Fertile nuclides, as opposed to fissile nuclides, are incapable of direct fission, nor are generally capable of undergoing induced fission, but are able to be converted into fissile nuclides by being energised by fast neutrons within a nuclear reactor. The high speed passage of a neutron through the nucleus without absorption leaves the nucleus in an excited state.

Whereas normally, such an excited metastable nucleus would, within a certain time depending upon the halflife, release this extra internal energy by undergoing an internal transition and emitting a gamma ray, a fertile nucleus also becomes liable to spontaneous fission when so excited. In books, such nuclides are designated by an 'f' as in U-236f, where the 'f' means it is in an excited fissionable state (as opposed to an 'm' meaning metastable from which only a radiative isomeric transition decay to the ground state is possible). Uranium-236f can decay by either spontaneous fission (12% probability) or by isomeric transition (88% probability).

The following are fertile nuclides : neptunium-237, protactinium-233, plutonium-238, plutonium-240, thorium-232, uranium-234, uranium-236 and uranium-238, all shown above.

See Fission Isomers.

Fissile nuclides, on the other hand, are nuclides that can readily be induced into fission by the absorption of a slow neutron. Examples are Plutonium-239 and Plutonium-241.

See Induced Fission.