Uranium tetrafluoride
Uranium tetrafluoride (UF4) is a green crystalline solid compound of uranium with an insignificant vapor pressure and very slight solubility in water. Uranium in its tetravalent (uranous) state is very important in different technological processes. In the uranium refining industry it is known as green salt.

UF4 is generally an intermediate in the conversion of uranium hexafluoride (UF6) to either uranium oxides (U3O8 or UO2) or uranium metal. It is formed by the reaction of UF6 with hydrogen gas in a vertical tube-type reactor or by the action of hydrogen fluoride (HF) on uranium dioxide. UF4 is less stable than the uranium oxides and reacts slowly with moisture at ambient temperature, forming UO2 and HF, the latter of which is very corrosive; it is thus a less favorable form for long-term disposal. The bulk density of UF4 varies from about 2.0 g/cm3 to about 4.5 g/cm3 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 UF4 as the core material. UF4 is generally chosen over other salts because of the usefulness of the elements without isotope separation, better neutron economy and moderating efficiency, lower vapor pressure and better chemical stability.

Like all uranium salts, UF4 is toxic and thus harmful by inhalation, ingestion, and through skin contact.