plutonium

American  

[ploo-toh-nee-uhm] / pluˈtoʊ ni əm /

noun

Chemistry, Physics.

1. a transuranic element with a fissile isotope of mass number 239 plutonium 239 that can be produced from non-fissile uranium 238, as in a breeder reactor. Pu; 94.

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plutonium British  

/ pluːˈtəʊnɪəm /

noun

1. a highly toxic metallic transuranic element. It occurs in trace amounts in uranium ores and is produced in a nuclear reactor by neutron bombardment of uranium-238. The most stable and important isotope, plutonium-239 , readily undergoes fission and is used as a reactor fuel in nuclear power stations and in nuclear weapons. Symbol: Pu; atomic no: 94; half-life of ²³⁹ Pu: 24 360 years; valency: 3, 4, 5, or 6; relative density (alpha modification): 19.84; melting pt: 640°C; boiling pt: 3230°C

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plutonium Scientific  

/ plo̅o̅-tō′nē-əm /

1. A silvery, radioactive metallic element of the actinide series that has the highest atomic number of all naturally occurring elements. It is found in minute amounts in uranium ores and is produced artificially by bombarding uranium with neutrons. It is absorbed by bone marrow and is highly poisonous. Plutonium is used in nuclear weapons and as a fuel in nuclear reactors. Its longest-lived isotope is Pu 244 with a half-life of 80 million years. Atomic number 94; melting point 640°C; boiling point 3,228°C; specific gravity 19.84; valence 3, 4, 5, 6.

2. See Periodic Table

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plutonium Cultural  

1. A radioactive chemical element that is artificially derived from uranium.

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Discover More

Plutonium is used in nuclear reactors.

Etymology

Origin of plutonium

1940–45; < Greek Ploútōn Pluto + -ium

Compare meaning

How does plutonium compare to similar and commonly confused words? Explore the most common comparisons:

• plutonium vs. uranium

Example Sentences

Decades ago, the U.S. chose the uranium and plutonium approaches to nuclear power at the urging of the military, which wanted to benefit from the parallel investment in civilian nuclear technology.

From The Wall Street Journal • Apr. 22, 2026

Under that program, American nuclear specialists and laboratory equipment, such as X-ray machines, scales and glove boxes for handling dangerous material, can be sent abroad to extract plutonium or highly enriched uranium.

From The Wall Street Journal • Apr. 19, 2026

After listing Oppenheimer’s wartime accomplishments, including overseeing the invention of the plutonium bomb, Rabi asked the inquisitors, “What more do you want, mermaids?”

From Los Angeles Times • Feb. 6, 2026

The site opened in the 1950s, when it was creating plutonium to be used in the core of nuclear bombs.

From BBC • Jul. 31, 2025

Over the next eighteen months, this indefatigable machine would produce two thousandths of a gram of plutonium, a quantity about the size of a grain of salt.

From "Big Science" by Michael Hiltzik