Plutonium

Plutonium is a chemical element with the symbol Pu and atomic number 94. It is a silvery-gray metal that belongs to a group called actinides. When exposed to air, plutonium changes and can form a dull coating. It is also radioactive, which means it gives off energy and particles, making it dangerous to handle.

Plutonium was first made in a laboratory in 1940 and 1941 by scientists at the University of California, Berkeley. They created it by changing uranium using a machine called a cyclotron. Because it was discovered around the same time as other elements named after planets, plutonium was named after the planet Pluto.

Plutonium is important because some of its forms can be used in nuclear reactors and weapons. One type, called plutonium-239, can keep a nuclear reaction going. Another type, plutonium-238, gives off heat and is used to power some spacecraft. However, handling plutonium is very careful because it can be harmful.

Characteristics

Plutonium is a shiny, silvery metal that quickly turns dull when exposed to air. It can exist in several different forms, changing shape with temperature and pressure. Unlike most metals, it does not conduct heat or electricity well.

Plutonium can be used in nuclear reactions because certain types of it can split easily when hit by small particles. This splitting releases a lot of energy, which is useful in both nuclear power plants and weapons. Different types of plutonium have different uses depending on how easily they split apart.

Occurrence

Tiny amounts of certain types of plutonium can be found in nature. Very small traces of one type, plutonium-239, are found in some uranium ores, like in a special place in Gabon called Oklo. These tiny bits of plutonium form when uranium atoms split apart and then change over time.

Another type of plutonium, plutonium-244, might have existed a very long time ago when our Solar System was young. Though we can't easily find it now, we see clues that it was once here, left behind in meteorites and deep in space. Very small amounts of plutonium can also be found in our bodies because of old nuclear tests in the air and water.

History

Discovery

Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered element 94 in 1934. Fermi called the element hesperium and mentioned it in his Nobel Lecture in 1938. The sample actually contained products of nuclear fission, primarily barium and krypton. Nuclear fission, discovered in Germany in 1938 by Otto Hahn and Fritz Strassmann, was unknown at the time.

Plutonium was first produced, isolated, and then chemically identified between December 1940 and February 1941 by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy, and Arthur Wahl by deuteron bombardment of uranium in the cyclotron at the Berkeley Radiation Laboratory at the University of California, Berkeley. Neptunium was created directly by the bombardment but decayed by beta emission with a half-life of a little over two days, which indicated the formation of element 94. The new element was first identified through oxidation in February 1941.

A paper documenting the discovery was prepared by the team and sent to the journal Physical Review in March 1941, but publication was delayed until a year after the end of World War II due to security concerns. At the Cavendish Laboratory in Cambridge, Egon Bretscher and Norman Feather realized that a slow neutron reactor fuelled with uranium would theoretically produce substantial amounts of plutonium as a by-product. They calculated that element 94 would be fissile, and had the added advantage of being chemically different from uranium, and could easily be separated from it.

McMillan had recently named the first transuranic element neptunium after the planet Neptune, and suggested that element 94, being the next element in the series, be named for what was then considered the next planet, Pluto. Nicholas Kemmer of the Cambridge team independently proposed the same name, based on the same reasoning as the Berkeley team. Seaborg originally considered the name "plutium", but later thought that it did not sound as good as "plutonium". He chose the letters "Pu" as a joke, in reference to the interjection "P U" to indicate an especially disgusting smell, which passed without notice into the periodic table. Alternative names considered by Seaborg and others were "ultimium" or "extremium" because of the erroneous belief that they had found the last possible element on the periodic table.

Early research

The chemistry of plutonium was found to resemble uranium after a few months of initial study. Early research was continued at the secret Metallurgical Laboratory of the University of Chicago. On August 20, 1942, a trace quantity of this element was isolated and measured for the first time. About 50 micrograms of plutonium combined with uranium and fission products was produced and only about 1 microgram was isolated. This procedure enabled chemists to determine the new element's atomic weight. On December 2, 1942, on a racket court under the west grandstand at the University of Chicago's Stagg Field, researchers headed by Enrico Fermi achieved the first self-sustaining chain reaction in a graphite and uranium pile known as CP-1. Using theoretical information garnered from the operation of CP-1, DuPont constructed an air-cooled experimental production reactor, known as X-10, and a pilot chemical separation facility at Oak Ridge. The separation facility, using methods developed by Glenn T. Seaborg and a team of researchers at the Met Lab, removed plutonium from uranium irradiated in the X-10 reactor. Information from CP-1 was also useful to Met Lab scientists designing the water-cooled plutonium production reactors for Hanford. Construction at the site began in mid-1943.

In November 1943 some plutonium trifluoride was reduced to create the first sample of plutonium metal: a few micrograms of metallic beads. Enough plutonium was produced to make it the first synthetically made element to be visible with the unaided eye.

The nuclear properties of plutonium were also studied; researchers found that when it is hit by a neutron it breaks apart (fissions) by releasing more neutrons and energy. These neutrons can hit other atoms of plutonium and so on in an exponentially fast chain reaction. This can result in an explosion large enough to destroy a city if enough of the isotope is concentrated to form a critical mass.

Production during the Manhattan Project

During World War II the U.S. government established the Manhattan Project, for developing an atomic bomb. The three primary research and production sites of the project were the plutonium production facility at what is now the Hanford Site, the uranium enrichment facilities at Oak Ridge, Tennessee, and the weapons research and design lab, now known as Los Alamos National Laboratory, LANL.

The first production reactor that made ²³⁹Pu was the X-10 Graphite Reactor. It went online in 1943 and was built at a facility in Oak Ridge that later became the Oak Ridge National Laboratory.

In January 1944, workers laid the foundations for the first chemical separation building, T Plant located in 200-West. Both the T Plant and its sister facility in 200-West, the U Plant, were completed by October. (U Plant was used only for training during the Manhattan Project.) The separation building in 200-East, B Plant, was completed in February 1945. The second facility planned for 200-East was canceled. Nicknamed Queen Mary by the workers who built them, the separation buildings were awesome canyon-like structures 800 feet long, 65 feet wide, and 80 feet high containing forty process pools. The interior had an eerie quality as operators behind seven feet of concrete shielding manipulated remote control equipment by looking through television monitors and periscopes from an upper gallery. Even with massive concrete lids on the process pools, precautions against radiation exposure were necessary and influenced all aspects of plant design.

On April 5, 1944, Emilio Segrè at Los Alamos received the first sample of reactor-produced plutonium from Oak Ridge. Within ten days, he discovered that reactor-bred plutonium had a higher concentration of ²⁴⁰Pu than cyclotron-produced plutonium. ²⁴⁰Pu has a high spontaneous fission rate, raising the overall background neutron level of the plutonium sample. The original gun-type plutonium weapon, code-named "Thin Man", had to be abandoned as a result—the increased number of spontaneous neutrons meant that nuclear pre-detonation (fizzle) was likely.

The entire plutonium weapon design effort at Los Alamos was soon changed to the more complicated implosion device, code-named "Fat Man". In an implosion bomb, plutonium is compressed to high density with explosive lenses—a technically more daunting task than the simple gun-type bomb, but necessary for a plutonium bomb. Uranium, by contrast, can be used with either method.

Construction of the Hanford B Reactor, the first industrial-sized nuclear reactor for the purposes of material production, was completed in March 1945. B Reactor produced the fissile material for the plutonium weapons used during World War II. B, D and F were the initial reactors built at Hanford, and six additional plutonium-producing reactors were built later at the site.

By the end of January 1945, the highly purified plutonium underwent further concentration in the completed chemical isolation building, where remaining impurities were removed successfully. Los Alamos received its first plutonium from Hanford on February 2. While it was still by no means clear that enough plutonium could be produced for use in bombs by the war's end, Hanford was by early 1945 in operation. Only two years had passed since Col. Franklin Matthias first set up his temporary headquarters on the banks of the Columbia River.

In 2004, a safe was discovered during excavations of a burial trench at the Hanford nuclear site. Inside the safe were various items, including a large glass bottle containing a whitish slurry which was subsequently identified as the oldest sample of weapons-grade plutonium known to exist. Isotope analysis by Pacific Northwest National Laboratory indicated that the plutonium in the bottle was manufactured in the X-10 Graphite Reactor at Oak Ridge during 1944.

Trinity and Fat Man atomic bombs

The first atomic bomb test, codenamed "Trinity "and detonated on July 16, 1945, near Alamogordo, New Mexico, used plutonium as its fissile material. The implosion design of "Gadget", as the Trinity device was codenamed, used conventional explosive lenses to compress a sphere of plutonium into a supercritical mass, which was simultaneously showered with neutrons from "Urchin", an initiator made of polonium and beryllium (neutron source: (α, n) reaction). Together, these ensured a runaway chain reaction and explosion. The weapon weighed over 4 tonnes, though it had just 6 kg of plutonium. About 20% of the plutonium in the Trinity weapon, fissioned; releasing an energy equivalent to about 20,000 tons of TNT.

An identical design was used in "Fat Man", dropped on Nagasaki, Japan, on August 9, 1945. Only after the announcement of the first atomic bombs was the existence and name of plutonium made known to the public by the Manhattan Project's Smyth Report.

Cold War use and waste

Large stockpiles of weapons-grade plutonium were built up by both the Soviet Union and the United States during the Cold War. The U.S. reactors at Hanford and the Savannah River Site in South Carolina produced 103 tonnes, and an estimated 170 tonnes of military-grade plutonium was produced in the USSR. Each year about 20 tonnes of the element is still produced as a by-product of the nuclear power industry. As much as 1000 tonnes of plutonium may be in storage with more than 200 tonnes of that either inside or extracted from nuclear weapons. SIPRI estimated the world plutonium stockpile in 2007 as about 500 tonnes, divided equally between weapon and civilian stocks.

Radioactive contamination at the Rocky Flats Plant primarily resulted from two major plutonium fires in 1957 and 1969. Much lower concentrations of radioactive isotopes were released throughout the operational life of the plant from 1952 to 1992. Prevailing winds from the plant carried airborne contamination south and east, into populated areas northwest of Denver. The contamination of the Denver area by plutonium from the fires and other sources was not publicly reported until the 1970s. According to a 1972 study, co-authored by Edward Martell, "In the more densely populated areas of Denver, the Pu contamination level in surface soils is several times fallout", and the plutonium contamination "just east of the Rocky Flats plant ranges up to hundreds of times that from nuclear tests". As noted by Carl Johnson in Ambio, "Exposures of a large population in the Denver area to plutonium and other radionuclides in the exhaust plumes from the plant date back to 1953." Weapons production at the Rocky Flats plant was halted after a combined FBI and EPA raid in 1989 and years of protests. The plant has since been shut down, with its buildings demolished and completely removed from the site.

In the U.S., some plutonium extracted from dismantled nuclear weapons is melted to form glass logs of plutonium oxide that weigh two tonnes. The glass is made of borosilicates mixed with cadmium and gadolinium. These logs are planned to be encased in stainless steel and stored as much as 4 km (2 mi) underground in bore holes that will be backfilled with concrete. The U.S. planned to store plutonium in this way at the Yucca Mountain nuclear waste repository, which is about 100 miles (160 km) north-east of Las Vegas, Nevada.

On March 5, 2009, Energy Secretary Steven Chu told a Senate hearing "the Yucca Mountain site no longer was viewed as an option for storing reactor waste". Starting in 1999, military-generated nuclear waste is being entombed at the Waste Isolation Pilot Plant in New Mexico.

In a Presidential Memorandum dated January 29, 2010, President Obama established the Blue Ribbon Commission on America's Nuclear Future. In their final report the Commission put forth recommendations for developing a comprehensive strategy to pursue, including:

"Recommendation #1: The United States should undertake an integrated nuclear waste management program that leads to the timely development of one or more permanent deep geological facilities for the safe disposal of spent fuel and high-level nuclear waste".

Applications

Explosives

²³⁹Pu is important for making nuclear weapons because it can easily split apart and is available. When the plutonium in a bomb is placed inside a special layer, it helps the bomb work with less material. This makes the bomb smaller and stronger.

The first plutonium bomb ever used, called Fat Man, used a special method to make the plutonium very dense. This let it work with just a small amount of plutonium and create a big explosion.

Mixed oxide fuel

Main articles: Nuclear reprocessing and Weapons-grade nuclear material

When nuclear reactors finish using their fuel, they leave behind a mix of different kinds of plutonium. This mix isn’t good for making weapons, but it can be used again as fuel in special reactors. This helps use the material better and reduces waste.

There is a common way to treat used fuel to take out plutonium and uranium, which can then be mixed to make new fuel. This mixed fuel has been used since the 1980s, especially in Europe. Some special reactors are made to create more useful material than they use.

Power and heat source

Plutonium-238 lasts a long time and gives off heat without making much radiation that is hard to shield. Because of this, it is great for powering machines that need to work for many years without anyone checking on them. It has been used in space probes like Voyager and Cassini, as well as on Mars rovers like Curiosity.

The Voyager spacecraft, launched in 1977, each had a power source made of plutonium-238. Even more than 30 years later, these sources still provided power to the spacecraft. This same kind of power has also been used in devices inside people’s bodies to help their hearts keep a steady beat.

Precautions

See also: Plutonium in the environment

Plutonium can be harmful because it is radioactive and acts like a heavy metal in the body. When plutonium gets inside the body, it can build up in bones and other parts, causing health problems over time. It can damage cells and increase the chance of cancer.

Plutonium is more dangerous when breathed in than when swallowed. Even small amounts can be risky, especially if breathed over a long time. Special care is needed to prevent plutonium from becoming concentrated enough to cause a dangerous reaction, as this can produce harmful radiation.

Metallic plutonium can catch fire, especially if it is in small pieces, and requires special handling to keep it safe.

Transportation

Plutonium is usually moved in a stable form called plutonium oxide, kept inside sealed packages. On land, a truck might carry one container with between 80 to 200 kg of this material. By sea, ships can carry several containers, each with sealed packages too. Special rules make sure the material is safe during travel.

Sometimes, plutonium is also moved by airplane. There are strict rules about how it is packed and where it is placed on the plane. In some cases, small amounts have been transported on regular passenger planes, following safety guidelines.