Centrus used the gaseous diffusion process to enrich uranium at its Paducah, Kentucky, plant until 2013. This process used uranium hexafluoride (UF6) as a feed material. UF6 is a solid at room temperature but becomes a gas when heated above 135 degrees Fahrenheit. Once heated to a gaseous state, the UF6 was fed into the plant's cascades to be enriched.
The process separated the lighter U235 isotopes from the heavier U238. The gas was forced through a series of porous membranes with microscopic openings. Because the U235 is lighter, it moved through the barriers more easily.
As the gas moved, the two isotopes were separated, increasing the U235 concentration and decreasing the concentration of U238.