|Jmol-3D images||Image 1
|Molar mass||241.05273 g mol−1|
|Density||10.95 g cm−3|
|Solubility in water||Reacts|
|Crystal structure||Cubic, cP32|
|Space group||Pm3n, No. 223|
|Lattice constant||a = 664.3 pm|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Uranium hydride is a highly toxic, brownish gray to brownish black pyrophoric powder or brittle solid. Its specific gravity at 20 °C is 10.95, much lower than that of uranium (19.1). It has a metallic conductivity, is slightly soluble in hydrochloric acid and decomposes in nitric 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. 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. Both α- and β-UH3 are ferromagnetic at temperatures below ~180 K. Above 180 K, they are paramagnetic.
Formation in uranium metal
Hydrogen gas reaction
Exposition of uranium metal to hydrogen leads to hydrogen embrittlement. Hydrogen diffuses through metal and forms a network of brittle hydride over the grain boundaries. Hydrogen can be removed and ductility renewed by annealing in vacuum.
Uranium metal heated to 250 to 300 °C (482 to 572 °F) reacts with hydrogen to form uranium hydride. Further heating to about 500 °C will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride, and halide compounds. The reversible reaction proceeds as follows:
- 2 U + 3 H2 ↔ 2 UH3
Uranium hydride expands considerably during formation and is therefore not an interstitial compound. In its lattice, each uranium atom is surrounded by 6 other uranium atoms and 12 atoms of hydrogen; each hydrogen atom occupies a large tetrahedral hole in the lattice. The density of hydrogen in uranium hydride is approximately the same as in liquid water or in liquid hydrogen. The U-H-U linkage through a hydrogen atom is present in the structure.
- 7 U + 6 H2O → 3 UO2 + 4 UH3
The formed uranium hydride is pyrophoric; when the metal (e.g. a damaged fuel rod) gets exposed to air afterwards, a lot of heat may be generated and the bulk uranium metal itself can be ignited. Hydride-contaminated uranium can be passivated by exposition to a gaseous mixture of 98% helium with 2% oxygen. Condensed moisture on uranium metal promotes formation of hydrogen and uranium hydride; pyrophoric surface may be formed in absence of oxygen. This poses a problem with underwater storage of spent nuclear fuel in spent fuel ponds. Depending on the size and distribution on the hydride particles, self-ignition occurs after indeterminate length of exposure to air. Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults.
Other chemical reactions
Uranium deuteride is said to be usable for design of some types of neutron initiators.
Uranium hydride enriched to about 5% uranium-235 is proposed as a combined nuclear fuel/neutron moderator for the Hydrogen Moderated Self-regulating Nuclear Power Module. According to the aforementioned patent application, the reactor design in question begins producing power when hydrogen 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. Uranium hydride is both a nuclear fuel and a neutron 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 chemical decomposition 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.
On heating with diborane, uranium hydride produces uranium boride. With bromine at 300 °C, uranium(IV) bromide is produced. With chlorine at 250 °C, uranium(IV) chloride is produced. Hydrogen fluoride at 20 °C produces uranium(IV) fluoride. Hydrogen chloride at 300 °C produces uranium(III) chloride. Hydrogen bromide at 300 °C produces uranium(III) bromide. Hydrogen iodide at 300 °C produces uranium(III) iodide. Ammonia at 250 °C produces uranium(III) nitride. Hydrogen sulfide at 400 °C produces uranium(IV) sulfide. Oxygen at 20 °C produces triuranium octoxide. Water at 350 °C produces uranium dioxide.
Uranium hydride and uranium deuteride were suggested as a fissile material for a uranium hydride bomb. The tests with uranium hydride and uranium deuteride during Operation Upshot-Knothole were disappointing, however. During the early phases of the Manhattan Project, in 1943, uranium hydride was investigated as a promising bomb material; however, it was abandoned by the spring of 1944 as it turned out that such a design would be inefficient.
Hydrogen, deuterium, and tritium can be purified by reacting with uranium, then thermally decomposing the resulting hydride/deuteride/tritide. Extremely pure hydrogen has been prepared from beds of uranium hydride for decades. Heating uranium hydride is a convenient way to introduce hydrogen into a vacuum system.
The swelling and pulverization at uranium hydride synthesis can be used for preparation of very fine uranium metal if the powdered hydride is thermally decomposed.
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