Plutonium.html



94 neptunium ← plutonium → americium Sm ↑ Pu ↓ (Uqq) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	plutonium, Pu, 94
Element category	actinides
Group, Period, Block	n/a, 7, f
Appearance	silvery white
Standard atomic weight	(244)  g·mol−1
Electron configuration	Rn 5f6 7s2
Electrons per shell	2, 8, 18, 32, 24, 8, 2
Physical properties
Phase	solid
Density (near r.t.)	19.816  g·cm−3
Liquid density at m.p.	16.63  g·cm−3
Melting point	912.5 K (639.4 °C, 1182.9 °F)
Boiling point	3505 K (3228 °C, 5842 °F)
Heat of fusion	2.82  kJ·mol−1
Heat of vaporization	333.5  kJ·mol−1
Specific heat capacity	(25 °C) 35.5  J·mol−1·K−1
Vapor pressure P(Pa) 1 10 100 1 k 10 k 100 k at T(K) 1756 1953 2198 2511 2926 3499
Atomic properties
Crystal structure	monoclinic
Oxidation states	6, 5, 4, 3 (amphoteric oxide)
Electronegativity	1.28 (Pauling scale)
Ionization energies	1st: 584.7 kJ/mol
Atomic radius	175  pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(0 °C) 1.460 µΩ·m
Thermal conductivity	(300 K) 6.74  W·m−1·K−1
Thermal expansion	(25 °C) 46.7  µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 2260 m/s
Young's modulus	96  GPa
Shear modulus	43  GPa
Poisson ratio	0.21
CAS registry number	7440-07-5
Most-stable isotopes
Main article: Isotopes of plutonium iso NA half-life DM DE (MeV) DP 238Pu syn 88 y SF - - α 5.5 234U 239Pu syn 2.41×104 y SF - - α 5.245 235U 240Pu syn 6.5×103 y SF - - α 5.256 236U 241Pu syn 14 y β− .02078 241Am SF - - 242Pu syn 3.73×105 y SF - - α 4.984 238U 244Pu trace 8.08×107 y α 4.666 240U SF - -
References

Plutonium (pronounced /pluːˈtoʊniəm/, symbol Pu, atomic number 94) is a rare radioactive, metallic chemical element. The most significant isotope of plutonium is ²³⁹Pu, with a half-life of 24,100 years; this isotope is fissile and is used in most modern nuclear weapons. Plutonium-239 can be synthesized from natural uranium.

The most stable isotope is ²⁴⁴Pu, with a half-life of approximately 80 million years, long enough to be found in extremely small quantities in nature, making ²⁴⁴Pu the most nucleon-rich atom that naturally occurs in the Earth's crust, albeit in small traces.¹

Pu-239 is suitable for use in nuclear weapons and nuclear reactors. Pu-238 is suitable for use in radioisotope thermoelectric generators.

Characteristics

Physical

The metal has a bright, silver-like appearance at first, much like nickel, but it oxidizes very quickly to a dull gray.² It is about as hard and brittle as gray cast iron unless it is alloyed with other metals to make it soft and ductile.² Although it is a metal, it is not a good conductor of heat or electricity like most other metals.² It has a low melting point (640°C) and an unusually high boiling point (3,327°C).²

Alpha particle emission, which is essentially the release of high-energy helium nuclei formed as a by-product of fission, is the most common form of radiation given off by plutonium.³ Heat given off by these alpha particles make a mass of plutonium the size of a softball hot 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

Electrical resistivity of plutonium at room temperature is very high for a metal and oddly decreases as temperature is increased. Resistivity in fact continues to increase as temperature decreases to a maximum at 100 K, after which it rapidly decreases with lower temperature.⁵ Radiation damage causes a steady increase of resistivity with time at around 20 K with the rate dictated by the isotopic composition of the sample.

Due to self-irradiation, plutonium fatigues throughout its crystal structure.⁶ However, self-irradiation can also lead to annealing which counteracts some of the fatigue effects.

Nuclear

Plutonium is a radioactive actinide metal that, with uranium, is one of the few elements with one or more fissile isotopes.

A ring of weapons-grade electrorefined plutonium.

The plutonium isotope ²³⁹Pu can undergo nuclear fission if its nucleus is struck by a neutron, particularly a thermal neutron.⁷ The fission of ²³⁹Pu itself releases neutrons that bombard other ²³⁹Pu 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. During fission, a tiny fraction of the nuclear material (i.e., the mass defect) is converted directly into a large amount of energy; a kilogram of ²³⁹Pu can produce an explosion equivalent to 20,000 tons of TNT.⁴ It is this energy that makes ²³⁹Pu useful in nuclear weapons and reactors.

The presence of the isotope ²⁴⁰Pu in a material limits its nuclear bomb potential since it emits neutrons randomly, increasing the difficulty of accurately initiating the chain reaction at the desired instant and thus reducing the bomb's reliability and power.⁸ Plutonium is identified as either weapon grade, fuel grade, or power reactor grade based on the percentage of ²⁴⁰Pu that is contained in the plutonium. Weapons grade plutonium contains less than 7% ²⁴⁰Pu. Fuel grade plutonium contains from 7 to less than 19% percent, and power reactor grade contains from 19% and greater ²⁴⁰Pu.⁹ The isotope ²³⁸Pu is not capable of undergoing nuclear fission.⁴

Isotopes and synthesis

Main article: Isotopes of plutonium

Twenty plutonium radioisotopes have been characterized.³ The longest-lived are ²⁴⁴Pu, with a half-life of 80.8 million years, ²⁴²Pu, with a half-life of 373,300 years, and ²³⁹Pu, with a half-life of 24,110 years.³ Because of its comparatively large half-life, minute amounts of ²⁴⁴Pu can be found in nature,¹⁰ All of the remaining radioactive isotopes have half-lives that are less than 7,000 years.³ This element also has eight meta states, though none are very 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 before the most stable isotope, ²⁴⁴Pu, are spontaneous fission and alpha emission; the primary mode after is beta emission.³ The primary decay products before ²⁴⁴Pu are uranium and neptunium isotopes (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are americium isotopes.^{3 241}Pu is the parent isotope of the neptunium decay series, decaying to americium-241 via β or electron emission.⁴

²³⁸Pu and ²³⁹Pu are the most-widely synthesized isotopes.⁴ Plutonium-239 is synthesized via the following reaction using uranium (U) and neutrons (n) via beta decay (β^-):¹¹

²³⁸U + n → ²³⁹U (half-life 23.5 min; β^-) → ²³⁹Np (2.36 days; β^-) → ²³⁹Pu (24,100 years)

In other words, neutrons from the fission of ²³⁵U are captured by ²³⁸U nuclei to form form ²³⁹U; a beta decay adds a proton to form ²³⁹Np (half-life 2.3 days) and another beta decay forms ²³⁹Pu, which has a half-life of 24,000 years.^{12 238}Pu is synthesized by bombarding ²³⁸U with deuterons (d, the nuclei of heavy hydrogen) in the following reaction:⁴

²³⁸U(d,2n)²³⁸Np → ²³⁸Pu + β^-

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

Allotropes

A diagram of the allotropes of plutonium at ambient pressure

Main article: Allotropes of plutonium

Plutonium normally has six allotropes¹³ but forms a seventh (ζ) under high temperature and a limited pressure range.¹⁴ The allotropes have very similar energy levels but significantly varying densities and crystal structures, making plutonium very sensitive to changes in temperature, pressure, or chemistry, and allowing for dramatic volume changes following phase transitions.⁶ Plutonium increases in density when it melts by 2.5% but the liquid metal exhibits a linear decrease in density as it is heated.⁵ Densities vary from 16.00 to 19.86 g/cm³.¹⁵

The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. For example, the unalloyed α phase exists at room temperature and has machining characteristics similar to cast iron but transitions to the plastic and easy to work β phase at slightly higher temperatures.¹⁶ The reasons for the complicated phase diagram are not entirely understood; recent research has focused on constructing accurate computer models of the phase transitions. The α phase also possesses a low-symmetry structure, causing it to become progressively more brittle over time.¹⁴

Plutonium in the delta phase normally exists in the 310 to 452 °C range but is often alloyed with a small percentage of gallium, aluminium, or cerium to increase phase stability at room temperature and thereby enhance workability and allows it to be welded.¹⁶

Compounds and chemistry

Image showing colors of various oxidation states of Pu in solution on the left and colors of only one Pu oxidation state (IV) on the right in solutions containing different anions.

At room temperature, pure plutonium is silvery in color but gives a yellow tarnish when oxidized.⁴ The element displays four common ionic oxidation states in aqueous solution¹⁵ and one rare one:

• 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 PuO₅^3- (dark red); the heptavalent ion is rare and prepared only under extreme oxidizing conditions.

The actual color shown by plutonium solutions depends on both the oxidation state and the nature of the acid anion.¹⁷ 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 fluoride with either barium or lithium at 1200 °C.¹² It is attacked by acids, oxygen, and steam but not by alkalis and dissolves easily in concentrated hydrochloric, hydroiodic and perchloric acids.¹⁸ Molten metal must be kept in a vacuum or an inert atmosphere to avoid reaction with air.¹⁶ At 135°C the metal will ignite in air and will explode if placed in carbon tetrachloride.¹²

Plutonium pyrophoricity can cause it to look like a glowing ember under certain conditions.

Plutonium is an active metal. In moist air or moist argon, the metal oxidizes rapidly, producing a mixture of oxides and hydrides.² If the metal is exposed long enough to a limited amount of water vapor, an olive-green powdery surface coating of PuO₂ is formed.² Also formed is plutonium hydride but an excess of water vapor forms just PuO₂.¹⁸

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.² Oxygen retards the effects of moisture and acts as a passivating agent.²

Plutonium reacts readily with oxygen, forming PuO and PuO₂, as well as intermediate oxides;¹⁵ plutonium oxide fills 40% more volume than plutonium metal.¹² 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.¹⁵ The following oxyhalides are observed: PuOCl, PuOBr and PuOI.¹⁵ It will react with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi₂.¹⁵

Crucibles used to contain plutonium need to be able to withstand its strongly reducing properties.¹⁶ Refractory metals such as tantalum and tungsten along with the more stable oxides, borides, carbides, nitrides and silicides can tolerate this, however.¹⁶ Melting in an Electric arc furnace can be used to produce small ingots of the metal without the need for a crucible.¹⁶

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.¹⁸ Partial exceptions include the refractory metals chromium, molybdenum, niobium, tantalum and tungsten, which, while soluble in liquid plutonium and insoluble or only slightly so in solid plutonium.¹⁸

Occurrence

While almost all plutonium is manufactured synthetically, a few parts per trillion of ²³⁹Pu are found naturally in uranium ores,¹⁹ for example the ratio of ²³⁹Pu/U in the Cigar Lake uranium deposit ranges from 2.4 × 10⁻¹² to 44 × 10⁻¹² for ²³⁹Pu/U .²⁰ A relatively high concentration of plutonium was discovered at the natural nuclear fission reactor in Oklo, Gabon in 1972.²¹

Minute traces are found in the human body due to the 550 above-ground nuclear tests and several major nuclear accidents.¹² 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 the result of radioactive decay of uranium ores, ²³⁹Pu is the most abundant isotope of plutonium.¹²

History

See also: Hesperium

Discovery

Glenn Seaborg at the Geiger Counter, 301 Gilman Hall, Berkeley, California, where he discovered plutonium.

Plutonium (²³⁸Pu) 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 (1,500 mm) cyclotron at the University of California, Berkeley.²² 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 94.¹² The basic chemistry of element 94 was found to resemble uranium after a few months of study.¹²

A paper documenting the discovery was prepared by the team and sent to the journal Physical Review in March 1941.¹² The paper was withdrawn before publication after the discovery that an isotope of the new element (²³⁹Pu) 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.⁷

Edwin McMillan 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.^{4note 2} Seaborg originally considered the name "plutium", but later thought that it didn't sound as good as "plutonium."²³ 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 "ultinium" or "extremium" because of the now-discredited belief that they had found the last possible element on the periodic table.²⁴

Chemists at the University of Chicago began to study the newly-manufactured radioactive element. The George Herbert Jones Laboratory at the university was the site where, on August 18, 1942, a trace quantity of this new 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.¹⁹ This procedure enabled chemists to determine the new element's atomic weight.^{note 425} In November 1943 some plutonium trifluoride was reduced to create the first sample of plutonium metal; a few micrograms of metallic beads.¹⁹ Enough plutonium was produced to make it the first synthetically-made element to be visible with the unaided eye.¹³

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 a critical mass of the isotope is assembled in one place.¹²

Industrial production

Workers use rods to push fresh uranium slugs into the X-10 Graphite Reactor's concrete loading face

During World War II the U.S. government set-up the Manhattan Project, which was tasked with developing an atomic bomb. 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, Tennessee that later became the Oak Ridge National Laboratory.^{12note 5} Later, large (200 MW) reactors were set up at the Hanford Site near Richland, Washington, for the mass production of plutonium. Plutonium was produced using uranium that was bombarded by neutrons. The fuel rods this reaction took place in were removed after six months and nitric acid was used to extract the plutonium as plutonium nitrate.

Six kilograms of plutonium were detonated in the first atomic bomb test, codenamed "Trinity", on July 16, 1945 near Alamogordo, New Mexico.¹⁹ Conventional explosives were used in the Trinity device to force pieces of plutonium into a critical mass that was immediately showered with neutrons from an initiator made of beryllium and polonium.¹² Together, this ensured a runaway chain reaction and explosion.

Hanford Site plutonium production reactors along the Columbia River during the Manhattan Project.

Plutonium was also used in the "Fat Man" atomic bomb dropped on Nagasaki, Japan on August 9, 1945. Fat Man's detonation was equivalent to several thousand tons of TNT and killed 70,000 and wounded another 100,000.^{12note 6} The "Little Boy" bomb dropped on Hiroshima three days earlier used uranium-235, not plutonium. On August 11th, a second plutonium-based bomb was scheduled to be dropped on Kumagaya but was not ready in time; 6000 tons of conventional explosives were used to destroy the city instead.¹² Japan capitulated on August 15th, effectively ending the war.

Proposed waste storage tunnel design for the Yucca Mountain nuclear waste depository.

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 103,000 kg²⁶ and an estimated 170,000 kg of military-grade plutonium was produced in Russia.^{27note 7} Each year about 20,000 kg of the element is still produced as a by-product of the nuclear power industry.¹⁵ 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.¹²

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.¹² The glass consists of borosilicates mixed with as cadmium and gadolinium, which are all powerful neutron absorbers.^{note 8} 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.¹² As of 2008, the only facility in the U.S. that is scheduled to store plutonium in this way is the Yucca Mountain nuclear waste repository, which is about 100 miles (160 km) north-east of Las Vegas, Nevada. A great deal of local and state opposition exists against this plan.

Medical experimentation

During the initial years after the discovery of plutonium, when its biological and physical properties were very poorly understood, a series of human radiation experiments were performed by the U.S. government and by private organizations acting on its behalf. 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. 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.

These eighteen injections were made without the informed consent of those patients and were not done with the belief that the injections would heal their conditions; rather, they were used to develop diagnostic tools for determining the uptake of plutonium in the body for use in developing safety standards for people working with plutonium during the course of developing nuclear weapons.²⁸

The episode is now considered to be a serious breach of medical ethics and of the Hippocratic Oath, and has been sharply criticised as failing "both the test of our national values and the test of humanity."²⁹ 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."³⁰

Applications

Nuclear weapons

The atomic bomb dropped on Nagasaki, Japan in 1945 had a plutonium core.

The isotope ²³⁹Pu is a key fissile component in nuclear weapons, due to its ease of fissioning and availability. Encasing the bomb's sphere of plutonium in a tamper decreases the amount of plutonium needed to reach critical mass by reflecting escaping neutrons back into the plutonium core. This reduces the amount of plutonium needed to reach criticality from 16 kg to 10 kg, which is a sphere with a diameter of 10 cm.³¹ This critical mass is about a third of ²³⁵U.⁴

The Manhattan Project's "Fat Man" type plutonium bombs, using explosive compression of Pu to significantly higher densities than normal, were able to function with plutonium cores of only 6.2 kg.^{note 9} Complete detonation may be achieved through the use of an additional neutron source (often from a small amount of fusion fuel). The Fat Man bomb had an explosive yield of 21 kilotons. (See also nuclear weapon design.)

At one time nuclear fusion explosives were seriously considered to economically excavate large inter-oceanic canals and in other large earth-moving operations.¹⁸ Under those plans, plutonium would likely have been used to initiate the fusion reaction. These plans were abandoned after a negative public perception of nuclear fallout grew.

Power source

The isotope plutonium-238 (²³⁸Pu) has a half-life of 88 years and emits a large amount of thermal energy as it decays without harmful gamma rays. Being an alpha emitter, it combines high energy radiation with low penetration and thereby requires minimal shielding. In fact, one kilogram of plutonium-238 can generate 22 million kilowatt-hours of heat energy.⁴

A pellet of plutonium-238, glowing due to blackbody radiation, used for radioisotope thermoelectric generators.

These characteristics make it well-suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human lifetime. It is therefore used in radioisotope thermoelectric generators such as those powering the Cassini, Voyager and New Horizons space probes. Earlier versions of the same technology powered the ALSEP and EASEP systems including seismic experiments on the Apollo 14 Moon mission.¹²

Plutonium-238 has also been used successfully to power artificial heart pacemakers, to reduce the risk of repeated surgery.³² 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.³³ This isotope was also used to heat Scuba suits.¹⁸ Plutonium-238 mixed with beryllium is used to generate neutrons for research purposes.¹²

Use of plutonium waste

In 2002, the United States Department of Energy took possession of 34 tonnes of excess weapons-grade plutonium stockpiles from the United States Department of Defense, and as of early 2003 was considering converting several nuclear power plants in the U.S. from enriched uranium fuel to mixed oxides (MOX) of uranium and plutonium as a means of disposing of plutonium stocks. Mox fuel is used in fast breeder reactors and consists of 60 kg of plutonium per tonne of fuel; after 4 years, three-quarters of the plutonium is burned (turned into other elements).¹²

Efficiencies are also attained through reprocessing: A fuel rod is reprocessed after 3 years of use to remove waste products, which by then account for 3% of the total weight of the rods.¹² Any uranium or plutonium isotopes produced during those 3 years are left and the rod goes back into production.^{note 10}

Precautions

See also: Plutonium in the environment

Toxicity

Isotopes and compounds of plutonium are toxic due to its radioactivity.³⁴ While plutonium is sometimes described in media reports as "the most toxic substance known to man", from the standpoint of actual chemical or radiological toxicity this is incorrect.³⁵³⁶ When taken in by mouth, plutonium is less poisonous than if inhaled, since it is not absorbed into the body efficiently when ingested; only 0.04% of plutonium oxide is absorbed after ingestion.¹² The U.S. Department of Energy estimates the increase in lifetime cancer risk for inhaled plutonium as 3×10⁻⁸ pCi⁻¹.^{37note 11}

When plutonium is absorbed into the body, it is excreted very slowly, with a biological half-life of 200 years.³⁸ Plutonium has a metallic taste.³⁹

Plutonium may be extremely dangerous when handled incorrectly. The alpha radiation it emits does not penetrate the skin, but can irradiate internal organs when plutonium is inhaled or ingested.¹² Particularly at risk is the skeleton, where it is likely to be absorbed by the bone surface, and the liver, where it will likely collect and become concentrated.¹⁸

Other substances, including ricin, tetrodotoxin, botulinum toxin, and tetanus toxin, are fatal in doses of (sometimes far) under one milligram, and others (the nerve agents, the amanita toxin) are in the range of a few milligrams. As such, plutonium is not unusual in terms of toxicity, even by inhalation. In addition, those substances are fatal in hours to days, whereas plutonium (and other cancer-causing radioactives) give an increased chance of illness decades in the future. Considerably larger amounts may cause acute radiation poisoning and death if ingested or inhaled; however, so far, no human is known to have immediately died because of inhaling or ingesting plutonium and many people have measurable amounts of plutonium in their bodies.³⁶

Disposal difficulties

In contrast to naturally occurring radioisotopes such as radium or C-14, plutonium was manufactured, concentrated, and isolated in large amounts (hundreds of metric tons) during the Cold War for weapons production. These stockpiles, whether or not in weapons form, pose a significant problem because, unlike chemical or biological agents, no chemical process can destroy them. One proposal to dispose of surplus weapons-grade plutonium is to mix it with highly radioactive isotopes (e.g., spent reactor fuel) to deter handling by potential thieves or terrorists. Another is to mix it with uranium and use it to fuel nuclear power reactors (the mixed oxide or MOX approach). This would not only fission (and thereby destroy) much of the Pu-239, but also transmute a significant fraction of the remainder into Pu-240 and heavier isotopes that would make the resulting mixture useless for nuclear weapons.⁴⁰

Criticality potential

A simulated sphere of plutonium surrounded by neutron-reflecting tungsten carbide blocks in a re-enactment of Harry Daghlian's 1945 experiment.

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's.⁴ A critical mass of plutonium emits lethal amounts of neutrons and gamma rays.⁴¹ Despite not being confined by external pressure as is required for a nuclear weapon, it will nevertheless heat itself and break whatever confining environment it is in. Shape is relevant; compact shapes such as spheres are to be avoided. Plutonium in solution is more likely to form a critical mass than the solid form due to moderation by the hydrogen in water.¹⁵ A weapon-scale nuclear explosion cannot occur accidentally, since it requires a greatly supercritical mass in order to explode rather than simply melt or fragment. However, a marginally critical mass will cause a lethal dose of radiation and has in fact done so in the past on several occasions.

Criticality accidents have occurred in the past, some of them with lethal consequences. Careless handling of tungsten carbide bricks around a 6.2 kg plutonium sphere resulted in a lethal dose of radiation at Los Alamos on August 21, 1945, when scientist Harry K. Daghlian, Jr. received a dose estimated to be 510 rems (5.1 Sv) and died 28 days later.⁴² 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.⁴³ These incidents were fictionalized in the 1989 film Fat Man and Little Boy. 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 crane operator.⁴⁴ Other accidents of this sort have occurred in the Soviet Union, Japan, and many other countries.⁴⁴ (See List of nuclear accidents.)

Flammability

Metallic plutonium is also a fire hazard, especially if the material is finely divided. It reacts chemically with oxygen and water, which may result in an accumulation of plutonium hydride, a pyrophoric substance; that is, a material that will ignite in air at room temperature. Plutonium expands considerably in size as it oxidizes and thus may break its container. The radioactivity of the burning material is an additional hazard. Magnesium-oxide sand is the most effective material for extinguishing a plutonium fire. It cools the burning material, acting as a heat sink, and also blocks off oxygen. There was a major plutonium-initiated fire at the Rocky Flats Plant near Boulder, Colorado in 1969.⁴⁵ To avoid these problems, special precautions are necessary to store or handle plutonium in any form; generally a dry inert atmosphere is required.⁴⁶

See also

• Nuclear engineering
• Nuclear fuel cycle
• Nuclear physics
• Nuclear reactor
• Plutonium in the environment

Notes

1. ^ The PuO₂⁺ ion is unstable in solution and will disproportionate into Pu⁴⁺ and PuO₂²⁺; the Pu⁴⁺ will then oxidize the remaining PuO₂⁺ to PuO₂²⁺, being reduced in turn to Pu³⁺. Thus, aqueous solutions of plutonium tend over time towards a mixture of Pu³⁺ and PuO₂²⁺. Reference: Crooks, William J. (2002). "Nuclear Criticality Safety Engineering Training Module 10 - Criticality Safety in Material Processing Operations, Part 1" (PDF). Retrieved on 2006-02-15.
2. ^ This wasn't the first time somebody suggested that an element be named "plutonium." A decade after barium was discovered, a Cambridge University professor suggested it be renamed to "plutonium" because the element wasn't (as suggested by the Greek root, barys, it was named for) heavy. He reasoned that, since it was produced by the relatively-new technique of electrolysis, then its name should refer to fire. Thus he suggested it be named for the Roman god of the underworld, [[Pluto (god)|]].(Heiserman 1992)
3. ^ As one article puts it, referring to information Seaborg gave in a talk: "The obvious choice for the symbol would have been Pl, but facetiously, Seaborg suggested Pu, like the words a child would exclaim, 'Pee-yoo!' when smelling something bad. Seaborg thought that he would receive a great deal of flak over that suggestion, but the naming committee accepted the symbol without a word." David L. Clark and David E. Hobart (2000). "Reflections on the Legacy of a Legend: Glenn T. Seaborg, 1912–1999" (PDF). Los Alamos Science 26: 56–61, on 57, http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818011.pdf. 
4. ^ Room 405 of the building was named a National Historic Landmark in May 1967.
5. ^ During the Manhattan Project, plutonium was also often referred to as simply "49":. the number 4 was for the last digit in 94 (atomic number of plutonium), and 9 was for the last digit in Pu-239, the weapon-grade fissile isotope used in nuclear bombs. Hammel, E.F. (2000). "The taming of "49" — Big Science in little time. Recollections of Edward F. Hammel, pp. 2-9. In: Cooper N.G. Ed. (2000). Challenges in Plutonium Science". Los Alamos Science 26 (1): 2–9, http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818010.pdf.  Hecker, S.S. (2000). "Plutonium: an historical overview. In: Challenges in Plutonium Science". Los Alamos Science 26 (1): 1–2, http://www.fas.org/sgp/othergov/doe/lanl/pubs/number26.htm. 
6. ^ Fat Man's original target was Kokura but the city was obscured by clouds so the back-up city of Nagasaki was used. Kokura was re-scheduled for nuclear destruction on August 17th but was spared when the war ended before that.
7. ^ Much of this plutonium was used to make the fissionable cores of a type of thermonuclear weapon employing the Teller-Ulam design. These so-called 'hydrogen bombs' are a variety of nuclear weapon that use what is in effect an atomic bomb to trigger the nuclear fusion of heavy hydrogen isotopes. Their destructive yield is commonly in the millions of tons of TNT equivalent compared with the thousands of tons of TNT equivalent of atomic devices.(Emsley 2001)
8. ^ Gadolinium zirconium oxide (Gd₂Zr₂O₇) has been studied because it could hold plutonium for up to 30 million years.(Emsley 2001)
9. ^ Much of the information about the plutonium in the Fat Man bomb comes from reports of the criticality accidents of Harry K. Daghlian, Jr. and Louis Slotin, both of whom died after conducting experiments with plutonium bomb cores. See http://members.tripod.com/~Arnold_Dion/Daghlian/accident.html.
10. ^ Breakdown of plutonium in a spent nuclear fuel rod: ²³⁹Pu (~58%), ²⁴⁰Pu (24%), ²⁴¹Pu (11%), ²⁴²Pu (5%), and ²³⁸Pu (2%). (Emsely 2001)
11. ^ This means that inhaling 1 μCi, or about 2.5 μg of reactor-grade plutonium is estimated to increase one's lifetime risk of developing cancer as a result of the exposure to 3%.

References