Neptunium (pronounce /nɛpˈtjuːniəm/) is a chemical element with the symbol Np and atomic number 93. A silvery radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores.