Plutonium

In 1934, Enrico Fermi and a team of scientists at the University of Rome reported that they had discovered a new element, element 94. Fermi called the element hesperium. However, the sample of material actually contained products of nuclear fission, primarily barium and krypton. Nuclear fission was unknown at the time and not discovered until 1938 (by Otto Hahn and Fritz Strassman).

Plutonium was first made in December 1940 at the University of California, Berkeley, by Glenn Seaborg, Arthur Wahl, Joseph Kennedy, and Edwin McMillan in a 60-inch cyclotron, a type of particle accelerator. They produced plutonium by bombarding uranium-238 with alpha particles (two protons and two neutrons, like helium but without the electrons). Within a couple of months, element 94 had been identified and its basic chemistry characterized. However, the discovery announcement was delayed until a year after the end of World War II because of security concerns.

McMillan had recently named the first transuranic (after uranium) element neptunium after the planet Neptune. He suggested that the new element be named for the next planet, Pluto, which was still considered a planet at the time (and remains so in our hearts).

At first, the amounts of plutonium produced were so minuscule that they were invisible to the naked eye. But by August 1942, there was enough to see—just three millionths of a gram. By 1945, enough plutonium was produced to make three atomic bombs: the Gadget, Fat Man, and Third Shot.

The Gadget was detonated in the world's first nuclear test—Trinity—on July 16, 1945, marking the beginning of the atomic age. Fat Man was one of two atomic bombs used against Japan during World War II; it was detonated above Nagasaki on August 9, 1945, killing over 40,000 people. Third Shot was another Fat Man-style bomb that was disassembled, with its plutonium core kept for testing, after Japan surrendered.

Had the war continued, the US had enough plutonium to make a new bomb about every ten days.

Plutonium is a radioactive metal with a silvery-gray appearance. It tarnishes when exposed to air and forms a dull coating with a gray, yellow, or olive-green color when oxidized. It is about as hard and brittle as cast iron, unless it is alloyed with another metal to make is soft and ductile. A large piece can feel warm to the touch. Large pieces will produce enough heat to boil water because of radioactive decay. It is highly radioactive.

Plutonium is used as an explosive ingredient in nuclear weapons and is a key material in the development of industrial nuclear reactors and nuclear power. It is also used as a power and heat source for instruments left on the Moon by Apollo astronauts, weather satellites, interplanetary probes, thermoelectric generators for spacecraft and extra-terrestrial rovers, and some medical equipment. It is also used in basic scientific research.

Plutonium can form alloys and intermediate compounds with most other metals. Gallium, aluminum, americium, scandium, and cerium can stabilize plutonium metal. Nuclear fuel pellets can be formed by alloying plutonium with various metals such as aluminum, zirconium, cerium, cerium-cobalt, uranium-titanium, uranium-zirconium, and uranium-molybdenum.

Plutonium also forms compounds with a variety of other elements. Plutonium reacts with pure hydrogen, forming plutonium hydrides. It also reacts readily with oxygen, forming PuO and PuO₂. The metal reacts with the halogens, giving rise to trivalent Pu compounds with the general formula PuX₃ where X can be F, Cl, Br, or I, and tetravalent plutonium compounds, such as PuF₄.

Plutonium metal is man-made. It has six crystal structures: alpha, beta, gamma, delta, delta prime, and epsilon. At room temperature, plutonium is in its alpha form, which is its most common structure.

Twenty isotopes have been characterized—all radioactive. The longest-lived are plutonium-244, with a half-life of 80.8 million years, plutonium-242, with a half-life of 373,300 years, and plutonium-239, with a half-life of 24,110 years. The remaining radioactive isotopes have half-lives that are less than 7,000 years.

Plutonium-244 has been found in interstellar space and has the longest half-life of any non-primordial radioisotopes. The known isotopes range from -228 to -247. Plutonium-244 is the most stable isotope.

Plutonium has the highest atomic number of any element to occur in nature. Trace quantities are found in natural uranium deposits and in the natural nuclear fission reactor in Oklo, Gabon. It is the heaviest primordial element in existence. There is also evidence of spontaneous fission of extinct plutonium in meteorites.

For the most part, plutonium is produced as a by-product in nuclear reactors. It can also be found in Earth’s atmosphere due to atmospheric weapons testing in the 1950s and 1960s.