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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 239 Np intermediary), which has a half-life of 24,100 years. This is also the process used to manufacture 239Pu in nuclear reactors.
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, acetominophen, 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). As such, it is debatable whether plutonium should even be classified as a poison.
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 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 as inherently dangerous and 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:
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 can and will cause a lethal dose of radiation.
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 (5.1 Sv), 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.
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 [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].
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