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.