Uranium oxide (U₃O₈)extracted from mines is converted to uranium hexafluoride (UF6) and then enriched by gaseous diffusion or centrifuge to separate the heavier U-238 atoms from U-235 (half-life 700 million years) atoms. Some of the U-238 is converted to Pu-239 (half-life 24,000 years). Some of the unused U-235 and Pu-239 in the spent fuel can be recovered and recycled. The radioactive waste is buried in deep, stable geologic structures.