The one known attempt to slip a small quantity of bomb-grade uranium out of Russia was foiled, but smuggling routes are being established. The most worrisome routes are not the well-monitored ones to the West, but those through Kazakhstan or Georgia to Iraq, Iran, Afghanistan, India, and Pakistan. U.S. officials have credible evidence that Iranian front companies have tried to acquire nuclear materials from Kazakhstan. To have a credible nuclear threat, terrorists do not need to produce the complex and efficient bombs like those in the U.S. and Russian arsenals. Their task would be cruder, i.e., to create a critical mass to make the nucleus of the plutonium or uranium atoms fission and release enormous energy. Either of two simple bomb designs could reproduce the Hiroshima bomb: a gun-type bomb in which a slug of uranium shot from a modified gun barrel boosts a chunk of uranium to critical mass, as well as an implosion bomb. A team might need no more than a year to master bomb design and obtain uranium. Then it could assemble a workable nuclear bomb in a few days, according to David Fischer of the International Atomic Energy Agency in Vienna. He indicates, however, that this scenario is extremely unlikely, since metallurgist terrorists and physicist terrorists are rare. Still, no one is doing much to choke off the potential supply. The world is not yet awash in the fuel for bombs, but the spigot has been turned.