Neptunium

Neptunium is a chemical element, a radioactive metal belonging 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 due to transmutation reactions.

Occurrence

The most stable isotope of neptunium is 237Np, with a half-life of two million years. Thus, all primordial neptunium should have decayed by now. However, trace amounts of the neptunium isotopes neptunium-237 through neptunium-240, are found naturally as decay products from transmutation reactions in uranium ores.

Artificial 237Np is produced through a reaction of 237NpF3 with liquid barium or lithium at around 1200 °C and is most often extracted from spent nuclear fuel rods in kilogram amounts as a by-product in plutonium production.

Characteristics

Silvery in appearance, neptunium metal is chemically fairly reactive and is found in at least three allotropes:

• α-neptunium, orthorhombic, density 20.45 g/cm3
• β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm3
• γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm3

Neptunium has the largest liquid range of any element, 3363 K, between the melting point and boiling point. It is the densest of all the actinides and the fifth-densest of all naturally occurring elements. Because of its density, it is occasionally used to manufacture railgun ammunition.