In the U. S. Department of Energy (DOE) plant at Paducah and the Comurhex plant at Pierrelatte [B5], UF4 is converted to UF6 by reaction with fluorine in a tower reactor. Solid UF4 and a slight excess of fluorine gas are fed at the top of a monel tower with walls cooled to around 500°C. Most of the UF4 reacts almost instantaneously with a flame temperature of around 1600°C. Small amounts of unreacted UF4 and uranium oxides are removed from the bottom of the tower and recycled to the hydrofluorination step.

The effluent gases containing UF6, fluorine, and diluent gases such as oxygen and nitrogen are cooled to around 150°C and passed through filters to remove entrained solids. Most of the UF6 is condensed as solid in cold traps cooled to -10° C. Residual fluorine in the gases leaving the cold trap is removed by reaction with additional UF4 in a fluid-bed reactor which forms additional UF6 and nonvolatile intermediate fluorides such as UF5. Solids from this bed are fed to the primary fluorination reactor.

Exhaust gases from the second reactor go to a second cold trap at — 50° C, which condenses most of the UF6. The last traces of UF6 are removed by a second UF4 fluid-bed reactor, which reduces the UF6 content of exhaust gases to less than 10 ppm.

UF6 produced in this way is exceptionally pure. The UF6 content is above 99.97 percent, and the overall process yield exceeds 99.5 percent. Table 5.28 summarizes U. S. DOE specifications that UF6 must meet to be fed to U. S. gaseous diffusion plants.