Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon, and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. Plutonium was first isolated and identified in 1940 by a team of scientists working under Glenn Seaborg at the University of California, Berkeley.
Seaborg’s team discovered plutonium while investigating what happened during neutron bombardment of uranium-238 atoms. Their work culminated in the separation of minute quantities of plutonium from uranium in 1941. They also determined that plutonium-239 has a half-life of 24,100 years—10 times longer than any previously known radioactive element—and thus may be suitable for use in nuclear weapons or reactors (which is its primary current use).
Plutonium isotopes are extremely unstable: the longest-lived isotope, plutonium-244, decays with a half-life of 80 million years; this compares with around 12 minutes for technetium-99m (the most widely used medical radioisotope), about 10^19 years for lead-212 (another common medical radioisotope with an alpha decay half life), 4.468 billion years for thorium-232 (one of the main actinide primordial nuclides), or 730 million years for uranium-235 (one of the fissionable isotopes used in nuclear reactors). All other isotopes have half lives less than 60 days except one nucleus: Pu–240 which has 95% spontaneous fission rate making it useless directly as fuel but still useful if burned as MOX fuel.
There are three naturally occurring isotopes of plutonium: Pu–238 (half life 87.7 years), Pu–239 (24100 y) and Pu–240 (6570 y); only these three have been found outside laboratories where they were produced artificially; however two more have been found inside natural uranium deposits: U–236 and U–234 both contain small amounts (~0.1%) of Pu as do thorium ores (~12 ppm). All artificial nuclei heavier than lead are highly unstable because their nuclei contain too many protons relative to neutrons leading to instability via beta decay where a proton emits an electron transforming into a neutron; this leaves too many neutrons relative to protons again so another proton will often decay soon afterwards emitting another electron etc.; eventually all decaying nuclei end up stable either as iron or lead after going through various alpha & beta decays plus some gamma emission along their way down through stability “valleys”.
The most important commercial application using plutonium is MOX fuel fabrication; mixed oxide or MOX fuel is made by blending powdered natural uranium dioxide UO2 with reprocessed weapons grade plutonium dioxide PUO2; it can then be used within light water reactors after being fabricated into pellets which are loaded into rods similar to those used for enriched Uranium oxide UOX fuel but without enrichment taking place first since Plutonium cannot be enriched like Uranium can be using gaseous diffusion methods nor centrifuges currently available; currently about 8% global electric power production comes from 1000 nuclear reactors including about 400 pressurized water reactors PWRs – two thirds using enriched Uranium oxide UOX fuel while one third use MOX giving rise to about 1200 tonnes/year demand globally for separated weapon grade Plutonium coming from civilian reactor spent Nuclear Fuel Reprocessing Plants NRPs such as Capenhurst UK & La Hague France though this could increase given China’s plans for massive increases in Nuclear Electric Power Generation NEP over coming decades – see World Nuclear Association WNA reports on projections re same