A material that will sustain a fission chain reaction so that it can be used as a source of nuclear energy. The fissile materials are uranium–235, uranium–233, plutonium–241, and plutonium–239. The first occurs in nature as 1 part in 140 of natural uranium, the others have to be made artificially. 233U is produced when thorium–232 captures a neutron and 239Pu is produced by neutron capture in 238U. 232Th and 238U are called fertile isotopes (see fertile material).