Uranium metal is very reactive. It tarnishes in air, with the oxide film preventing further oxidation of massive metal at room temperature. However, finely divided uranium ignites spontaneously at room temperature, and massive uranium bums steadily at 700°C, forming U308.

Water attacks massive uranium slowly at room temperature and rapidly at higher temperatures. U02 and UH3 are formed, heat is evolved, and the metal swells and disintegrates. In water-cooled reactors uranium metal must be clad or canned in nonreacting metals such as aluminum, stainless steel, or zirconium. Nitric acid dissolves uranium readily.