Nuclear fission

Nuclear fission occurs when an atom of a heavy element (such as uranium) breaks into two or more atoms of a lighter element (such as barium and krypton). Fission of elements heavier than iron will release energy, while fission of elements lighter than iron will always consume energy. Efficient production of energy using nuclear fission requires heavy, fissile isotopes (such as uranium-235 or plutonium-239) that will decay relatively easily to liberate a relatively large amount of energy.