Fast fission

Fast fission is

thermal neutrons

, which move more slowly.

Fast reactors vs. thermal reactors

thermal reactor, and fast fission produces a higher average number of neutrons per fission, so fast reactors have better neutron economy making a plutonium breeder reactor possible. However, a fast neutron reactor must use relatively highly enriched uranium or plutonium

so that the neutrons have a better chance of fissioning atoms.

fissile

Some atoms, notably

hydrogen bomb by fusion of deuterium and tritium have even higher energy than the fast neutrons produced in a nuclear reactor. This makes it possible to increase the yield of any given fusion weapon by the simple expedient of adding layers of cheap natural (or even depleted) uranium. Fast fission of uranium-238 provides a large part of the explosive yield, and fallout

, in many designs of hydrogen bomb.

Differences in fission product yield

A graph of fission product yield against the mass 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 as ^113mCd, ^119mSn, ^121mSn, ¹²³Sn, ¹²⁵Sb, ¹²⁶Sn, and ¹²⁷Sb are very low.

The higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the

actinides tend to make even more shallow valleys. In extreme cases such as ²⁵⁹Fm, only one peak is seen.^{[citation needed}

]

References

^ H.W.B. Skinner, The A B C of nuclear weapons // The New Scientist, 26 February 1959, page 473-475: "(3) Dirty H-bombs"

[1] H.W.B. Skinner, The A B C of nuclear weapons // The New Scientist, 26 February 1959, page 473-475: "(3) Dirty H-bombs"