On December 14, 1940, chemist Glenn Seaborg and his colleagues at the Berkeley Radiation Laboratory used a 60-inch cyclotron to bombard the element uranium with deuterons. Initially, the reaction produced an isotope of the element neptunium (neptunium-238), which has a two-day half-life and decayed quickly into an isotope of an element with 94 protons. Two months later, scientists would confirm that this was in fact a new element. They named it after then-planet Pluto, and their ability to create it changed the course of human history.
Plutonium is “one of the most exotic metals in the periodic table—maybe the most,” Seaborg said in an interview with the Atomic Heritage Foundation in 1965. “It undergoes change in ways that are different: in expansion, in contraction, in heating and the effect of temperature on electrical conductivity, and things of that sort are all anomalies.” Learning how to produce the element in usable quantities was a feat in and of itself; the first identifiable plutonium existed only in trace amounts. Learning how to harness and use such a volatile element has been a monumental effort that continues to this day.
The following timeline highlights notable developments in plutonium history, along with plutonium-related milestones in the history of Los Alamos National Laboratory.
December 14, 1940: Chemists Glenn Seaborg, Joseph Kennedy, Edwin McMillan, and Arthur Wahl, all of the University of California, Berkeley, use a 60-inch cyclotron to produce the (yet unnamed) isotope pluntoium-238, element 94.
February 23–24, 1941: Seaborg’s team performs the first chemical identification of the new element, confirming the discovery of plutonium. Plutonium-239, the isotope that will be of major importance for use in nuclear weapons, is discovered as the decay product of neptunium-239, produced by cyclotron neutrons.
March 28, 1941: Seaborg, Kennedy, and physicist Emilio Segrè determine the fissionability of plutonium-239 with slow neutrons. They keep the discovery a secret for fear that an adversary might use it to develop a weapon.
March 21, 1942: The element discovered before plutonium was neptunium, named after the planet Neptune, so Seaborg, Wahl, Kennedy, and McMillan propose the new element 94 be named “plutonium” after then-planet Pluto.
June 1942: Scientists at the University of Chicago’s Metallurgical Laboratory (Met Lab) begin designing the first full-scale plutonium production reactor, in support of the United States’ wartime effort to create an atomic weapon.
August 20, 1942: At the Met Lab, chemists Burris Cunningham and Louis Werner isolate an approximately 1-microgram sample of plutonium-239, making plutonium the first man-made element obtained in visible quantity.
September 10, 1942: The first weighing of plutonium is carried out by Cunningham, who measures a 2.77-microgram sample of plutonium oxide.
December 2, 1942: At the University of Chicago, physicist Enrico Fermi leads a team that achieves the world’s first self-sustaining fission chain reaction with the Pile-1 reactor at the Met Lab.
January 1, 1943: Los Alamos, New Mexico, is established as Project Y of the Manhattan Project, the U.S. government’s top-secret effort to design and build an atomic bomb—possibly by using plutonium—to help end World War II.
July 1943: The first physical experiment completed at Los Alamos is the observation of neutrons from the fissioning of plutonium-239. The neutron number is measured from an almost invisible speck of plutonium and is greater than the number of neutrons from fissioning uranium-235, which justifies the already-made decision to construct a plutonium reactor at Hanford.
October 1943: Construction begins on the B Reactor, the world’s first large-scale plutonium production reactor, at the Manhattan Project’s Hanford Site in Washington.
December 1943: The D-Building is constructed at Project Y for plutonium chemistry, metallurgy, and processing.
1943–1945: Manhattan Project scientists code-name all plutonium “49,” a combination of the end of plutonium’s atomic number, 94, and the end of the isotope plutonium-239.
1944: At the Hanford T Plant, scientists scale up the bismuth phosphate process, which is used to separate plutonium from uranium and fission products.
March 23, 1944: At Los Alamos, metallurgists Ted Magel and Nick Dallas use a centrifuge to isolate plutonium in a pure metallic form. This makes it possible to use solid plutonium in potential weapons.
April 5, 1944: Los Alamos receives the first sample of reactor-produced plutonium-239 from Oak Ridge, Tennessee. Segrè discovers plutonium-240 in the sample. Plutonium-240 spontaneously fissions at a much higher rate than plutonium-239, so the presence of this contaminant makes the Thin Man weapon design, a gun-type weapon, impossible. The spontaneous fission of plutonium-240 would have caused the plutonium to pre-detonate— to lose its explosive potential before the pieces could be brought together.
June–August 1944: Manhattan Project Director J. Robert Oppenheimer reorganizes Project Y to focus on creating an implosion-type plutonium weapon rather than a gun-type weapon. This implosion-type weapon uses high explosives to compress a plutonium core to create nuclear yield.
1944: Los Alamos researchers realize the scarcity of plutonium and build the Concrete Bowl, a 200-foot catchment, to recover plutonium if an experiment were to go wrong. A giant steel container called Jumbo was built, but never used, for the same purpose at the Trinity site.
September 26, 1944: The Hanford Site’s B Reactor goes critical at 10:48 p.m. The reactor produces plutonium239, specifically for implosion-type atomic bombs. Los Alamos receives the first shipment in February 1945.
January 24, 1945: Manhattan Project engineer Robert Henderson writes to Lieutenant Colonel Robert Lockridge, leader of the Project Y procurement group, suggesting “pit assembly” as the code phrase for the plutonium cores being developed for atomic weapons at Project Y. Over time, these cores will come to be known simply as “pits.”
April 1945: Scientists at Los Alamos create the first plutonium–gallium alloy, which stabilizes plutonium and allows it to more easily be shaped into the hemispheres that form a weapon’s pit.
July 16, 1945: The world’s first atomic device, codenamed the Gadget, is successfully tested at the Trinity site near Alamogordo, New Mexico. The Gadget employs 13 pounds of plutonium239.
August 9, 1945: The plutonium implosion bomb Fat Man is dropped on Nagasaki, Japan.
August 21, 1945: Los Alamos physicist Harry Daghlian receives a fatal dose of radiation during an accident while handling a plutonium core. He dies 25 days later. On May 21, 1946, physicist Louis Slotin is exposed to a fatal dose of radiation during an experiment accident. He dies nine days later. As a result of these criticality incidents, the types of plutonium cores involved in these accidents are nicknamed “demon cores.”
November 1945: Plutonium processing at Los Alamos shifts from the D-Building to the newly completed DP West Site, in what will later be known as Technical Area 21.
1946: The world’s first fast plutonium nuclear reactor, code-named Clementine, achieves criticality at Los Alamos. Fast nuclear reactors are more energy efficient than reactors that rely on slower-moving thermal neutrons. In developing a fast reactor, scientists hope to design a reactor whose waste products can be recycled as fuel.
June 30, 1946: The United States begins conducting atmospheric tests of nuclear weapons in the South Pacific Ocean. The first test, designed by Los Alamos, is named Crossroads-Able.
August 1, 1946: President Harry Truman signs the Atomic Energy Act, which transfers the control of atomic energy from the military to civilians. The Atomic Energy Commission is established to control nuclear materials and develop nuclear weapons.
January 27, 1951: Nuclear testing begins at the Nevada Proving Ground (later the Nevada Test Site, now the Nevada National Security Site). The first test is the Los Alamos–designed Ranger-Able test.
July 10, 1951: Ground is broken on the Rocky Flats Plant near Denver, Colorado. From 1952 to 1989, the plant will be the United States’ primary plutonium pit manufacturing facility.
November 1, 1952: The first full-scale test of a thermonuclear device, code-named Ivy Mike, leads to Los Alamos scientists’ discovery of plutonium-244 among the debris. The discovery, which demonstrates uranium-238’s capacity to absorb neutrons, will lead Lawrence Berkeley National Laboratory physicist Albert Ghioroso to seek out and discover elements 99 and 100, einsteinium and fermium, respectively, among Ivy Mike’s fallout.
December 8, 1953: President Dwight Eisenhower delivers to the United Nations General Assembly what will become known as the “Atoms for Peace” speech. In it, Eisenhower discusses the potential non-warfare uses of nuclear energy.
1954: The PUREX (plutonium uranium reduction extraction) process is employed at the Savannah River Site to separate plutonium from spent reactor fuel. This process makes it easier to purify plutonium for use in nuclear weapons and power reactors.
1957: The International Conference on Plutonium is held in Chicago, Illinois. This is followed by other plutonium conferences in 1960, 1965, 1970, and 1975. In 1997, Los Alamos revives the concept with the “Plutonium Futures—the Science” conference, held in Santa Fe, New Mexico. This is followed by 10 more, in different locations, and an 11th planned for 2022 in Avignon, France.
1958: Construction begins on the Nevada Test Site’s Nuclear Rocket Development Station, which will explore the use of atomic energy for spacecraft propulsion.
December 30, 1958: Chemical operator Cecil Kelley is exposed to a lethal dose of radiation while working with residual plutonium-239 at Los Alamos. He dies 35 hours later.
1961: Powered by plutonium-238, SNAP-3 is the first radioisotope thermoelectric generator (RTG)—a kind of nuclear battery—deployed in spacecraft. First used in U.S. Navy satellites, RTGs will power many subsequent spacecraft, including NASA’s research probes Voyager 1, Voyager 2, Ulysses, and Cassini.
1963: Mixed uranium and plutonium oxide (MOX) fuel, recovered plutonium from used reactor fuel that is mixed with uranium, is used in a Belgian reactor. MOX fuel provides a means of repurposing weapons-grade plutonium for the production of electricity.
January 17, 1966: During a mid-air refueling, a U.S. Air Force B-52 bomber collides with a tanker aircraft above the Mediterranean Sea. The B-52’s four thermonuclear bombs land near Palomares, Spain. No nuclear explosions occur, but the area is contaminated with radioactive plutonium. The incident results in increased safety measures to ensure that no nuclear yield will ever occur in the event of an accident.
1967: The Plutonium Handbook, the authoritative source on plutonium science and technology edited by metallurgist O.J. Wick, is published.
April 27, 1970: The first plutonium-powered pacemaker is implanted in a human. Nuclear pacemakers are used until the mid-1980s, when improved lithium-ion battery technology renders them obsolete.
1971: Los Alamos chemist Darleane Hoffman discovers naturally occurring plutonium-244 among a phosphate mineral deposit from the Precambrian era, a discovery that demonstrates that plutonium can be found in nature.
August 4, 1977: The Department of Energy (DOE) is created and replaces the Energy Research and Development Administration, which had replaced the Atomic Energy Commission two years earlier.
1978: Los Alamos’ state-of-the-art Plutonium Facility (PF-4) becomes fully operational.
1983: At Los Alamos, physicists James Smith and Edward Kmetko design a binary phase diagram, charting how temperature and pressure affect the atomic structure of actinides, which are the radioactive elements with atomic numbers 89 to 103. This diagram makes plutonium’s behavior more predictable for researchers and metallurgists.
1989: The FBI raids the Rocky Flats Plant amid violations of environment safety regulations. Rocky Flats stops plutonium production, and the plant closes in 1992. The site undergoes a years-long cleanup effort, after which the area is split into two areas: 1,300 acres of restricted area and the 5,237-acre Rocky Flats National Wildlife Refuge.
July 31, 1991: Soviet President Mikhail Gorbachev and U.S. President George H.W. Bush sign the Strategic Arms Reduction Treaty, which eliminates approximately half of the nuclear warheads carried by ballistic missiles.
July 13, 1992: President Bush announces that the United States will stop making fissionable materials for weapons. Plutonium production is suspended.
September 23, 1992: At the Nevada Test Site, the United States conducts its final nuclear test, Divider.
October 2, 1992: President Bush signs legislation that establishes a unilateral nine-month moratorium on U.S. nuclear testing. Bush’s successors extend this moratorium.
January 3, 1993: President Bush and Russian President Boris Yeltsin sign the Strategic Arms Reduction Treaty (START II), which reduces each nation’s arsenal of long-range nuclear weapons.
1993: DOE instructs Los Alamos to begin producing a limited number of plutonium pits.
1994: The National Defense Authorization Act, which requires a program be put in place to maintain the nation’s nuclear stockpile, results in the establishment of the Stockpile Stewardship Program.
1995: President Bill Clinton announces that the United States will remove 200 metric tons of fissile materials from its stockpile. The announcement spurs the development of Los Alamos’ Advanced Recovery and Integrated Extraction System (ARIES), which will convert surplus plutonium weapons pits to plutonium oxide—a compound of plutonium and oxygen—that can be processed into MOX fuel for use in nuclear power reactors. In 1999, the ARIES program demonstrates its feasibility, and by 2021, the ARIES program disposes of one metric ton of plutonium.
September 24, 1996: President Clinton signs the Comprehensive Nuclear-Test-Ban Treaty, which prohibits nuclear testing of any kind. As of 2021, the treaty has yet to be ratified by the U.S. Senate.
1997: The United States begins subcritical experiments at the Nevada Test Site. During these experiments, nuclear materials are tested without bringing them to the point of criticality.
1999: The Waste Isolation Pilot Plant (WIPP), near Carlsbad, New Mexico, opens. WIPP is a long-term storage facility for transuranic radioactive waste, which contains man-made elements heavier than uranium, including plutonium.
2000: The United States and the Russian Federation mutually agree to convert 34 metric tons of weapons-grade plutonium to MOX fuel before the end of 2019. But the agreement never reaches fruition. In 2015, Russia suspends the agreement, and, in 2016, the United States ends construction of its MOX fuel facility at Savannah River.
2000: Congress establishes the National Nuclear Security Administration (NNSA) to enhance national security through the military application of nuclear science. NNSA oversees Los Alamos and several other DOE laboratories, plants, and sites.
2000s: The Los Alamos Neutron Science Center (LANSCE) measures plutonium’s cross-sections, which dictate the probability that a reaction will occur under certain circumstances—in this case, the likelihood that plutonium-239 will convert to plutonium-238 when bombarded with neutrons. These measurements allow researchers to better understand how plutonium pits will perform when detonated.
2003: The Joint Actinide Shock Physics Experimental Research (JASPER) facility is used as part of the Stockpile Stewardship Program to test the effects that changing temperature and pressure can have on aging plutonium.
2000–2004: Los Alamos leads a series of experiments, in collaboration with Lawrence Livermore National Laboratory, that artificially age plutonium 60 years. The results indicate that plutonium pits change over time, which might necessitate their eventual replacement.
May 2003: Los Alamos completes Qual-1, the first nuclear weapons pit that meets or exceeds the quality of the pits produced at Rocky Flats. Unlike Rocky Flats’ pits, which were produced by rolling plutonium flat and pressing it into shape, Qual-1 is created by casting molten plutonium.
March 23, 2005: The Jason group, a panel of experts that advises the U.S. government on scientific matters, stresses the importance of quantification of margins and uncertainties (QMU) for plutonium aging and weapons systems. QMU helps those working in the post-testing era make informed risk assessments when evaluating the stockpile.
2005: A study of plutonium compounds at Lawrence Berkeley National Laboratory, conducted using scanning transmission x-ray microscopy (STXM), opens the door to further investigations using STXM. STXM helps researchers study very small quantities of substances such as plutonium dioxide, which can be produced from dismantled weapon pits and used in MOX fuel.
2006: The Jason group concludes there isn’t enough proof to support a plutonium pit aging issue.
2007: Los Alamos reaches the ability to produce 10 pits per year, a requirement handed down by NNSA and Congress.
2007: Los Alamos completes Prod-1, the first of 31 replacement pits for W88 warheads. All 31 will be delivered by 2011.
2009: NNSA designates Los Alamos National Laboratory as the Nation’s Plutonium Center of Excellence for Research and Development.
March 11, 2011: The Tohoku earthquake and tsunami begin a sequence of events resulting in the meltdowns of three nuclear reactors at the Fukushima Daiichi Nuclear Power Plant in Okuma, Japan. One of the reactors used a MOX core, and small amounts of plutonium are released from the damaged reactor. The disaster is the worst nuclear accident since the meltdown of a uranium-fueled reactor at the Soviet Chernobyl Nuclear Power Plant in 1986.
2012: Los Alamos conducts the first nuclear magnetic resonance measurement of plutonium-239. This measurement allows for the detection of plutonium’s magnetic resonance signature, which is produced when a magnetic field perturbs the element’s nucleus.
August 6, 2012: The Curiosity rover, which is powered by a plutonium heat source designed at Los Alamos, lands on Mars.
August 25, 2012: Voyager 1, launched by NASA in 1977 and powered by plutonium-238, becomes the first man-made object to reach interstellar space, the space between the Milky Way galaxy and other star systems.
November 2015: DOE and the U.S. Department of the Interior establish the Manhattan Project National Historical Park. With sites in New Mexico, Tennessee, and Washington, the park tells the story of the people, events, science, and engineering that led to the creation of the atomic bomb.
2015: A team of scientists led by Los Alamos’ Marc Janoschek confirm plutonium’s long-theorized magnetism. Because plutonium’s magnetic field is in a state of constant flux, its magnetism—a quality that arises from the interaction of its electrons—had previously been impossible to measure. This breakthrough helps explain how small changes in temperature and pressure affect plutonium’s volume.
2018: Los Alamos leads a multiagency portfolio that will include the development of Scorpius, a 20 megaelectron-volt linear accelerator, at the Nevada National Security Site. The accelerator will allow for the subcritical study of plutonium’s behavior during the final stages of implosion. Scorpius is expected to be operational by 2025.
February 2018: The Department of Defense’s Nuclear Posture Review states that by 2030, the United States will produce no fewer than 80 plutonium pits per year. The NNSA directs that 30 pits per year will be produced at Los Alamos, starting in 2026. Fifty pits per year will be produced at the Savannah River Site, starting in 2030.
2019: The Jason group releases a study that assesses plutonium pit lifetimes. In the unclassified summary, the authors “urge that pit manufacturing be re-established as expeditiously as possible in parallel with the focused program to understand Pu [plutonium] aging, to mitigate against potential risks posed by Pu aging on the stockpile.”
2019: The American Nuclear Society publishes a second edition of the Plutonium Handbook. The update, coordinated by a team at Los Alamos and edited by David Clark, David Geeson, and Robert Hanrahan, comprises seven volumes and features contributions from 215 authors from 13 countries.
2019: Los Alamos produces five research and development pits, kicking off the 30-pits-per-year mission. At the same time, Los Alamos delivers its official plan to reach that level of production.
July 30, 2020: The Perseverance rover is launched; it lands on Mars nearly six months later. The rover is powered by a Los Alamos–manufactured multi-mission radioisotope thermoelectric generator that contains plutonium-238.
August 19, 2020: Much of the Rocky Flats archive arrives in Los Alamos. The archived documents will aid the Laboratory’s pit production mission. For more, see “Raiders of the lost archive” in the summer 2020 issue of this magazine.
May 2021: Los Alamos provides eight plutonium-powered heat sources for NASA’s Dragonfly, which will fly to Titan, one of Saturn’s moons, in 2026.
July 2021: Nightshade, a series of subcritical experiments, begins at the Nevada National Security Site.
Subject matter expertise was contributed by David Clark, Franz Freibert, and Bob Putnam. Jake Bartman, Brenda Fleming, and Sierra Sweeney contributed to this feature.