Plutonium (pronounced /pluːˈtoʊniəm/, symbol Pu, atomic number—or element—94) is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and silicon. When exposed to moist air, it forms oxides and hydrides that expand the sample up to 70% in volume, which in turn flake off as a powder that can spontaneously ignite. It is also a radioactive poison that accumulates in bone marrow. These and other properties make the handling of plutonium dangerous, although its overall toxicity is sometimes overstated.
The most important isotope of plutonium is plutonium-239, with a half-life of 24,100 years. Plutonium-239 is fissile, meaning that the nuclei of its atoms can break apart by being bombarded by slow moving neutrons, releasing energy, gamma radiation and more neutrons. It can therefore sustain a nuclear chain reaction after reaching a critical mass, leading to applications in nuclear weapons and use in some nuclear reactors. The most stable isotope of plutonium is plutonium-244, with a half-life of about 80 million years, long enough to be found in trace quantities in nature. Plutonium-238 has a half-life of 88 years and emits alpha particles. It is a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft. Plutonium-240 has a high rate of spontaneous fission, raising the background neutron rate of any sample it is contained in. The presence of Pu-240 ends up limiting a sample's weapon and power potential and determining its grade: weapons (< 7%), fuel (7–19%) and reactor grade (> 19%). Pu-238 and Pu-239 are synthesized by bombarding uranium-238 with deuterons and neutrons, respectively.
Element 94 was first synthesized in 1940 by a team led by Glenn T. Seaborg and Edwin McMillan at a University of California, Berkeley laboratory. McMillan named the new element after Pluto, and Seaborg suggested the symbol Pu as a joke. Trace amounts of plutonium were subsequently discovered in nature. Discovery of plutonium became a classified part of the Manhattan Project to develop an atomic bomb during World War II. The first nuclear test, "Trinity" (July 1945), and the second atomic bomb used to destroy a city (Nagasaki, Japan, in August 1945), "Fat Man", both had cores of Pu-239. Human radiation experiments studying plutonium were conducted without informed consent, and a number of criticality accidents, some lethal, occurred during and after the war. Disposal of plutonium waste from nuclear power plants and dismantled nuclear weapons built during the Cold War is a major nuclear-proliferation, health, and environmental concern. Other sources of plutonium in the environment are fallout from numerous above-ground nuclear tests (now banned) and several nuclear accidents.