Uranium pyrophoric

Although pyrophoric metal is normally absent in the case of uranium, pyrophoricity can be introduced into the product by the presence of uranium hydride. This is formed if traces of moisture are present in the system, e.g. introduced in an imperfectly dried reactor lining. 

Uranium exhibits three crystallographic modifications as follows alpha —(688C)—> beta —(776C)—> gamma. Uranium is a heavy, silvery-white metal which is pyrophoric when finely divided. 

Finely divided uranium metal, being pyrophoric, presents a fire hazard. 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Layers of uranium or thorium hydrides ignited spontaneously after exposure to ambient air for a few min [1], Thermal decomposition yields pyrophoric uranium [2],... 

Mellor, 1940, Vol. 8, 436 1943, Vol. 11, 162 1942, Vol. 12, 32 Pyrophoric chromium attains incandescence in the oxide, while calcium, potassium and uranium need heating before ignition occurs, when combustion is brilliant in the 53% oxygen content. 

Storage of uranium foil in closed containers in presence of air and water may produce a pyrophoric surface [1], Uranium must be machined in a fume hood because, apart from the radioactivity hazard, the swarf is easily ignited. The massive metal ignites at 600-700°C in air [2]. The finely divided reactive form of uranium produced by pyrolysis of the hydride is pyrophoric [3], while that produced as a slurry by reduction of uranium tetrachloride in dimethoxyethane by potassium-sodium alloy is not [4],... 

Alloys of reactive metals are often more pyrophoric than the parent metals. Examples are alloys of titanium with zirconium thorium with copper, silver or gold uranium with tin, lead or gold magnesium with aluminium hafnium with iron [1], Cerium amalgams and thorium-silver alloys are spontaneously flammable when dry [2], Individually indexed alloys are ... 

Uranium and its compunds are highly toxic. These substances also present a radiation hazard. Finely-divided metal is pyrophoric. 

Uranium Borohydride [Uranium (III) tetrahydro-borate]. U(BH4)3 mw 28258 green pyrophoric crysts, volatile at RT mp >70° (22mm Hg) bp, explds on heating. Reacts with water and methanol. Prepn is by reacting U (IV) fluorides with an excess of A1 borohydride... 

Uranium Hydrida. UH3 mw 241.05 blk-brn pyrophoric cubic crysts or powder d 10.95 11.42g/cc (separate values). SI sol in dil HC1 insol in acet, ethanol and liq ammonia. Prepn is by passing H2 over coarsely ground U at 250° the yield is quant. The hydride ignites spontaneously in air, and must be stored and handled under N2 or C02. It reacts very violently with w, and explosively with organic halides. The hydride is also violently attacked by halogens to form U halides... 

Uranium Arsenide, U3As4, may be obtained i by passing hydrogen over a fused mixture of sodium uranous chloride and sodium arsenide. It is a greyish powder which readily burns in the air. Sometimes it is obtained in a pyrophoric condition. An aluminium-containing product results when the aluminothermic process, using an oxide of uranium and arsenious oxide, is employed. The purest arsenide is obtained, in the crystalline form, when a mixture of hydrogen and arsenic vapour is passed over sodium uranium chloride. It is rapidly decomposed by nitric acid. 

Occupational Protection and Radiation Consideration. The main adverse factor during the mining and processing of uranium and uranium-containing minerals is airborne dust. Personal protection should be used. Finely divided uranium metal, some alloys, and uranium hydride are pyrophoric, therefore such materials should be handled in an inert atmosphere glovebox. 

Uranium hydride, UH3, is a reactive black powder. It is a powerful reducing agent and is pyrophoric. A mixture of uranium and zirconium hydrides is used as the fuel... 

See Magnesium Water, Uranium See other metals, pyrophoric metals... 

Magnesium—nickel hydride, 4458 Plutonium(III) hydride, 4504 Poly(germanium dihydride), 4409 Poly(germanium monohydride), 4407 Potassium hydride, 4421 Rubidium hydride, 4444 Sodium hydride, 4438 f Stibine, 4505 Thorium dihydride, 4483 Thorium hydride, 4535 Titanium dihydride, 4484 Titanium—zirconium hydride, 4485 Trigermane, 4415 Uranium(III) hydride, 4506 Uranium(IV) hydride, 4536 Zinc hydride, 4486 Zirconium hydride , 4487 See COMPLEX HYDRIDES, PYROPHORIC MATERIALS See entry LANTHANIDE—TRANSITION METAL ALLOY HYDRIDES... 

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