Fissile - Nuclear Fuel

Nuclear Fuel

To be a useful fuel for nuclear fission chain reactions, the material must:

  • Be in the region of the binding energy curve where a fission chain reaction is possible (i.e. above radium)
  • Have a high probability of fission on neutron capture
  • Release two or more neutrons on average per neutron capture (which means a higher average number of them on each fission, to compensate for nonfissions, and absorptions in the moderator)
  • Have a reasonably long half life
  • Be available in suitable quantities
Capture-fission ratios of fissile nuclides
Thermal neutrons Epithermal neutrons
σF σγ % σF σγ %
531 46 8.0% 233U 760 140 16%
585 99 14.5% 235U 275 140 34%
750 271 26.5% 239Pu 300 200 40%
1010 361 26.3% 241Pu 570 160 22%

Fissile nuclides in nuclear fuels include:

  • Uranium-235 which occurs in natural uranium and enriched uranium
  • Plutonium-239 bred from uranium-238 by neutron capture
  • Plutonium-241 bred from plutonium-240 by neutron capture. The Pu-240 comes from Pu-239 by the same process.
  • Uranium-233 bred from thorium-232 by neutron capture

Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture cross sections for fission (σF), the cross section for neutron capture with emission of a gamma ray (σγ), and the percentage of non-fissions are in the table at right.

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