Plutonium
Plutonium, the element with atomic number 94 and the symbol Pu, is warm to the touch. A 5 kilogram mass of its main fissile isotope gives off about 9.68 watts, because the atoms keep decaying and the slowed alpha particles turn into heat. That quiet warmth is the least strange thing about it.
No one had made this element before the last months of 1940, and within five years it sat at the center of a bomb. Why does a metal that melts at a modest temperature defeat the people who try to machine it? What did scientists do to hospital patients to learn how dangerous it is? And what is anyone supposed to do now with stockpiles measured in hundreds of tonnes?
Plutonium normally comes in six solid forms, called allotropes, and a seventh appears at high temperature within a narrow range of pressures. Allotropes are different crystal arrangements of the same element. In plutonium they carry almost identical internal energies but very different densities, from 16.00 to 19.86 grams per cubic centimetre. A small change in temperature, pressure or chemistry can therefore swell or shrink a piece of the metal dramatically. Nobody fully understands why its phase diagram is so tangled.
The alpha form is the one found on a bench at room temperature. It is about as hard and brittle as gray cast iron, and its low-symmetry monoclinic structure also makes it a poor conductor of heat. Warm it slightly and it turns into the plastic, malleable beta form. That eagerness to change state is what makes machining so difficult.
Metalworkers get around the problem by alloying. Add a small percentage of gallium, aluminium or cerium and the delta form, normally stable only between 310 and 452 degrees Celsius, stays put at room temperature. Delta is roughly as strong and malleable as aluminium, and it can be welded. The gallium alloy is used mainly in the cores of implosion bombs.
The melting point is 640 degrees Celsius, while the boiling point is an unusually high 3228. That leaves the metal liquid across a range wider than 2,500 kelvin. Most materials grow less dense as they melt, but plutonium becomes 2.5% denser. Unlike most metals, it also conducts heat and electricity badly. Its own radiation slowly fatigues the crystal structure from the inside, disrupting the orderly arrangement of atoms.
Behind all of this lies its electronic structure. The 5f electrons in plutonium sit right on the border between roaming freely and staying bound to their own atoms. The energy gap between the 6d and 5f subshells is tiny, so several electron configurations compete at nearly equal energy. That competition makes plutonium one of the most complex elements known.
One alloy of plutonium, gallium and cobalt even behaves as an unconventional superconductor below 18.5 kelvin, far warmer than the best heavy fermion systems.
In 1934, Enrico Fermi and a team at the University of Rome reported that they had found element 94. Fermi called it hesperium and mentioned it in his Nobel Lecture in 1938. The claim was wrong. The sample actually held fission products, mainly barium and krypton, and fission had not yet been discovered. Otto Hahn and Fritz Strassmann would find it in Germany in 1938.
The real discovery came at Berkeley between December 1940 and February 1941. Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy and Arthur Wahl fired deuterons, the nuclei of heavy hydrogen, at uranium in a cyclotron. The first bombardment took place on the 14th of December 1940. It created neptunium-238, which decayed by beta emission with a half-life of a little over two days. That decay showed that element 94 was forming.
The team identified the new element through oxidation on the night of the 23rd to the 24th of February 1941. The isotope was plutonium-238, with a half-life of about 88 years. A paper went to the journal Physical Review in March 1941, but wartime security kept it from print for years.
At the Cavendish Laboratory in Cambridge, Egon Bretscher and Norman Feather were thinking along parallel lines. They realized that a slow neutron reactor fuelled with uranium would produce plutonium-239 as a by-product. They calculated that it would be fissile. Because it differs chemically from uranium, it could also be separated from it easily. Bretscher, working on the British Tube Alloys project, had predicted the underlying reaction theoretically in 1940.
McMillan had recently named the first transuranic element neptunium, after the planet Neptune, just as uranium had been named for Uranus. He suggested that element 94 take the name of the next planet, Pluto, which was then counted as a planet. Nicholas Kemmer of the Cambridge team proposed the same name independently, with the same reasoning.
Seaborg first considered the name plutium but decided it did not sound as good as plutonium. He chose the symbol Pu as a joke, a nod to the interjection P U for an especially disgusting smell. It passed into the periodic table without anyone noticing. He and others also weighed the names ultimium and extremium, because they wrongly believed they had found the last possible element.
In Berlin, Hahn and Strassmann, and independently Kurt Starke, were also working on transuranic elements. They reported element 93 in 1942 but never pursued element 94, probably discouraged by the earlier failure of McMillan and Abelson to isolate it. Hahn's group had access to the stronger cyclotron in Paris. Had they tried, they could probably have detected plutonium, though only a few becquerels of it.
On the 20th of August 1942, chemists at the secret Metallurgical Laboratory in Chicago isolated about 1 microgram of plutonium-239. The batch had held roughly 50 micrograms, mixed with uranium and fission products. Even that speck let them determine the new element's atomic weight.
On the 2nd of December 1942, researchers led by Enrico Fermi achieved the first self-sustaining chain reaction. They did it in a stack of graphite and uranium known as CP-1, built on a racket court under the west grandstand of the University of Chicago's Stagg Field. Lessons from CP-1 guided DuPont as it built X-10, an air-cooled experimental reactor at Oak Ridge, with a pilot separation plant beside it. That plant used methods developed by Seaborg and his Chicago team to pull plutonium out of irradiated uranium.
In November 1943, some plutonium trifluoride was reduced to make the first sample of plutonium metal. It was a few micrograms of tiny beads, yet it was enough to make plutonium the first synthetic element visible to the unaided eye.
The water-cooled production reactors at Hanford came next, designed by Chicago scientists using what CP-1 had taught them. Construction began in mid-1943. In January 1944, workers laid the foundations of T Plant, the first chemical separation building. The men who built these structures nicknamed them Queen Mary. Each was 800 feet long, 65 feet wide and 80 feet high, and held forty process pools.
Inside, operators worked a gallery above the pools, behind seven feet of concrete. They steered remote control equipment while watching through television monitors and periscopes. The B Reactor was the first industrial-sized reactor built to produce material, and it made the fissile material for the plutonium weapons used in the war. Los Alamos received its first Hanford plutonium on the 2nd of February 1945. Only two years earlier, Colonel Franklin Matthias had set up a temporary headquarters on the banks of the Columbia River.
In 2004, excavators digging a burial trench at Hanford found a safe. Inside was a large glass bottle of whitish slurry, later identified as the oldest known sample of weapons-grade plutonium. Isotope analysis showed it had been made in the Oak Ridge X-10 reactor during 1944.
On the 5th of April 1944, Emilio Segrè at Los Alamos received the first sample of reactor-produced plutonium from Oak Ridge. Within ten days he discovered that it contained more plutonium-240 than the cyclotron-made samples did. That isotope fissions spontaneously at a high rate, which raises the background level of neutrons in the metal.
The consequence was severe. The original gun-type plutonium weapon, code-named Thin Man, had to be abandoned, because the extra neutrons made a premature chain reaction, a fizzle, likely. The whole design effort at Los Alamos switched to a more complicated implosion device, code-named Fat Man. In it, explosive lenses squeeze the plutonium to high density. That is a harder engineering task than a gun-type bomb, but a plutonium bomb required it. Uranium, by contrast, can be used with either method.
The first atomic bomb test, code-named Trinity, was detonated on the 16th of July 1945 near Alamogordo, New Mexico. Its device, called Gadget, used explosive lenses to compress a sphere of plutonium into a supercritical mass. At the same moment, an initiator called Urchin, made of polonium and beryllium, showered the sphere with neutrons.
The weapon weighed over 4 tonnes but held just 6 kg of plutonium. About 20% of that plutonium fissioned, releasing energy equivalent to about 20,000 tons of TNT.
An identical design was dropped on Nagasaki, Japan, on the 9th of August 1945. The bomb killed 35,000-40,000 people and destroyed 68%-80% of the city's war production. Only after the first atomic bombs were announced did the public learn that plutonium existed, through the Manhattan Project's Smyth Report.
Wrapping the core in a tamper, a layer of dense material that reflects neutrons back inward, cuts the critical mass from 16 kg to 10 kg, a sphere about 10 cm across. Designers say that with very sophisticated assembly, as little as 4 kg might one day suffice.
Metallic plutonium can burn, especially when finely divided. In moist air its surface forms hydrides that may ignite at room temperature. As the metal oxidizes it expands by up to 70% in volume, enough to split its container. Magnesium oxide sand is probably the best way to put out a plutonium fire, since it both cools the burning metal and blocks the oxygen. Storage and handling generally require a dry, inert gas atmosphere.
A second hazard is accidental criticality. At Los Alamos on the 21st of August 1945, careless handling of tungsten carbide bricks around a 6.2 kg plutonium sphere gave scientist Harry Daghlian a fatal dose. He received an estimated 5.1 sievert and died 25 days later.
Nine months later, another Los Alamos scientist, Louis Slotin, died in a similar accident. It involved a beryllium reflector and the same plutonium core, now known as the demon core. In December 1958, while plutonium was being purified at Los Alamos, a critical mass formed in a mixing vessel and killed the chemical operator Cecil Kelley. Plutonium in solution is more likely to go critical than the solid, because hydrogen in the water slows neutrons.
The poison itself works in two ways, as radioactivity and as a heavy metal. Alpha radiation cannot cross the dead outer layer of human skin. Inside the body it is a different matter, and the skeleton, where plutonium accumulates, and the liver, where it concentrates, are at risk. The body absorbs only 0.04% of ingested plutonium oxide, and absorbed plutonium leaves with a biological half-life of 200 years.
Donald Mastick once swallowed a small amount of plutonium(III) chloride. It stayed detectable for the next thirty years of his life, yet he appeared to suffer no ill effects. Inhalation is worse. The United States Department of Energy estimates that breathing in 5,000 particles, each about 3 micrometres wide, adds 1% to a person's lifetime cancer risk.
A commonly cited quote from Ralph Nader holds that a pound of plutonium dust spread in the air could kill 8 billion people. Bernard Cohen disputed that and estimated no more than 2 million deaths, putting plutonium roughly on a par with nerve gas. He also pointed to about 25 Los Alamos workers who inhaled considerable plutonium dust in the 1940s, none of whom developed lung cancer. No human being is known to have died from inhaling or swallowing plutonium.
Much of what scientists first learned about its effects on the body, however, came from experiments on people who never agreed to take part.
In 1944, Joseph G. Hamilton at the University of California's Radiation Laboratory began giving rats soluble microgram portions of plutonium-239 compounds. He varied the chemical state of the metal and the method, including oral and intravenous doses. He wanted to know how plutonium spread through the body, how long it stayed and where it lodged. The Chicago laboratory later ran its own injections on mice, rabbits, fish and even dogs.
The results alarmed them. Plutonium behaved quite differently from radium, with significant deposits in the liver and in the actively metabolizing part of bone. Excretion rates varied between species by as much as a factor of five, which made predicting the rate for humans extremely hard. Animal studies also showed that a few milligrams per kilogram of tissue is a lethal dose.
Human testing followed. Doctors injected solutions that typically held 5 micrograms of plutonium into hospital patients believed to be terminally ill, or to have under ten years left to live. After animal work showed that plutonium settles in bone more dangerously than radium, the dose dropped to 1 microgram in July 1945. Journalist Eileen Welsome says most of the subjects were poor, powerless and sick.
In 1945-47, eighteen people were injected without informed consent. The aim was to build diagnostic tools for measuring plutonium uptake and then set safety standards for workers. One of them was Ebb Cade, who was injected with 4.7 micrograms of plutonium on the 10th of April 1945 at Oak Ridge, Tennessee. Harold Hodge supervised the experiment, and Cade was an unwilling participant.
Experiments directed by the Atomic Energy Commission and the Manhattan Project continued into the 1970s. A book called The Plutonium Files later named the people involved and described the secret research. The episode is now regarded as a serious breach of medical ethics and of the Hippocratic Oath.
The government hid most of this until 1993, when President Bill Clinton ordered a change of policy and agencies released the records. The Advisory Committee on Human Radiation Experiments investigated. Its controversial 1995 report said that wrongs were committed, but it stopped short of condemning those who committed them.
The twin Voyager spacecraft were launched in 1977, each carrying a plutonium power source rated at 500 watts. More than thirty years later, each still produced about 300 watts, enough for limited operation. They run on plutonium-238, which has a half-life of 87.74 years and gives off a large amount of heat. One kilogram yields about 570 watts. Because it emits alpha particles, a sheet of paper is enough to shield against them.
Those traits suit machines that must run unattended for roughly a human lifetime. Plutonium-238 has powered generators and heater units aboard Cassini, Galileo, New Horizons and both Voyagers, as well as the Curiosity and Perseverance rovers. Five Apollo Lunar Surface Experiment Packages used an earlier version, starting with Apollo 12 in 1969. It also drove heart pacemakers, though lithium cells largely replaced it, and by the end of 2007 only nine plutonium-powered pacemakers were reported still working.
Spent reactor fuel offers another use. The PUREX process extracts plutonium and uranium so they can be blended into mixed oxide fuel, called MOX, which has been used since the 1980s and is widely used in Europe. MOX fuel carries 60 kg of plutonium per tonne, and after four years three-quarters of that plutonium has been burned.
The Cold War left far larger quantities. The reactors at Hanford and the Savannah River Site in South Carolina produced 103 tonnes of military plutonium, while the Soviet Union produced an estimated 170 tonnes. In 2007, the research institute SIPRI put the world stockpile at about 500 tonnes, split equally between weapon and civilian stocks.
The pollution was heavy. According to Kate Brown, over four decades both Hanford and Mayak in Russia released more than 200 million curies of radioactive isotopes, twice the amount expelled at Chernobyl in each case. At the Rocky Flats Plant, plutonium fires in 1957 and 1969 sent contamination south and east toward populated areas near Denver. The public did not hear of it until the 1970s. A combined FBI and EPA raid in 1989 and years of protest ended weapons production there.
Disposal remains unsettled. Some plutonium from dismantled weapons is melted into two-tonne glass logs of plutonium oxide, made with borosilicates, cadmium and gadolinium. The plan was to seal them in stainless steel and bury them in boreholes at Yucca Mountain, about 100 miles north-east of Las Vegas. On the 5th of March 2009, Energy Secretary Steven Chu told a Senate hearing that the site was no longer viewed as an option. Since 1999, military-generated waste has been entombed at the Waste Isolation Pilot Plant in New Mexico.
Fallout adds a thin layer everywhere. About 550 atmospheric and underwater nuclear tests, plus a few major accidents, leave minute traces of plutonium in most human bodies. The Limited Test Ban Treaty of 1963 stopped most such testing. France kept testing in the atmosphere until 1974 and China until 1980.
Nature makes a little of its own. Small traces occur in rich uranium ores, such as the natural nuclear reactor at Oklo in Gabon. Evidence of extinct plutonium-244 turns up in meteorites, and tiny amounts of live interstellar plutonium-244 have been found on the deep sea floor. Beyond Earth, astronomers have detected plutonium in the spectrum of the chemically peculiar Przybylski's Star.
Updated
Common questions
Who discovered plutonium and when?
Plutonium was first produced and chemically identified between December 1940 and February 1941 by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy and Arthur Wahl at Berkeley. The first bombardment took place on the 14th of December 1940.
Why is plutonium named after Pluto?
Edwin McMillan had named neptunium after Neptune, so he suggested element 94 take the name of the next planet, Pluto, then counted as a planet. Nicholas Kemmer of the Cambridge team independently proposed the same name.
Why does plutonium symbol Pu come from a joke?
Glenn Seaborg chose the letters Pu as a joke, referring to the interjection P U for an especially disgusting smell. The choice passed into the periodic table without anyone noticing.
How was plutonium used in the first atomic bombs?
Plutonium was the fissile core of the Trinity test device, detonated on the 16th of July 1945, and of the Fat Man bomb dropped on Nagasaki on the 9th of August 1945. The Trinity weapon held 6 kg of plutonium, and about 20% of it fissioned.
Why is plutonium so hard to machine?
Plutonium has six allotropes that change state very readily, with densities from 16.00 to 19.86 grams per cubic centimetre. The room-temperature alpha form is as hard and brittle as gray cast iron, and gallium alloying stabilizes the softer delta form.
What is plutonium-238 used for?
Plutonium-238 is used as a heat source in radioisotope thermoelectric generators for spacecraft such as Cassini, Voyager, Galileo and New Horizons, and for the Curiosity and Perseverance rovers. One kilogram produces about 570 watts of heat.
Is plutonium dangerous to humans?
Plutonium is dangerous because it is radioactive and accumulates in the skeleton and liver, with a biological half-life of 200 years. Inhaling it is more hazardous than swallowing it, though no human is known to have died from inhaling or ingesting it.
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- 145BookThe Encyclopedia of the Chemical ElementsWilliam N. Miner et al. — Reinhold Book Corporation — 1968
- 146BookNuclear forensic analysisKenton James Moody et al. — CRC Press — 2005
- 147BookThe Making of the Atomic BombRichard Rhodes — Simon & Schuster — 1986
- 148BookAdventures in the Atomic Age: From Watts to WashingtonG. T. Seaborg et al. — Farrar, Straus and Giroux — 2001
- 149BookNuke-Rebuke: Writers & Artists Against Nuclear Energy & WeaponsMorty Sklar — The Spirit That Moves Us Press — 1984
- 150BookSIPRI Yearbook 2007: Armaments, Disarmament, and International SecurityStockholm International Peace Research Institute — Oxford University Press — 2007
- 151BookPlentiful Energy: The Story of the Integral Fast Reactor, the Complex History of a Simple Reactor Technology, with Emphasis on Its Scientific Basis for Non-specialistsC. E. Till et al. — Charles E. Till and Yoon Il Chang — 2011
- 152BookHistory of 100-B AreaR. K. Wahlen — Westinghouse Hanford Company — 1989
- 153BookThe Plutonium Files: America's Secret Medical Experiments in the Cold WarEileen Welsome — Random House — 2000