Plutonium

History

In 1934, Enrico Fermi and his University of Rome team thought they had found element 94, which they called hesperium, but what they actually had were fission products like barium and krypton. The real discovery of plutonium-238 came later, between December 1940 and February 1941, when Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy, and Arthur Wahl bombarded uranium with deuterons in Berkeley’s 60-inch cyclotron. That process first created neptunium-238, which then gave off beta radiation over roughly two days to reveal element 94.

The initial bombardment happened on December 14, 1940, and the new element was identified on the night of February 23–24, 1941. A paper describing the work went to Physical Review in March 1941, but wartime secrecy kept it from being published until a year after the war ended. Meanwhile, at Cambridge’s Cavendish Laboratory, Egon Bretscher and Norman Feather figured that a slow neutron reactor using uranium would yield significant plutonium-239, which would be fissile and easy to separate from uranium chemically.

McMillan had already named neptunium after Neptune, so element 94 got its name from Pluto. Seaborg briefly liked “plutium” but settled on “plutonium,” and chose the symbol “Pu” as a joke about a bad smell. Other names considered included “ultimium” and “extremium,” based on the mistaken idea that this was the last element possible.

Characteristics

Plutonium initially has a bright silvery appearance like nickel but quickly oxidizes to a dull gray, with yellow and olive green also reported. At room temperature it exists in the α allotrope, which is as hard and brittle as gray cast iron. Alloying with other metals stabilizes the δ allotrope at room temperature, making it soft and ductile.

Unlike most metals, plutonium is a poor conductor of heat and electricity. It has a low melting point of 640 °C and an unusually high boiling point of 3228 °C, giving a liquid range over 2500 kelvin wide. The low melting point and reactivity of the native metal make plutonium oxides the preferred form for nuclear fission reactor fuel such as MOX-fuel. Alpha decay is the most common radioactive decay mode; a 5 kg mass of plutonium produces about 9.68 watts of power from emitted alpha particles, making it warm to the touch.

The resistivity of plutonium at room temperature is very high for a metal and increases as temperature decreases, which is unusual. Below 100 K, resistivity rapidly drops for fresh samples but then increases with time around 20 K due to radiation damage. Self-irradiation fatigues the crystal structure, though annealing can counteract some effects above 100 K.

Applications

Plutonium-239 is a key fissile component in nuclear weapons due to its ease of fission and availability. Encasing the plutonium pit in a tamper reduces the critical mass from 16 kg to 10 kg, a sphere about 10 cm in diameter. The Fat Man bomb used explosive compression and a central neutron source, requiring only 6 kg of plutonium for a 20-kiloton yield. Spent nuclear fuel from light water reactors contains a mixture of plutonium isotopes not sufficiently enriched for weapons but usable as MOX fuel.

The PUREX process reprocesses spent fuel to extract plutonium and uranium for MOX fuel, which is used in light water reactors and consists of 60 kg of plutonium per tonne of fuel; after four years, three-quarters of the plutonium is burned. Breeder reactors are designed to create more fissionable material than they consume. Plutonium-238, with a half-life of 87.7 years, emits alpha particles and serves as a heat source in radioisotope thermoelectric generators for spacecraft.

Precautions

Plutonium poses hazards from both radioactivity and heavy metal poisoning. Its compounds are radioactive and accumulate in bone marrow. Contamination by plutonium oxide has occurred from nuclear disasters and military accidents. The decay of plutonium releases alpha, beta, and gamma radiation, with acute or long-term exposure causing radiation sickness, genetic damage, cancer, and death.

Alpha radiation cannot penetrate the outer dead layer of skin, but inhaled or ingested plutonium irradiates internal organs; alpha particles from inhaled plutonium have caused lung cancer in European nuclear workers. The skeleton and liver are at risk. Only 0.04% of plutonium oxide is absorbed after ingestion, and absorbed plutonium is excreted very slowly, with a biological half-life of 200 years. Donald Mastick accidentally swallowed a small amount of plutonium(III) chloride, which remained detectable for thirty years without apparent ill effects.

Did You Know?

• The ratio of plutonium-239 to uranium at the Cigar Lake Mine uranium deposit ranges from 2.5 × 10⁻¹² to 44 × 10⁻¹².
• U.S. Government air transport regulations permit plutonium transport by air, with restrictions on other dangerous materials and stowage in the rearmost part of the aircraft.