Neptunium

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93 uranium ← neptunium → plutonium
Pm
↑
Np
↓
(Uqt)
General
Name, Symbol, Number neptunium, Np, 93
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance silvery metallic
Standard atomic weight (237)  g·mol−1
Electron configuration [Rn] 5f4 6d1 7s2
Electrons per shell 2, 8, 18, 32, 22, 9, 2
Physical properties
Phase solid
Density (near r.t.) 20.45 [2]  g·cm−3
Melting point 910 K
(637 °C, 1179 °F)
Boiling point 4273 K
(4000 °C, 7232 °F)
Heat of fusion 3.20  kJ·mol−1
Heat of vaporization 336  kJ·mol−1
Specific heat capacity (25 °C) 29.46  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2194 2437        
Atomic properties
Crystal structure 3 forms: orthorhombic,
tetragonal and cubic
Oxidation states 6, 5, 4, 3
(amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ/mol
Atomic radius 175  pm
Miscellaneous
Magnetic ordering  ?
Electrical resistivity (22 °C) 1.220 µΩ·m
Thermal conductivity (300 K) 6.3  W·m−1·K−1
CAS registry number 7439-99-8
Selected isotopes
Main article: Isotopes of neptunium
iso NA half-life DM DE (MeV) DP
235Np syn 396.1 d α 5.192 231Pa
ε 0.124 235U
236Np syn 1.54×105 y ε 0.940 236U
β- 0.940 236Pu
α 5.020 232Pa
237Np syn 2.144×106 y SF & α 4.959 233Pa
References

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.

Contents

[edit] Notable characteristics

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

  • alpha-neptunium, orthorhombic, density 20.45 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

[edit] 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.

NpF5 is volatile like uranium hexafluoride. See fluoride volatility and uranium enrichment.

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

[edit] Uses

[edit] Precursor in Plutonium-238 Production

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

[edit] Weapons applications

Neptunium is fissile, 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 2004, planned to move the nation's supply of separated neptunium to a site in Nevada.

[edit] 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.

[edit] 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]

[edit] 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. This only happens with very energetic neutrons.
  • 237Np is the product of alpha decay of 241Am.

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

[edit] Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment.[4] This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation.[5]

[edit] 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 (t½ 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. This decay chain is known as the neptunium series.

[edit] 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.

[edit] 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
  4. ^ http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818052.pdf
  5. ^ [1]

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