Nuclides that tend to absorb a neutron and either fissionable fuels and have a corresponding " fission cross section ".
2.
Uranium-238, for example, has a near-zero fission cross section for neutrons of less than one MeV energy.
3.
In general, as the energy of the neutrons increases, the ratio of the fission cross section to the neutron capture cross section changes in favour of fission.
4.
As an example, the plot on the right shows that the fission cross section of the uranium 235 is low at high neutron energies but becomes higher at low energies.
5.
The isotope has about the same thermal neutron capture cross section as ( 289.5 ?1.4 vs 269.3 ?2.9 barns ), but only a tiny thermal neutron fission cross section ( 0.064 barns ).
6.
Although the thermal neutron fission cross section ( ? f ) of the resulting is comparable to and, it has a much lower capture cross section ( ? ? ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy.
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