Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources.
Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.
Yellow-colored glass, containing more than 1% uranium oxide and dating back to 79 A.D., has been found near Naples, Italy. Klaproth recognized an unknown element in pitchblende and attempted to isolate the metal in 1789. The metal apparently was first isolated in 1841 by Peligot, who reduced the anhydrous chloride with potassium.
Uranium is a heavy, silvery-white metal which is pyrophoric when finely divided. It is a little softer than steel, and is attacked by cold water in a finely divided state. It is malleable, ductile, and slightly paramagnetic. In air, the metal becomes coated with a layer of oxide. Acids dissolve the metal, but it is unaffected by alkalis. Finely divided uranium metal, being pyrophoric, presents a fire hazard. Working with uranium requires the knowledge of the maximum allowable concentrations that may be inhaled or ingested. Recently, the natural presence of uranium in many soils has become of concern to homeowners because of the generation of radon and its daughters.
Uranium is used in inertial guidance devices, in gyro compasses, as counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and as a shielding material. Uranium metal is used for X-ray targets for production of high-energy X-rays; the nitrate has been used as a photographic toner, and the acetate is used in analytical chemistry.
Uranium-235, while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone.
Uranium gun-assembled weapons are the easiest of all nuclear devices to design and build. It is generally conceded to be impossible to prevent any nation having the requisite amount of HEU from building one or more gun-assembled weapons. Therefore, the acquisition of significant quantities of 235 U or a facility in which to separate the fissile material is an indicator that the acquiring state could be in the process of gaining a rudimentary nuclear capability.
Because HEU is used in certain research reactors, another interpretation is possible. Because of the weapons potential, the United States and France have sought to replace HEU-fueled reactors with ones using a lower grade (<20% 235 U, for example) of uranium which cannot be so readily converted to weapons use. The uranium gun-bomb route was successfully taken by South Africa. Any nation having uranium ore in sufficient quantity, a sufficiently well-developed technological and industrial infrastructure, sufficient electric power, and the desire to acquire nuclear weapons might well choose the uranium gun technology.
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