Plutonium:

Plutonium is an artificial element generates through the transmutation of ²³⁸U.  It does exist in small amounts (5 categories per trillion) in uranium ore, other than this concentration is not high sufficient to be mined commercially.

Plutonium is generated through the conversion of ²³⁸U into ²³⁹Pu according to the following reaction.

²³⁸₉₂U +¹₀n→²³⁹₉₂ U^β-→²³⁹₉₃Np^β-→²³⁹₉₄Pu

This reaction occurs in reactors designed specifically to generate fissionable fuel. These reactors are often known as breeder reactor since they generates more fissionable fuel than is used in the reaction. Plutonium is also generates in thermal ²³⁵U reactors which contain ²³⁸U.  Plutonium could be acquired through the processing of spent fuel elements. To be meaningful as a fuel, plutonium must be alloyed to be in a stable phase as a ceramic or a metal.

Plutonium dioxide (PuO₂) is the most general form used as a reactor fuel.   PuO₂ is not used alone as a reactor fuel; it is mixed along with uranium dioxide.   That mixture ranges from 20 percent plutonium dioxide for fast reactor fuel to 3 percent to 5 percent for thermal reactors.

Plutonium-239 could serve as the fissile material within both thermal and fast reactors. Within thermal reactors, a plutonium-239 generates from uranium-238 could gives a partial replacement for uranium-235. A use of plutonium-239 in fast reactors is much more economical, since breeding takes place those results in the production of more plutonium-239 after that is consumed through fission.