Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-white metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all its isotopes are unstable (with half-lives of the 6 naturally known isotopes, uranium-233 to uranium-238, varying between 69 years and 4 1⁄2 billion years).
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In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%).[4] Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years,[5] making them useful in dating the age of the Earth.
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The major application of uranium in the military sector is in high-density penetrators. This ammunition consists of depleted uranium (DU) alloyed with 1–2% other elements. At high impact speed, the density, hardness, and pyrophoricity of the projectile enable destruction of heavily armored targets. Tank armor and other removable vehicle armor are also hardened with depleted uranium plates.
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The most common forms of uranium oxide are triuranium octoxide (U3O8) and UO2. Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions.
http://en.wikipedia.org/wiki/Uranium#Nuclear_weaponry

L'uranium appauvri, un sous-produit de l'enrichissement de l'uranium, est très prisé pour sa dureté et sa densité. Il est pyrophorique, employé comme arme antichar dotée d'un fort pouvoir à la fois pénétrant et incendiaire : à très haute vitesse, il perfore aisément les blindages en s'enflammant lors de l'impact, provoquant un incendie qui fait exploser le véhicule touché. Ainsi, des munitions à base d'uranium appauvri (obus de 20 à 30 mm des avions ou hélicoptères chasseurs de chars) ont été utilisées lors des guerres du Golfe (guerre du Koweït et guerre en Irak) et du Kosovo.
Dans ses emplois militaires, l'uranium appauvri est également utilisé pour faire des plaques de blindages
http://fr.wikipedia.org/wiki/Uranium#Applications