Plutonium

Plutonium, element 94, may be the most physically and chemically complicated metal ever studied in bulk. Its electron configuration [Rn] 7s2 5f6 places six electrons in the 5f subshell, close to the half-filled seven-electron configuration that would be especially stable, giving plutonium a set of nearly degenerate electronic states that produce remarkable consequences: the metal exists in six distinct allotropic phases between room temperature and its melting point, some expanding and some contracting as it heats, making it one of very few materials that shrinks on melting. The most important isotope, Pu-239, has a half-life of 24,100 years — long enough to persist across thousands of years of geological storage but short enough to make even gram quantities intensely radioactive and warm to the touch from decay heat. Named after Pluto, the outermost planet known when it was discovered, plutonium carries both immense destructive potential and genuine utility in space exploration.

Plutonium-239 is the primary fissile material in most modern nuclear warheads, where its critical mass is far smaller than that of U-235 and its production from reactor-irradiated uranium is efficient. Beyond weapons, plutonium-238 — a different isotope produced by neutron irradiation of neptunium-237 — is the heat source in radioisotope thermoelectric generators that power spacecraft unable to use solar panels. The 87.7-year half-life of Pu-238 and its high alpha-emission power density of about 0.54 watts per gram make it ideal for missions lasting decades. Cassini, New Horizons, and the Curiosity and Perseverance Mars rovers all rely on Pu-238 RTGs. Mixed oxide fuel (MOX), which blends plutonium dioxide with uranium dioxide, is used in commercial nuclear reactors in France, Japan, and elsewhere to burn down stockpiles of reactor-grade plutonium recovered from spent fuel reprocessing. Plutonium-238 heat sources are also used in cardiac pacemakers implanted in patients in the 1970s, some of which are still functioning.

Glenn T. Seaborg, Edwin McMillan, Joseph W. Kennedy, and Arthur C. Wahl synthesized plutonium at the University of California, Berkeley in December 1940, bombarding uranium-238 with deuterons in the 60-inch cyclotron. They produced neptunium-238, which beta-decayed to plutonium-238. The discovery was kept secret during World War II as part of the Manhattan Project. Pu-239 was identified as fissile in early 1941, making it a candidate weapon material. Enrico Fermi's team produced the first microgram quantities of plutonium in the Chicago Pile-1 reactor in 1942. The Hanford Site in Washington State was built specifically to produce kilogram quantities of Pu-239 in large production reactors; the plutonium it yielded powered the Trinity test device and the Fat Man bomb dropped on Nagasaki in August 1945. Seaborg named the element plutonium, following the Neptune-Pluto planetary sequence. He received the Nobel Prize in Chemistry in 1951 for his work on transuranium elements.

Plutonium is essentially nonexistent in nature under current conditions. Trace quantities of Pu-239 — on the order of one part per 10 to the 11 — have been detected in uranium ores from the Oklo natural nuclear reactor in Gabon, where conditions about 2 billion years ago briefly sustained a natural fission chain reaction, producing plutonium in situ. Outside such extraordinary environments, the half-life of Pu-244 (the longest-lived plutonium isotope at 81 million years) is short enough that any plutonium present when Earth formed has long since decayed away. The global inventory of plutonium today is almost entirely anthropogenic, produced in the roughly 500 nuclear reactor-years of operation worldwide and in weapons testing. Atmospheric nuclear tests between 1945 and 1980 dispersed about 4 tonnes of plutonium into the global environment as fine oxide particles, traces of which are still detectable in soil samples worldwide and serve as stratigraphic markers for the start of the Anthropocene.

Plutonium forms a rich set of compounds across its +3, +4, +5, +6, and +7 oxidation states, though +4 and +6 are most common. Plutonium dioxide (PuO2) is the most stable and widely handled compound — a black, high-melting ceramic used in MOX fuel fabrication and as the feedstock for most plutonium processing. Plutonium tetrafluoride (PuF4) is the precursor used to reduce plutonium to metal by reaction with calcium or barium. Plutonium hexafluoride (PuF6), analogous to UF6, is a volatile compound studied in isotope separation contexts. In aqueous solution, the plutonyl(VI) ion PuO22+ resembles the uranium(VI) species UO22+ and is highly soluble, making Pu(VI) a concern for groundwater mobility near waste sites. Plutonium nitrate solutions are the standard form in reprocessing operations, where the PUREX (Plutonium Uranium Reduction EXtraction) solvent-extraction process separates plutonium from uranium and fission products using tributyl phosphate in a hydrocarbon diluent.

• A sphere of weapons-grade plutonium about 10 centimeters in diameter — roughly the size of a large orange — contains enough fissile material for a nuclear weapon, yet that same sphere sits warm to the touch from the heat of its own radioactive decay.
• Plutonium metal is so mechanically and dimensionally quirky — contracting when it melts, expanding and contracting through phase transitions as it heats — that early metallurgists at Los Alamos described it as 'the most complex metal in the periodic table.'
• Some patients implanted with plutonium-238 powered cardiac pacemakers in the 1970s are still alive, their devices generating electricity from radioactive decay heat after more than 50 years.
• The Pu-239 detonated at the Trinity test in July 1945 was produced by just a few months of reactor operation at Hanford — demonstrating how rapidly a newly discovered element could be industrialized when military urgency drove the effort.
• Atmospheric weapons testing spread detectable traces of plutonium oxide particles into soils worldwide; the global layer of anthropogenic plutonium in sediment cores is now used as a geologic marker for the beginning of the Anthropocene epoch.