Neptunium is a ductile, silvery, radioactive metal. Neptunium forms numerous chemical compounds. Chemically it is extremely reactive and is attacked by oxygen, steam and acids, but not by alkalis. It can exist in many oxidation states, from neptunium (II) to neptunium (VII). Neptunium has been used in neutron detectors. It does not have any commercial application. Neptunium occurs naturally in Earth, being present in minute quantities in uranium ores. Today neptunium-237 is extracted in kg quantities from the spent uranium fuel rods of nuclear reactors. Neptunium is used mainly for research purposes. When bombarded with neutrons neptunium-237 is used to produce plutonium-238 which is used for spacecraft generators and terrestrial navigation beacons.