Plutonium (Pu)

Plutonium (symbol Pu) is the chemical element with atomic number 94: an actinide, solid at 0 °C, radioactive, with an atomic mass of 244 u. Discovered in 1940 (Glenn T. Seaborg, Edwin McMillan et al.).

Plutonium is a synthetic metal produced in nuclear reactors from uranium. Fissile plutonium-239 was used in the first nuclear test in history and in the Nagasaki bomb; today, France partly recycles it as fuel (MOX). Its cousin, plutonium-238, gives off so much heat that it supplies electricity to probes like Voyager and the Perseverance rover. Its most stable isotope, plutonium-244, has a half-life of 80 million years.

Key facts

Atomic number: 94
Symbol: Pu
French name: Plutonium
Atomic mass: 244 u
Molar mass: [244] g/mol
Family: Actinides
Position: period 7, f-block
State at 0 °C: Solid
Melting point: 639.4 °C
Boiling point: 3,231.9 °C
Density: 19.85 g/cm³
Electronegativity (Pauling): 1.28
First ionization energy: 585 kJ/mol
Atomic radius: 175 pm
Electron configuration: [Rn] 5f⁶ 7s²
Indicative price: 6.5 million $/kg (order of magnitude)
Radioactive: yes
Discovery: 1940, Glenn T. Seaborg, Edwin McMillan et al.

Origin of the name

From Pluto, the body that lies beyond Neptune.

What does plutonium look like?

A silvery metal that tarnishes to yellow and then olive green in air. It is warm to the touch from its radioactivity, and a lump of plutonium-238 can even glow red from its own heat. It is a very strange metal: it has six different crystal structures between room temperature and its melting point, and in some of them it shrinks when heated.

Where is plutonium found?

Tiny traces exist in uranium ores, where neutrons transform uranium-238. Virtually all plutonium on Earth is man-made: about 1% of spent power plant fuel is plutonium, and atomic tests have scattered a few tonnes of it into the environment.

Natural sources

Traces in uraninite

How is plutonium produced?

In a reactor, uranium-238 captures a neutron and turns into plutonium-239 within a few days. The plutonium is then separated from spent fuel by chemical reprocessing, as at the La Hague plant in France. Plutonium-238 is made separately, by irradiating neptunium-237.

Top producers

France (La Hague)
Russia
United Kingdom (stockpiles)
United States (plutonium-238)

What is plutonium used for?

MOX fuel (uranium–plutonium mix) for nuclear power plants
Nuclear weapons
Radioisotope thermoelectric generators (RTGs) for space probes
Formerly: pacemaker batteries (plutonium-238)

In everyday life

Part of France's nuclear electricity (MOX fuel)
The images sent back by the Mars rovers Curiosity and Perseverance
The signals from Voyager 1 and 2, still powered by their RTGs

Role in living things

No biological role. If inhaled, it settles in the lungs, then migrates to the bones and liver, where it stays for decades.

Hazards of plutonium

Its main danger is the inhalation of fine particles, which sharply raises the risk of lung cancer; swallowed, by contrast, it is poorly absorbed by the gut. Contrary to popular belief, it is not “the most toxic substance known,” but it demands sealed glove boxes. It also poses a criticality risk: too much plutonium in one place can set off a chain reaction.

Cosmic origin

Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.

History of plutonium

In late 1940 and early 1941, Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl produced it at Berkeley by bombarding uranium with deuterons in a cyclotron. Its discovery stayed secret until 1946. The Manhattan Project then built huge reactors at Hanford to make it, leading to the Trinity test and the Nagasaki bomb in 1945.

Did you know?

Seaborg chose the symbol “Pu” as a joke, because it sounds like “pee-yoo,” the English exclamation at a bad smell. He expected protests that never came.
The Nagasaki bomb held about 6 kg of plutonium, a sphere no bigger than a grapefruit.
The RTGs of the Voyager probes, launched in 1977, are still producing electricity nearly half a century later, more than 20 billion kilometers from Earth.
The first plutonium compound ever weighed, in 1942 in Chicago, came to only about 3 micrograms.

Isotopes of plutonium

No stable isotopes; plutonium is made in reactors from uranium. Plutonium-239 is fissile, while plutonium-238 runs hot enough to power space probes.

plutonium-238: radioactive, half-life 87.7 years. The heat source in the generators of the Voyager and Cassini probes and the Perseverance rover. The first isotope synthesized, in 1940.
plutonium-239: radioactive, half-life 24,100 years. The fissile material of nuclear weapons and MOX fuel.
plutonium-240: radioactive, half-life 6,560 years. Unwanted in weapons: its spontaneous fission can trigger a premature explosion.
plutonium-241: radioactive, half-life 14.3 years. Turns into americium-241 in spent fuel.
plutonium-244: radioactive, half-life 8 × 10⁷ years. The longest-lived; scientists search sediments for traces of it delivered by exploding stars.

Related elements

Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).