Actinides

Neutrons captures in the fuel of nuclear reactors can either trigger fissions or produce nuclei heavier than uranium, known as actinides. The best known actinide is plutonium-239, which is fissile. Other actinides are generated, in particular isotopes of neptunium, americium and curium, which are called minor actinides. These radioactive nuclei, have much longer lifetimes and lower activity in general than fission products. Their relative abundance, and these long lifetimes are a problem for storage of radioactive waste from reactors.