Nuclear energy (also called atomic energy) results from the conversion of mass into energy according to Albert Einstein's formula E=mc². (This is read "E equals m c squared." E represents energy, m mass, and c the speed of light. If the mass is measured in kilograms and the speed of light in meters per second, the result is energy in joules.) The conversion of one kilogram (2.2 pounds) of any substance into energy would produce about 9 X 1016 joules, or 25 billion kilowatt-hours, of energy.
Nuclear energy is released when the particles that make up the nucleus (core) of an atom are rearranged in some manner. As the particles are rearranged, a small portion of the mass of the nucleus is converted into energy. Nuclear energy in large amounts has been produced by two processes -- fission and fusion. Fission refers to the splitting (fissioning) of a large nucleus into two or more smaller ones. Fusion refers to the building up of a nucleus by combining smaller nuclei or individual protons and neutrons.