Plutonium – the 94th element in the classification of elements – was discovered in 1940 by the American researchers Seaborg, McMillan, Wahl and Kennedy as the second transuranium element upon bombardment of uranium-238 with deuterons, thus forming Pu-238. Today 15 Pu-isotopes are known. Due to its property as fissile material, the isotope Pu-239 (half-life 24,110 years) is of specific importance. The elements 93 and 94 following the 92nd elementuranium – in the classification of elements have been named analogously to uranium, which is named after the planet Uranus, ‘neptunium’ and ‘plutonium’, the planets Neptune and Pluto following Uranus. Plutonium is generated by neutron capture in uranium-238 and two subsequent beta decays according to the following scheme:

U-238 + n ==> U-239 ==> ß-decay ==> Np-239 ==> ß-decay ==> Pu-239.

In nature, plutonium-239 occurs in tiny quantities in minerals containing uranium (pitchblende, carnotite) – one Pu atom per 1 trillion and more uranium atoms. It is formed from U-238 by neutron capture released upon the spontaneous fission of U-238. In above-ground nuclear weapon tests, approx. six tonnes Pu-239 were released into the atmosphere and distributed all over the world, so that in Central Europe for example, about 60 Bq Pu-239 per m2 have been deposited. Plutonium is a radiotoxic substance and its chemical toxicity as a heavy metal is therefore negligible. The radiotoxic effect of plutonium is very serious in the case of inhalation of the finest Pu aerosols; ingestion of plutonium is about 10,000 times less dangerous, since only 1/100 percent of plutonium is absorbed by the intestinal mucosa, 99.99% is excreted immediately.