Plutonium (Pu)
Plutonium (Pu) is a silvery-white, radioactive metal recognized as element number 94 in the periodic table. It has fifteen known isotopes, with plutonium-239 being the most prevalent and significant for both energy production and nuclear weaponry due to its long half-life of 24,390 years. While plutonium is toxic and radiotoxic, it is not easily absorbed by the body, posing a relatively low risk when handled properly, though fine airborne particles can be particularly dangerous if inhaled. This element plays a crucial role in nuclear reactors, where it can be generated from uranium and contributes significantly to the energy output. Additionally, plutonium has been historically pivotal in nuclear weapons, with notable examples such as the bomb used in the Nagasaki attack during World War II. Beyond weaponry, plutonium-238 is utilized in radioisotope thermoelectric generators (RTGs) to power spacecraft and other devices, demonstrating its diverse applications. The potential for plutonium to be misused by terrorists remains a concern, though its use as a radiological weapon is considered unlikely due to its low immediate toxicity compared to other chemical agents.
Plutonium (Pu)
Plutonium is both very useful and very dangerous. It contributes a significant percentage of the power produced in nuclear reactors, but it is also a radiological poison and a nuclear explosive.
Background
Plutonium (abbreviated Pu), element number 94 in the periodic table, is a silvery-white metal that oxidizes readily. Normally hard and brittle, it can be molded and machined if it is alloyed with gallium (0.9 percent by weight). Plutonium has fifteen known isotopes, ranging from plutonium 232 to plutonium 246; all of them are radioactive. Half-lives range from twenty-one minutes for plutonium 233 to eighty-one million years for plutonium 244. The most abundant isotope is plutonium 239, which has a half-life of 24,390 years. Minuscule amounts of plutonium 244 occur naturally in uranium ore, but the only way to obtain usable amounts is to make plutonium in a nuclear reactor.
Plutonium is radiotoxic: It harms by radiation. Plutonium primarily decays by emission of an alpha particle (a helium 4 nucleus, a grouping of two neutrons and two protons). Nonetheless, a grape-sized plutonium sample could be safely held in the hand, even though it would feel warm because of its radioactivity. Plutonium’s alpha particle radiation is easily blocked by the outermost layers of a person’s skin. Furthermore, plutonium is not easily absorbed by the body. If plutonium were ingested with food or water, almost all of it would be excreted. However, 4 parts per 10,000 might be absorbed and eventually settle in the liver or bones, where the plutonium might produce cancer tens of years later. The maximum long-term body burden of plutonium 239 believed to be safe is less than one microgram.
The most toxic form of plutonium is thought to be fine (10 microns in diameter) airborne particles of plutonium oxide. If inhaled, a significant fraction of such particles could be expected to lodge in the lungs. Estimates based on animal studies suggest that 10 milligrams of plutonium particles lodged in the lungs could cause death in about one month. For comparison, doses a thousand times smaller of anthrax spores, botulism, or coral snake venom will cause death within a few hours or days. With proper precautions, plutonium can be handled safely. More than fifty years of monitoring plutonium workers at United States nuclear weapons plants has not found any workers who have suffered serious consequences.
Plutonium as a Fuel
A common type of nuclear power reactor contains natural uranium that has been enriched in the isotope uranium 235. When struck by a neutron, the rare uranium 235 may fission to produce two daughter nuclei, a few neutrons, and energy. Plutonium 239 is formed when the more common uranium 238 isotope absorbs a neutron. If left in the reactor, plutonium 239 may also absorb a neutron and either fission or become plutonium 240. Over half of the plutonium produced in a power reactor does fission, and this fission contributes about one-third of the total energy produced in the reactor.
It takes nearly 3 million metric tons of coal to produce the same amount of energy as 1 metric ton of plutonium 239. The world stock of civilian plutonium is approximately 1,000 metric tons. Eighty percent of it is tied up in used reactor fuel elements.
Plutonium in Nuclear Weapons
Thebomb dropped on Nagasaki, Japan, by the United States in World War II contained 6.1 kilograms of plutonium and had an explosive yield equal to almost 20,000 metric tons of dynamite. The much greater yield of a hydrogen bomb is triggered by detonating a small plutonium bomb.
The military organizations of the world possess about 250 metric tons of plutonium. With the ending of the Cold War, the U.S. Department of Energy and Department of Defense declared that 38 metric tons of weapons-grade plutonium (nearly one-half of its stockpile) was surplus plutonium. The two ways that this plutonium is most likely to be disposed of are to use it as fuel in nuclear reactors or to mix it with radioactive waste and molten glass in a vitrification process.
Other Uses
Radioisotope thermoelectric generators (RTGs) use plutonium 238 (with a half-life of 86 years) to heat silicon-germanium junctions which then produce electricity. Having no moving parts, RTGs can be made to be very sturdy and reliable. RTGs are used to provide electrical power for interplanetary spacecraft such as Galileo and the Voyagers, since solar panels are too inefficient beyond the orbit of Mars. RTGs may also be used to power navigational beacons, remote weather stations, and even cardiac pacemakers. Americium 241 formed by the decay of plutonium 241 is a vital constituent of household smoke detectors.
Nuclear Terrorism
It seems unlikely that terrorists would use plutonium as a radiological poison, because its toxicity is relatively low. Bernard L. Cohen has calculated that 0.45 kilogram of plutonium particles dispersed in a large city in the most effective way might produce twenty-seven fatalities ten to forty years later. Chemical or biological weapons are probably easier for terrorists to obtain. The nerve gas sarin, for example, was used by terrorists in the 1995 Tokyo subway attack in which thousands were immediately overcome; fifty-five hundred people were injured, and twelve died.
Following the breakup of the Soviet Union, there were several cases of smuggling small amounts of high-grade plutonium and uranium for sale in the West. After the terrorist attacks in the United States on September 11, 2001, fears of terrorists acquiring and using nuclear weapons were heightened. To lesson the chances of plutonium being diverted for weapons use, weapons-grade plutonium can be oxidized and mixed with uranium oxide to form mixed oxide fuel (MOX), which is suitable for use in some nuclear reactors. Terrorists might conceivably construct an inefficient bomb from MOX, but it would require hundreds to thousands of kilograms.
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