|Name, Symbol, Number ||Plutonium, Pu, 94 |
|Chemical series ||Actinides |
|Period, Block ||7 , f |
|Density, Hardness ||19816 kg/m3, no data |
|Appearance ||silvery white metal |
|Atomic properties |
|Atomic weight ||244.06 amu |
|Atomic radius (calc.) ||175 (no data) pm |
|Covalent radius ||no data |
|van der Waals radius ||no data |
|Electron configuration ||[Rn]5f67s2 |
|e-'s per energy level ||2,8,18,32,24,8,2 |
|Oxidation states (Oxide) ||6,5,4,3 (amphoteric) |
|Crystal structure ||Monoclinic |
|Physical properties |
|State of matter ||Solid (__) |
|Melting point ||912.5 K (1182.9 °F) |
|Boiling point ||3503 K (5846 °F) |
|Molar volume ||12.29 ×10-6 m3/mol |
|Heat of vaporization ||344 kJ/mol |
|Heat of fusion ||2.84 kJ/mol |
|Vapor pressure ||ND Pa at 298 K |
|Velocity of sound ||2260 m/s at 293.15 K |
|Electronegativity ||1.28 (Pauling scale) |
|Specific heat capacity ||ND J/(kg*K) |
|Electrical conductivity ||0.666 106/m ohm |
|Thermal conductivity ||6.74 W/(m*K) |
|1st ionization potential ||584.7 kJ/mol |
|Most stable isotopes |
|SI units & STP are used except where noted. |
Plutonium is a radioactive, metallic, chemical element. It has the symbol Pu and the atomic number 94. Its atomic weight is 244.06, its density 19,816 kg/m3. It is the element used in most modern nuclear weapons. The most important isotope of plutonium is 239Pu, with a half-life of 24,200 years.
Plutonium is silvery in pure form, but has a yellow tarnish when oxidized. Peculiarly, the metal goes through phases of contraction as its temperature is increased.
The heat given off by alpha particle emission makes plutonium warm to the touch in reasonable quantities; larger amounts can boil water. It displays four ionic oxidation states in aqueous solution:
- Pu3+ (blue lavender)
- Pu4+ (yellow brown)
- PuO2+ (pink orange)
- PuO+ (thought to be pink; this ion is unstable in solution and will disproportionate into Pu4+ and PuO2+; the Pu4+ will then oxidize the remaining PuO+ to PuO2+, being reduced in turn to Pu3+. Thus, aqueous solutions of plutonium tend over time towards a mixture of Pu3+ and PuO2+.)
Plutonium is a key fissile component in modern nuclear weapons, due to its ease of fissioning and availability. The critical mass for an unreflected sphere of plutonium is 16 kg, but through the use of a neutron reflecting tamper the pit of plutonium in a fission bomb is reduced to 10 kg, which is a sphere with a diameter of 10 cm. Complete detonation of plutonium will produce an explosion of 20 kiloton per kilogram. (See also Nuclear_weapon_design#Enriched_materials.)
Plutonium could also be used to manufacture radiological weapons or as a (not particularly deadly) poison.
The plutonium isotope 238Pu is an alpha emitter with a half life of 87 years. These characteristics make it well suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human life time. It is therefore used in RTGs such as those powering the Galileo and Cassini space probes; earlier versions of the same technology powered seismic experiments on the Apollo Moon missions.
238Pu has been used successfully to power artificial heart pacemakers, to reduce the risk of repeated surgery. It has been largely replaced by lithium-based batteries, but as of 2003 there were somewhere between 50 and 100 plutonium-powered pacemakers still implanted and functioning in living patients.
Plutonium was discovered in 1940 by Dr. Glenn T. Seaborg, Edwin M. McMillan, J. W. Kennedy, and A. C. Wahl by deuteron bombardment of uranium in the 60-inch cyclotron of the Berkeley Radiation Laboratory at the University of California, Berkeley, but the discovery was kept secret. It was named after the planet Pluto, having been discovered directly after neptunium (which itself was one higher on the periodic table than uranium), by analogy with the ordering of the planets in the solar system. During the Manhattan Project, large reactors were set up in Hanford, Washington for the production of plutonium, which was used in two of the first atomic bombs (the first was tested at Trinity site, the second dropped on Nagasaki, Japan).
Large stockpiles of plutonium were built up by both the old Soviet Union and the United States during the Cold War—it was estimated that 300,000 kg of plutonium had been accumulated by 1982. Since the end of the Cold War, these stockpiles have become a focus of nuclear proliferation concerns. In 2002, the United States Department of Energy took possession of 34 metric tons 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 US from enriched uranium fuel to MOX fuel as a means of disposing of these.
While almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores. These come about by a process of neutron capture by 238U nuclei, initially forming 239U; two subsequent beta decays then form 239Pu (with a 239Np intermediary), which has a half-life of 24,100 years. This is also the process used to manufacture 239Pu in nuclear reactors. Some traces of 244Pu is also still left since birth of solar system from waste of supernovae, because its half-life (80 million yrs) is so long. A relative high concentration of plutonium had been discovered at the natural fission reactor of Oklo, Gabon in 1972. Since 1945, about 10 tons of plutonium have been released onto Earth through nuclear explosions.
Plutonium reacts readily with oxygen, forming PuO and PuO2, as well as intermediate oxides. It reacts with the halides, 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.
21 plutonium radioisotopes have been characterized, with the most stable being Pu-244 with a half-life of 80.8 million years, Pu-242 with a half-life of 373,300 years and Pu-239 with a half-life of 24,100 years. All of the remaining radioactive isotopes have half-lifes that are less than 7,000 years. This element also has 8 meta states, though none are very stable (all have half-lives less than 1s).
The isotopes of plutonium range in atomic weight from 228.0387 u (Pu-228) to 247.074 u (Pu-247). The primary decay modes before the most stable isotope, Pu-244, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before Pu-244 are uranium and neptunium isotopes (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are americium isotopes.
All isotopes and compounds of plutonium are toxic and radioactive. While plutonium is sometimes described in media reports as "the most toxic substance known to man", there is general agreement among experts in the field that this is incorrect. As of 2003, there has yet to be a single human death officially attributed to plutonium exposure. Naturally-occurring radium is about 200 times more radiotoxic than plutonium, and some organic toxins like botulism toxin are still more toxic. Botulism toxin, in particular, has a lethal dose in the hundreds of pg per kg, far less than the quantity of plutonium that poses a significant cancer risk. In addition, beta and gamma emitters (including the C-14 and K-40 in nearly all food) can cause cancer on casual contact, which alpha emitters cannot.
Orally, plutonium is less toxic than several common substances, including caffeine, acetaminophen, some vitamins, pseudoephedrine, and any number of plants and fungi. It is perhaps somewhat more toxic than pure ethanol, but less so than tobacco and many illegal drugs (some such as LSD and marijuana are negligibly toxic). Considering the pure chemical toxicity it probably ranks with lead and other heavy metals.
That said, there is no doubt that 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; particularly at risk are the skeleton, which it is liable to be absorbed onto the surface of, and the liver, where it will collect and become concentrated. Extremely small particles of plutonium on the order of micrograms have a (small) chance to cause lung cancer if inhaled into the lungs.
Other substances including ricin, botulinum toxin and tetanus toxin are fatal in doses of (sometimes far) under one milligram, and others (the nerve agents, nutmeg by injection, the amanita toxin, the fugu 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.
The chemical and radiological toxicity of plutonium should be distinguished from each other and further, from the potential danger of a runaway fission reaction or "criticality". Many, both in the anti-nuclear movement and in the continuing green politics movement, refer to plutonium as the most dangerous substance known to man because of its use in nuclear power plants which are seen by them as inherently dangerous, and for its potential as a catalyst for nuclear weapons proliferation.
Possibly it is the confusion of these two issues that has led to sensational exaggerations of plutonium toxicity. A 1989 paper by Bernard L. Cohen states:
- Pu hazards are far better understood than [those from insecticides or food additives], and the one fatality per 300 years they may someday cause is truly trivial by comparison. In spite of the facts we have cited here, facts well known in the scientific community, the myth of Pu toxicity lingers on. (MS Word (http://www.environmental.usace.army.mil/info/technical/hp/hpfaq/THE_MYTH_OF_PLUTONIUM_TOXICITY.doc)) (html (http://russp.org/BLC-3.html))
It must however be noted, that in contrast to naturally occuring radioisotopes such as radium or C-14, Plutonium has been manufactured, concentrated and isolated in large amounts (100s of metric tons) during the Cold War for weapons production. These piles (whether in weapons form or otherwise) could pose a significant toxicologic risk—not least due to the fact that there is no feasible known way to destroy them (whereas that can be easily done with biological poisons).
Toxicity issues aside, care must be taken to avoid the accumulation of amounts of plutonium which approach critical mass, the amount of plutonium which will self-generate a nuclear reaction. 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. 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.
Multiple criticality accidents have occurred in the past at least in the US and the former USSR, some of them with lethal consequences. Careless handling of a 6.2 kg plutonium sphere resulted in a lethal dose of radiation at Los Alamos on August 21, 1945, when scientist Harry Daghlian received a dose estimated to be 510 rems (5.1 Sv) and died four weeks later. Nine months later, another Los Alamos scientist, Louis Slotin, died from a similar accident. In 1958, during a process of purifying plutonium at Los Alamos, a critical mass was formed in a mixing vessel, which ended in the death of a crane operator. Other accidents of this sort have occured in the Soviet Union, Japan, and many other countries. (See List of nuclear accidents)
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 burn 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 of course an additional hazard. Magnesium oxide sand is the most effective material for extinguishing a plutonium fire. It both cools the burning material, acting as a heat sink, and also blocks off oxygen. Water is also effective. There was a major plutonium initiated fire at the Rocky Flats Plant near Boulder, Colorado in 1969  (http://tis.eh.doe.gov/techstds/standard/hdbk1081/hbk1081f.html#ZZ39). To avoid these problems, special precautions are necessary to store or handle plutonium in any form; generally a dry inert atmosphere is required  (http://tis.eh.doe.gov/techstds/standard/hdbk1081/hbk1081d.html#ZZ28).