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Description: artificial heavy metal

Isotopes: fifteen isotopes including plutonium 238, 239, and 241

Production: irradiation of uranium 238

Use: plutonium 239: component of nuclear warheads and of Mox fuel; plutonium 238: source of neutrons and of heat

Radioactivity: emitter of alpha articles and of weak gamma radiation, except for plutonium 241, a beta emitter

Commentary: plutonium 239 and 241 are fissile materials

Plutonium, a heavy metal, only exists as traces in nature. Virtually all existing plutonium is a twentieth-century human creation.

There are fifteen known isotopes of plutonium. Those that are of interest for the use of plutonium are plutonium 238, 239, 240, 241, and 242. All are radioactive and all are what is called fissionable (see Natural Uranium). Both plutonium 239 and 241 are fissile; but plutonium 239 is normally used for the production of energy and of weapons. Plutonium 241 is rarely used separately because of the difficulty of producing it in large quantity, the high cost of its production, its brief half-life, and its higher radioactivity than plutonium 239. Plutonium 238 has commercial and military applications, because it is an exceptionally strong emitter of alpha radiation. Plutonium 240 and 242 are only neutron poisons.

Production

The irradiation of uranium 238 in nuclear reactors creates plutonium 239. When the fuel undergoes longer and longer periods of irradiation, higher isotopes accumulate because the plutonium 239 and its products absorb neutrons. The isotopes plutonium 240, plutonium 241, plutonium 242 are thus also formed. At the same time, plutonium 238 is created by the decay chain that begins with uranium 235.

For this reason, when a specific reactor is used for the fabrication of military plutonium, the fuel used for the production of the plutonium and also the targets and cover, if they exist, are removed after only a brief stay (a few weeks) in the reactor in order to ensure that the plutonium 239 is as pure as possible. A brief irradiation never extracts all the energy that the fuel could produce. Therefore fuel is removed from an electricity-generating reactor after a much longer stay (three or four years).

After the fuel, the targets, and the cover have been removed from the reactor in which they were irradiated, they undergo a chemical treatment, known as the reprocessing of irradiated fuel, in a plant or a workshop to separate the plutonium.

Plutonium is classified according to the percentage of the contaminant plutonium 240 that it contains:

Super grade 2-3%

Military grade less than 7%

Fuel grade 7-18%

Reactor grade 18% or more.

Nevertheless, this classification is deceptive. According to the International Atomic Energy Agency (AIEA), "even highly irradiated fuel of reactor grade can be used for the fabrication of very powerful nuclear weapons. With the exception of plutonium that was created to serve as a source of heat and that contains 80% or more of the 238 isotope, we consider, for the purpose of IAEA guarantees that all plutonium in the various nations not equipped with nuclear weapons has an equally sensitive character [Shea 93].

For makers of bombs, the presence of isotopes other than plutonium 239 poses some problems, but they are not insurmountable. Plutonium 238 and plutonium 240 increase the heat present, but a device that resolves this difficulty can be constructed. Plutonium 240 has an elevated rate of spontaneous fission and, the more plutonium 240 there is, the greater the possibility of pre-ignition. However, even with a pre-ignition at the worst moment in a device as simple as the Nagasaki bomb, the yield would be on the order of one or several kilotons. A descendent of plutonium 241, americium 241, emits gamma radiation, but the radioactivity would not stop manipulation of the contaminated plutonium by resolute fabricators, using protective screens .

Theodore Taylor, a former weapons designer at Los Alamos, has stated that it is possible to replace military quality plutonium with reactor quality plutonium in every weapon that has been fabricated at any time. One would need between zero and two times more plutonium and some other changes [Taylor 94].

Another American expert, J. Carson Mark, estimates that a group of terrorists could fabricate a bomb of significant yield very rapidly by using about five to ten kilograms of plutonium. A nation, with time and means, would only need a few kilos [Mark 90]. Others estimate that for a yield of one kiloton, only three kilograms of military plutonium would be necessary [Cochran 94].

Plutonium oxide is used in a mixture of natural uranium or depleted uranium in Mox fuel (mixed uranium and plutonium oxides) for fast breeder and light water reactors.

Plutonium 238, with a half-life of 86.41 years, is a very powerful alpha emitter. Because of its high level of alpha activity, it is used as a source of neutrons (by alpha reaction with light elements), as a source of heat, and as a source of electrical energy (by conversion of the heat into electricity). As the source of electricity, the applications include cardiac stimulators and uses in space.

Plutonium 238 is prepared by the irradiation of neptunium 237, a fission product recovered during reprocessing or by the irradiation of americium in a reactor. In both cases, the targets are subjected to a chemical treatment, including dissolution in nitric acid to extract the plutonium 238. A ton of light water fuel that has been irradiated for three years contains only about 700 grams of neptunium 237, and the neptunium must be extracted selectively.

As a heavy metal, plutonium is chemically toxic, but its radiotoxicity is the determining factor. All the isotopes of plutonium that concern us emit alpha particles and weak gamma radiation, except plutonium 241, which emits beta particles.

Uranium 235 and uranium 238 are also alpha emitters. Plutonium is more harmful than uranium 235 and uranium 238, in great part because of the differences in their half-lives and, consequently, in their specific activity. The half-lives of the five isotopes of plutonium that concern us here are much shorter than those of uranium 235 and 238. Plutonium 239 with a half-life of 24,400 years, has a specific activity about 200,000 times greater than that of uranium 238 and about 30,000 times greater than that of uranium 235. The alpha particles emitted in the disintegration of plutonium 239 are about 25% more energetic than those emitted by the disintegration of uranium 238 and of uranium 235. Therefore, plutonium 239 is about 250,000 times more harmful per gram than uranium 238 and about 39,000 times more harmful per gram from the radiological point of view than uranium 235 [see IPPNW 92 and NAS 95].

These statistics do not measure biological damage, which can only be calculated by taking into account the pathway and length of time in the body as well as the specific activity and energy. The dilution volume index, which takes into account these factors, shows that, by gram, plutonium of military quality represents a potential risk from inhalation 23,000 times greater and a risk from ingestion 130,000 times greater than that posed by highly enriched uranium [NAS 95]. Plutonium metal and its compounds can enter the body by ingestion. However, as is the case with uranium, the greatest part of the plutonium ingested is quickly eliminated. The most dangerous pathway is inhalation. As with uranium, the small particles are the most dangerous. The particles of plutonium can lodge in the lunges where they may cause cancer; they can be carried from the lungs to the lymphatic pulmonary ganglions; they can also pass by means of the blood to other parts of the body. The plutonium has a tendency to concentrate in the liver and in the bones as well as in the lungs. In the bones, the plutonium is deposited on non-calcified and non-cartilaginous areas [IPPNW 92].

As with the compounds of uranium, the impact on the body of plutonium compounds depends on the solubility of the compound. The soluble compounds, including plutonium nitrate, pass rapidly to the liver and bones and are a proven cause of bone and liver cancer. The insoluble or little soluble compounds, including plutonium oxide, remain in the lungs for years. Inhalation of about 30 micrograms of plutonium in insoluble or relatively insoluble form, given a homogenous disposition of the plutonium, is almost certain to cause lung cancer, animal experiments have shown. The effects of exposure on the human body are relatively unknown, however, in great part because governments have not systematically collected and analyzed data on workers exposed to plutonium [IPPNW 92; IPPNW 95].

Plutonium 241 (half-life 13.2 years), which is found in varying quantities in all forms of plutonium disintegrates into americium 241, which emits gamma as well as alpha radiation. Americium 241 accumulates in tissues, in particular the kidneys and bones, creating a danger similar to that of plutonium [Benedict 81]. Moreover, it decays with a period of 458 years by alpha decay into neptunium 237, itself an alpha and gamma emitter with a period of 2 million years. Plutonium 239 disintegrates into uranium 235.

Pyrophoric character. Plutonium metal, when it is finely divided, is pyrophoric. Particles of less than a millimeter in diameter are pyrophoric at about 150 degrees Centigrade; particles of more than 1 millimeter in diameter are pyrophoric at about 500 degrees C [Science no. 2, 94]. Plutonium fires produce a smoke of fine and insoluble particles of plutonium dioxide.

Unexpected criticality. About 5-6 kg of plutonium surrounded by an excellent reflector such as a mass of water constitute a critical mass; but, as with uranium, the quantity necessary to produce a critical mass varies according to such factors as the geometry of the mass and the presence of neutron moderators or reflectors. Plutonium in liquid solution, such as plutonium nitrate, is more likely to become critical than solid plutonium [IPPNW 95].

Difficulty of detection. Because plutonium 239 emits only alpha particles, which only travel short distances and emit weak gamma radiation, plutonium within a receptacle is difficult to detect. It is measured by indirect means, usually calculations based on the gamma radiation present, which is not an exact measure. P. Nicolai of Valduc praised a method that permits plutonium in waste to be measured with "a level of error close to 15%" [Nicolai 93].