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What are the common forms of uranium?

Uranium can take many chemical forms. In nature, uranium is generally found as an oxide, such as in the olive-green-colored mineral pitchblende. Uranium oxide is also the chemical form most often used for nuclear fuel. Uranium-fluorine compounds are also common in uranium processing, with uranium hexafluoride (UF6) and uranium tetrafluoride (UF4) being the two most common. In its pure form, uranium is a silver-colored metal.

The most common forms of uranium oxide are U3O8 and UO2. Both oxide forms have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octaoxide (U3O8) is the most stable form of uranium and is the form most commonly found in nature. Uranium dioxide (UO2) is the form in which uranium is most commonly used as a nuclear reactor fuel. At ambient temperatures, UO2 will gradually convert to U3O8. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.

Uranium hexafluoride (UF6) is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt. Uranium hexafluoride does not react with oxygen, nitrogen, carbon dioxide, or dry air, but it does react with water or water vapor (including humidity in the air). When UF6 comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive hydrogen fluoride (HF) and a uranium-fluoride compound called uranyl fluoride (UO2F2). For this reason, UF6 is always handled in leaktight containers and processing equipment. Although very convenient for processing, UF6 is not considered a preferred form for long-term storage or disposal because of its relative instability.

Uranium tetrafluoride (UF4) is often called green-salt because of its characteristic color. It is generally an intermediate in the conversion of UF6 to either uranium oxide (U3O8 or UO2) or uranium metal. UF4 can be readily converted to either uranium metal or uranium oxide. Depleted UF4 is a solid composed of agglomerating particles with a texture similar to baking soda. It is non-volatile, non-hydroscopic, but only slightly soluble in water. After exposure to water, UF4 slowly dissolves and undergoes hydrolysis, forming several possible uranium compounds and hydrogen fluoride (HF). The time for hydrolysis can be significant. Although not as stable as the uranium oxides, several recent studies have indicated that UF4 may be suitable for disposal.

Uranium metal is heavy, silvery white, malleable, ductile, and softer than steel. It is one of the densest materials known (19 g/cm3), being 1.6 times more dense than lead. Uranium metal is not as stable as U3O8 or UO2 because it is subject to surface oxidation. It tarnishes in air, with the oxide film preventing further oxidation of massive metal at room temperature. Water attacks uranium metal slowly at room temperature and rapidly at higher temperatures. Uranium metal powder or chips will ignite spontaneously in air at ambient temperature.

Additional information can be found in the "Properties of Uranium and Its Compounds" section of this web site:

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