Neptunium is a synthetic chemical element with the symbol Np and atomic number 93.
A radioactive metallic element, neptunium is the first of the transuranics and belongs to the actinide family. Its most stable isotope, neptunium 237, is produced in nuclear reactors. It is also found in trace amounts in uranium ore.
It was discovered in 1940 at the University of California. As it comes after uranium in the periodic table, it was named after the planet Neptune, which comes after Uranus in the solar system.
Neptunium was discovered by Edwin McMillan and Philip Abelson in 1940. The discovery was made at the Berkeley Radiation Laboratory at the University of California at Berkeley, where the team produced the 239 isotope of neptunium, with a half-life of 2.4 days, bombarding uranium 238 with neutrons. This is the intermediate step leading to the production of plutonium 239.
It is named after the planet Neptune, possessing chemical properties similar to those of uranium, previously named after the planet Uranus which precedes Neptune in the Solar System.
Neptunium is prepared in its metallic form by reducing the compound NpF3 in lithium or barium vapors at 1200 ° C. Silver in appearance, the metal is chemically quite reactive, and it has at least 3 allotropic structures:
Alpha-neptunium (at room temperature), orthorhombic, specific gravity: 20.25;
Beta-neptunium (above 280 ° C), tetragonal, specific gravity (at 313 ° C): 19.36;
Gamma-neptunium (above 577 ° C), cubic, density (at 600 ° C): 18.00. Neptunium forms halide compounds such as NpF3, NpF4, NpCl4, NpBr3, NpI3. It also forms oxides with valences similar to uranium oxides, in particular Np3O8 and NpO2.
In an aqueous medium, this element can be found under four degrees of oxidation:
Np3 +: n.o. +3 (pale purple), analogous to the rare ion Pm3 +; Np4 +: n.o. +4 (yellow-green); NpO2 +: n.o. +5 (blue-green); NpO22 +: n.o. +6 (pale pink).
20 radioisotopes of neptunium have been identified. The most stable is 237Np with a half-life of 2.14 million years, while 236Np has a half-life of 154,000 years, and 235Np of 396.1 days. The half-life of all other isotopes is less than 4.5 days, and in the majority of cases less than 50 minutes.
The atomic weight of neptunium isotopes ranges from 225.0339 u for 225Np to 244.068 u for 244Np.
Neptunium 236 is thermal neutron fissile, the fission cross section is close to 2,800 barns (according to HBPC), which is therefore a fairly high value.
Neptunium 237 is weakly fissile in thermal neutrons, the fission cross section is 19 millibarns according to HBPC; the fission cross section in high energy neutrons is undoubtedly higher.
Neptunium 237 can therefore theoretically be used as fuel in a reactor or to manufacture a fission weapon system. This information was made public by the US DOE in 1992. The actual use of neptunium to produce a weapon is however not established to date.
In September 2002, researchers at the University of California (Los Alamos National Laboratory) working for a US weapons of mass destruction project, indicated that a mixture of neptunium 237 and enriched uranium could allow the manufacture of 'a fission weapon with a lower quantity of neptunium 237 than previously imagined. It can be noted that uranium 233 (fissile) has 4 nucleons less than neptunium 237.
This is the first nuclear critical mass based on the use of neptunium 237, mixed with enriched uranium, rather than pl