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Old 06-11-2019, 10:24 AM
Gray Ghost is offline
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Join Date: Apr 2008
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Section from the Nuclear Weapon Archive on Neptunium: TL;DR: It's too expensive to make relative to Plutonium or HEU, and the critical mass is much greater than either of those two. Further, though not stated in the quoted material, if it has anything like Pu's ridiculous number of state changes and physcial property changes, while a device could be made from it, the required material science research would be prohibitively expensive.

Quote:
6.2.4.2 Neptunium
Neptunium was the first transuranium element discovered, and the first synthetic transuranium element to be prepared. Though traces of neptunium have subsequently been found in nature, it was discovered by Edwin M. McMillan and Philip H. Abelson in May 1940 at Berkeley, California, USA, who bombarded uranium with neutrons produced from a cyclotron, producing Np-239. Since it was the next element in the periodic table after uranium, by analogy it was named after the planet Neptune, which is the next planet out from the Sun after Uranus (this pattern was subsequently followed in the naming of plutonium).

As is also true of plutonium, trace quantities of the element are actually found in nature due to neutron-induced transmutation reactions in uranium ores produced by spontaneous fission neutrons.

Neptunium is a silvery metal, with a density of 20.45 (Np-237, 25 C), neptunium is the fifth densest element. It has at least three allotropic (crystalline) forms -- the alpha form which is the densest (stable below 280 C, orthorhombic, density 20.45 at 25 C), beta (above 280C, tetragonal, density 19.36 at 313 C), and gamma (above 577 C, cubic, density 18.0 at 600 C). Its melting point is 637 C, and its boiling point is estimated at 5235 C.

Neptunium is chemically reactive and similar to uranium with oxidation states from +3 (III) to +7 (VII). The metal is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200C. Neptunium of the five ionic oxidation states the pentavalent is the most stable in solution. Neptunium ions in aqueous solution possess characteristic colours: Np3+, pale purple; Np4+, pale yellow-green; NpO+2, green-blue; NpO2+2, varying from colourless to pink or yellow-green, depending on the anion present. The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides for the various compositions such as are found in the uranium-oxygen system, including Np3O8 and NpO2. Its electronic configuration is 2-8-18-32-22-9-2 or (Rn)5f4-6d1-7s2.

Fifteen isotopes of neptunium are now recognized. The isotope neptunium-237 (discovered in 1942 by Glenn T. Seaborg and Arthur C. Wahl) is the longest lived, with a half-life of 2.144 x 10^6 years, and is fissile. It is most easily produced by the following reactions:

U-235 + n -> U-236

U-236 + n -> U-237

U-237 -> (6.8 days, beta) -> Np-237
It is also produced by the decay of Am-241, but the 433 year half-life of this latter isotope makes the production rate very small.

Published estimates for the critical mass of neptunium-237 are around 90 kg (range of estimates is 75-105 kg), my one-group lower bound calculation is 19 kg. It has a very low spontaneous fission rate (< 5x10^-2 F/sec-kg). The high critical mass value (almost double that of HEU) and the high cost of manufacture makes it unattractive for weapons use. A certain amount Np-237 is produced normally by U-235 capture in reactors. A typical power reactor can discharge about 0.4 kg of Np-237 per ton of fuel. Fast reactors can discharge a significantly higher amount.

Np-237 is used in some types of neutron detection instruments

Last edited by Gray Ghost; 06-11-2019 at 10:28 AM.