plutonium

Silvery-white, radioactive, metallic element of the actinide series, atomic number 94, relative atomic mass 239.13. It occurs in nature in minute quantities in pitchblende and other ores, but is produced in quantity only synthetically. It has six allotropic forms (see allotropy) and is one of the fissile elements (elements capable of splitting into other elements - others include thorium and uranium). Plutonium dioxide, PuO₂, a yellow crystalline solid, is the compound most widely used in the nuclear industry. It was believed to be inert until US researchers discovered in 1999 that it reacts very slowly with oxygen and water to form a previously unknown green crystalline compound that is soluble in water.

Its fissile isotope Pu-239 is usually made in breeder reactors by bombarding uranium-238 with neutrons. Because Pu-239 is so easily synthesized from abundant uranium, it has been produced in large quantities (hundreds of thousands of pounds) by the weapons industry. It has a long half-life (24,000 years) during which time it remains highly toxic. It is dangerous to handle, difficult to store, and impossible to dispose of - it has become a human-made menace of global proportions. According to a US Department of Energy report December 1994, the conditions in which it is stored across the USA pose a threat to both workers in the nuclear weapons industry and the general public.

Plutonium was first produced in 1940 by Glenn Seaborg and his team at the University of California at Berkeley, by bombarding uranium with deuterons; this was the second transuranic element synthesized (after neptunium). It was named by them after the dwarf planet Pluto, since it comes after neptunium as Pluto came after Neptune as a planet in our Solar System before its reclassification to dwarf planet in 2006.