Actinide
Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric (ignites spontaneously in air). It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.
Plutonium was first synthesized and isolated in late 1940 and early 1941, by deuteron bombardment of uranium-238 in the 1.5-metre (60 in) cyclotron at the University of California, Berkeley. First, neptunium-238 (half-life 2.1 days) was synthesized, which then beta-decayed to form the new element with atomic number 94 and atomic weight 238 (half-life 88 years). Since uranium had been named after the planet Uranus and neptunium after the planet Neptune, element 94 was named after Pluto, which at the time was also considered a planet. Wartime secrecy prevented the University of California team from publishing its discovery until 1948.
Plutonium is the element with the highest atomic number known to occur in nature. Trace quantities arise in natural uranium deposits when uranium-238 captures neutrons emitted by decay of other uranium-238 atoms. …
Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered element 94 in 1934. Fermi called the element hesperium and mentioned it in his Nobel Lecture in 1938. The sample actually contained products of nuclear fission, primarily barium and krypton. Nuclear fission, discovered in Germany in 1938 by Otto Hahn and Fritz Strassmann, was unknown at the time.
Plutonium (specifically, plutonium-238) was first produced, isolated, and then chemically identified between December 1940 and February 1941 by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy, and Arthur Wahl by deuteron bombardment of uranium in the 60-inch (150 cm) cyclotron at the Berkeley Radiation Laboratory at the University of California, Berkeley.
Neptunium-238 was created directly by the bombardment but decayed by beta emission with a half-life of a little over two days, which indicated the formation of element 94. The first bombardment took place on December 14, 1940, and the new element was first identified through oxidation on the night of February 23–24, 1941.
A paper documenting the discovery was prepared by the team and sent to the journal Physical Review in March 1941, but publication was delayed until a year after the end of World War II due to security concerns. …
Plutonium, like most metals, has a bright silvery appearance at first, much like nickel, but it oxidizes very quickly to a dull gray, though yellow and olive green are also reported. At room temperature plutonium is in its α (alpha) form. This allotrope is about as hard and brittle as gray cast iron. When plutonium is alloyed with other metals, the high-temperature δ allotrope is stabilized at room temperature, making it soft and ductile. Unlike most metals, it is not a good conductor of heat or electricity. It has a low melting point (640 °C, 1,184 °F) and an unusually high boiling point (3,228 °C, 5,842 °F). This gives a large range of temperatures (over 2,500 kelvin wide) at which plutonium is liquid, but this range is neither the greatest among all actinides nor among all metals, with neptunium theorized to have the greatest range in both instances. The low melting point as well as the reactivity of the native metal compared to the oxide leads to plutonium oxides being a preferred form for applications such as nuclear fission reactor fuel (MOX-fuel).
Alpha decay, the release of a high-energy helium nucleus, is the most common form of radioactive decay for plutonium. A 5 kg mass of 239Pu contains about 12.5×1024 atoms. With a half-life of 24,100 years, about 11.5×1012 of its atoms decay each second by emitting a 5.157 MeV alpha particle. This amounts to 9.68 watts of power. Heat produced by the deceleration of these alpha particles makes it warm to the touch. …
Trace amounts of plutonium-238, plutonium-239, plutonium-240, and plutonium-244 can be found in nature. Small traces of plutonium-239, a few parts per trillion, and its decay products are naturally found in some concentrated ores of uranium, such as the natural nuclear fission reactor in Oklo, Gabon. The ratio of plutonium-239 to uranium at the Cigar Lake Mine uranium deposit ranges from 2.4×10−12 to 44×10−12. These trace amounts of 239Pu originate in the following fashion: on rare occasions, 238U undergoes spontaneous fission, and in the process, the nucleus emits one or two free neutrons with some kinetic energy. When one of these neutrons strikes the nucleus of another 238U atom, it is absorbed by the atom, which becomes 239U. With a relatively short half-life, 239U decays to 239Np, which decays into 239Pu. Finally, exceedingly small amounts of plutonium-238, attributed to the extremely rare double beta decay of uranium-238, have been found in natural uranium samples.
Due to its relatively long half-life of about 80 million years, it was suggested that plutonium-244 occurs naturally as a primordial nuclide, but early reports of its detection could not be confirmed. Based on its likely initial abundance in the Solar System, present experiments as of 2022 are likely about an order of magnitude away from detecting live primordial 244Pu. …
Large stockpiles of weapons-grade plutonium were built up by both the Soviet Union and the United States during the Cold War. The U.S. reactors at Hanford and the Savannah River Site in South Carolina produced 103 tonnes, and an estimated 170 tonnes of military-grade plutonium was produced in the USSR. Each year about 20 tonnes of the element is still produced as a by-product of the nuclear power industry. As much as 1000 tonnes of plutonium may be in storage with more than 200 tonnes of that either inside or extracted from nuclear weapons.
SIPRI estimated the world plutonium stockpile in 2007 as about 500 tonnes, divided equally between weapon and civilian stocks.
Radioactive contamination at the Rocky Flats Plant primarily resulted from two major plutonium fires in 1957 and 1969. Much lower concentrations of radioactive isotopes were released throughout the operational life of the plant from 1952 to 1992. Prevailing winds from the plant carried airborne contamination south and east, into populated areas northwest of Denver. The contamination of the Denver area by plutonium from the fires and other sources was not publicly reported until the 1970s. According to a 1972 study, co-authored by Edward Martell, "In the more densely populated areas of Denver, the Pu contamination level in surface soils is several times fallout", and the plutonium contamination "just east of the Rocky Flats plant ranges up to hundreds of times that from nuclear tests". …
Twenty-two radioisotopes of plutonium have been characterized, from 226Pu to 247Pu. The longest-lived are 244Pu, with a half-life of 80.8 million years; 242Pu, with a half-life of 373,300 years; and 239Pu, with a half-life of 24,110 years. All other isotopes have half-lives of less than 7,000 years. This element also has eight metastable states, though all have half-lives less than a second. 244Pu has been found in interstellar space and it has the longest half-life of any non-primordial radioisotope. The main decay modes of isotopes with mass numbers lower than the most stable isotope, 244Pu, are spontaneous fission and alpha emission, mostly forming uranium (92 protons) and neptunium (93 protons) isotopes as decay products (neglecting the wide range of daughter nuclei created by fission processes). The main decay mode for isotopes heavier than 244Pu, along with 241Pu and 243Pu, is beta emission, forming americium isotopes (95 protons). Plutonium-241 is the parent isotope of the neptunium series, decaying to americium-241 via beta emission.
Plutonium-238 and 239 are the most widely synthesized isotopes. …
There are two aspects to the harmful effects of plutonium: radioactivity and heavy metal poisoning. Plutonium compounds are radioactive and accumulate in bone marrow. Contamination by plutonium oxide has resulted from nuclear disasters and radioactive incidents, including military nuclear accidents where nuclear weapons have burned. Studies of the effects of these smaller releases, as well as of the widespread radiation poisoning sickness and death following the atomic bombings of Hiroshima and Nagasaki, have provided considerable information regarding the dangers, symptoms and prognosis of radiation poisoning, which in the case of the Japanese survivors was largely unrelated to direct plutonium exposure.
The decay of plutonium, releases three types of ionizing radiation: alpha (α), beta (β), and gamma (γ). Either acute or longer-term exposure carries a danger of serious health outcomes including radiation sickness, genetic damage, cancer, and death. The danger increases with the amount of exposure. α-radiation can travel only a short distance and cannot travel through the outer, dead layer of human skin. β-radiation can penetrate human skin, but cannot go all the way through the body. γ-radiation can go all the way through the body.
Even though α radiation cannot penetrate the skin, ingested or inhaled plutonium does irradiate internal organs. α-particles generated by inhaled plutonium have been found to cause lung cancer in a cohort of European nuclear workers. …