Uranium nitride

Uranium nitride[1]
Names
IUPAC name
Uranium nitride
Identifiers
12033-83-9 Yes
Properties
U2N3
Molar mass 518.078 g/mol
Appearance crystalline solid
Density 11300 kg·m−3, solid
Melting point 900 to 1,100 °C (1,650 to 2,010 °F; 1,170 to 1,370 K) (decomposes to UN)
Boiling point Decomposes
0.08 g/100 ml (20 °C)
Structure
Crystal structure Hexagonal, hP5
Space group P-3m1, No. 164
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Uranium nitride refers to a family of several ceramic materials: uranium mononitride (UN), uranium sesquinitride (U2N3) and uranium dinitride (UN2). The word nitride refers to the −3 oxidation state of the nitrogen bound to the uranium.

Uranium nitride has been considered as a potential fuel for nuclear reactors and also motor vehicles, which would only have to be refueled every 3–5 years. It is said to be safer, stronger, denser, more thermally conductive and having a higher temperature tolerance.[2] Uranium nitride, like other nuclear fuels, isn't used as a fuel for cars because it is radioactive.

Synthesis

Carbothermic reduction

The common techniques for generating UN is carbothermic reduction of uranium oxide (UO2) in a 2 step method illustrated below.[3][4]

3UO2 + 6C → 2UC + UO2 + 4CO (in argon, > 1450 °C for 10 to 20 hours)
4UC + 2UO2 +3N2 → 6UN + 4CO

Sol-gel

Sol-gel methods and arc melting of pure uranium under nitrogen atmosphere can also be used.[5]

Ammonolysis

Another common technique for generating UN2 is the ammonolysis of uranium tetrafluoride. Uranium tetrafluoride is exposed to ammonia gas under high pressure and temperature which replaces the fluorine with nitrogen and generates hydrogen fluoride.[6] Hydrogen fluoride gas is a colourless gas at this temperature and mixes with the ammonia gas.

Hydriding-nitriding

An additional method of UN synthesis employs fabrication directly from metallic uranium. By exposing metallic uranium to hydrogen gas at temperatures in excess of 280 °C, UH3 can be formed.[7] Furthermore, since UH3 has a lower specific volume than the metallic phase, hydridation can be used to physically decompose otherwise solid uranium. Following hydridation, UH3 can be exposed to a nitrogen atmosphere at temperatures around 500 °C, thereby forming U2N3. By additional heating to temperatures above 1150 °C, the sesquinitride can then be decomposed to UN.

2U + 3H2 → 2UH3
2UH3 + 1.5N2 → U2N3
U2N3 → UN + 0.5N2

Use of the isotope 15N (which constitutes around 0.37% of natural nitrogen) is preferable because the predominant isotope, 14N, has a not insignificant neutron absorption cross section which affects neutron economy and, in particular, it undergoes an (n,p) reaction which produces significant amounts of radioactive 14C which would need to be carefully contained and sequestered during reprocessing or permanent storage.[8]

Decomposition

Each uranium dinitride complex is considered to have three distinct compounds present simultaneously because of decomposing of uranium dinitride (UN2) into uranium sesquinitride (U2N3), and the uranium mononitride (UN). Uranium dinitrides decompose to uranium mononitride by the following sequence of reactions:[9]

4UN2 → 2U2N3+ N2
2U2N3 → 4UN +N2

Decomposition of UN2 is the most common method for isolating uranium sesquinitride (U2N3).

Uses

Uranium mononitride is being considered as a potential fuel for generation IV reactors such as the Hyperion Power Module reactor created by Hyperion Power Generation.[10] It has also been proposed as nuclear fuel in some fast neutron nuclear test reactors. UN is considered superior because of its higher fissionable density, thermal conductivity and melting temperature than the most common nuclear fuel, uranium oxide (UO2), while also demonstrating lower release of fission product gases and swelling, and decreased chemical reactivity with cladding materials.[11] It also has a superior mechanical, thermal and radiation stability compared to standard metallic uranium fuel.[9][12] The thermal conductivity is on the order of 4-8 times higher than that of uranium dioxide, the most commonly used nuclear fuel, at typical operating temperatures. Increased thermal conductivity results a lower thermal gradient between inner and outer sections of the fuel,[8] potentially allowing for higher operating temperatures and reducing macroscopic restructuring of the fuel, which limits fuel lifetime.[4]

Molecular and crystal structure

Example structure of the uranium dinitride crystal

The uranium dinitride (UN2) compound has a face-centered cubic crystal structure of the calcium fluoride (CaF2) type with a space group of Fm3m.[13] Nitrogen forms triple bonds on each side of uranium forming a linear structure.[14]

Example structure of the uranium sesquinitride crystal

α-(U2N3) has a body centered cubic crystal structure of the (Mn2O3) type with a space group of Ia3 .[13]

Example crystal structure of uranium mononitride

UN has a face centered cubic crystal structure of the NaCl type. .[15] The metal component of the bond uses the 5f orbital of the uranium but forms a relatively weak interaction but is important for the crystal structure. The covalent portion of the bonds forms from the overlap between the 6d orbital and 7s orbital on the uranium and the 2p orbitals on the nitrogen.[16] N forms a triple bond with uranium creating a linear structure.[14]

Trophy molecule

Scientists have found a stable version of a uranium molecule, which they are calling the trophy molecule. Scientists have searched for decades for this trophy compound. It is stable at room temperature and therefore can be stored in the form of crystals or in powder form. This trophy molecule is very important because it could help lead to learning how to extract and separate the 2-3% of the highly radioactive material in nuclear waste.[17]

Uranium nitrido derivatives

Recently, there have been many developments in the synthesis of complexes with terminal uranium nitride (–U≡N) bonds. In addition to radioactive concerns common to all uranium chemistry, production of uranium nitrido complexes has been slowed by harsh reaction conditions and solubility challenges. Nonetheless, syntheses of such complexes have been reported in the past few years, for example the three shown below among others.[18][19] Other U≡N compounds have also been synthesized or observed with various structural features, such as bridging nitride ligands in di-/polynuclear species, and various oxidation states.[20][21]

[N(n-Bu)4] [(C6F5)3B−N≡U(Nt-BuAr)3][22]
N≡UF3[23]
[Na(12-crown-4)2] [N≡U−N(CH2CH2Ntips)3][24]

See also

References

  1. ↑ R. B. Matthews, K. M. Chidester, C. W. Hoth, R. E. Mason, R. L. Petty (1988). "Fabrication and testing of uranium nitride fuel for space power reactors". Journal of Nuclear Materials 151 (3): 345. doi:10.1016/0022-3115(88)90029-3.
  2. ↑ New Kind of Uranium Could Power Your Car, ABC News, 11 October 2010, Eric Bland
  3. ↑ Minato, Kazuo; Akabori, Mitsuo; Takano, Masahide; Arai, Yasuo; Nakajima, Kunihisa; Itoh, Akinori; Ogawa, Toru (2003). "Fabrication of nitride fuels for transmutation of minor actinides". Journal of Nuclear Materials 320 (1-2): 18–24. doi:10.1016/S0022-3115(03)00163-6. ISSN 0022-3115.
  4. ↑ 4.0 4.1 Carmack, W. J. (2004). "Internal Gelation as Applied to the Production of Uranium Nitride Space Nuclear Fuel". AIP Conference Proceedings 699: 420–425. doi:10.1063/1.1649601. ISSN 0094-243X.
  5. ↑ Ganguly, C.; Hegde, P. J. Sol-Gel Sci. Technol.. 1997, 9, 285.
  6. ↑ Silva, G. W. C.; Yeamans, C. B.; Ma, L.; Cerefice, G. S.; Czerwinski, K. R.; Sarrelberger, A. P. Chem. Mater.. 2008, 20, 3076.
  7. ↑ urn:nbn:se:kth:diva-35249: Manufacturing methods for (U-Zr)N-fuels
  8. ↑ 8.0 8.1 Matthews, R. B.; Chidester, K. M.; Hoth, C. W.; Mason, R. E.; Petty, R. L. Journal of Nuclear Materials. '1988, 151(3), 345.
  9. ↑ 9.0 9.1 Silva, G. W. Chinthaka; Yeamans, Charles B.; Sattelberger, Alfred P.; Hartmann, Thomas; Cerefice, Gary S.; Czerwinski, Kenneth R. (2009). "Reaction Sequence and Kinetics of Uranium Nitride Decomposition". Inorganic Chemistry 48 (22): 10635–10642. doi:10.1021/ic901165j. ISSN 0020-1669.
  10. ↑ Staff (2009-11-20). "Hyperion launches U2N3-fuelled, Pb-Bi-cooled fast reactor". Nuclear Engineering International (Global Trade Media, a division of Progressive Media Group Ltd.).
  11. ↑ "Simple method for producing a stable form of uranium nitride". Advanced Ceramics Report (International Newsletters). August 1, 2012. [R]esearcher ... Stephen Liddle, says: '... it could help...extract and separate the 2-3% of the highly radioactive material in nuclear waste.' Check date values in: |accessdate= (help);
  12. ↑ Mizutani, A.; Sekimoto, H. Ann. Nucl. Energy. 2005, 25(9), 623–638.
  13. ↑ 13.0 13.1 Rundle, R. E.; Baenziger, N. C.; Wilson, A. S.; McDonald, R. A. J. Am. Chem Soc.. 1948, 70, 99.
  14. ↑ 14.0 14.1 Wang, X.; Andrews, L.; Vlaisavljevich, B.; Gagliardi, L. Inorganic Chemistry. 2011, 50(8), 3826–3831. doi:10.1021/ic2003244
  15. ↑ Mueller, M. H.; Knott, H. W.Acta Cryst.. 1958, 11, 751–752. doi:10.1107/S0365110X58002061 ]
  16. ↑ Évarestov, R. A., Panin, A. I., & Losev, M. V. Journal Of Structural Chemistry. 2008, 48, 125–135.
  17. ↑ Scientists Make a Big Breakthrough in Nuclear Research, Oil Price, 4 July 2012, Brian Westenhaus
  18. ↑ Nocton, G.; Pécaut, J.; Mazzanti, M. A Nitrido-Centered Uranium Azido Cluster Obtained from a Uranium Azide. Angew. Chem. Int. Ed. 2008, 47 (16), 3040–3042. doi:10.1002/anie.200705742
  19. ↑ Thomson, R. K.; Cantat, T.; Scott, B. L.; Morris, D. E.; Batista, E. R.; Kiplinger, J. L. Uranium azide photolysis results in C–H bond activation and provides evidence for a terminal uranium nitride. Nature Chemistry 2010, 2, 723–729. doi:10.1038/nchem.705
  20. ↑ Fox, A. R.; Arnold, P. L.; Cummins, C. C. Uranium−Nitrogen Multiple Bonding: Isostructural Anionic, Neutral, and Cationic Uranium Nitride Complexes Featuring a Linear U═N═U Core. J. Am. Chem. Soc. 2010, 132 (10), 3250–3251. doi:10.1021/ja910364u
  21. ↑ Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Molecular Octa-Uranium Rings with Alternating Nitride and Azide Bridges. Science 2005, 309 (5742), 1835–1838. doi:10.1126/science.1116452
  22. ↑ Fox, A.; Cummins, C. Uranium−Nitrogen Multiple Bonding: The Case of a Four-Coordinate Uranium(VI) Nitridoborate Complex. J. Am. Chem. Soc., 2009, 131 (16), 5716–5717. doi:10.1021/ja8095812
  23. ↑ Andrew, L.; Wang, X.; Lindh, R.; Roos, B.; Marsden, C. Simple N≡UF3 and P≡UF3 Molecules with Triple Bonds to Uranium. Angew. Chem. Int. Ed. 2008, 47 (29), 5366-5370. doi:10.1002/anie.200801120
  24. ↑ King, D.; Tuna, F.; McInnes, E.; McMaster, J.; Lewis, W.; Blake, A.; Liddle, S. T. Synthesis and Structure of a Terminal Uranium Nitride Complex. Science 2012, 337 (6095), 717–720. doi:10.1126/science.1223488

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