Page "Nuclear engineering" Paragraph 2

The common types of nuclear fission include thermal fission, which is fission caused by the absorption of a relatively slow thermal neutron with kinetic energy approximately 0. 125 eV.

Fast fission is fission caused by the absorption of a more energetic neutron, with kinetic energy on the order of MeV.

Also, in especially heavy nuclei, spontaneous fission may occur.

Nuclei that are fissionable by neutrons typically carry at least a very small chance of spontaneous fission occurring.