Fission cross sections are a measurable property related to the probability that fission will occur in a nuclear reaction. Cross sections are a function of incident neutron energy, and those for  and  are a million times higher than  at lower neutron energy levels. Absorption of any neutron makes available to the nucleus binding energy of about 5.3 MeV.  needs a fast neutron to supply the additional 1 MeV needed to cross the critical energy barrier for fission. In the case of  however, that extra energy is provided when  adjusts from an odd to an even mass. In the words of Younes and Lovelace, "...the neutron absorption on a  target forms a  nucleus with excitation energy greater than the critical fission energy, whereas in the case of n + , the resulting  nucleus has an excitation energy below the critical fission energy."
About 6 MeV of the fission-input energy is supplied by the simple binding of an extra neutron to the heavy nucleus via the strong force; however, in many fissionable isotopes, this amount of energy is not enough for fission. Uranium-238, for example, has a near-zero fission cross section for neutrons of less than 1 MeV energy. If no additional energy is supplied by any other mechanism, the nucleus will not fission, but will merely absorb the neutron, as happens when  absorbs slow and even some fraction of fast neutrons, to become . The remaining energy to initiate fission can be supplied by two other mechanisms: one of these is more kinetic energy of the incoming neutron, which is increasingly able to fission a fissionable heavy nucleus as it exceeds a kinetic energy of 1 MeV or more (so-called fast neutrons). Such high energy neutrons are able to fission  directly (see thermonuclear weapon for application, where the fast neutrons are supplied by nuclear fusion). However, this process cannot happen to a great extent in a nuclear reactor, as too small a fraction of the fission neutrons produced by any type of fission have enough energy to efficiently fission  (fission neutrons have a mode energy of 2 MeV, but a median of only 0.75 MeV, meaning half of them have less than this insufficient energy).
Among the heavy actinide elements, however, those isotopes that have an odd number of neutrons (such as U with 143 neutrons) bind an extra neutron with an additional 1 to 2 MeV of energy over an isotope of the same element with an even number of neutrons (such as U with 146 neutrons). This extra binding energy is made available as a result of the mechanism of neutron pairing effects, which itself is caused by the Pauli exclusion principle, allowing an extra neutron to occupy the same nuclear orbital as the last neutron in the nucleus. In such isotopes, therefore, no neutron kinetic energy is needed, for all the necessary energy is supplied by absorption of any neutron, either of the slow or fast variety (the former are used in moderated nuclear reactors, and the latter are used in fast-neutron reactors, and in weapons).
According to Younes and Loveland, "Actinides like  that fission easily following the absorption of a thermal (0.25 meV) neutron are called fissile, whereas those like  that do not easily fission when they absorb a thermal neutron are called fissionable."