a steady, controllable manner, as in devices called nuclear reactors, or in the form of a violent explosion, as in nuclear weapons. Properly speaking, the energy released by the process of radioactive decay is also a form of nuclear energy. In radioactive decay, elements such as uranium produce radiation when the nucleus of the element loses protons or neutrons, thus changing into a different kind of nucleus. However, this type of energy is generally not included in discussions of nuclear energy. This article deals with nuclear energy in the usual sense.

Nuclear energy is sometimes called atomic energy. However, atoms can be the source of both nuclear and chemical energy. Nuclear energy involves the atom's nucleus; chemical energy involves the atom's electrons—subatomic particles that surround the nucleus. Pound for pound, a nuclear fuel (a material used as a source of nuclear energy) will produce several million times as much energy as a chemical fuel such as gasoline. Nuclear fuels yield so much energy that even a heavy ship such as an aircraft carrier powered by nuclear energy can operate many years without refueling.