A uranium enrichment process used to prepare uranium for use in fabricating fuel for nuclear reactors by separating its isotopes (as gases) based on their slight difference in velocity. (Lighter isotopes diffuse faster through a porous membrane or vessel than do heavier isotopes. ) This process involves filtering uranium hexafluoride (UF₆) gas to separate uranium-234 and uranium-235 from uranium-238, in order to increase the percentage of uranium-235 from 1 to 3 percent. The only gaseous diffusion plant in operation in the United States is in Paducah, KY. A similar plant near Piketon, OH, was closed in March 2001. Both plants are leased by the United States Enrichment Corporation (USEC) from the DOE and regulated by the NRC since March 4, 1997. For additional information, see Gaseous Diffusion and the Fact Sheet on Gaseous Diffusion.