What Is Plutonium?
QUICK ANSWER
Plutonium is a chemical element with atomic number 94 and the symbol Pu. It's a radioactive, silvery-gray metal that's mostly created artificially in nuclear reactors. Plutonium is famous as a key material in nuclear weapons and is also used to fuel some nuclear reactors and power deep-space probes.
Plutonium is one of the most dangerous elements known to science and the second human-made element after neptunium. It barely exists in nature, but humans have produced about 500 metric tons of it since 1940, mostly for nuclear weapons. The same element that powered the bomb dropped on Nagasaki also powers spacecraft heading to the outer solar system, where solar panels won't work.
Where is plutonium on the periodic table?
Plutonium has atomic number 94, the symbol Pu, and sits in the actinide series at the bottom of the periodic table. It was discovered in 1940 by Glenn Seaborg and colleagues at the University of California, Berkeley, the first transuranic element created in significant quantities. Plutonium was named after the dwarf planet Pluto, continuing the pattern set by uranium (Uranus) and neptunium (Neptune). The atomic mass varies by isotope; Pu-239 is the most important isotope for both weapons and reactor fuel, with a half-life of 24,100 years.
What are the properties of plutonium?
Plutonium is a silvery-gray metal that tarnishes to dull yellow-brown in air. It has six different solid phases, each with different densities, the most of any element, which makes plutonium metallurgy unusually difficult. The metal is unusual in that it's a poor conductor of heat and electricity. Density at room temperature is 19.86 g/cm³, similar to gold. The melting point is 639°C. Plutonium is pyrophoric in finely divided form, meaning small particles can spontaneously ignite in air, and it's chemically reactive, forming compounds with oxygen, halogens, and many other elements.
How is plutonium made?
Trace amounts of plutonium exist in nature from natural reactions in uranium ores, but essentially all plutonium is produced artificially in nuclear reactors. When uranium-238 absorbs a neutron in a reactor, it briefly becomes uranium-239, which decays through neptunium-239 into plutonium-239. Reprocessing of spent reactor fuel separates plutonium from other byproducts. The United States and several other countries produced military plutonium during the Cold War, and reactors continue to produce smaller amounts as byproducts. Global stockpiles include hundreds of tons of weapons-grade material today.
What is plutonium used for?
Nuclear weapons are the most consequential historical use. A plutonium core implodes to reach critical mass and explode through fission. Some commercial nuclear reactors use mixed-oxide (MOX) fuel containing plutonium recycled from spent fuel. Radioisotope thermoelectric generators (RTGs) use plutonium-238 to generate electricity for deep-space missions like Voyager, the Curiosity rover, and New Horizons through heat from radioactive decay. Pacemakers historically used small plutonium sources, though this has been discontinued. Almost no civilian applications exist outside specialized nuclear and space programs.
Plutonium is the element humans made to power both their most destructive weapons and their farthest-reaching spacecraft. The same atom that ended World War II is now generating electricity for probes exploring Pluto's neighborhood. Few elements have such consequential and contradictory legacies.
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