Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Effects of isotopes

Natural uranium contains three isotopes: U-234 (0.006%), U-235 (0.7%), and U-238 (99.3%). The speed required for a fission event vs. a non-fission capture event is different for different isotopes.

Related Topics:
U-235 - U-238

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U-238 will fission with neutrons at energies >1 MeV such as those produced in a nuclear fusion explosion, but uranium fission does not produce such extremely energetic neutrons. U-238 merely captures less energetic neutrons without fissioning, so it cannot support a chain reaction as can U-235; U-238 has no critical mass. However, when U-238 absorbs slow neutrons, the resulting U-239 is not stable. It decays first to Np-239, which in turn decays to Pu-239, which will fission with slow neutrons just as U-235 does. Therefore, some of the output of a reactor fueled only with uranium comes from plutonium fission.

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U-235 fissions over a much wider range of neutron energies than U-238. U-235 has the greatest cross section (i.e., is most likely to fission) when hit with very low speed ("thermal") neutrons much less energetic than those produced when U-235 fissions. A moderator, usually water or graphite, is often used to slow down fission-generated neutrons so they can fission more U-235. Since U-238 just gets in the way of the chain reaction, most reactors use uranium enriched in U-235. If the U-235 fraction is high enough, a chain reaction can be sustained even without a moderator.

Related Topics:
Moderator - Enriched

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U-235 is present in natural uranium only to the extent of about one part in 140. Also, the relatively small difference in mass between the two isotopes makes isotope separation difficult. Nevertheless, the possibility of separating U-235 was recognized early on in the Manhattan Project as being of the greatest importance to their success.

Related Topics:
Isotope separation - Manhattan Project

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