Telling Atomic Plowshares from Nuclear Swords

Whether you want nuclear power or a nuclear bomb, you start off with the same basic material: uranium. In both civilian and military nuclear programs, mined uranium is converted into a gas and then enriched in centrifuges to increase the proportion of U-235--the uranium atoms that start and continue a nuclear chain reaction. Uranium that feeds a power plant needs only 3% enrichment, but a nuclear warhead requires at least 90% enrichment, and more centrifuges. The difference is so significant that international inspectors would probably detect the enrichment change unless Iran chose to enrich its uranium covertly, slowing the process. A country with civilian nuclear plants could choose to reprocess spent atomic fuel into plutonium, which can also be used for bombmaking. That would require the construction of a separate reprocessing facility. You need to be smart enough to actually design a weapon, but as Edwin Lyman of the Union of Concerned Scientists points out, "If you can make fissile material, you can make a warhead."