Uranium enrichment is the industrial process of increasing the proportion of the fissile uranium-235 isotope relative to the more common uranium-238. Natural uranium contains about 0.7% U-235. Enrichment to 3-5% produces fuel for civilian nuclear reactors. Enrichment to 20% or above is defined by the IAEA as highly enriched uranium (HEU). Weapons-grade uranium is typically enriched to 90% or higher. Iran's enrichment to 60% purity is well above civilian reactor levels and is generally considered a threshold with no legitimate peaceful purpose. Converting 60%-enriched uranium to 90% weapons-grade requires relatively few additional centrifuge steps. Iran has accumulated enough 60%-enriched uranium to potentially produce multiple nuclear devices if it chooses to enrich further.