Over one third of the energy produced in most nuclear power plants comes from plutonium. It is created in the reactor as a by-product.
Plutonium has occurred naturally, but except for trace quantities it is not now found in the Earth's crust.
There are several tonnes of plutonium in our biosphere, a legacy of atmospheric weapons testing in the 1950s and 1960s.
In practical terms, there are two different kinds of plutonium to be considered: reactor-grade and weapons-grade. The first is recovered as a by-product of typical used fuel from a nuclear reactor, after the fuel has been irradiated ('burned') for about three years. The second is made specially for the military purpose, and is recovered from uranium fuel that has been irradiated for only 2-3 months in a plutonium production reactor. The two kinds differ in their isotopic composition but must both be regarded as a potential proliferation risk, and managed accordingly.
Plutonium, both that routinely made in power reactors and that from dismantled nuclear weapons, is a valuable energy source when integrated into the nuclear fuel cycle. In a conventional nuclear reactor, one kilogram of Pu-239 can produce sufficient heat to generate nearly 10 million kilowatt-hours of electricity.
Plutonium and nuclear power
Plutonium is formed in nuclear power reactors from uranium. When operating, a typical 1000 MWe nuclear power reactor contains within its uranium fuel load several hundred kilograms of plutonium. World Nuclear Association
"It may seem incredible to you who are accustomed to the idea of density in matter, and of the difficulty of ordinary light to penetrate even its crust, to know that the white fluorescent rays of plutonium or neptunium can penetrate several feet into solid lead after having become "de-electrolyzed" by their impact with niton. It is the accepted belief of the entire scientific world that radium eventually becomes harmless by "decaying" into lead. This is an impossibility of Nature for radium is male, and lead is female, in their sex divisions. Such a transition is one effect which is entirely outside of Nature." [Atomic Suicide, page 262]