Plutonium

Plutonium is a chemical element, represented by the symbol Pu and carrying the atomic number 94. This silvery-gray metal belongs to the actinide series and quickly develops a dull coating when it oxidizes in air. It normally exists in six different allotropes and can take on four oxidation states. The element reacts with carbon, halogens, nitrogen, silicon, and hydrogen. In moist air, it forms oxides and hydrides that can cause a sample to swell by up to 70% in volume; the resulting flakes are pyrophoric, meaning they can ignite spontaneously in air. Plutonium is radioactive and tends to accumulate in bones, making its handling hazardous.

The element was first synthesized and isolated between late 1940 and early 1941. Scientists at the University of California, Berkeley bombarded uranium-238 with deuterons in a 1.5-meter cyclotron. This produced neptunium-238, which has a half-life of 2.1 days and then beta-decayed into the new element with atomic number 94 and an atomic weight of 238 (half-life 88 years). Because uranium was named after the planet Uranus and neptunium after Neptune, element 94 was named after Pluto, which was then considered a planet. Due to wartime secrecy, the discovery was not published until 1948.

Plutonium is the element with the highest atomic number that occurs naturally. Trace amounts appear in natural uranium deposits when uranium-238 captures neutrons released by the decay of other uranium-238 atoms. The heavy isotope plutonium-244 has a half-life long enough that extremely small quantities from the Earth's formation might still exist, but experiments have not yet been sensitive enough to detect them.

Both plutonium-239 and plutonium-241 are fissile, capable of sustaining a nuclear chain reaction, which makes them useful in nuclear weapons and reactors. Plutonium-240 has a high rate of spontaneous fission, raising the neutron flux of any sample containing it. This limits a sample's suitability for weapons or as reactor fuel, and the percentage of plutonium-240 determines its grade: weapons-grade, fuel-grade, or reactor-grade. Plutonium-238, with a half-life of 87.7 years, emits alpha particles and serves as a heat source in radioisotope thermoelectric generators, which power some spacecraft. Separating plutonium isotopes is expensive and inconvenient, so specific isotopes are usually produced in specialized reactors.

Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II, which developed the first atomic bombs. The Fat Man bombs used in the Trinity nuclear test in July 1945 and the bombing of Nagasaki in August 1945 had plutonium cores. After the war, human radiation experiments studying plutonium were conducted without informed consent, and several criticality accidents, some lethal, occurred. Disposing of plutonium waste from nuclear power plants and dismantled Cold War nuclear weapons raises concerns about nuclear proliferation and environmental harm. Other sources of plutonium in the environment include fallout from many above-ground nuclear tests, which are now banned.

**Characteristics**

**Physical properties** Plutonium initially has a bright, silvery appearance similar to nickel, but it oxidizes quickly to a dull gray; yellow and olive green have also been reported. At room temperature, it exists in its alpha form, which is about as hard and brittle as gray cast iron. When alloyed with other metals, the high-temperature delta allotrope becomes stable at room temperature, making the material soft and ductile. Unlike most metals, plutonium is a poor conductor of heat and electricity. It has a low melting point of 640 °C (1,184 °F) and an unusually high boiling point of 3,228 °C (5,842 °F), giving it a liquid range over 2,500 kelvin wide. This range is not the greatest among all actinides or all metals—neptunium is theorized to have the widest range in both cases. Because the native metal has a low melting point and is reactive compared to its oxide, plutonium oxides are preferred for applications like nuclear fission reactor fuel (MOX fuel).

Alpha decay, which releases a high-energy helium nucleus, is the most common radioactive decay mode for plutonium. A 5 kg mass of plutonium-239 contains about 12.5 × 10²⁴ atoms. With a half-life of 24,100 years, roughly 11.5 × 10¹² of its atoms decay each second, each emitting a 5.157 MeV alpha particle. This produces about 9.68 watts of power. The heat from the deceleration of these alpha particles makes the metal warm to the touch. Plutonium-238, with its much shorter half-life, heats up to much higher temperatures and glows red hot with blackbody radiation if left without external heating or cooling. This heat is used in radioisotope thermoelectric generators.

At room temperature, plutonium has very high electrical resistivity for a metal, and this resistivity increases as temperature drops—an unusual behavior. This trend continues down to 100 K, below which resistivity rapidly decreases in fresh samples. Around 20 K, resistivity then begins to increase over time due to radiation damage, with the rate depending on the sample's isotopic composition.

Because of self-irradiation, a plutonium sample fatigues throughout its crystal structure, meaning the ordered arrangement of atoms becomes disrupted over time. Self-irradiation can also cause annealing, which counteracts some of the fatigue effects as the temperature rises above 100 K.

Unlike most materials, plutonium becomes denser when it melts—by 2.5%—but the liquid metal's density decreases linearly with temperature. Near its melting point, liquid plutonium has very high viscosity and surface tension compared to other metals.