Plutonium

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 19,800 kg/m³. It is widely used in nuclear weapons. The most important isotope of plutonium is ²³⁹Pu, with a half-life of 24,200 years.

Notable characteristics

Plutonium is silvery in pure form, but has a yellow tarnish when oxidized.

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⁺.)

Applications

Plutonium is a key fissile component in modern nuclear weapons, due to its ease of fissioning and availability; complete detonation of one kilogram of plutonium will produce a 20 kiloton explosion.

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.

History

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, by analogy with the ordering of the planets in the solar system.

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.

Occurrence

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.

Compounds

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.

Isotopes

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 (Np-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.

Precautions

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.

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 can 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, so plutonium is not unusual in this regard. 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 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 the danger of plutonium. 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 crucial role in the production of nuclear weapons.

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. [1] (html-ized version )

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.

Criticality accidents have occurred in the past. Careless handling of a 6.2 kg plutonium sphere resulted in a lethal dose of radiation at Los Alamos on August 21, 1945. Harry Daghlian received a dose estimated to be 510 rems, he died four weeks later. Another death occurred in 1958 at the Los Alamos uranium enrichment plant. Plutonium accumulated inside a mixing vessel. A new batch was transferred and all 8 kg of plutonium came together in the vessel's center. A worker exposed to the radiation died less than two days later.

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 compound; 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 fire at the Rocky Flats Plant near Boulder, Colorado in 1969 [2] . To avoid these problems, special precautions are necessary to store or handle plutonium in any form; generally a dry inert atmosphere is required [3] .

References

• Los Alamos National Laboratory - Plutonium
• It's Elemental - Plutonium

External links

• WebElements.com - Plutonium (also used as a reference)
• EnvironmentalChemistry.com - Plutonium (also used as a reference)
• The Dangers of Plutonium - Anti-nuclear viewpoint
• collection of articles on plutonium at the Canadian Coalition for Nuclear Responsibility
• The Myth of Plutonium Toxicity
• Criticality Accidents Report Issued
• Nuclear Accidents Timeline