Uranium in its natural state contains less than 1 percent of U-235, the isotope needed for reactors and bombs. Most nuclear reactors need uranium enriched to just 3 to 5 percent. Highly enriched uranium is enriched to 20 percent or more. Although 20 percent enrichment can be used for crude nuclear devices, in addition to some types of nuclear reactors, weapons-grade uranium is enriched to 90 percent or above.
