What Is Plutonium?

Plutonium is a radioactive metallic element with atomic number 94, belonging to the actinide series of the periodic table. Designated by the chemical symbol Pu, it is the heaviest naturally occurring element in trace amounts, though almost all commercial and industrial quantities are produced synthetically inside nuclear reactors. First isolated in 1941 by Glenn Seaborg and colleagues at the University of California, Berkeley, plutonium became central to both nuclear weapons development and civilian nuclear power generation. Its unique combination of fissile isotopes, high energy density, and complex physical behavior makes it one of the most studied and carefully regulated materials in nuclear engineering.

Nuclear Properties and Isotopes

Plutonium has 15 known isotopes, with mass numbers ranging from 232 to 246. Among these, plutonium-239 and plutonium-241 are fissile, meaning they can sustain a neutron chain reaction when struck by slow neutrons. Plutonium-239, with a half-life of approximately 24,100 years, is produced when uranium-238 absorbs a neutron inside a reactor and then undergoes two successive beta decays. This transmutation process makes plutonium-239 a natural byproduct of conventional uranium-fueled reactors. According to the Institute for Energy and Environmental Research's plutonium factsheet, a bare sphere of pure Pu-239 metal requires roughly 10 kilograms to reach critical mass, though various geometric and reflective configurations reduce this considerably. Plutonium-240 is problematic for weapons applications because its high spontaneous fission rate generates unwanted neutrons; weapons-grade material therefore requires Pu-239 concentrations above 93 percent. Plutonium-238, with a half-life of 87.7 years, produces steady thermal output from radioactive decay, making it the preferred isotope for radioisotope thermoelectric generators used in deep-space missions.

Nuclear Fuel Applications

Plutonium plays a significant role in the civilian nuclear fuel cycle. As the World Nuclear Association reports, over one-third of the energy produced in most operating nuclear power plants already derives from plutonium, generated in situ as uranium-238 absorbs neutrons during normal reactor operation. Spent fuel contains recoverable quantities of plutonium that can be separated through chemical reprocessing and blended with depleted uranium oxide to form mixed-oxide (MOX) fuel assemblies. MOX fuel is used in light-water reactors in France, Japan, and several European countries as a strategy for reducing the inventory of separated plutonium while generating additional electricity. Fast neutron reactors can exploit plutonium even more efficiently, because all plutonium isotopes contribute to fission under fast-spectrum conditions, making these reactor designs theoretically capable of breeding more fissile material than they consume.

Physical Characteristics and Metallurgy

Plutonium metal exhibits an unusual range of physical behavior. It is one of the densest metals, with a density near 19.8 grams per cubic centimeter in its alpha phase, and its melting point of 641 degrees Celsius is low for an actinide. Uniquely, plutonium exists in six allotropic forms at ambient pressure, with density varying by as much as 25 percent across those phases. This extreme allotropic complexity makes fabrication and machining particularly demanding. Plutonium also oxidizes readily in air, forming plutonium dioxide particles that can be inhaled and represent a significant radiological hazard; finely divided plutonium metal is pyrophoric, igniting spontaneously near 150 degrees Celsius. Handling therefore requires rigorous containment in gloveboxes under inert atmospheres, and all facilities working with the material must meet regulatory requirements set by national nuclear authorities.

Applications

Plutonium has applications in a range of fields, including:

• Nuclear power generation via in-reactor fission and MOX fuel recycling
• Nuclear weapons and defense applications using weapons-grade Pu-239
• Deep-space power systems using Pu-238 in radioisotope thermoelectric generators for planetary probes
• Scientific research on actinide chemistry, nuclear criticality safety, and materials science