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Mar25-06, 06:00 AM
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U235 is considered 'fissile', which means the nucleus is readily or more easily fissioned upon the absorption of a neutron, and it tends to more readily fission with neutrons of low energy (< 1 ev). U238 is considered 'fissionable', which means it may fission but that it requires an energetic neutron in order to fission - generally several MeV.

The other fissile nuclei also have odd masses, U233, Pu239 and Pu241, as well as some heavier nuclei.

The stability, or rather instability, to fission has to do with the binding energy of the excited nuclear once the neutron is absorbed. For example, the binding energy (excess energy) of the last neutron in U236 is approximately 6.4 MeV, but the critical energy for fission is only 5.3 MeV, so when U235 absorbs a neutron, the resulting U236 nucleus has an excess of 1.1 MeV above the energy needed to cause fission.

U238 will absorb a neutron to form U239, which will eventually decay by beta emission to Np239, and this nuclide then subsequently decays by beta emission to Pu239, which is fissile. This is the process by which Pu239 is produced.