A radioactive element with atomic number 93 and symbol Np. The isotope ²³⁷Np is both fissionable and fissile; it is formed during the irradiation of fuel in reactors and may be separated from high level waste and reprocessing streams. While not defined under the IAEA Statute as source material or special fissionable material, separated neptunium is monitored by the IAEA under voluntary arrangements with relevant States. Neptunium has sometimes been referred to as an 'alternative nuclear material.'