Plutonium (Pu) is a radioactive, silvery, metallic element, occurring in uranium ores and produced artificially by neutron bombardment of uranium. Plutonium has 15 isotopes with masses ranging from 232 to 246 and half-lives from 20 minutes to 76 million years. Plutonium is a radiological poison, specifically absorbed by bone marrow, and is used, especially the highly fissionable isotope plutonium-239, as a nuclear fuel.
Plutonium 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. The elements 93 and 94 following the 92nd element – uranium – 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.
|melting point||639.5°C (1,183°F)|
|boiling point||3,227°C (5,841°F)|
Due to its property as fissile material, the isotope Pu-239 (half-life 24,110 years) is of specific importance. It is generated by neutron capture in uranium-238 and two subsequent beta decays according to the following scheme:
U-238 + n → U-239 → β-decay → Np-239 → β-decay → Pu-239.
In nature, plutonium-239 occurs in tiny quantities in minerals containing uranium (pitchblende, carnotite) – one plutonium 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, approximately six tonnes of 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 plutonium 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.