A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. At the present time, all commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium (the fuel) are broken apart into smaller nuclei, with the release of energy. It is theoretically possible to construct reactors that operate on the principle of nuclear fusion, in which small nuclei are combined with each other with the release of energy. But after a half century of research on fusion reactors, no practicable device has yet been developed.

When neutrons strike the nucleus of a large atom, they cause that nucleus to split apart into two roughly equal pieces known as fission products. In that process, additional neutrons and very large amounts of energy are also released. Only three...