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Uranium(III) hydride
Uranium hydride, also called uranium trihydride (UH3), is an inorganic compound and a hydride of uranium.
Uranium hydride is a brownish black pyrophoric powder. 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 (482 °F). 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 (212 °F). Both α- and β-UH3 are ferromagnetic at temperatures below ~180 K (−136 °F; −93 °C). Above 180 K (−136 °F; −93 °C), they are paramagnetic.
Exposure of uranium metal to hydrogen at 250 °C (482 °F) gives the trihydride:
Bulk uranium metal crumbles into a fine powder during the course of the reaction.
The process is reminiscent of hydrogen embrittlement but uranium hydride is not an interstitial compound. Instead, according to X-ray crystallography, each uranium atom is surrounded by 12 atoms of hydrogen (defect perovskite structure). 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.
Uranium hydride forms when uranium metal (e.g. in Magnox fuel with corroded cladding) becomes exposed to water or steam, with uranium dioxide as byproduct:
The resulting uranium hydride is pyrophoric; if the metal (e.g. a damaged fuel rod) is exposed to air afterwards, excessive heat may be generated and the bulk uranium metal itself can ignite. Hydride-contaminated uranium can be passivated by exposure to a gaseous mixture of 98% helium with 2% oxygen. Condensed moisture on uranium metal promotes formation of hydrogen and uranium hydride; a pyrophoric surface may be formed in absence of oxygen. This poses a problem with underwater storage of very special spent nuclear fuel in spent 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. Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults.
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Uranium(III) hydride
Uranium hydride, also called uranium trihydride (UH3), is an inorganic compound and a hydride of uranium.
Uranium hydride is a brownish black pyrophoric powder. 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 (482 °F). 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 (212 °F). Both α- and β-UH3 are ferromagnetic at temperatures below ~180 K (−136 °F; −93 °C). Above 180 K (−136 °F; −93 °C), they are paramagnetic.
Exposure of uranium metal to hydrogen at 250 °C (482 °F) gives the trihydride:
Bulk uranium metal crumbles into a fine powder during the course of the reaction.
The process is reminiscent of hydrogen embrittlement but uranium hydride is not an interstitial compound. Instead, according to X-ray crystallography, each uranium atom is surrounded by 12 atoms of hydrogen (defect perovskite structure). 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.
Uranium hydride forms when uranium metal (e.g. in Magnox fuel with corroded cladding) becomes exposed to water or steam, with uranium dioxide as byproduct:
The resulting uranium hydride is pyrophoric; if the metal (e.g. a damaged fuel rod) is exposed to air afterwards, excessive heat may be generated and the bulk uranium metal itself can ignite. Hydride-contaminated uranium can be passivated by exposure to a gaseous mixture of 98% helium with 2% oxygen. Condensed moisture on uranium metal promotes formation of hydrogen and uranium hydride; a pyrophoric surface may be formed in absence of oxygen. This poses a problem with underwater storage of very special spent nuclear fuel in spent 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. Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults.