Small pieces of plutonium. (Source: Wikipedia.org)

A chemical element with atomic number 94. Plutonium is a poisonous, radioactive silver-gray metal from the transuranic group. It is found in trace amounts in nature, where it is formed by neutron capture in natural uranium. It can be produced in large quantities in breeder reactors. Plutonium isotope ²³⁸Pu is fissile and can be used as a nuclear fuel as the MOX compound. Because it generates a lot of heat during alpha decay, it is used in thermoelectric generators. These supply electricity to, for example, space probes.