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 more than one neutron on average per neutron capture. (Enough of them on each fission, to compensate for non-fissions and absorptions in non-fuel material)
* Have a reasonably long half-life
* Be available in suitable quantities
Fissile nuclides in nuclear fuels include:
* Uranium-233, bred from thorium-232 by neutron capture with intermediate decays steps omitted.
* Uranium-235, which occurs in natural uranium and enriched uranium
* Plutonium-239, bred from uranium-238 by neutron capture with intermediate decays steps omitted.
* Plutonium-241, bred from plutonium-240 directly 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 (σ), the cross section for neutron capture with emission of a gamma ray (σ), and the percentage of non-fissions are in the table at right.
Fertile nuclides in nuclear fuels include:
* Thorium-232, which breeds uranium-233 by neutron capture with intermediate decays steps omitted.
* Uranium-238, which breeds plutonium-239 by neutron capture with intermediate decays steps omitted.
* Plutonium-240, bred from plutonium-239 directly by neutron capture.