The early generations of nuclear fission technology depended on the thermal neutron (neutrons with an energy ~ 0.025 eV) fission of the natural isotope 235U, present in nature at 0.72 atom%. The dominant uranium isotope, 238U, is a so — called ‘‘fertile’’ isotope since it can be converted into artificial isotopes, especially the fissile 239Pu, by neutron irradiation. Thermally fissile plutonium isotopes can also be exploited in energy production. So, as plutonium and other fissile iso­topes are produced through irradiation of 238U, they can also be exploited in energy production, either through consumption in situ, or through recycling into new fuel materials. In a uranium-fuelled thermal reactor, about 40% of total energy is derived from the fission of plutonium isotopes produced in situ.