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31 janvier 2012 2 31 /01 /janvier /2012 08:55

 

Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu andatomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. Sony A1258274A Battery

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 radioactive and can accumulate in thebones. These properties make the improper handling of plutonium dangerous. Sony VGP-BPL10 Battery

Plutonium is the heaviest primordial element by virtue of its most stable isotope, plutonium-244, whose half-life of about 80 million years is just long enough for the element to be found in trace quantities in nature.[3] Plutonium is mostly a byproduct of nuclear fission in reactors: Some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium.[4] Sony VGP-BPS10 Battery

One utilized isotope of plutonium is plutonium-239, which has a half-life of 24,100 years. Plutonium-239 along with Plutonium-241 are both fissile, meaning the nuclei of their atoms can split when bombarded by thermal neutrons, releasing energy, gamma radiation and more neutrons. These neutrons can sustain a nuclear chain reaction, leading to applications innuclear weapons and nuclear reactors. Sony VGP-BPS10/S Battery

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 theneutron flux of any sample it is in. The presence of plutonium-240 limits a sample's usability for weapons or reactor fuel, and determines its grade. Sony VGP-BPS10A/B Battery

Plutonium isotopes are expensive and inconvenient to separate, so particular isotopes are usually manufactured in specialized reactors.

Plutonium was first synthesized in 1940 by a team led by Glenn T. Seaborg and Edwin McMillan at the University of California, Berkeley laboratory by bombarding uranium-238 with deuterons. Sony VGP-BPS10B Battery

Trace amounts of plutonium were subsequently discovered in nature. Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II, which developed the first atomic bombs. 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 plutonium-239. Sony VGP-BPS10A Battery

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 nuclear-proliferation and environmental concern. Sony VGP-BPS11 Battery

Other sources of plutonium in the environment arefallout from numerous above-ground nuclear tests (now banned).

Physical properties

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.[5][6] At room temperature plutonium is in its ? form (alpha). Sony VGP-BPL11 Battery

This, the most common structural form of the element (allotrope), is about as hard and brittle as grey cast iron unless it isalloyed with other metals to make 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) and an unusually high boiling point (3,327 °C).[5]

Alpha decay, the release of a high-energy helium nucleus, is the most common form of radioactive decay for plutonium.[7] Sony VGP-BPL12 Battery

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.[8][9] Sony VGP-BPS12 Battery

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.[10]This trend continues down to 100 K, below which resistivity rapidly decreases for fresh samples.[10] Sony VGP-BPL13 Battery

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.[10]

Because of self-irradiation, a sample of plutonium fatigues throughout its crystal structure, meaning the ordered arrangement of its atoms becomes disrupted by radiation with time.[11] Sony VGP-BPS13 Battery

Self-irradiation can also lead to annealing which counteracts some of the fatigue effects as temperature increases above 100 K.[12]

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.[10] Sony VGP-BPS13/B Battery

Near the melting point, the liquid plutonium has also very high viscosity and surface tensionas compared to other metals.[11]

Allotropes

Plutonium normally has six allotropes and forms a seventh (zeta, ?) at high temperature within a limited pressure range.[13]These allotropes, which are different structural modifications or forms of an element, have very similar internal energiesbut significantly varying densities and crystal structures. Sony VGP-BPS13/S Battery

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.[11] Densities of the different allotropes vary from 16.00 g/cm3 to 19.86 g/cm3.[14]

The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. Sony VGP-BPS13/S Battery

For example, the ? form exists at room temperature in unalloyed plutonium. It has machining characteristics similar to cast ironbut changes to the plastic and malleable ? form (beta) at slightly higher temperatures.[15] 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.[13] Sony VGP-BPS13A/B Battery

Plutonium in the ? form 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.[15] The delta form has more typical metallic character, and is roughly as strong and malleable as aluminium.[13] Sony VGP-BPS13A/S Battery

In fission weapons, the explosive shock waves used to compress a plutonium core will also cause a transition from the usual delta phase plutonium to the denser alpha form, significantly helping to achieve supercriticality.[16] The ? phase, the highest temperature solid allotrope, exhibits anomalously high atomic self-diffusion compared to other elements.[11] Sony VGP-BPS13AS Battery

Nuclear fission

Plutonium is an element in which the 5f electrons are the transition border between delocalized and localized; it is therefore considered one of the most complex elements.[17] It is a radioactive actinide metal whose isotope, plutonium-239, is one of the three primary fissileisotopes[18] (uranium-233 and uranium-235 ); Sony VGP-BPS13B/B Battery

[19] plutonium-241 is also highly fissile. To be considered fissile, an isotope's atomic nucleus must be able to break apart or fission when struck by a slow moving neutron, and to release enough additional neutrons in the process to sustain the nuclear chain reaction by splitting further nuclei. Sony VGP-BPS13B/Q Battery

Plutonium-239 has a multiplication factor (k) larger than one, 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.[20] During fission, a fraction of the binding energy, which holds a nucleus together, is released as a large amount of electromagnetic and kinetic energy (much of the latter being quickly converted to thermal energy). Sony VGP-BPS13B/S Battery

Fission of a kilogram of plutonium-239 can produce an explosion equivalent to 21,000 tons of TNT.[8]It is this energy that makes plutonium-239 useful in nuclear weapons and reactors. Sony VGP-BPS13Q Battery

The presence of the isotope plutonium-240 in a sample limits its nuclear bomb potential, as plutonium-240 has a relatively high spontaneous fission rate (~440 fissions per second per gram—over 1,000 neutrons per second per gram[21]), raising the background neutron levels and thus increasing the risk of predetonation.[22] Sony VGP-BPS13S Battery

Plutonium is identified as either weapons-grade, fuel grade, or power reactor grade based on the percentage of plutonium-240 that it contains. Weapons-grade plutonium contains less than 7% plutonium-240. Fuel grade plutonium contains from 7% to less than 19%, and power reactor grade contains 19% or more plutonium-240. Sony VGP-BPL7 Battery

Supergrade plutonium, with less than 4% of plutonium-240, is used in U.S. Navy weapons stored in proximity to ship and submarine crews, due to its lower radioactivity.[23] The isotope plutonium-238 is not fissile but can undergo nuclear fission easily with fast neutrons as well as alpha decay.[8] Sony VGP-BPS7 Battery

Isotopes and synthesis

Twenty radioactive isotopes of plutonium have been characterized. The longest-lived are plutonium-244, with a half-life of 80.8 million years, plutonium-242, with a half-life of 373,300 years, and plutonium-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. Sony VGP-BPL8 Battery

This element also has eight metastable states, though none are stable and all have half-lives less than one second.[7]

The isotopes of plutonium range in mass number from 228 to 247. The primary decay modes of isotopes with mass numbers lower than the most stable isotope, plutonium-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).Sony VGP-BPL8A Battery

The primary decay mode for isotopes with mass numbers higher than plutonium-244 is ? emission, mostly forming americium (95 protons) isotopes as decay products. Plutonium-241 is the parent isotope of the neptunium decay series, decaying to americium-241 via ? or electron emission.[7][24] Sony VGP-BPL8B Battery

Plutonium-238 and 239 are the most widely synthesized isotopes.[8] Plutonium-239 is synthesized via the following reaction using uranium (U) and neutrons (n) via beta decay (??) with neptunium (Np) as an intermediate:[25]

Neutrons from the fission of uranium-235 are captured by uranium-238 nuclei to form uranium-239; Sony VGP-BPS8 Battery

a beta decay converts a neutron into a proton to form Np-239 (half-life 2.36 days) and another beta decay forms plutonium-239.[26]Workers on the Tube Alloys project had predicted this reaction theoretically in 1940.

Plutonium-238 is synthesized by bombarding uranium-238 with deuterons (D, the nuclei of heavy hydrogen) in the following reaction:[27] Sony VGP-BPS8A Battery

In this process, a deuteron hitting uranium-238 produces two neutrons and neptunium-238, which spontaneously decays by emitting negative beta particles to form plutonium-238.

Decay heat and fission properties

Plutonium isotopes undergo radioactive decay, which produces decay heat. Different isotopes produce different amounts of heat per mass. Sony VGP-BPS8B Battery

The decay heat is usually listed as watt/kilogram, or milliwatt/gram. In case of larger pieces of plutonium (e.g. a weapon pit) and inadequate heat removal the resulting self-heating may be significant. All isotopes produce weak gamma on decay.

Americium-241, the decay product of plutonium-241, has half-life of 430 years, 1.2 spontaneous fissions per gram per second, and decay heat of 114 watts per kilogram. Sony VGP-BPL9 Battery

As its decay produces highly penetrative gamma rays, its presence in plutonium, determined by the original concentration of plutonium-241 and the sample age, increases the radiation exposure of surrounding structures and personnel.

Compounds and chemistry

At room temperature, pure plutonium is silvery in color but gains a tarnish when oxidized.[29] Sony VGP-BPS9 Battery

The element displays four common ionic oxidation states in aqueous solution and one rare one:[14]

  • 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.[30] It is the acid anion that influences the degree of complexing—how atoms connect to a central atom—of the plutonium species. Sony VGP-BPS9/B Battery

Metallic plutonium is produced by reacting plutonium tetrafluoride with barium, calcium or lithium at 1200 °C.[31] It is attacked by acids, oxygen, and steam but not by alkalis and dissolves easily in concentrated hydrochloric,hydroiodic and perchloric acids.[32] Molten metal must be kept in a vacuum or an inert atmosphere to avoid reaction with air.[15] At 135 °C the metal will ignite in air and will explode if placed in carbon tetrachloride.[33] Sony VGP-BPS9/S Battery

Plutonium is a reactive metal. In moist air or moist argon, the metal oxidizes rapidly, producing a mixture of oxides and hydrides.[5]If the metal is exposed long enough to a limited amount of water vapor, a powdery surface coating of PuO2 is formed.[5] Also formed isplutonium hydride but an excess of water vapor forms only PuO2.[32] Sony VGP-BPS9A Battery

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. Oxygen retards the effects of moisture and acts as a passivating agent.[5]

Plutonium shows enormous, and reversible, reaction rates with pure hydrogen, forming plutonium hydride.[11] Sony VGP-BPS9A/B Battery

It also reacts readily with oxygen, forming PuO and PuO2 as well as intermediate oxides; plutonium oxide fills 40% more volume than plutonium metal. 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. The following oxyhalides are observed: PuOCl, PuOBr and PuOI. It will react with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi2.[14][33] Sony VGP-BPS9A/S Battery

Crucibles used to contain plutonium need to be able to withstand its strongly reducing properties. Refractory metals such as tantalum andtungsten along with the more stable oxides, borides, carbides, nitrides and silicides can tolerate this. Melting in an electric arc furnace can be used to produce small ingots of the metal without the need for a crucible.[15] Sony VGP-BPS9B Battery

Cerium is used as a chemical simulant of plutonium for development of containment, extraction, and other technologies.[35]

Electronic structure

The anomalous behavior of plutonium is caused by its electronic structure. The energy difference between the 6d and 5f subshells is very low. Sony VGP-BPX11 Battery

The size of the 5f shell is just enough to allow the electrons to form bonds within the lattice, on the very boundary between localized and bonding behavior. The proximity of energy levels leads to multiple low-energy electron configurations with near equal energy levels. This leads to competing 5fn7s2 and 5fn-17s26d1 configurations, which causes the complexity of its chemical behavior. Sony VGP-BPL18 Battery

The highly directional nature of 5f orbitals is responsible for directional covalent bonds in molecules and complexes of plutonium.[11]

Alloys

Plutonium can form alloys and intermediate compounds with most other metals. Exceptions include lithium, sodium, potassium, rubidium andcaesium of the alkali metals; Sony VGP-BPS18 Battery

and magnesium, calcium, strontium, and barium of the alkaline earth metals; and europium and ytterbium of the rare earth metals.[32] 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.[32] Sony VGP-BPL20 Battery

Gallium, aluminium, americium, scandium and cerium can stabilize the ? phase of plutonium for room temperature. Silicon, indium, zinc and zirconium allow formation of metastable ? state when rapidly cooled. High amounts of hafnium,holmium and thallium also allows retaining some of the ? phase at room temperature. Neptunium is the only element that can stabilize the ? phase at higher temperatures.[11] Sony VGP-BPS20/B Battery

Plutonium alloys can be produced by adding a metal to molten plutonium. If the alloying metal is sufficiently reductive, plutonium can be added in the form of oxides or halides. The ? phase plutonium-gallium and plutonium-aluminium alloys are produced by adding plutonium(III) fluoride to molten gallium or aluminium, which has the advantage of avoiding dealing directly with the highly reactive plutonium metal.[36] Sony VGP-BPS20/S Battery

  • Plutonium-gallium is used for stabilizing the ? phase of plutonium, avoiding the ?-phase and ?-? related issues. Its main use is in pits of implosion nuclear weapons.[37]

Plutonium-aluminium is an alternative to the Pu-Ga alloy. It was the original element considered for ? phase stabilization, but its tendency to react with the alpha particles and release neutrons reduces its usability for nuclear weapon pits. Sony VGP-BPL21 Battery

  • Plutonium-aluminium alloy can be also used as a component of nuclear fuel.[38]
  • Plutonium-gallium-cobalt alloy (PuCoGa5) is an unconventional superconductor, showing superconductivity below 18.5 kelvin, an order of magnitude higher than the highest between heavy fermion systems, and has large critical current.[17][39]
  • Plutonium-zirconium alloy can be used as nuclear fuel.[40]

Plutonium-cerium and plutonium-cerium-cobalt alloys are used as nuclear fuels.[41] Sony VGP-BPS21 Battery

Plutonium-uranium, with about 15–30 mol.% plutonium, can be used as a nuclear fuel for fast breeder reactors. Its pyrophoric nature and high susceptibility to corrosion to the point of self-igniting or disintegrating after exposure to air require alloying with other components. Addition of aluminium, carbon or copper did not improve disintegration rates markedly, zirconium and iron alloys have better corrosion resistance but they disintegrate in several months in air as well. Sony VGP-BPS21/S Battery

  • Addition of titanium and/or zirconium significantly increases the melting point of the alloy.[42]

Plutonium-uranium-titanium and plutonium-uranium-zirconium were investigated for use as nuclear fuels. The addition of the third element increases corrosion resistance, reduces flammability, and improves ductility, fabricability, strength, and thermal expansion. Sony VGP-BPS21A Battery

  • Plutonium-uranium-molybdenum has the best corrosion resistance, forming a protective film of oxides, but titanium and zirconium are preferred for physics reasons.[42]

Thorium-uranium-plutonium was investigated as a nuclear fuel for fast breeder reactors.[42] Sony VGP-BPS21A/B Battery

Occurrence

Almost all the plutonium occurring on this planet has been produced in nuclear reactors , the rest from nuclear reprocessing.These sources have been well documented. However it is difficult to be definitive as to how much plutonium exists,but it is clear that several hundred tonnes have been manufactured.This plutonium occurs in local areas where it is stored under security due to its hazardous nature. Sony VGP-BPS21B Battery

During the manufacture and testing of nuclear weapons a certain amount of plutonium has been released into the wider environment,an estimate of 12.7 tonnes from the U.S weapons programme alone [43].

In addition during plutonium's manufacture in civil nuclear reactors some plutonium has by accident and design escaped into the biosphere and has been found in sediment layers and aquatic species sellafield. Sony VGP-BPL22 Battery

Trace amounts of at least two plutonium isotopes (plutonium-239 and 244) can be found in nature. Small traces of plutonium-239, a few parts per trillion, and itsdecay products are naturally found in some concentrated ores of uranium,[44] such as the natural nuclear fission reactor in Oklo, Gabon.[45] The ratio of plutonium-239 to uranium at the Cigar Lake Mine uranium deposit ranges from 2.4 × 10?12 to 44 × 10?12.[46] Sony VGP-BPS22 Battery

Even smaller amounts of primordial plutonium-244 occur naturally due to its relatively long half-life of about 80 million years.[47]These trace amounts of Pu-239 originate in the following fashion: On rare occasions, U-238 undergoes spontaneous fission, and in the process, the nucleus emits one or two free neutrons with some kinetic energy. Sony VGP-BPS22/A Battery

When one of these neutrons strikes the nucleus of another U-238 atom, it is absorbed by the atom, which becomes U-239. With quite-short half-lives, U-239 decays to neptunium-239 (Np-239), and then Np-239 decays into Pu-239.

Since the relatively long-lived isotope plutonium-240 occurs in the decay chain of plutonium-244 it should also be present, albeit 10,000 times rarer still. Sony VGP-BPS22A Battery

Finally, exceedingly small amounts of plutonium-238, attributed to the incredibly rare double beta decay of uranium-238, have been found in natural uranium samples.[48]

Minute traces of plutonium are usually found in the human body due to the 550 atmospheric and underwater nuclear tests that have been carried out, and to a small number of major nuclear accidents. Sony VGP-BPL23 Battery

Most atmospheric and underwater nuclear testing was stopped by the Limited Test Ban Treaty in 1963, which was signed and ratified by the United States, the United Kingdom, the Soviet Union, and other nations. Continued atmospheric nuclear weapons testing since 1963 by non-treaty nations included those by China (atomic bomb test above the Gobi Desert in 1964, hydrogen bomb test in 1967, and follow-on tests), and France (tests as recently as the 1980s). Sony VGP-BPS23 Battery

Sony VGP-BPS23/B Battery

Because it is purposely manufactured for nuclear weapons and nuclear reactors, plutonium-239 is the most abundant isotope of plutonium by far.[33]

It is also hypothetically possible for minute quantities of plutonium to be produced by the natural bombardment of uranium ores with cosmic rays. Sony VGP-BPS23/D Battery

Discovery

Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered element 94 in 1934.[49] Fermi called the element hesperium and mentioned it in his Nobel Lecture in 1938.[50] The sample was actually a mixture of barium, krypton, and other elements, but this was not known at the time becausenuclear fission had not been discovered yet.[51]Sony VGP-BPS23/G Battery

Plutonium (specifically, plutonium-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, and A. C. Wahl by deuteron bombardment of uranium in the 60-inch (150 cm) cyclotron at the University of California, Berkeley.[52][53] ISony VGP-BPS23/P Battery

n 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.[33]

A paper documenting the discovery was prepared by the team and sent to the journal Physical Review in March 1941.[33] TSony VGP-BPS23/W Battery

he paper was withdrawn before publication after the discovery that an isotope of the new element (plutonium-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.[18] Sony Limited Edition 007 Battery

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.[8][note 2] Seaborg originally considered the name "plutium", but later thought that it did not sound as good as "plutonium."[54] Sony VAIO VGC-LB15 Battery

He chose the letters "Pu" as a joke, which passed without notice into the periodic table.[note 3] Alternative names considered by Seaborg and others were "ultimium" or "extremium" because of the erroneous belief that they had found the last possible element on the periodic table.[55] Sony VGN-AR Series Battery

Early research

The basic chemistry of plutonium was found to resemble uranium after a few months of initial study.[33] 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. Sony VGN-CR Series Battery

About 50 micrograms of plutonium-239 combined with uranium and fission products was produced and only about 1 microgram was isolated.[44] This procedure enabled chemists to determine the new element's atomic weight.[56][note 4]

In November 1943 some plutonium trifluoride was reduced to create the first sample of plutonium metal: a few micrograms of metallic beads.[44] Sony VIAO VGN-G Series Battery

Enough plutonium was produced to make it the first synthetically made element to be visible with the unaided eye.[57]

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. Sony VIAO VGN-G118 Battery

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.[33]

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.Sony VIAO VGN-G118CN/B Battery

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.[58] Sony VIAO VGN-G118CN/T Battery

The 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.[33][note 5]

On April 5, 1944, Emilio Segrè at Los Alamos received the first sample of reactor-produced plutonium from Oak Ridge.[59] Sony VIAO VGN-G218 Battery

Within ten days, he discovered that reactor-bred plutonium had a higher concentration of the isotope plutonium-240 than cyclotron-produced plutonium. Plutonium-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. Sony VGN-TZ121 Battery

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 (or, in later designs, hollow) 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. Sony VGN-TZ13 Battery

(Enriched uranium, by contrast, can be used with either method.)[59]

Construction of the Hanford B Reactor, the first industrial-sized nuclear reactor for the purposes of material production, was completed in March 1945. 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.[60] Sony VGN-TZ131 Battery

In 2004, a safe was discovered during excavations of a burial trench at the Hanford nuclear site. Inside the safe were various items, including a large glass bottle containing a whitish slurry which was subsequently identified as the oldest sample of weapons-grade plutonium known to exist. Isotope analysis by Pacific Northwest National Laboratory indicated that the plutonium in the bottle was manufactured in the X-10 reactor at Oak Ridge during 1944.[ Sony VGN-TZ132N Battery

Trinity and Fat Man atomic bombs

The first atomic bomb test, codenamed "Trinity" and detonated on July 16, 1945, near Alamogordo, New Mexico, used plutonium as its fissile material.[44] 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 simultaneously showered with neutrons from the"Urchin", an initiator made of polonium and beryllium (neutron source: (?, n) reaction).[33] Sony VGN-TZ132/N Battery

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.[64]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.[65][note 7] Sony VGN-TZ17N Battery

An identical design was used in the "Fat Man" atomic bomb dropped on Nagasaki, Japan, on August 9, 1945, killing 70,000 people and wounding another 100,000.[33] The "Little Boy" bomb dropped on Hiroshima three days earlier used uranium-235, not plutonium. Japan capitulated on August 15 to General Douglas MacArthur. Only after the announcement of the first atomic bombs was the existence of plutonium made public. Sony VGN-TZ16N Battery

Cold War use and waste

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,[66] and an estimated 170 tonnes of military-grade plutonium was produced in Russia. Sony VGN-TZ18N Battery

Each year about 20 tonnes of the element is still produced as a by-product of the nuclear power industry.[14] 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.[33]SIPRI estimated the world plutonium stockpile in 2007 as about 500 tons, divided equally between weapon and civilian stocks.[68]Sony VAIO VGN-AR47G/E1 Battery

Since the end of the Cold War these stockpiles have become a focus of nuclear proliferation concerns. In the U.S., some plutonium extracted from dismantled nuclear weapons is melted to form glass logs of plutonium oxide that weigh two tonnes.[33] The glass is made of borosilicates mixed with cadmium and gadolinium.[note 9] Sony VAIO VGN-AR520E Battery

These logs are planned to be encased in stainless steel and stored as much as 4 km underground in bore holes that will be back-filled with concrete.[33] As of 2008, the only facility in the U.S. that was scheduled to store plutonium in this way was the Yucca Mountain nuclear waste repository, which is about 100 miles (160 km) north-east of Las Vegas, Nevada.[69] Sony VAIO VGN-AR53DB Battery

Local and state opposition to this plan delayed efforts to store nuclear waste at Yucca Mountain. In March 2010, the Department of Energy withdrew its license application for the Yucca Mountain repository "with prejudice" and eliminated funding for the Office of Civilian Radioactive Waste Management, which had managed the Yucca Mountain site for 25 years, canceling the program.[70] Sony VAIO VGN-AR54DB Battery

Medical experimentation

During and after the end of World War II, scientists working on the Manhattan Project and other nuclear weapons research projects conducted studies of the effects of plutonium on laboratory animals and human subjects.[71] Animal studies found that a few milligrams of plutonium per kilogram of tissue is a lethal dose.[72] Sony VAIO VGN-AR550E Battery

In the case of human subjects, this involved injecting solutions containing (typically) five micrograms of plutonium into hospital patients thought to be either terminally ill, or to have a life expectancy of less than ten years either due to age or chronic disease condition.[71] This was reduced to one microgram in July 1945 after animal studies found that the way plutonium distributed itself in bones was more dangerous than radium.[72] Sony VAIO VGN-AR55DB Battery

Eighteen human test subjects were injected with plutonium without informed consent. The tests were used to create diagnostic tools to determine the uptake of plutonium in the body in order to develop safety standards for working with plutonium.[71]

The episode is now considered to be a serious breach of medical ethics and of the Hippocratic Oath. Sony VAIO VGN-AR630E Battery

More sympathetic commentators have noted that while it was definitely a breach in trust and ethics, "the effects of the plutonium injections were not as damaging to the subjects as the early news stories painted, nor were they so inconsequential as many scientists, then and now, believe."[73] Sony VAIO VGN-AR64DB Battery

Explosives

The isotope plutonium-239 is a key fissile component in nuclear weapons, due to its ease of fission and availability. Encasing the bomb'splutonium pit in a tamper (an optional layer of dense material) decreases the amount of plutonium needed to reach critical mass byreflecting escaping neutrons back into the plutonium core. Sony VAIO VGN-AR65DB Battery

This reduces the amount of plutonium needed to reach criticality from 16 kg to 10 kg, which is a sphere with a diameter of about 10 centimeters (4 in).[74] This critical mass is about a third of that for uranium-235.[8] Sony VAIO VGN-AR660U Battery

The "Fat Man"-type plutonium bombs produced during the Manhattan Project used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. Thus only 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT.[65][75] (See also Nuclear weapon design.) Sony VAIO VGN-AR690U Battery

Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.[75]

Mixed oxide fuel

Spent nuclear fuel from normal light water reactors contains plutonium, but it is a mixture of plutonium-242, 240, 239 and 238.Sony VAIO VGN-AR71ZU Battery

The mixture is not sufficiently enriched for efficient nuclear weapons, but can be used once as MOX fuel. Accidental neutron capture causes the amount of plutonium-242 and 240 to grow each time the plutonium is irradiated in a reactor with low-speed "thermal" neutrons, so that after the second cycle, the Plutonium can only be consumed by fast neutron reactors. Sony VAIO VGN-AR730E/B Battery

If fast neutron reactors are not available (the normal case), excess Plutonium is usually discarded, and forms the longest-lived component of nuclear waste. The desire to consume this Plutonium and other transuranic fuels and reduce the radiotoxicity of the waste is the usual reason nuclear engineers give to make fast neutron reactors. Sony VAIO VGN-AR73DB Battery

The most common chemical process, PUREX (Plutonium–URanium EXtraction) reprocesses spent nuclear fuel to extract plutonium and uranium which can be used to form a mixed oxide "MOX fuel" for reuse in nuclear reactors. Weapons grade plutonium can be added to the fuel mix. Sony VAIO VGN-AR74DB Battery

MOX fuel is used in light water reactors and consists of 60 kg of plutonium per tonne of fuel; after four years, three-quarters of the plutonium is burned (turned into other elements).[33] Breeder reactors are specifically designed to create more fissionable material than they consume.

MOX fuel has been in use since the 1980s and is widely used in Europe.[76] Sony VAIO VGN-AR75UDB Battery

In September 2000, the United States and the Russian Federation signed a Plutonium Management and Disposition Agreement by which each agreed to dispose of 34 tonnes of weapon grade plutonium.[77] The U.S. Department of Energy plans to dispose of 34 tonnes of weapon grade plutonium in the United States before the end of 2019 by converting the plutonium to a MOX fuel to be used in commercial nuclear power reactors.[77] Sony VAIO VGN-AR790U/B Battery

MOX fuel improves total burnup. A fuel rod is reprocessed after three years of use to remove waste products, which by then account for 3% of the total weight of the rods.[33] Any uranium or plutonium isotopes produced during those three years are left and the rod goes back into production.[note 10] The presence of up to 1% galliumper mass in weapon grade plutonium alloy has the potential to interfere with long-term operation of a light water reactor.[78] Sony VAIO VGN-AR820E Battery

Plutonium recovered from spent reactor fuel poses a less significant proliferation hazard, because of excessive contamination with non-fissile plutonium-240 andplutonium-242. Separation of the isotopes is not feasible. A dedicated reactor operating on very low burnup (hence minimal exposure of newly-formed Pu-239 to additional neutrons which causes it to be transformed to heavier isotopes of plutonium) is generally required to produce material suitable for use in efficientnuclear weapons. Sony VAIO VGN-AR830E Battery

While 'weapons-grade' plutonium is defined to contain at least 92% plutonium-239 (of the total plutonium), the United States have managed to detonate an under-20Kt device using plutonium believed to contain only about 85% plutonium-239, so called 'fuel-grade' plutonium.[79] Sony VAIO VGN-AR83S Battery

The 'reactor grade' plutonium produced by a regular LWR burnup cycle typically contains less than 60% Pu-239, with up to 30% parasitic Pu-240/Pu-242, and 10-15% fissile Pu-241.[79] It's unknown if a device using plutonium obtained from reprocessed civil nuclear waste can be detonated, however such a device could hypothetically fizzle and spread radioactive materials over a large urban area. Sony VAIO VGN-AR83US Battery

The IAEA conservatively classifies plutonium of all isotopic vectors as "direct-use" material, that is, "nuclear material that can be used for the manufacture of nuclear explosives components without transmutation or further enrichment".[79]

241Am has recently been suggested for use as a denaturing agent in plutonium reactor fuel rods to further limit its proliferation potential.[80] Sony VAIO VGN-AR84S Battery

Power and heat source

The isotope plutonium-238 has a half-life of 87.74 years.[81] It emits a large amount of thermal energy with low levels of bothgamma rays/particles and spontaneous neutron rays/particles.[82] Being an alpha emitter, it combines high energy radiation with low penetration and thereby requires minimal shielding. Sony VAIO VGN-AR84US Battery

A sheet of paper can be used to shield against the alpha particles emitted by plutonium-238 while one kilogram of the isotope can generate about 570 watts of heat.[8][82]

These characteristics make it well-suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human lifetime. Sony VAIO VGN-AR85S Battery

It is therefore used in radioisotope thermoelectric generators andradioisotope heater units such as those in the Cassini, Voyager and New Horizons space probes.

The twin Voyager spacecraft were launched in 1977 with each containing a 500 watt plutonium power source. Over 30 years later each source is still producing about 300 watts which allows limited operation of each spacecraft.[83] Sony VAIO VGN-AR85US Battery

An earlier version of the same technology powered five Apollo Lunar Surface Experiment Packages, starting with Apollo 12 in 1969.[33]

Plutonium-238 has also been used successfully to power artificial heart pacemakers, to reduce the risk of repeated surgery.[84][85]Sony VAIO VGN-AR870EA Battery

It has been largely replaced by lithium-based primary cells, but as of 2003 there were somewhere between 50 and 100 plutonium-powered pacemakers still implanted and functioning in living patients.[86] Plutonium-238 was studied as way to provide supplemental heat to scuba diving.[87]Plutonium-238 mixed with beryllium is used to generate neutrons for research purposes.[33]Sony VAIO VGN-AR890U Battery

Toxicity

Isotopes and compounds of plutonium are radioactive and accumulate in bone marrow. Contamination by plutonium oxide has resulted from a number of nuclear disasters and radioactive incidents including military nuclear accidents where nuclear weapons have burned.[88] Sony VAIO VGN-AR93S Battery

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.[89] Sony VAIO VGN-AR93US Battery

During the decay of plutonium, three types of radiation are released—alpha, beta, and gamma. Alpha radiation can travel only a short distance and cannot travel through the outer, dead layer of human skin. Beta radiation can penetrate human skin, but cannot go all the way through the body. Gamma radiation can go all the way through the body.[90] Sony VAIO VGN-AR94S Battery

Alpha, beta, and gamma radiation are all forms of ionizing radiation. 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.Sony VAIO VGN-AR94US Battery

Even though alpha radiation cannot penetrate the skin, ingested or inhaled plutonium does irradiate internal organs.[33]The skeleton, where plutonium is absorbed, and the liver, where it collects and becomes concentrated, are at risk.[32]Plutonium is not absorbed into the body efficiently when ingested; only 0.04% of plutonium oxide is absorbed after ingestion.[33]Sony VAIO VGN-AR95S Battery

Plutonium absorbed by the body is excreted very slowly, with a biological half-life of 200 years.[91] Plutonium passes only slowly through cell membranes and intestinal boundaries, so absorption by ingestion and incorporation into bone structure proceeds very slowly.[92][93]

Plutonium is more dangerous when inhaled than when ingested. Sony VAIO VGN-AR95US Battery

The risk of lung cancer increases once the total radiation dose equivalent of inhaled plutonium exceeds 400 mSv.[94] The U.S. Department of Energy estimates that the lifetime cancer risk from inhaling 5,000 plutonium particles, each about 3 microns wide, to be 1% over the background U.S. average.[95] Ingestion or inhalation of large amounts may cause acute radiation poisoning and death;

Sony VAIO VGN-AW110J/H Battery

no human is known to have died because of inhaling or ingesting plutonium, and many people have measurable amounts of plutonium in their bodies.[79]

The "hot particle" theory in which a particle of plutonium dust radiates a localized spot of lung tissue has been tested and found false—such particles are more mobile than originally thought and toxicity is not measurably increased due to particulate form.[92] Sony VAIO VGN-AW11M/H Battery

However, when inhaled, plutonium can pass into the bloodstream. Once in the bloodstream, plutonium moves throughout the body and into the bones, liver, or other body organs. Plutonium that reaches body organs generally stays in the body for decades and continues to expose the surrounding tissue to radiation and thus may cause cancer.[96] Sony VAIO VGN-AW11S/B Battery

A commonly cited quote by Ralph Nader, states that a pound of plutonium dust spread into the atmosphere would be enough to kill 8 billion people. However, the math shows that one pound of plutonium could kill no more than 2 million people by inhalation. This makes the toxicity of plutonium roughly equivalent with that of nerve gas.[97] Sony VAIO VGN-AW11XU/Q Battery

Several populations of people who have been exposed to plutonium dust (e.g. people living down-wind of Nevada test sites, Hiroshima survivors, nuclear facility workers, and "terminally ill" patients injected with Pu in 1945–46 to study Pu metabolism) have been carefully followed and analyzed. These studies generally do not show especially high plutonium toxicity or plutonium-induced cancer results.[92] Sony VAIO VGN-AW11Z/B Battery

"There were about 25 workers from Los Alamos National Laboratory who inhaled a considerable amount of plutonium dust during the 1940's; according to the hot-particle theory, each of them has a 99.5% chance of being dead from lung cancer by now, but there has not been a single lung cancer among them."[97][98] Sony VAIO VGN-AW120J/H Battery

Criticality potential

Toxicity issues aside, care must be taken to avoid the accumulation of amounts of plutonium which approach critical mass, particularly because plutonium's critical mass is only a third of that of uranium-235.[8] A critical mass of plutonium emits lethal amounts of neutrons and gamma rays.[100] Sony VAIO VGN-AW150Y/H Battery

Plutonium in solution is more likely to form a critical mass than the solid form due to moderation by the hydrogen in water.[14]

Criticality accidents have occurred in the past, some of them with lethal consequences. Careless handling of tungsten carbidebricks around a 6.2 kg plutonium sphere resulted in a fatal dose of radiation at Los Alamos on August 21, 1945, when scientistHarry K. Daghlian, Jr. Sony VAIO VGN-AW160J/Q Battery

received a dose estimated to be 5.1 Sievert (510 rems) and died 25 days later.[101][102] Nine months later, another Los Alamos scientist, Louis Slotin, died from a similar accident involving a beryllium reflector and the same plutonium core (the so-called "demon core") that had previously claimed the life of Daghlian.[103] These incidents were fictionalized in the 1989 film Fat Man and Little Boy. Sony VAIO VGN-AW170Y/Q Battery

In December 1958, during a process of purifying plutonium at Los Alamos, a critical mass was formed in a mixing vessel, which resulted in the death of a chemical operator named Cecil Kelley.[104] Other nuclear accidents have occurred in the Soviet Union,Japan, the United States and many other countries.[104] Sony VAIO VGN-AW180Y/Q Battery

Flammability

Metallic plutonium is a fire hazard, especially if the material is finely divided. In a moist environment, plutonium forms hydrides on its surface, which arepyrophoric and may ignite in air at room temperature. Plutonium expands up to 70% in volume as it oxidizes and thus may break its container.[105] Sony VAIO VGN-AW190JAH Battery

The radioactivity of the burning material is an additional hazard. Magnesium oxide sand is probably the most effective material for extinguishing a plutonium fire. It cools the burning material, acting as a heat sink, and also blocks off oxygen. Special precautions are necessary to store or handle plutonium in any form; generally a dry inert gasatmosphere is required. Sony VAIO VGN-AW190NAB Battery

Americium is a transuranic radioactive chemical element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide elementeuropium, and thus by analogy was named after another continent, America.[2] Sony VAIO VGN-AW190NBB Battery

Americium was first produced in 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. Although it is the third element in the transuranic series, it was discovered fourth, after the heavier curium. The discovery was kept secret and only released to the public in November 1945. Sony VAIO VGN-AW190NCB Battery

Most americium is produced by bombarding uranium or plutonium withalpha particles in nuclear reactors – one tonne of spent nuclear fuel contains about 100 grams of americium. It is widely used in commercial ionization chamber smoke detectors, as well as in neutron sources and industrial gauges. Sony VAIO VGN-AW190YAB Battery

Several unusual applications, such as a nuclear battery or fuel for space ships with nuclear propulsion, have been proposed for the isotope242mAm, but they are as yet hindered by the scarcity and high price of this nuclear isomer.

Americium is a relatively soft radioactive metal with silvery-white appearance. Its most common isotopes are 241Am and 243Am.Sony VAIO VGN-AW190YBB Battery

In chemical compounds, they usually assume the oxidation state +3, especially in solutions. Several other oxidation states are known, which range from +2 to +7 and can be identified by their characteristic optical absorption spectra. The crystal lattice of solid americium and its compounds contains intrinsic defects, which are induced by self-irradiation with alpha particles and accumulate with time; this results in a drift of some material properties. Sony VAIO VGN-AW190YCB Battery

Although americium was likely produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in late autumn 1944, at the University of California, Berkeley by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso. They used a 60-inch cyclotron at the University of California, Berkeley.[4] Sony VAIO VGN-AW190YDB Battery

The element was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) of theUniversity of Chicago. Following the lighter neptunium, plutonium, and heavier curium, americium was the fourth transuranium element to be discovered. At the time, the periodic table had been restructured by Seaborg to its present layout, containing the actinide row below the lanthanideone. Sony VAIO VGN-AW220J/B Battery

This led to americium being located right below its twin lanthanide element europium; it was thus by analogy named after another continent, America: "The name americium (after the Americas) and the symbol Am are suggested for the element on the basis of its position as the sixth member of the actinide rare-earth series, analogous to europium, Eu, of the lanthanide series."[5][6][7] Sony VAIO VGN-AW230J/H Battery

The new element was isolated from its oxides in a complex, multi-step process. Firstplutonium-239 nitrate (239PuNO3) solution was coated on a platinum foil of about 0.5 cm2 area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO2) by annealing. Sony VAIO VGN-AW235J/B Battery

After cyclotron irradiation, the coating was dissolved with nitric acid, and then precipitated as the hydroxide using concentrated aqueous ammonia solution. The residue was dissolved inperchloric acid. Further separation was carried out by ion exchange, yielding a certain isotope of curium. Sony VAIO VGN-AW290JFQ Battery

The separation of curium and americium was so painstaking that those elements were initially called by the Berkeley group as pandemonium (from Greek for all demons or hell) anddelirium (from Latin for madness).[8][9][10][11]

Initial experiments yielded four americium isotopes: 241Am, 242Am, 239Am and 238Am. Sony VAIO VGN-AW41JF Battery

Americium-241was directly obtained from plutonium upon absorption of one neutron. It decays by emission of a?-particle to 237Np; the half-life of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years.[12]

The times are half-lives

The second isotope 242Am was produced upon neutron bombardment of the already-created 241Am. Sony VAIO VGN-AW41JF/H Battery

Upon rapid ?-decay, 242Am converts into the isotope of curium 242Cm (which had been discovered previously). The half-life of this decay was initially determined at 17 hours, which was close to the presently accepted value of 16.02 h.[12]

The discovery of americium and curium in 1944 was closely related to the Manhattan Project; the results were confidential and declassified only in 1945. Sony VAIO VGN-AW41MF Battery

Seaborg leaked the synthesis of the elements 95 and 96 on the U.S. radio show for children, theQuiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945, when one of the listeners asked whether any new transuranium element beside plutonium and neptunium had been discovered during the war.[8] Sony VAIO VGN-AW41MF/H Battery

After the discovery of americium isotopes 241Am and 242Am, their production and compounds were patented listing only Seaborg as the inventor.[13] The initial americium samples weighed a few micrograms; they were barely visible and were identified by their radioactivity. The first substantial amounts of metallic americium weighing 40–200 micrograms were not prepared until 1951 by reduction of americium(III) fluoride with barium metal in high vacuum at 1100 °C.[14] Sony VAIO VGN-AW41XH Battery

Occurrence

The longest-lived and most common isotopes of americium, 241Am and 243Am, have half-lives of 432.2 and 7,370 years, respectively. Therefore, all primordial americium (americium that was present on Earth during its formation) should have decayed by now. Sony VAIO VGN-AW41XH/Q Battery

Existing americium is concentrated in the areas used for the atmospheric nuclear weapons tests conducted between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster. For example, the analysis of the debris at the testing site of the first U.S. hydrogen bomb, Ivy Mike, (November 1, 1952, Enewetak Atoll), revealed high concentrations of various actinides including americium; Sony VAIO VGN-AW41ZF Battery

due to military secrecy, this result was published only in 1956.[15] Trinitite, the glassy residue left on the desert floor near Alamogordo, New Mexico, after the plutonium-based Trinity nuclear bomb test on July 16, 1945, contains traces of americium-241. Elevated levels of americium were also detected at the crash site of a US B-52 bomber, which carried four hydrogen bombs, in 1968 in Greenland.[16] Sony VAIO VGN-AW41ZF/B Battery

In other regions, the average radioactivity due to residual americium is only about 0.01 picocuries (0.37 mBq). Atmospheric americium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 1,900 higher concentration of americium inside sandy soil particles than in the water present in the soil pores; an even higher ratio was measured in loam soils.[17] Sony VAIO VGN-AW50DB/H Battery

Americium is produced mostly artificially in small quantities, for research purposes. A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes, mostly 241Am and 243Am.[18] Their prolonged radioactivity is undesirable for the disposal, and therefore americium, together with other long-lived actinides, have to be neutralized. Sony VAIO VGN-AW51JGB Battery

The associated procedure may involve several steps, where americium is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure is well known as nuclear transmutation, but it is still being developed for americium.[19][20]

A few atoms of americium can be produced by neutron capture reactions and beta decay in very highly concentrateduranium-bearing deposits.[21] Sony VAIO VGN-AW52JGB Battery

Isotope nucleosyntheses

Americium has been produced in small quantities in nuclear reactors for decades, and kilograms of its 241Am and 243Am isotopes have been accumulated by now.[22] Nevertheless, since it was first offered for sale in 1962, its price, about 1,500 USD per gram of 241Am, remains almost unchanged owing to the very complex separation procedure.[23] Sony VAIO VGN-AW53FB Battery

The heavier isotope 243Am is produced in much smaller amounts; it is thus more difficult to separate, resulting in a higher cost of the order 100–160 USD/mg.[24][25]

Americium is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope 239Pu. The latter needs to be produced first, according to the following nuclear process: Sony VAIO VGN-AW70B/Q Battery

The capture of two neutrons by 239Pu (a so-called (n,?) reaction), followed by a ?-decay, results in 241Am:

The plutonium present in spent nuclear fuel contains about 12% of 241Pu. Because it spontaneously converts to 241Am, 241Pu can be extracted and may be used to generate further 241Am.[23] Sony VAIO VGN-AW71JB Battery

However, this process is rather slow: half of the original amount of 241Pu decays to 241Am after about 15 years, and the 241Am amount reaches a maximum after 70 years.[26]

The obtained 241Am can be used for generating heavier americium isotopes by further neutron capture inside a nuclear reactor. In alight water reactor (LWR), 79% of 241Am converts to 242Am and 10% to its nuclear isomer 242mAm:[note 1][27] Sony VAIO VGN-AW72JB Battery

Americium-242 has a half-life of only 16 hours, which makes its further up-conversion to 243Am, extremely inefficient. The latter isotope is produced instead in a process where 239Pu captures four neutrons under high neutron flux:

Metal generation

Most synthesis routines yield a mixture of different actinide isotopes in oxide forms, from which isotopes of americium need to be separated. Sony VAIO VGN-AW73FB Battery

In a typical procedure, the spent reactor fuel (e.g. MOX fuel) is dissolved in nitric acid, and the bulk of uranium and plutonium is removed using a PUREX-type extraction (Plutonium –URanium EXtraction) with tributyl phosphate in a hydrocarbon. The lanthanides and remaining actinides are then separated from the aqueous residue (raffinate) by adiamide-based extraction, to give, after stripping, a mixture of trivalent actinides and lanthanides. Sony VAIO VGN-AW80NS Battery

Sony VAIO VGN-AW80S Battery

Americium compounds are then selectively extracted using multi-step chromatographic and centrifugation techniques[28] with an appropriate reagent. A large amount of work has been done on the solvent extraction of americium. For example, a recent EU funded project codenamed "EUROPART" studied triazines and other compounds as potential extraction agents. Sony VAIO VGN-AW80US Battery

Bis-triazinyl bipyridine complex has been recently proposed as such reagent as highly selective to americium (and curium).[34]Separation of americium from the highly similar curium can be achieved by treating a slurry of their hydroxides in aqueous sodium bicarbonate with ozone, at elevated temperatures. Both Am and Cm are mostly present in solutions in the +3 valence state; Sony VAIO VGN-AW81DS Battery

whereas curium remains unchanged, americium oxidizes to soluble Am(IV) complexes which can be washed away.[35]

Metallic americium is obtained by reduction from its compounds. Americium(III) fluoride was first used for this purpose. The reaction was conducted using elementalbarium as reducing agent in a water- and oxygen-free environment inside an apparatus made of tantalum and tungsten.[14][36][37] Sony VAIO VGN-AW81JS Battery

An alternative is the reduction of americium dioxide by metallic lanthanum or thorium:[37][38]

Physical properties

In the periodic table, americium is located right to plutonium, left to curium, and below the lanthanide europium, with which it shares many similarities in physical and chemical properties. Americium is a highly radioactive element. Sony VAIO VGN-AW81YS Battery

When freshly prepared, it has a silvery-white metallic lustre, but then slowly tarnishes in air. With a density of 12 g/cm3, americium is lighter than both curium (13.52 g/cm3) and plutonium (19.8 g/cm3); but is heavier than europium (5.264 g/cm3)—mostly because of its higher atomic mass. Sony VAIO VGN-AW82DS Battery

Americium is relatively soft and easily deformable and has a significantly lower bulk modulus than the actinides before it: Th, Pa, U, Np and Pu.[39] Its melting point of 1173 °C is significantly higher than that of plutonium (639 °C) and europium (826 °C), but lower than for curium (1340 °C).[38][38][40] Sony VAIO VGN-AW82JS Battery

At ambient conditions, americium is present in its most stable ? form which has a hexagonal crystal symmetry, and a space groupP63/mmc with lattice parameters a = 346.8 pm and c = 1124 pm, and four atoms per unit cell. The crystal consists of a double-hexagonal close packing with the layer sequence ABAC and so is isotypic with ?-lanthanum and several actinides such as ?-curium.[36][40] Sony VAIO VGN-AW82YS Battery

The crystal structure of americium changes with pressure and temperature. When compressed at room temperature to 5 GPa, ?-Am transforms to the ? modification, which has a face-centered cubic (fcc) symmetry, space group Fm3m and lattice constant a = 489 pm. This fcc structure is equivalent to the closest packing with the sequence ABC.[36][40] Sony VAIO VGN-AW83FS Battery

Upon further compression to 23 GPa, americium transforms to an orthorhombic ?-Am structure similar to that of ?-uranium. There are no further transitions observed up to 52 GPa, except for an appearance of a monoclinic phase at pressures between 10 and 15 GPa.[39] There is no consistency on the status of this phase in the literature, which also sometimes lists the ?, ? and ? phases as I, II and III. Sony VAIO VGN-AW83GS Battery

The ?-? transition is accompanied by a 6% decrease in the crystal volume; although theory also predicts a significant volume change for the ?-? transition, it is not observed experimentally. The pressure of the ?-? transition decreases with increasing temperature, and when ?-americium is heated at ambient pressure, at 770 °C it changes into an fcc phase which is different from ?-Am, and at 1075 °C it converts to a body-centered cubic structure. Sony VAIO VGN-AW83HS Battery

The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum, praseodymium and neodymium.[41]

As with many other actinides, self-damage of the crystal lattice due to alpha-particle irradiation is intrinsic to americium. Sony VAIO VGN-AW90NS Battery

It is especially noticeable at low temperatures, where the mobility of the produced lattice defects is relatively low, by broadening of X-ray diffraction peaks. This effect makes somewhat uncertain the temperature of americium and some of its properties, such as electrical resistivity.[42] Sony VAIO VGN-AW90S Battery

So for americium-241, the resistivity at 4.2 K increases with time from about 2 µOhm·cm to 10 µOhm·cm after 40 hours, and saturates at about 16 µOhm·cm after 140 hours. This effect is less pronounced at room temperature, due to annihilation of radiation defects; also heating to room temperature the sample which was kept for hours at low temperatures restores its resistivity. Sony VAIO VGN-AW90US Battery

In fresh samples, the resistivity gradually increases with temperature from about 2 µOhm·cm at liquid helium to 69 µOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium and protactinium, but is different from plutonium and curium which show a rapid rise up to 60 K followed by saturation. Sony VAIO VGN-AW91CDS Battery

The room temperature value for americium is lower than that of neptunium, plutonium and curium, but higher than for uranium, thorium and protactinium.[1]

Americium is paramagnetic in a wide temperature range, from that of liquid helium, to room temperature, and above. Sony VAIO VGN-AW91CJS Battery

This behavior is markedly different from that of its neighbor curium which exhibit antiferromagnetic transition at 52 K.[43]The thermal expansion coefficient of americium is slightly anisotropic and amounts to (7.5 ± 0.2)×10?6/°C along the shorter a axis and (6.2 ± 0.4)×10?6/°C for the longer c hexagonal axis.[40] TSony VAIO VGN-AW91CYS Battery

he enthalpy of dissolution of americium metal inhydrochloric acid at standard conditions is ?620.6 ± 1.3 kJ/mol, from which the standard enthalpy change of formation (?fH°) of aqueous Am3+ ion is ?621.2 ± 2.0 kJ/mol?1. The standard potential Am3+/Am0is 2.08 ± 0.01 V.[44]

Chemical properties

Americium readily reacts with oxygen and dissolves well in acids. Sony VAIO VGN-AW91DS Battery

The most common oxidation state for americium is +3,[45] in which americium compounds are rather stable against oxidation and reduction. In this sense, americium is chemically similar to most lanthanides. The trivalent americium forms insoluble fluoride, oxalate, iodate, hydroxide, phosphate and other salts.[45] Sony VAIO VGN-AW91JS Battery

Other oxidation states have been observed between +2 and +7, which is the widest range among the actinide elements. Their color in aqueous solutions varies as follows: Am3+ (colorless to yellow-reddish), Am4+ (yellow-reddish), AmVO2+; (yellow), AmVIO22+(brown) and AmVIIO65- (dark green).[46][47] Sony VAIO VGN-AW91YS Battery

All oxidation states have their characteristic optical absorption spectra, with a few sharp peaks in the visible and mid-infrared regions, and the position and intensity of these peaks can be converted into the concentrations of the corresponding oxidation states.[48][49][50] For example, Am(III) has two sharp peaks at 504 and 811 nm, Am(V) at 514 and 715 nm, and Am(VI) at 666 and 992 nm.[51] Sony VAIO VGN-AW92CDS Battery

Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to the permanganate ion (MnO4-) in acidic solutions.[52] Whereas the Am4+ ions are unstable in solutions and readily convert to Am3+, the +4 oxidation state occurs well in solids, such as americium dioxide (AmO2) and americium(IV) fluoride (AmF4). Sony VAIO VGN-AW92CJS Battery

All pentavalent and hexavalent americium compounds are complex salts such as KAmO2F2, Li3AmO4 and Li6AmO6, Ba3AmO6, AmO2F2. These high oxidation states Am(IV), Am(V) and Am(VI) can be prepared from Am(III) by oxidation with ammonium persulfate in dilute nitric acid,[53] with silver(I) oxide in perchloric acid,[51] or with ozone or sodium persulfate in sodium carbonate solutions.[50] Sony VAIO VGN-AW92CYS Battery

The pentavalent oxidation state of americium was first observed in 1951.[54] It is present in aqueous solution in the form of AmO2+ions (acidic) or AmO3- ions (alkaline) which are however unstable and subject to several rapid disproportionation reactions:

Oxygen compounds

Two americium oxides are known, with the oxidation states +3 (Am2O3) and +4 (AmO2). Sony VAIO VGN-AW92DS Battery

Americium(III) oxide is a red-brown solid with a melting point of 2205 °C.[58]Americium(IV) oxide is the main form of solid americium which is used in nearly all its applications. As most other actinide dioxides, it is a black solid with a cubic (fluorite) crystal structure.[59]

The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula Am2(C2O4)3·7H2O. Sony VAIO VGN-AW92JS Battery

Upon heating in vacuum, it loses water at 240 °C and starts decomposing into AmO2 at 300 °C, the decomposition completes at about 470 °C.[45] The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L.[60]

Halides

Halides of americium are known for the oxidation states +2, +3 and +4,[61] where the +3 is most stable, especially in solutions.[62]Sony VAIO VGN-AW92YS Battery

Sony VAIO VGN-AW93FS Battery

Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides AmCl2, AmBr2 and AmI2. They are very sensitive to oxygen and oxidize in water, releasing hydrogen and converting back to the Am(III) state. Specific lattice constants are:

  • Orthorhombic AmCl2: a = 896.3 ± 0.8 pm, b = 757.3 ± 0.8 pm and c = 453.2 ± 0.6 pm
  • Tetragonal AmBr2: a = 1159.2 ± 0.4 and c = 712.1 ± 0.3 pm.[63]

They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX2, where X = Cl, Br or I:[64] Sony VAIO VGN-AW93GS Battery

Americium(III) fluoride (AmF3) is poorly soluble and precipitates upon reaction of Am3+ and fluoride ions in weak acidic solutions:

The tetravalent americium(IV) fluoride (AmF4) is obtained by reacting solid americium(III) fluoride with molecular fluorine:[65][66] Sony VAIO VGN-AW93HS Battery

Sony VAIO VGN-AW93ZFS Battery

Another known form of solid tetravalent americium chloride is KAmF5.[65][67] Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)4 was dissolved in 15-M NH3F with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of AmF4 but differed from other oxidation states of americium. Sony VAIO VGN-AW93ZGS Battery

Sony VAIO VGN-AW93ZHS Battery

Heating the Am(IV) solution to 90 °C did not result in its disproportionation or reduction, however a slow reduction was observed to Am(III) and assigned to self-irradiation of americium by alpha particles.[49]

Most americium(III) halides form hexagonal crystals with slight variation of the color and exact structure between the halogens.Sony VAIO VGN-BZ11EN Battery

So, chloride (AmCl3) is reddish and has a structure isotypic to uranium(III) chloride (space group P63/m) and the melting point of 715 °C.[61] The fluoride is isotypic to LaF3 (space group P63/mmc) and the iodide to BiI3 (space group R3). The bromide is an exception with the orthorhombic PuBr3-type structure and space group Cmcm.[62] Sony VAIO VGN-BZ11MN Battery

Crystals of americium hexahydrate (AmCl3·6H2O) can be prepared by dissolving americium dioxide in hydrochloric acid and evaporating the liquid. Those crystals are hygroscopic and have yellow-reddish color and a monoclinic crystal structure.[68] Sony VAIO VGN-BZ11XN Battery

Oxyhalides of americium in the form AmVIO2X2, AmVO2X, AmIVOX2 and AmIIIOX can be obtained by reacting the corresponding americium halide with oxygen or Sb2O3, and AmOCl can also be produced by vapor phase hydrolysis:[64] Sony VAIO VGN-BZ12VN Battery


 

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