Plutonium

Plutonium is a silvery radioactive metal named after the planetoid Pluto. It is a man-made element discovered in 1940 after uranium-238 was bombarded with neutrons in a device called a cyclotron. Plutonium-238, as well as uranium-235 and uranium-233, are among the few materials whose atoms can be split (or "fissioned") to create a nuclear explosion, which releases massive amounts of energy almost instantaneously.

Plutonium is used in the manufacture of nuclear weapons. It was first made in significant quantities during the World War II Manhattan Project. Plutonium can also be used as a fuel in certain types of nuclear reactors where the fission process is controlled so that large quantities of energy are released more slowly to make steam and produce electricity. The split plutonium atoms become lighter elements such as cesium-137 and strontium, many of which are highly radioactive. These "fission products" are radioactive waste, although some are useful for nuclear medicine and other purposes.