there are 258 shuttered nuclear reactors worldwide that have not been properly decommissioned. They contain between 50 and 100 metric tons of HEU, enough for 1,000 bombs -- and most are not under proper guard.
"Many of these facilities are in urban locations with only modest security, presenting potential targets to would-be nuclear terrorists. A large fraction are in Russia, which has yet to give adequate priority to cleaning out facilities containing HEU that is no longer needed. At several sites, there is enough HEU to make more than 10 gun-type weapons," the paper states.
Russia, which accounts for about one-third of the world's HEU-fueled reactors and more than half of the world's civilian HEU, has yet to make a commitment to convert or decommission any of its own HEU-fueled research reactors. President George W. Bush took pressure off of Russia to do so at a February 2005 summit with Russian President Vladimir Putin. The two leaders agreed to limit to "third countries" U.S.-Russian cooperative efforts to deal with the danger from HEU-fueled reactors.
If I were Iran and wanted The Bomb all the highly enriched uranium I'd need for a nuke program is sitting over the border under light security. I wouldn't waste precious technical resources developing a fuel cycle if all I wanted was The Bomb.