LYCOS RETRIEVER 
Uranium: Neutrons
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Uranium atoms are unstable; that is, their nuclei tend spontaneously to fission or break down into smaller nuclei, fast particles (including neutrons), and high-energy photons. The fission of an isolated uranium nucleus is a randomly timed event; ... collision with a neutron may trigger a uranium nucleus to fission immediately. Crowding large numbers of uranium atoms together can enable the neutrons emitted by a few nuclei undergoing fission to cause other nuclei to fission, whose released neutrons in turn trigger still other nuclei, and so on. If this chain reaction proceeds at a constant rate, it may be used to generate electricity; if it proceeds at an exponentially increasing rate, a nuclear explosion results.
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Uranium-238, uranium's most common isotope, can be converted into plutonium-239, a fissionable material that can ... be used as a fuel in nuclear reactors. To produce plutonium-239, atoms of uranium-238 are exposed to neutrons. Uranium-239 forms when uranium-238 absorbs a neutron. Uranium-239 has a half-life of about 23 minutes and decays into neptunium-239 through beta decay. Neptunium-239 has a half-life of about 2.4 days and decays into plutonium-239, also through beta decay.
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Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. Natural uranium, slightly enriched with 235U by a small percentage, is used to fuel nuclear power reactors to generate electricity. Natural thorium can be irradiated with neutrons as follows to produce the important isotope 233U:
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Neutrons