Plutonium

Plutonium was not discovered; it was manufactured. Unlike the elements found in nature, element 94 had to be created atom by atom in a cyclotron, identified through painstaking chemistry, and then scaled to kilogram quantities in the largest industrial project ever attempted. The adjacent possible had opened just eighteen months earlier, when Otto Hahn, Lise Meitner, and Fritz Strassmann announced nuclear fission in uranium. From that moment, the path to plutonium became inevitable.

The discovery unfolded at the University of California, Berkeley, home of the 60-inch cyclotron—the most powerful particle accelerator on Earth. In 1940, physicist Edwin McMillan bombarded uranium with neutrons and found a new element, number 93, which he named neptunium. McMillan predicted that neptunium would undergo beta decay to produce element 94, but before he could confirm this, he was called away to war work on radar.

Glenn Seaborg, a young chemist, took over McMillan's project. On December 14, 1940, Seaborg's team—including Joseph Kennedy, Arthur Wahl, and Emilio Segrè—began bombarding uranium with deuterons in the cyclotron. On the night of February 23-24, 1941, they chemically identified a new element through oxidation reactions. They had created plutonium-238.

Within weeks, they produced a more significant isotope: plutonium-239. Tests showed it was fissionable with slow neutrons—meaning it could sustain a chain reaction and power an atomic bomb. Until that moment, only uranium-235, which constitutes less than 1% of natural uranium and requires laborious separation, could serve this purpose. Plutonium could be bred from abundant uranium-238 in a nuclear reactor, then chemically separated. This changed everything.

The publication was suppressed until 1946. What had been a scientific discovery became a military secret, feeding directly into the Manhattan Project. In April 1942, Seaborg moved to the University of Chicago's Metallurgical Laboratory, where his team developed the chemical processes to extract microscopic quantities of plutonium from irradiated uranium. Enrico Fermi's reactor—the first controlled nuclear chain reaction on December 2, 1942—produced the plutonium that Seaborg's chemists would extract.

The industrial scale-up remains staggering. The Hanford Site in Washington state built reactors and separation plants covering 586 square miles—the size of a small city—to produce the plutonium for the atomic bomb. The processes Seaborg developed at the laboratory bench were translated into massive chemical plants within three years. On August 9, 1945, a plutonium bomb destroyed Nagasaki.

Seaborg and McMillan shared the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements." Seaborg would go on to discover nine more elements, more than any other scientist in history. But plutonium remained his most consequential creation—the element that demonstrated humanity could synthesize nature's building blocks and, in doing so, reshape geopolitics permanently.

The cascade from plutonium extends to nuclear power, where plutonium bred in reactors provides fuel, and to space exploration, where plutonium-238's radioactive decay powers spacecraft like Voyager and Curiosity. The element that emerged from a Berkeley cyclotron in 1940 now orbits distant planets.