Uranium Primer - Glossary of Uranium Industry Terms
Fission: The splitting of a heavy nucleus into two, accompanied by the release of a relatively large amount of energy and usually two or more neutrons. It may be spontaneous, but usually is due to a nucleus absorbing a neutron and thus becoming unstable.
Nuclear reactor: A device in which a nuclear fission chain reaction occurs under controlled conditions so that the heat yield can be harnessed or the neutron beams utilized. All commercial reactors are thermal reactors, using a moderator to slow down the neutrons.
Uranium (U): A mildly radioactive element with two isotopes that are fissile (U-235 and U-233) and two isotopes that are fertile (U-238 and U-234). Uranium is the basic fuel of nuclear energy.