Плутоний-239: қасиеттері мен қолданылуы

Isotope of plutonium
an isotope of plutonium

Plutonium 239 (239Pu or Pu 239) is an isotope of plutonium. Plutonium 239 is the primary fissile isotope used for the production of nuclear weapons, although uranium 235 is also used for that purpose. Plutonium 239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium 235 and uranium 233. Plutonium 239 has a half life of 24,110 years.

The "supergrade" fission fuel, which has less radioactivity, is used in the primary stage of US Navy nuclear weapons in place of the conventional plutonium used in the Air Force's versions. "Supergrade" is industry parlance for plutonium alloy bearing an exceptionally high fraction of 239Pu (>95%), leaving a very low amount of 240Pu, which is a high spontaneous fission isotope (see above). Such plutonium is produced from fuel rods that have been irradiated a very short time as measured in MW day/ton burnup. Such low irradiation times limit the amount of additional neutron capture and therefore buildup of alternate isotope products such as 240Pu in the rod, and also by consequence is considerably more expensive to produce, needing far more rods irradiated and processed for a given amount of plutonium. Plutonium 240, in addition to being a neutron emitter after fission, is a gamma emitter, and so is responsible for a large fraction of the radiation from stored nuclear weapons. Whether out on patrol or in port, submarine crew members routinely live and work in very close proximity to nuclear weapons stored in torpedo rooms and missile tubes, unlike Air Force missiles where exposures are relatively brief. The need to reduce radiation exposure justifies the additional costs of the premium supergrade alloy used on many naval nuclear weapons. Supergrade plutonium is used in W80 warheads.

In any operating nuclear reactor containing 238U, some plutonium 239 will accumulate in the nuclear fuel. Unlike reactors used to produce weapons grade plutonium, commercial nuclear power reactors typically operate at a high burnup that allows a significant amount of plutonium to build up in irradiated reactor fuel. Plutonium 239 will be present both in the reactor core during operation and in spent nuclear fuel that has been removed from the reactor at the end of the fuel assembly's service life (typically several years). Spent nuclear fuel commonly contains about 0.8% plutonium 239. Plutonium 239 present in reactor fuel can absorb neutrons and fission just as uranium 235 can. Since plutonium 239 is constantly being created in the reactor core during operation, the use of plutonium 239 as nuclear fuel in power plants can occur without reprocessing of spent fuel; the plutonium 239 is fissioned in the same fuel rods in which it is produced. Fissioning of plutonium 239 provides more than one third of the total energy produced in a typical commercial nuclear power plant. Reactor fuel would accumulate much more than 0.8% plutonium 239 during its service life if some plutonium 239 were not constantly being "burned off" by fissioning. A small percentage of plutonium 239 can be deliberately added to fresh nuclear fuel. Such fuel is called MOX (mixed oxide) fuel, as it contains a mixture of uranium dioxide (UO2) and plutonium dioxide (PuO2). The addition of plutonium 239 reduces the need to enrich the uranium in the fuel.

Plutonium 239 emits alpha particles to become uranium 235. As an alpha emitter, plutonium 239 is not particularly dangerous as an external radiation source, but if it is ingested or breathed in as dust it is very dangerous and carcinogenic. It has been estimated that a pound (454 grams) of plutonium inhaled as plutonium oxide dust could give cancer to two million people. However, ingested plutonium is by far less dangerous as only a tiny fraction is absorbed in gastrointestinal tract; 800 mg would be unlikely to cause a major health risk as far as radiation is concerned. As a heavy metal, plutonium is also chemically toxic. See also Plutonium#Precautions. Weapons grade plutonium (with greater than 90% 239Pu) is used to make nuclear weapons and has many advantages over other fissile material for that purpose. Lower proportions of 239Pu would make a reliable weapon design difficult or impossible; this is due to the spontaneous fission (and thus neutron production) of the undesirable 240Pu.