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Fission occurs when the nucleus of an atom splits into smaller parts. Only a few atoms naturally undergo fission. These are the heavy atoms which contain an unstable nucleus. In the case of energy production it is the uranium atom that is mostly used. Uranium as found in nature is primarily U-238, this isotope rarely undergoes fission, however the U-235 isotope readily undergoes fission. So, the first thing that must be done is to take the U-238 which is very common and enrich it with U-235. That is to say that in natural uranium ore is 99.7% U-238 and 0.3% of the desired U-235. The fuel pellets used in nuclear power plants contain about 3% U-235.

Fission occurs when a neutron (emitted from another atom) smashes into the nucleus and is absorbed. After being absorbed the nucleus then becomes very unstable and spits forming two daughter nucleids and releasing about 3 neutrons (which then can split three more atoms and on and on and on....). However, in order for the nucleus to "absorb" the neutron it must be "slow moving." That is to say a fast moving neutron will not be absorbed by the nucleus and fission will not occur. Therefore a substance must be placed in the reactor vessel to slow down the neutrons. This is called a moderator, in the US it is usually heavy water. Click here to see a simulation of nuclear fission. (Macromedia Flash plugin required)


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