In February 1941 the American physicist Glenn Seaborg discovered the ninety-fourth element of the classification of Mendeleev: plutonium. The best known isotope of this "transuranic" element is plutonium-239, which is the nucleus the most abundantly produced in nuclear reactors. Generated by neutron captures in uranium-238, plutonium-239 is fissile.
Fission of plutonium-239 provides more secondary neutrons than uranium-235: 2.91 on average, of which 2.30 may trigger a further fission when captured. The fission probability is the highest when slow neutrons are used. Fast neutrons are able to split all the plutonium isotopes and allow "breeding".
Because of the role played by plutonium-239, one often forgets that the plutonium contains other isotopes among which plutonium-241 is also fissile. Within conventional pressurized water reactor burning enriched uranium fuel, plutonium 239 represents 58% of the plutonium found in the discharged spent fuel. Adding plutonium-241, fissile isotopes represent about 70% of this plutonium.
Weapon-grade plutonium should contain more than 90 % of pluonium-239. For that reason plutonium produced by civilian reactiors are unfit for atomic bombs.
When plutonium is not recycled in a reactor, it becomes a waste that will require a deep underground storage in repositories. The quantity of plutonium present today in the world is estimated to be around 1000 tons. According to some experts “fissionable uranium reserves (easily exploitable) would not exceed 72 years". It may be therefore useful to have plutonium available in a remote future if oil reserves also begin to run out, since it offers the potential with breeder reactors of an almost inexhaustible energy source.
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