![]() ![]() Enormous amounts of energy are released in a very short time, a fact that becomes visible with the explosion of a nuclear weapon.Īrranging for the uncontrolled, large –scale release of energy produced during nuclear fission is a relatively simple task. During a nuclear chain reaction, many billions of uranium nuclei may fission in less than a second. However, the chain –reaction principle is used in practice.)Ī reaction of this type that continues on its own once under way is known as a chain reaction. (In practice, a single neutron cannot begin a chain reaction even in a pure 1 –g block of uranium –235 too many neutrons will simply escape from the sample without causing a fission event. As long as more neutrons are being released, the fission of uranium nuclei can continue. In the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. Each of those neutrons, then, is available for the fission of three more uranium nuclei. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. 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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