- Fast fission
Fast fission is fission that occurs when a heavy
atom absorbs a high-energyneutron , called afast neutron , and splits. Most fissionable materials needthermal neutron s, which move slower.Fast reactors vs. thermal reactors
Fast neutron reactor s use fast fission to produce energy, unlike mostnuclear reactor s. In a conventional reactor, amoderator is needed to slow down the neutrons so that they are more likely to fission atoms. A fast neutron reactor uses fast neutrons, so it does not use a moderator. Moderators may absorb a lot of neutrons in athermal reactor , and fast fission produces a higher average number of neutrons per fission, so fast reactors have better neutron economy making a plutoniumbreeder reactor possible. However, a fast neutron reactor must use relatively highly enricheduranium orplutonium so that the neutrons have a better chance of fissioning atoms.Fissionable but notfissile Some atoms, notably
uranium-238 , do not usually undergo fission when struck by slow neutrons, but do split when struck with neutrons of high enough energy. The fast neutrons produced in ahydrogen bomb by fusion ofdeuterium andtritium have still higher energy than the fast neutrons produced in a nuclear reactor. This makes it very easy to increase the yield of any given fusion weapon almost indefinitely by the simple expedient of adding layers of cheap natural uranium. In fact, fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.Fast fission of uranium-238 does also occur in
fission bomb s but to a much lesser extent than inhydrogen bombs .Differences in fission product yield
A graph of
fission product yield against themass number of the fission fragments has two pronounced but fairly flat peaks, at around 90 to 100, and 130 to 140. With thermal neutrons, yields of fission products with mass between the peaks, such asCd-113m ,Sn-119m ,Sn-121m ,Sn-123 ,Sb-125 ,Sn-126 , andSb-127 are very low.The higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the
neutron energy increases and/or the energy of the fissioning atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass forPu-239 has a more shallow valley than that observed forU-235 , when the neutrons arethermal neutron s. The curves for the fission of the lateractinides tend to make even more shallow valleys. In extreme cases such as 259Fm, only one peak is seen.
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