Plutonium (pronounced /plu?'to?ni?m/, 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 radiological 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 it can break apart by being bombarded by thermal 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 neutrons and deuterons, 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. 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.

Contents [hide]
1 Characteristics 
1.1 Physical 
1.2 Allotropes 
1.3 Nuclear fission 
1.4 Isotopes and synthesis 
1.5 Compounds and chemistry 
1.6 Occurrence 
2 History 
2.1 Discovery 
2.2 Early research 
2.3 Production during the Manhattan Project 
2.4 Trinity and Fat Man atomic bombs 
2.5 Cold War use and waste 
2.6 Medical experimentation 
3 Applications 
3.1 Explosives 
3.2 Use of nuclear waste 
3.3 Power and heat source 
4 Precautions 
4.1 Toxicity 
4.2 Criticality potential 
4.3 Flammability 
5 See also 
6 Notes 
7 References 
8 Bibliography 
9 External links 
 


[edit] Characteristics

[edit] Physical
Plutonium, like most metals, has a bright silvery appearance at first, much like nickel, but it oxidizes very quickly to a dull gray, although yellow and olive green are also reported.[1][2] At room temperature plutonium is in its ? form (alpha). This, the most common structural form of the element (allotrope), is about as hard and brittle as gray cast iron unless it is alloyed with other metals to make it soft and ductile.[1] Unlike most metals, it is not a good conductor of heat or electricity.[1] It has a low melting point (640 °C) and an unusually high boiling point (3,327 °C).[1]

Alpha particle emission, which is the release of high-energy helium nuclei, is the most common form of radiation given off by plutonium.[3] Heat given off by the release of and deceleration of these alpha particles make a mass of plutonium the size of a softball warm to the touch while a somewhat larger mass can boil a liter of water in a few minutes, although this varies with isotopic composition.[4][5]

Resistivity is a measure of how strongly a material opposes the flow of electric current. The resistivity of plutonium at room temperature is very high for a metal and it gets even higher with lower temperatures, which is unusual for metals.[6] This trend continues down to 100 K, below which resistivity rapidly decreases for fresh samples.[6] Resistivity then begins to increase with time at around 20 K due to radiation damage, with the rate dictated by the isotopic composition of the sample.[6]

Due to self-irradiation, a sample of plutonium fatigues throughout its crystal structure, meaning the ordered arrangement of its atoms becomes disrupted by radiation with time.[7] However, self-irradiation can also lead to annealing which counteracts some of the fatigue effects as temperature increases above 100 K.[8]


[edit] Allotropes
Main article: Allotropes of plutonium
 
Plutonium has six allotropes at ambient pressure: alpha (?), beta (ß), gamma (?), delta (?), delta prime (?'), & epsilon (?)[9]Plutonium normally has six allotropes, and forms a seventh (zeta, ?) under high temperature and a limited pressure range.[9] These allotropes, which are different structural modifications or forms of an element, have very similar energy levels but significantly varying densities and crystal structures. This makes plutonium very sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following phase transitions from one allotropic form to another.[7] Unlike most materials, plutonium increases in density when it melts, by 2.5%, but the liquid metal exhibits a linear decrease in density with temperature.[6] Densities of the different allotropes vary from 16.00 g/cm3 to 19.86 g/cm3.[10]

The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. For example, the ? form exists at room temperature in unalloyed plutonium. It has machining characteristics similar to cast iron but changes to the plastic and easy to work ß form (beta) at slightly higher temperatures.[11] The reasons for the complicated phase diagram are not entirely understood. The ? form has a low-symmetry monoclinic structure, hence its brittleness, strength, compressibility and poor conductivity.[9]

Plutonium in the ? form (delta) normally exists in the 310 °C to 452 °C range but is stable at room temperature when alloyed with a small percentage of gallium, aluminium, or cerium, enhancing workability and allowing it to be welded.[11] The delta form has more typical metallic character, and is roughly as strong and malleable as aluminium.[9]


[edit] Nuclear fission
Plutonium is a radioactive actinide metal whose isotope, plutonium-239 (Pu-239), is one of the three primary fissile isotopes (uranium-233 and uranium-235 are the other two).[12] To be considered fissile, an isotope's atomic nucleus must be able to break apart or fission when struck by a neutron, and to release enough additional neutrons in the process to sustain the reaction.

 
Weapons-grade electrorefined plutoniumThe isotope Pu-239 can undergo nuclear fission if its nucleus is struck by a thermal neutron.[13] The fission of Pu-239 itself releases neutrons that bombard other Pu-239 atoms, which fission and release more neutrons and so on in a nuclear chain reaction. This isotope has a positive multiplication factor (k), which means that if the metal is present in sufficient mass and with an appropriate geometry (e.g., a compressed sphere), it can form a critical mass.[14]

During fission, a fraction of the binding energy, which holds a nucleus together, is released as a large amount of thermal, electromagnetic and kinetic energy; a kilogram of Pu-239 can produce an explosion equivalent to 20,000 tons of TNT.[4] It is this energy that makes Pu-239 useful in nuclear weapons and reactors.

The presence of the isotope plutonium-240 (Pu-240) in a material limits its nuclear bomb potential, as Pu-240 has a relatively high spontaneous fission rate (~440 fissions per second per gram—over 1,000 neutrons per second per gram[15]), raising the background neutron levels and thus increasing the risk of predetonation.[16] Plutonium is identified as either weapon grade, fuel grade, or power reactor grade based on the percentage of Pu-240 that it contains. Weapon grade plutonium contains less than 7% Pu-240. Fuel grade plutonium contains from 7 to less than 19%, and power reactor grade contains 19% or more Pu-240.[17] The isotope plutonium-238 (Pu-238) is not capable of undergoing nuclear fission easily, although it will undergo alpha decay.[4]


[edit] Isotopes and synthesis
Main article: Isotopes of plutonium
Twenty radioactive isotopes of plutonium have been characterized.[3] The longest-lived are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years.[3] All of the remaining radioactive isotopes have half-lives that are less than 7,000 years.[3] This element also has eight metastable states, though none are stable and all have half-lives less than one second.[3]

The isotopes of plutonium range in mass number from 228 to 247.[3] The primary decay modes of isotopes with mass numbers lower than the most stable isotope, Pu-244, are spontaneous fission and ? 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).[3] The primary decay mode for isotopes with mass numbers higher than Pu-244 is ß emission, mostly forming americium (95 protons) isotopes as decay products.[3] Pu-241 is the parent isotope of the neptunium decay series, decaying to americium-241 via ß or electron emission.[4]

Pu-238 and Pu-239 are the most-widely synthesized isotopes.[4] Pu-239 is synthesized via the following reaction using uranium (U) and neutrons (n) via beta decay (ß-) with neptunium (Np) as an intermediate:[18]


In other words, neutrons from the fission of U-235 are captured by U-238 nuclei to form U-239; a beta decay adds a proton to form Np-239 (half-life 2.36 days) and another beta decay forms Pu-239.[19] Workers on the Tube Alloys project had predicted this reaction theoretically in 1940.

Pu-238 is synthesized by bombarding U-238 with deuterons (D, the nuclei of heavy hydrogen) in the following reaction:[20]


In this equation, a deuteron hitting U-238 produces two neutrons and Np-238. The Np-238 spontaneously decays by emitting negative beta particles to form Pu-238.


[edit] Compounds and chemistry
See also: Plutonium compounds 
 
Various oxidation states of Pu in solutionAt room temperature, pure plutonium is silvery in color but gains a tarnish when oxidized.[4] The element displays four common ionic oxidation states in aqueous solution and one rare one:[10]

Pu(III), as Pu3+ (blue lavender) 
Pu(IV), as Pu4+ (yellow brown) 
Pu(V), as PuO2+ (pink?)[note 1] 
Pu(VI), as PuO22+ (pink orange) 
Pu(VII), as PuO53- (green)–the heptavalent ion is rare 
The color shown by plutonium solutions depends on both the oxidation state and the nature of the acid anion.[21] It is the acid anion that influences the degree of complexing—how atoms connect to a central atom—of the plutonium species.

Metallic plutonium is produced by reacting plutonium(IV) fluoride with barium, calcium or lithium at 1200 °C.[22] It is attacked by acids, oxygen, and steam but not by alkalis and dissolves easily in concentrated hydrochloric, hydroiodic and perchloric acids.[23] Molten metal must be kept in a vacuum or an inert atmosphere to avoid reaction with air.[11] At 135 °C the metal will ignite in air and will explode if placed in carbon tetrachloride.[24]

 
Plutonium pyrophoricity can cause it to look like a glowing ember under certain conditions.Plutonium is a reactive metal. In moist air or moist argon, the metal oxidizes rapidly, producing a mixture of oxides and hydrides.[1] If the metal is exposed long enough to a limited amount of water vapor, a powdery surface coating of PuO2 is formed.[1] Also formed is plutonium hydride but an excess of water vapor forms only PuO2.[23]

With this coating, the metal is pyrophoric, meaning it can ignite spontaneously, so plutonium metal is usually handled in an inert, dry atmosphere of nitrogen or argon.[1] Oxygen retards the effects of moisture and acts as a passivating agent.[1]

Plutonium reacts readily with oxygen, forming PuO and PuO2 as well as intermediate oxides;[10] plutonium oxide fills 40% more volume than plutonium metal.[24] It reacts with the halogens, giving rise to compounds such as PuX3 where X can be F, Cl, Br or I; PuF4 is also seen.[10] The following oxyhalides are observed: PuOCl, PuOBr and PuOI.[10] It will react with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi2.[10]

Crucibles used to contain plutonium need to be able to withstand its strongly reducing properties.[11] Refractory metals such as tantalum and tungsten along with the more stable oxides, borides, carbides, nitrides and silicides can tolerate this.[11] Melting in an electric arc furnace can be used to produce small ingots of the metal without the need for a crucible.[11]

Plutonium can form alloys and intermediate compounds with most other metals. Exceptions include lithium, potassium and sodium of the alkali metals; barium, calcium and strontium of the alkaline earth metals; and europium and ytterbium of the rare earth metals.[23] Partial exceptions include the refractory metals chromium, molybdenum, niobium, tantalum and tungsten, which are soluble in liquid plutonium, but insoluble or only slightly soluble in solid plutonium.[23]


[edit] Occurrence
Trace amounts of two plutonium isotopes (Pu-239 and Pu-244) can be found in nature. Tiny amounts of Pu-244 occur naturally because it is formed as a minor decay product in uranium ores and it has a comparatively long half-life of about 80 million years.[25] Even smaller traces of Pu-239, a few parts per trillion, and its decay products are naturally found in some concentrated ores of uranium,[26] such as the natural nuclear fission reactor in Oklo, Gabon.[27] The ratio of Pu-239 to U at the Cigar Lake Mine uranium deposit ranges from 2.4 × 10-12 to 44 × 10-12.[28]

Minute traces are found in the human body due to the 550 above-ground nuclear tests and several major nuclear accidents.[24] Most atmospheric nuclear testing was stopped in 1963 by the Limited Test Ban Treaty but France continued to test into the 1980s and several other nations also conducted tests after 1963. Because it is specifically manufactured and is the result of radioactive decay of uranium ores, Pu-239 is the most abundant isotope of plutonium.[24]


[edit] History

[edit] Discovery
Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered element 94 in 1934.[29] Fermi called the element hesperium and mentioned it in his Nobel Lecture in 1938.[30] However, the sample was actually a mixture of barium, krypton, and other elements, but this was not known at the time because nuclear fission had not been discovered yet.[31]

 
Glenn T. Seaborg and his team at Berkeley were the first to produce plutonium.Plutonium (specifically, Pu-238) was first produced and isolated on December 14, 1940, and chemically identified on February 23, 1941, by Dr. Glenn T. Seaborg, Edwin M. McMillan, J. W. Kennedy, Z. M. Tatom, and A. C. Wahl by deuteron bombardment of uranium in the 60-inch (150 cm) cyclotron at the University of California, Berkeley.[32] In the 1940 experiment, neptunium-238 was created directly by the bombardment but decayed by beta emission two days later, which indicated the formation of element 94.[24]

A paper documenting the discovery was prepared by the team and sent to the journal Physical Review in March 1941.[24] The paper was withdrawn before publication after the discovery that an isotope of the new element (Pu-239) could undergo nuclear fission in a way that might be useful in an atomic bomb. Publication was delayed until a year after the end of World War II due to security concerns.[13]

Edwin McMillan had recently named the first transuranium element after the planet Neptune and suggested that element 94, being the next element in the series, be named for what was then considered the next planet, Pluto.[4][note 2] Seaborg originally considered the name "plutium", but later thought that it did not sound as good as "plutonium."[33] He chose the letters "Pu" as a joke, which passed without notice into the periodic table.[note 3] Alternate names considered by Seaborg and others were "ultimium" or "extremium" because of the now-discredited belief that they had found the last possible element on the periodic table.[34]


[edit] Early research
The basic chemistry of plutonium was found to resemble uranium after a few months of initial study.[24] Early research was continued at the secret Metallurgical Laboratory of the University of Chicago. On August 18, 1942, a trace quantity of this element was isolated and measured for the first time. About 50 micrograms of plutonium-239 combined with uranium and fission products was produced and only about 1 microgram was isolated.[26] This procedure enabled chemists to determine the new element's atomic weight.[35][note 4]

In November 1943 some plutonium trifluoride was reduced to create the first sample of plutonium metal: a few micrograms of metallic beads.[26] Enough plutonium was produced to make it the first synthetically-made element to be visible with the unaided eye.[36]

The nuclear properties of plutonium-239 were also studied; researchers found that when it is hit by a neutron it breaks apart (fissions) by releasing more neutrons and energy. These neutrons can hit other atoms of plutonium-239 and so on in an exponentially-fast chain reaction. This can result in an explosion large enough to destroy a city if enough of the isotope is concentrated to form a critical mass.[24]


[edit] Production during the Manhattan Project
During World War II the U.S. government established the Manhattan Project, which was tasked with developing an atomic bomb. The three primary research and production sites of the project were the plutonium production facility at what is now the Hanford Site, the uranium enrichment facilities at Oak Ridge, Tennessee, and the weapons research and design laboratory, now known as Los Alamos National Laboratory.[37]

 
The Hanford B Reactor face under construction—the first plutonium-production reactorThe first production reactor that made plutonium-239 was the X-10 Graphite Reactor. It went online in 1943 and was built at a facility in Oak Ridge that later became the Oak Ridge National Laboratory.[24][note 5]

On April 5, 1944, Emilio Segre at Los Alamos received the first sample of reactor-produced plutonium from Oak Ridge.[38] Within ten days, he discovered that reactor-bred plutonium had a higher concentration of the isotope Pu-240 than cyclotron-produced plutonium. Pu-240 has a high spontaneous fission rate, raising the overall background neutron level of the plutonium sample. The original gun-type plutonium weapon, code-named "Thin Man", had to be abandoned as a result—the increased number of spontaneous neutrons meant that nuclear pre-detonation (a fizzle) would be likely.

The entire plutonium weapon design effort at Los Alamos was soon changed to the more complicated implosion device, code-named "Fat Man." With an implosion weapon, a solid sphere of plutonium is compressed to a high density with explosive lenses—a technically more daunting task than the simple gun-type design, but necessary in order to use plutonium for weapons purposes. (Enriched uranium, by contrast, can be used with either method.)[38]

Construction of the Hanford B Reactor, the first industrial-sized nuclear reactor for the purposes of material production, was completed in March 1945.[39] B Reactor produced the fissile material for the plutonium weapons used during World War II.[note 6] B, D and F were the initial reactors built at Hanford, and six additional plutonium-producing reactors were built later at the site.[39]


[edit] Trinity and Fat Man atomic bombs
 
Because of the presence of Pu-240 in reactor-bred plutonium, the implosion design was developed for the "Fat Man" and Trinity" weaponsThe first atomic bomb test, codenamed "Trinity" and detonated on July 16, 1945, near Alamogordo, New Mexico, used plutonium as its fissile material.[26] The implosion design of "the Gadget", as the Trinity device was code-named, used conventional explosive lenses to compress a sphere of plutonium into a supercritical mass, which was then simultaneously showered with neutrons from an initiator made of beryllium and polonium.[24] Together, these ensured a runaway chain reaction and explosion. The overall weapon weighed over 4 tonnes, although it used just 6.2 kg of plutonium in its core.[40] About 20% of the plutonium used in the Trinity weapon underwent fission, resulting in an explosion with an energy equivalent to approximately 20,000 tons of TNT.[41][note 7]

An identical design was used in the "Fat Man" atomic bomb dropped on Nagasaki, Japan on August 9, 1945, killing 70,000 people and wounded another 100,000.[24] The "Little Boy" bomb dropped on Hiroshima three days earlier used uranium-235, not plutonium. Japan capitulated on August 15, effectively ending the war. Only after the announcement of the first atomic bombs was the existence of plutonium made public.


[edit] Cold War use