About Plutonium [Pu] Element

Characteristics

An: 94 N: 150

Am: [244] g/mol

Group Name: Actinoid

Block: f-block Period: 7 (actinoid)

State: solid at 298 K

Colour: silvery white Classification: Metallic

Boiling Point: 3503K (3230^oC)

Melting Point: 912.5K (639.4^oC)

Density: 19.816g/cm³

Discovery Information

Who: G.T.Seaborg, J.W.Kennedy, E.M.McMillan, A.C.Wahl

When: 1940

Where: United States

Name Origin

From planet Pluto. "Plutonium" in different languages.

Sources

Almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores. Most plutonium is made synthetically by bombarding uranium with neutrons.

Annual production is around 20 tons, it is thought that world reserves are around 500 tons.

A small pellet of plutonium glowing under its own light.

Uses

Used in bombs and reactors. Complete detonation of plutonium will produce an explosion equivalent to 20 kilotons of Trinitrotoluene (TNT) per kilogram (of plutonium).

History

The production of plutonium and neptunium by bombarding uranium-238 with neutrons was predicted in 1940 by two teams working independently: Edwin M. McMillan and Philip Abelson at Berkeley Radiation Laboratory at the University of California, Berkeley and by Egon Bretscher and Norman Feather at the Cavendish Laboratory at University of Cambridge. Coincidentally both teams proposed the same names to follow on from uranium, like the sequence of the outer planets.

Plutonium was first produced and isolated on February 23, 1941 by Dr. Glenn T. Seaborg, Dr. Michael Cefola, Edwin M. McMillan, J. W. Kennedy, and A. C. Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley. The discovery was kept secret due to the war. It was named after Pluto, having been discovered directly after neptunium (which itself was one higher on the periodic table than uranium), by analogy to solar system planet order as Pluto was considered to be a planet at the time (though technically it should have been "plutium", Seaborg said that he did not think it sounded as good as "plutonium"). Seaborg chose the letters "Pu" as a joke, which passed without notice into the periodic table. Originally, Seaborg and others thought about naming the element "ultinium" or "extremium" because they believed at the time that they had found the last possible element on the periodic table.

Chemists at the University of Chicago began to study the newly manufactured radioactive element. The George Herbert Jones Laboratory at the university was the site where, for the first time, a trace quantity of this new element was isolated and measured in September 1942. This procedure enabled chemists to determine the new element’s atomic weight. Room 405 of the building was named a National Historic Landmark in May 1967. During the Manhattan Project, the first production reactor was built at the Oak Ridge, Tennessee site that later became Oak Ridge National Laboratory. Later, large reactors were set up in Hanford, Washington, for the production of plutonium, which was used in the first atomic bomb used at the "Trinity" test at White Sands, New Mexico in July 1945. Plutonium was also used in the "Fat Man" bomb dropped on Nagasaki, Japan in August 1945. The "Little Boy" bomb dropped on Hiroshima utilized uranium-235, not plutonium.

During the initial years after the discovery of plutonium, when its biological and physical properties were very poorly understood, a series of human radiation experiments were performed by the U.S. government and by private organizations acting on its behalf. During and after the end of World War II, scientists working on the Manhattan Project and other nuclear weapons research projects conducted studies of the effects of plutonium on laboratory animals and human subjects. In the case of human subjects, this involved injecting solutions containing (typically) five micrograms of plutonium into hospital patients thought to be either terminally ill, or to have a life expectancy of less than ten years either due to age or chronic disease condition. These eighteen injections were made without the informed consent of those patients and were not done with the belief that the injections would heal their conditions; rather, they were used to develop diagnostic tools for determining the uptake of plutonium in the body for use in developing safety standards for people working with plutonium during the course of developing nuclear weapons.

Notes

The heat given off by alpha particle emission makes plutonium warm to the touch in reasonable quantities; larger amounts can boil water.

All isotopes and compounds of plutonium are toxic and radioactive.

Hazards

Plutonium is radioactive. When taken in by mouth, plutonium is less poisonous (except for risk of causing cancer) than several common substances including caffeine, acetaminophen, some vitamins, pseudoephedrine, and any number of plants and fungi. It is perhaps somewhat more poisonous than pure ethanol (C₂H₅OH), but less so than tobacco; and many illegal drugs. From a purely chemical standpoint, it is about as poisonous as lead and other heavy metals.

Plutonium Compounds

Plutonium reacts readily with oxygen, forming PuO and PuO₂, as well as intermediate oxides. It reacts with the halogens, giving rise to compounds such as PuX₃ where X can be F, Cl, Br or I; PuF₄ and PuF₆ are 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.

Plutonium like other actinoids readily forms a dioxide plutonyl core (PuO₂). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH^-, NO₂^-, NO₃^-, and SO₄^-2) to form charged complexes which can be readily mobile with low affinities to soil.

PuO₂ formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO₂ which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium.

Reactions of Plutonium

Reactions with air

Plutonium reacts readily with oxygen, forming PuO and PuO₂, as well as intermediate oxides.

Reactions with halogens

It reacts with the halogens, giving rise to compounds such as PuX₃ where X can be F, Cl, Br or I; PuF₄ and PuF₆ are also seen.

Other reactions

It will react with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi₂.

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Occurrence and Production of Plutonium

Occurence

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 ²³⁹Np intermediary), which has a half-life of 24,110 years. This is also the process used to manufacture ²³⁹Pu in nuclear reactors. Some traces of ²⁴⁴Pu remain from the birth of the solar system from the waste of supernovae, because its half-life of 80 million years is fairly long.

A relatively high concentration of plutonium was discovered at the natural nuclear fission reactor in Oklo, Gabon in 1972. Since 1945, approximately 7700 kg has been released onto Earth through nuclear explosions.

Manufacture of Pu-240, Pu-241 and Pu-242

The activation cross section for ²³⁹Pu is 270 barns while the fission cross section is 747 barns for thermal neutrons. The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain bomb grade plutonium.

Manufacture of Pu-239

Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.

Manufacture of Pu-238

There are small amounts of Pu-238 in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U-235 atom captures a neutron, it is converted to an excited state of U-236. Some of the excited U-236 nuclei undergo fission, but some decay to the ground state of U-236 by emitting gamma radiation. Further neutron capture creates U-237 which has a half-life of 7 days and thus quickly decays to Np-237. Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np-237 by chemical separation of neptunium. After this chemical separation, Np-237 is again irradiated by reactor neutrons to be converted to Np-238 which decays to Pu-238 with a half-life of 2 days.

Isotopes of Plutonium

²³⁸Pu [144 neutrons]

Abundance: synthetic

Half life: 88 years [ Alpha Decay ]

Decay Energy: 5.5 MeV

Decays to ²³⁴U.

²³⁹Pu [145 neutrons]

Abundance: trace

Half life: 24.1 x 10³ years [ Alpha Decay ]

Decay Energy: 5.245 MeV

Decays to ²³⁵U.

²³⁹Pu can undergo nuclear fission if its nucleus is struck by a neutron, particularly a thermal neutron. The fission of ²³⁹Pu itself releases neutrons that bombard other ²³⁹Pu atoms, which fission and release more neutrons and so on in a nuclear chain reaction. This isotope has a positive multiplication factor (k), which means that if the metal is present in sufficient mass and with an appropriate geometry (e.g., a compressed sphere), it can form a critical mass. During fission, a tiny fraction of the nuclear material (i.e., the mass defect) is converted directly into a large amount of energy; a kilogram of ²³⁹Pu can produce an explosion equivalent to 20,000 tons of TNT. It is this energy that makes ²³⁹Pu useful in nuclear weapons and reactors.

²⁴⁰Pu [146 neutrons]

Abundance: synthetic

Half life: 6.5 x 10³ years

Decay Energy: 0.005 MeV

Decays to ²⁴⁰Am.

The presence of the isotope ²⁴⁰Pu in a material limits its nuclear bomb potential since it emits neutrons randomly, increasing the difficulty of accurately initiating the chain reaction at the desired instant and thus reducing the bomb’s reliability and power.

Plutonium is identified as either weapon grade, fuel grade, or power reactor grade based on the percentage of ²⁴⁰Pu that is contained in the plutonium. Weapons grade plutonium contains less than 7% ²⁴⁰Pu. Fuel grade plutonium contains from 7 to less than 19% percent, and power reactor grade contains from 19% and greater ²⁴⁰Pu. The isotope ²³⁸Pu is not capable of undergoing nuclear fission.

²⁴¹Pu [147 neutrons]

Abundance: synthetic

Half life: 14 years [ Alpha Decay ]

Decay Energy: 4.9 MeV

Decays to ²³⁷U.

²⁴²Pu [148 neutrons]

Abundance: synthetic

Half life: 3.73x 10⁵ years [ Alpha Decay ]

Decay Energy: 4.984 MeV

Decays to ²³⁸U.

²⁴⁴Pu [150 neutrons]

Abundance: synthetic

Half life: 8.08 x 10⁷ years [ Alpha Decay ]

Decay Energy: 4.66 MeV

Decays to ²⁴⁰U.

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