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Uranium

Background

Dating back to 79 A.D. yellow-coloured glass was discovered containing more than 1% uranium oxide, near Naples, Italy. Klaproth in 1789 recognised an unknown element in pitchblende and attempted to isolate the metal, however, it was in 1841 that Peligot, who reduced the anhydrous chloride with potassium, apparently first isolated the metal.

Uranium (U) occurs in many minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, davidite and torbernite. It can also be found in phosphate rock, lignite, and monazite sands where it may be recovered commercially from these sources.

Uranium can be prepared by the reduction of uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminium, or carbon at high temperatures. The metal may also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2NaCl. Uranium, high purity, may be prepared by the thermal decomposition of uranium halides on a hot filament.

Uranium has three crystallographic modifications (forms). The alpha (α) phase, or orthorhombic crystal, is stable to 660°C (1220°F); the beta (β) phase, or the tetragonal crystal, exists from 660°C to 760°C (1220°F to 1400°F); while the gamma (γ) phase, or the body-centred cubic, stable from 760°C (1400°F) to the melting point.

Uranium is a heavy, silver-white metal resembling nickel. It is pyrophoric when finely divided. It is also malleable, ductile and slightly paramagnetic. Uranium in the finely divided state is attacked by water. In air, the metal becomes coated with a layer of oxide. The metal is dissolved in acids but it is not affected in alkalis. Uranium has sixteen (16) isotopes, all of which are radioactive. Natural uranium (99.2830% by weight 238U, 0.7110% 235U, and 0.0054% 234U) is sufficiently radioactive to expose a photographic plate in approximately an hour.

Uranium has found great importance as a nuclear fuel, where 238U can be converted into fissionable plutonium through the following reaction:

238U(nγ)239U → 239Np → 239Pu

This nuclear conversion can be brought about in so called “breeder” reactors. However, 235U is of greater importance, as it is the key to the utilisation of Uranium. 235U is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made to occur in a reactor purely constructed from natural uranium and a suitable moderator (such as heavy water or graphite). 235U may also be concentrated by gaseous diffusion or other physical processes, and then may be used directly as a nuclear fuel or as an explosive. Natural uranium slightly enriched with 235U is used to fuel nuclear power reactors for the generation of electricity.

Applications

Uranium’s main uses are as a nuclear fuel to generate electrical power, to produce isotopes for peaceful intentions (e.g. in nuclear medicine) and to make explosives.

Uranium metal is used for X-ray targets for production of high-energy X-rays. It is alloyed with iron to make “ferrouranium” that imparts special properties to steels by increasing elastic limit and tensile strength and as a cathode in photoelectric tubes responsive to ultraviolet radiation.

It is a more powerful deoxidiser than vanadium and will denitrogenise steel.

It is used in high-speed steels as an alloying agent to improve strength and toughness.

Depleted uranium (uranium with the percentage of 235U lowered to 0.2%) has found use in inertial guidance devices, gyro compasses, counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.

Uranium compounds are used in the following applications:

        Uranium nitrate is used as a photographic toner.

        Uranium acetate is used for applications in analytical chemistry.

        Uranium dioxide (UO2) is used for making glazes in the ceramic industry producing a yellowish-green fluorescent glass and a red with yellow tinge for pottery glazes, pigment in paints. In the sintered form it is used as fuel for power reactors.

        Uranium yellow (termed also as yellow oxide) with formula Na2U2O7.6H2O is used for yellow and greenish glazing enamels and to impart an opalescent yellow to glass, which is green in reflected light.

        Uranium oxide (U3O8) an olive green powder is used as pigment.

        Uranium trioxide (also called uranic oxide) with formula UO3, is an orange-yellow powder and is used as a pigment for ceramics. In glasses it produces a beautiful greenish-yellow “uranium glass”.

        Uranium pentoxide (U2O5) a black powder is used in glass to give a fine black colour.

        Sodium uranate (Na2UO4) a yellow orange powder is used in pottery to produce ivory to yellow shades in glazes.

Primary author: AZoM.com

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