Neptunium

Neptunium ( /nɛpˈtjuːniəm/ nep-tew-nee-əm) is a chemical element with the symbol Np and atomic number 93. A radioactive metal, 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 due to transmutation reactions.^[3]

History

The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also a publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium.^[4]

At least three times, discoveries of the element 93 were falsely reported, as bohemium and ausonium in 1934 and then sequanium in 1939. The name neptunium has previously been considered for other elements.

The search for element 93 in minerals was encumbered by the fact that the predictions on the chemical properties of element 93 were based on a periodic table which lacked the actinides series and therefore placed thorium below hafnium, protactinium below tantalum and uranium below tungsten. This periodic table suggested that element 93, at that point often named eka-rhenium, should be similar to manganese or rhenium. With this misconception it was impossible to isolate element 93 from minerals although later neptunium was found in uranium ore in 1952.^[5]

Enrico Fermi believed that bombarding uranium with neutrons and subsequent beta decay would lead to the formation of element 93. Chemical separation of the new formed elements from the uranium yielded material with low half-life and therefore Fermi announced the discovery of a new element in 1934,^[6] though this was soon found to be mistaken. Soon it was speculated^[7] and later proven^[8] that most of the material is created by nuclear fission of uranium by neutrons. Small quantities of neptunium had to be produced in Otto Hahn's experiments in late 1930s as a result of decay of ²³⁹U. Hahn and his colleagues experimentally confirmed production and chemical properties of ²³⁹U, but were unsuccessful at isolating and detecting neptunium.^[9]

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was discovered by Edwin McMillan and Philip H. Abelson in 1940 at the Berkeley Radiation Laboratory of the University of California, Berkeley. 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.^[10]

Occurrence

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores.^[3] Artificial 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.

2 NpF₃ + 3 Ba → 2 Np + 3 BaF₂

By weight, neptunium-237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges.^[11]

Characteristics

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three allotropes:^[3]

• α-neptunium, orthorhombic, density 20.45 g/cm^3[12]
• β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm^3[12]
• γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm^3[12]

Neptunium has the largest liquid range of any element, 3363 K, between the melting point and boiling point. It is the densest element of all actinoids.

Isotopes

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-lives that are less than 4.5 days, and the majority of these have half-lives 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 fissionable.^{[13] 237}Np eventually decays to form bismuth-209, unlike most other common heavy nuclei which decay to make isotopes of lead. This decay chain is known as the neptunium series.

Synthesis

Chemically, neptunium is prepared by the reduction of NpF₃ with barium or lithium vapor at about 1200 °C.^[3] Most Np is produced in nuclear reactions:

• 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 through beta decay. During beta decay, the excited ²³⁷U emits an electron, while the atomic weak interaction converts a neutron to a proton, thus creating ²³⁷Np.

• ²³⁷U is also produced via an (n,2n) reaction with ²³⁸U. This only happens with very energetic neutrons.
• ²³⁷Np is the product of alpha decay of ²⁴¹Am.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure ²³⁷Np.

Chemistry

This element has four ionic oxidation states while in solution:

• Np³⁺ (pale purple), analogous to the rare earth ion Pm³⁺
• Np⁴⁺ (yellow green)
• NpO₂⁺ (green blue)
• NpO₂²⁺ (pale pink)

Neptunium(III) hydroxide is not soluble in water and does not dissolve in excess alkali. Neptunium(III) is susceptible to oxidation in contact to air forming neptunium(IV).^[14][15]

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(VI) fluoride, NpF₆, is volatile like uranium hexafluoride.

Neptunium, like protactinium, uranium, plutonium, and americium readily forms a linear dioxo neptunyl core (NpO₂ⁿ⁺), in its 5+ and 6+ oxidation states, which readily complexes with hard O-donor ligands such as OH^–, NO₂^–, NO₃^–, and SO₄^2– to form soluble anionic complexes which tend to be readily mobile with low affinities to soil.

• NpO₂(OH)₂^–
• NpO₂(CO₃)^–
• NpO₂(CO₃)₂^3–
• NpO₂(CO₃)₃^5–

Uses

Precursor in plutonium-238 production

²³⁷Np is irradiated with neutrons to create ²³⁸Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. ²³⁷Np will capture a neutron to form ²³⁸Np and beta decay with a half-life of two days to ²³⁸Pu.^[16]

²³⁸Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

Weapons applications

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that neptunium-237 "can be used for a nuclear explosive device".^[17] It is not believed that an actual weapon has ever been constructed using neptunium. As of 2009, the world production of neptunium-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

In September 2002, researchers at the University of California's Los Alamos National Laboratory briefly created the first known nuclear critical mass using neptunium in combination with shells of enriched uranium (U-235), discovering that the critical mass of a bare sphere of neptunium-237 "ranges from kilogram weights in the high fifties to low sixties,"^[1] showing that it "is about as good a bomb material as U-235."^[13] The United States Federal government made plans in March 2004 to move America's supply of separated neptunium to a nuclear-waste disposal site in Nevada.

Physics applications

²³⁷Np is used in devices for detecting high-energy (MeV) neutrons.^[18]

Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment.^[19] This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation.^[20] Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium-241 in a smoke detector includes about 3% neptunium after 20 years, and about 15% after 100 years.

Due to its long half-life neptunium becomes the major contributor of the total radiation in 10,000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.^[21][22]

References

Literature

• Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
• Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): The Chemistry of the Actinide and Transactinide Elements, Springer-Verlag, Dordrecht 2006, ISBN 1-4020-3555-1.
• Ida Noddack (1934). "Über das Element 93". Zeitschrift für Angewandte Chemie 47 (37): 653. doi:10.1002/ange.19340473707. http://www.chemteam.info/Chem-History/Noddack-1934.html. 

External links

• WebElements.com – Neptunium (also used as a reference)
• Lab builds world's first neptunium sphere, U.S. Department of Energy Research News
• NLM Hazardous Substances Databank – Neptunium, Radioactive
• Neptunium: Human Health Fact Sheet
• C&EN: It's Elemental: The Periodic Table – Neptunium