Enriched or depleted uranium is usually produced in the form of UF6, but is used as metallic uranium or U02. This requires conversion of UF6 to UF4 or U02. UF6 is converted to UF4 by vapor-phase reduction with hydrogen. Because the heat of reaction is small, the mixture must be heated. In small reactors used for converting highly enriched uranium, heat is provided internally by reacting fluorine with hydrogen.

Three processes have been used for converting UF6 to U02. In one, UF6 is reduced to UF4, which is then hydrolyzed by steam,

UF4 + 2H20 -► U02 + 4HF

the reverse of the reaction used to make UF4. In a second process, UF6 is hydrolyzed to U02F2 by solution in water, after which ammonia is added to precipitate ammonium diuranate,

2U02F2 + 6NH4OH -+ 4NH4F + (NH4)2U207 + 3H20

The diuranate is then reduced to U02 with hydrogen at 820°C. In the third, AUC (ammonium uranyl carbonate) process, developed in West Germany by Nukem, streams of gaseous UF6, C02, and NH3 are fed batchwise into demineralized water, whereby (NH4)4U02(C03)3 is precipitated. The AUC is converted batchwise to U02 by contacting it with steam and hydrogen at 500° C in a fluidized bed, with recovery of C02 and NH3. Subsequently, steam at 650°C is supplied to the fluidized bed to reduce fluorine content to 50 to 60 ppm by pyrohydrolysis.