Source: Appendix A of the PEIS (DOE/EIS-0269)
Uranium tetrafluoride (UF4) is a green crystalline solid that melts at about 1,760°F (960°C) and has an insignificant vapor pressure. It is very slightly soluble in water. It is generally an intermediate in the conversion of UF6 to either uranium oxide (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 can be readily converted to either uranium metal or uranium oxide. UF4 is less stable than the uranium oxides and produces hydrofluoric acid in reaction with water; 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.
Uranium tetrafluoride (UF4) reacts slowly with moisture at ambient temperature, forming UO2 and HF, which are very corrosive.