Uranium tetrafluoride is the inorganic compound with the formula UF<sub>4</sub>. It is a green solid with an insignificant vapor pressure and low solubility in water. Uranium in its tetravalent (uranous) state is important in various technological processes. In the uranium refining industry it is known as green salt.
UF<sub>4</sub> is prepared from UO<sub>2</sub> in a fluidized bed by reaction with hydrogen fluoride. The UO<sub>2</sub> is derived from mining operations. Around 60,000 tonnes are prepared in this way annually. A common impurity is UO<sub>2</sub>F<sub>2</sub>. UF<sub>4</sub> is susceptible to hydrolysis as well.
UF<sub>4</sub> is formed by the reaction of UF<sub>6</sub> with hydrogen gas in a vertical tube-type reactor. The bulk density of UF<sub>4</sub> varies from about 2.0 g/cm<sup>3</sup> to about 4.5 g/cm<sup>3</sup> depending on the production process and the properties of the starting uranium compounds.
A molten salt reactor design, a type of nuclear reactor where the working fluid is a molten salt, would use UF<sub>4</sub> as the core material. UF<sub>4</sub> is generally chosen over related compounds because of the usefulness of the elements without isotope separation, better neutron economy and moderating efficiency, lower vapor pressure and better chemical stability.
Uranium tetrafluoride reacts stepwise with fluorine, first to give uranium pentafluoride and then volatile UF<sub>6</sub>:
UF<sub>4</sub> is reduced by magnesium to give the metal:
UF<sub>4</sub> reacts slowly with moisture at ambient temperature, forming UO<sub>2</sub> and HF.
Like most binary metal fluorides, UF<sub>4</sub> is a dense highly crosslinked inorganic polymer. As established by X-ray crystallography, the U centres are eight-coordinate with square antiprismatic coordination spheres. The fluoride centres are doubly bridging.
Like all uranium salts, UF<sub>4</sub> is toxic and thus harmful by inhalation, ingestion, and through skin contact.