Plutonium is a chemical element heavier than uranium that does not exist in Nature. All plutonium isotopes are radioactive. The main and best known isotope is plutonium-239. Plutonium-239 is manufactured in reactors from uranium-238 and is fissile. It is used in atomic bombs and as fuel in reactors. Reactors known as fast neutron breeder regenerate as much fissile plutonium as they consumed it. When not burned in a reactor, plutonium becomes a major radioactive waste. In order to prevents the dispersion of its important radiotoxicity, it should be conditioned and made up of of non mobile oxides.