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| Names | |
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| IUPAC name Uranium nitride | |
| Identifiers | |
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3D model (JSmol) |
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| ChemSpider |
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| 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 | |
| Hexagonal,hP5 | |
| P-3m1, No. 164 | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Uranium nitrides refers to any of a family of severalceramic materials:uranium mononitride (UN), uranium sesquinitride (U2N3) and uranium dinitride (UN2). The wordnitride refers to the −3oxidation state of the nitrogen bound to theuranium.
Uranium nitride has been considered as a potentialnuclear fuel and will be used as such in theBREST-300 nuclear reactor currently under construction in Russia. It is said to be safer, stronger, denser, more thermally conductive and having a higher temperature tolerance. Challenges to implementation of the fuel include a complex conversion route from enriched UF6, the need to prevent oxidation during manufacturing and the need to define and license a final disposal route. The necessity to use expensive, highly isotopically enriched15N is a significant factor to overcome. This is necessary due to the (relatively) high neutron capture cross-section of the far-more-common14N, which affects theneutron economy of a reactor.[2]
The common technique for generating UN iscarbothermic reduction ofuranium oxide (UO2) in a 2 step method illustrated below.[3][4]
Sol-gel methods and arc melting of pureuranium undernitrogen atmosphere can also be used.[5]
Another common technique for generating UN2 is theammonolysis ofuranium tetrafluoride. Uranium tetrafluoride is exposed toammonia gas under high pressure and temperature, which replaces thefluorine with nitrogen and generateshydrogen fluoride.[6] Hydrogen fluoride is a colourless gas at this temperature and mixes with the ammonia gas.
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 higher 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.
Use of theisotope15N (which constitutes around 0.37% of natural nitrogen) is preferable because the predominant isotope,14N, has a significantneutronabsorption cross section which affects neutron economy and, in particular, it undergoes an (n,p) reaction which produces significant amounts of radioactive14C which would need to be carefully contained and sequestered during reprocessing or permanent storage.[8]
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 then uranium mononitride (UN). Uranium dinitrides decompose to uranium mononitride by the following sequence of reactions:[9]
Decomposition of UN2 is the most common method for isolating uranium sesquinitride (U2N3).
Uranium mononitride is being considered as a potential fuel forgeneration IV reactors such as theHyperion Power Module reactor created byHyperion Power Generation.[10] It has also been proposed asnuclear fuel in somefast neutron nuclear test reactors. UN is considered superior because of its higher fissionable density,thermal conductivity, andmelting 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 standardmetallic 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 in a smallerthermal gradient between inner and outer sections of the fuel,[8] potentially allowing for higher operating temperatures and reducingmacroscopic restructuring of the fuel, which limits fuel lifetime.[4]
The uranium dinitride (UN2) compound has aface-centered cubiccrystal structure of thecalcium fluoride (CaF2) type with aspace group of Fm3m.[13]Nitrogen formstriple bonds on each side of uranium forming alinear structure.[14][15]
α-(U2N3) has abody-centered cubiccrystal structure of the (Mn2O3) type with aspace group of Ia3 .[13]
UN has aface-centered cubiccrystal structure of theNaCl type.[14][16]Themetal component of the bond uses the 5f orbital of the uranium but forms a relatively weak interaction but is important for thecrystal structure. Thecovalent 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.[14][17] N forms atriple bond with uranium creating a linear structure.[15]
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]
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[R]esearcher ...Stephen Liddle, says: '... it could help... extract and separate the 2-3% of the highly radioactive material in nuclear waste.'
