Neptunium

Neptunium (pronounced /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.

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Notable characteristics

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

• alpha-neptunium, orthorhombic, density 20.26 Mg/m³,
• beta-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 Mg/m³, and
• gamma-neptunium (above 577 °C), cubic, density (600 °C) 18 Mg/m³

Compounds

This element has four ionic oxidation states while in solution:

• Np⁺³ (pale purple), analogous to the rare earth ion Pm⁺³,
• Np⁺⁴ (yellow green);
• NpO₂⁺ (green blue): and
• NpO₂⁺⁺ (pale pink).

Neptunium forms tri- and tetrahalides such as NpF₃, NpF₄, NpCl₄, NpBr₃, NpI₃, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np₃O₈ and NpO₂.

Neptunium like other actinides readily forms a dioxide neptunyl core (NpO₂). In the environment, this neptunyl core readily complexes with carbonate as well as other oxygen moieties (OH^-, NO₂^-, NO₃^-, and SO₄^-2) to form charged complexes which tend to be readily mobile with low affinities to soil.

• NpO₂(OH)₂^-1
• NpO₂(CO₃)^-1
• NpO₂(CO₃)₂^-3
• NpO₂(CO₃)₃^-5

See also: Actinides in the environment

Uses

Precursor in Plutonium-238 Production

²³⁷Np is irradiated with neutrons to create ²³⁸Pu, a rare and valuable isotope for spacecraft and military applications.

Weapons applications

Neptunium is fissionable, and could theoretically be used as reactor fuel or to create a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that Np-237 "can be used for a nuclear explosive device".^[1] It is not believed that an actual weapon has ever been constructed using neptunium.

In September 2002, researchers at the University of California Los Alamos National Laboratory created the first known nuclear critical mass using neptunium in combination with enriched uranium, discovering that the critical mass of neptunium is less than previously predicted^[2]. US officials in March 2008, planned to move the nation's supply of separated neptunium to a site in Nevada.

History

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan and Philip H. Abelson in the year 1940 in Berkeley, California. Initially predicted by Walter Russell's "spiral" organization of the periodic table, it was found at the Berkeley Radiation Laboratory of the University of California, Berkeley where the team produced the neptunium isotope ²³⁹Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

Occurrence

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. ²³⁷Np is produced through the reduction of ²³⁷NpF₃ with barium or lithium vapor at around 1200 °C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.

By weight, neptunium-237 discharges are about five percent as great as plutonium discharges and about 0.05 percent of spent nuclear fuel discharges. ^[3]

Nuclear synthesis

• When an ²³⁵U atom captures a neutron, it is converted to an excited state of ²³⁶U. About 81% of the excited ²³⁶U nuclei undergo fission, but the remainder decay to the ground state of ²³⁶U by emitting gamma radiation. Further neutron capture creates ²³⁷U which has a half-life of 7 days and thus quickly decays to ²³⁷Np.
• ²³⁷U is also produced via an (n,2n) reaction with ²³⁸U.
• ²³⁷Np is the decay product of ²⁴¹Am.

Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure ²³⁷Np by chemical separation of neptunium.

Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment.[1] This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation.[2]

Isotopes

Main article: isotopes of neptunium

19 neptunium radioisotopes have been characterized, with the most stable being ²³⁷Np with a half-life of 2.14 million years, ²³⁶Np with a half-life of 154,000 years, and ²³⁵Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lifes that are less than 4.5 days, and the majority of these have half lifes that are less than 50 minutes. This element also has 4 meta states, with the most stable being ^236mNp (t_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (²²⁵Np) to 244.068 u (²⁴⁴Np). The primary decay mode before the most stable isotope, ²³⁷Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

²³⁷Np is both fissionable and fissile^[2]. ²³⁷Np eventually decays to form bismuth, unlike most other common heavy nuclei which decay to make lead.

Neptunium in Popular Culture

• When Confederate scientists in Harry Turtledove's Southern Victory Series of alternate-history novels, discover the 93rd element they dub it "saturnium". When American and British scientists learn of the element, they call it "Neptunium" and "Mosleyium" (After Oswald Mosley) respectively.

References

1. ^ "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006
2. ^ ^{a b} Weiss, P. (October 26, 2002). Little-studied metal goes critical - Neptunium Nukes?. Science News. Retrieved on 2006-09-29.
3. ^ http://www.isis-online.org/publications/fmct/book/New%20chapter%205.pdf

• Los Alamos National Laboratory's Chemistry Division: Periodic Table - Neptunium
• Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1