Nuclear energy is presently provided by nuclear fission, and is expected to be provided by nuclear fusion sometime during the 21st century. Nuclear fission is the process in which a large, unstable nucleus decays into two smaller fragments. It depends on a finite supply of fissionable material. Nuclear fusion is the combination, or fusing, of two small nuclei into a single larger nucleus. Fusion reactions are the source of energy supplied by the Sun. Attempts to harness and commercialize fusion energy have so far been unsuccessful because of the technical difficulties involved in igniting and controlling a fusion reaction.
Nevertheless, fusion energy is expected to contribute significantly to the energy mix by the end of the 21st century, even though a prototype commercial-scale nuclear reactor is not expected to exist until 2015 at the earliest12. Both fission and fusion reactions release large amounts of energy. The decay products of the fission process can be highly radioactive for long periods of time, while the byproducts of the fusion process are relatively safe.