Weapon-grade and civilian uranium
Natural uranium is poor in the fissile isotope, containing as it does only 0.70% of uranium 235. It must be enriched before it can be used as a fuel in any commercial reactor. These reactors are powered by uranium which is enriched to have anywhere between 3 and 4% of uranium 235. In order to have an atomic bomb, the uranium has to be enriched to above 90%. The boundary between the uranium meant for civilian uses (LEU or low-enriched uranium) and that uranium meant for military use (HEU or highly-enriched uranium) is generally fixed at 20%.