| Names | |
|---|---|
| Other names | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| Properties | |
| UH 3 | |
| Molar mass | 241.05273 g mol−1 |
| Appearance | brownish grey to brownish blackpyrophoric powder |
| Density | 10.95 g cm−3 |
| Reacts | |
| Structure | |
| Cubic,cP32 | |
| Pm3n, No. 223 | |
a = 664.3 pm[4] | |
| Hazards | |
| Flash point | Pyrophoric |
| Safety data sheet (SDS) | ibilabs.com |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Uranium hydride, also calleduranium trihydride (UH3), is aninorganic compound and ahydride ofuranium.
Uranium hydride is a brownish blackpyrophoric powder. Its density at 20 °C is 10.95 g cm−3, much lower than that of uranium (19.1 g cm−3). It has a metallic conductivity, is slightly soluble inhydrochloric acid and decomposes innitric acid.
Two crystal modifications of uranium hydride exist, both cubic: an α form that is obtained at low temperatures and a β form that is grown when the formation temperature is above 250 °C.[5] After growth, both forms are metastable at room temperature and below, but the α form slowly converts to the β form upon heating to 100 °C.[3] Both α- and β-UH3 areferromagnetic at temperatures below ~180 K. Above 180 K, they are paramagnetic.[6]
Exposure of uranium metal to hydrogen at 250 °C gives the trihydride:
Bulk uranium metal crumbles into a fine powder during the course of the reaction.[7][2]
The process is reminiscent ofhydrogen embrittlement but uranium hydride is not aninterstitial compound. Instead, according toX-ray crystallography, each uranium atom is surrounded by 12 atoms ofhydrogen (defect perovskite structure). Each hydrogen atom occupies a large tetrahedral hole in the lattice.[8] The density of hydrogen in uranium hydride is approximately the same as in liquid water or inliquid hydrogen.[9] The U-H-U linkage through a hydrogen atom is present in the structure.[10]
Uranium hydride forms when uranium metal (e.g. inMagnox fuel with corrodedcladding) becomes exposed to water or steam, withuranium dioxide as byproduct:[8]
The resulting uranium hydride is pyrophoric; if the metal (e.g. a damagedfuel rod) is exposed to air afterwards, excessive heat may be generated and the bulk uranium metal itself can ignite.[11] Hydride-contaminated uranium can bepassivated by exposure to a gaseous mixture of 98%helium with 2%oxygen.[12] Condensed moisture on uraniummetal promotes formation of hydrogen and uranium hydride; a pyrophoric surface may be formed in absence of oxygen.[13] This poses a problem with underwater storage of very specialspent nuclear fuel inspent fuel ponds (nuclear fuel from commercial nuclear plants does not contain any uranium metal). Depending on the size and distribution on the hydride particles, self-ignition can occur after an indeterminate length of exposure to air.[14] Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults.[15]
Uranium hydride exposed to water evolves hydrogen. In contact with strong oxidizers this may cause fire and explosions. Contact withhalocarbons may cause a violent reaction.[16]
UH3 releases hydrogen upon heating to near 400 °C. In this way bulk uranium can be transformed to a powder with high surface area. The resulting powder is extremely reactive toward H2 even at -80 °C.[17]
Hydrogen,deuterium, andtritium can be purified by reacting with uranium, then thermally decomposing the resulting hydride/deuteride/tritide.[18] Extremely pure hydrogen has been prepared from beds of uranium hydride for decades.[19] Heating uranium hydride is a convenient way to introduce hydrogen into a vacuum system.[20] Uranium tritide (UT) is used for the safe and efficient storage of tritium, since gaseous tritium is harder to contain and work with. UT is formed by combining tritium and uranium at room temperature. The tritium can be later extracted by heating the UT. Tritium and its decay product3He are extracted at different temperatures.[21]
On heating withdiborane, uranium hydride producesuranium boride.[22] Withbromine at 300 °C,uranium(IV) bromide is produced. Withchlorine at 250 °C,uranium(IV) chloride is produced.Hydrogen fluoride at 20 °C producesuranium(IV) fluoride.Hydrogen chloride at 300 °C producesuranium(III) chloride.Hydrogen bromide at 300 °C producesuranium(III) bromide.Hydrogen iodide at 300 °C producesuranium(III) iodide.Ammonia at 250 °C producesuranium(III) nitride.Hydrogen sulfide at 400 °C producesuranium(IV) sulfide.Oxygen at 20 °C producestriuranium octoxide. Water at 350 °C producesuranium dioxide.[23]
Polystyrene-impregnated uranium hydride powder is non-pyrophoric and can be pressed, however its hydrogen-carbon ratio is unfavorable. Hydrogenated polystyrene was introduced in 1944 instead.[24]
Uranium hydrideenriched to about 5%uranium-235 has been proposed as a combinednuclear fuel/neutron moderator for theHydrogen Moderated Self-regulating Nuclear Power Module. According to the aforementioned patent application, the reactor design in question begins producing power whenhydrogen gas at a sufficient temperature and pressure is admitted to the core (made up of granulated uranium metal) and reacts with the uranium metal to form uranium hydride.[25] Uranium hydride is both anuclear fuel and aneutron moderator; apparently it, like other neutron moderators, will slow neutrons sufficiently to allow for fission reactions to take place; the uranium-235 atoms within the hydride also serve as the nuclear fuel. Once the nuclear reaction has started, it will continue until it reaches a certain temperature, approximately 800 °C (1,500 °F), where, due to the chemical properties of uranium hydride, it chemically decomposes and turns into hydrogen gas and uranium metal. The loss of neutron moderation due to the chemicaldecomposition of the uranium hydride will consequently slow — and eventually halt — the reaction. When temperature returns to an acceptable level, the hydrogen will again combine with the uranium metal, forming uranium hydride, restoring moderation and the nuclear reaction will start again.[25]
Uranium hydride ion may interfere with somemass spectrometry measurements, appearing as a peak at mass 239, creating false increase of signal for plutonium-239.[26]
Uranium hydride slugs were used in the "tickling the dragon's tail" series of experiments to determine thecritical mass of uranium.[27]
Uranium hydride and uraniumdeuteride were suggested as afissile material for auranium hydride bomb. The tests with uranium hydride and uranium deuteride duringOperation Upshot–Knothole were disappointing, however. During the early phases of theManhattan Project, in 1943, uranium hydride was investigated as a promising bomb material; it was abandoned by early 1944 as it turned out that such a design would be inefficient.[28]
