![]() ![]() Suppose that a single neutron strikes a one –gram block of uranium –235. The release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Thus far, however, this isotope has not been put to practical use in nuclear reactors. Uranium –233 can also be produced synthetically by the bombardment of thorium with neutrons. Since uranium –238 always outweighs uranium –235 in commercial nuclear reactor fuel, plutonium –239 is made as a byproduct in all commercial reactors now in operation. Plutonium –239 is produced synthetically when nuclei of uranium –238 are struck by neutrons and transformed into plutonium. Of these, only the first, uranium –235, occurs naturally. Only three isotopes are known to be fissionable, uranium –235, uranium –233, and plutonium –239. In that process, additional neutrons and very large amounts of energy are also released. When neutrons strike the nucleus of a large atom, they can cause that nucleus to split apart into two roughly equal pieces known as fission products. ![]() But after a half century of research on fusion reactors, no practicable device has yet been developed. 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. All commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium are broken apart into smaller nuclei with attendant release of energy. A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion.
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