Among U.S. counterterrorism officials, it is the ultimate nightmare scenario: al-Qaeda detonating a nuclear bomb in a U.S. city. Osama bin Laden says it is a religious duty to obtain a bomb, and most experts believe that if al-Qaeda were to succeed, the group wouldn't hesitate to use it. Though building even a crude nuclear weapon is time consuming, the wide availability of raw material and scientific expertise means that it is plausible for terrorists someday to get their hands on one. "The simplest nuclear bomb," says Ivan Oelrich, director of the security project at the Federation of American Scientists, "is very simple indeed."
The biggest hurdle is getting the material that causes the nuclear explosion. For a basic nuclear weapon, terrorists would need about 100 lbs. of highly enriched, weapons-grade uranium (HEU). Fortunately, manufacturing HEU is extremely difficult. Refining it requires vast industrial facilities, top-flight engineers and the kinds of resources available to a government but not to rogue terrorist groups. Unfortunately, many states have already done the hard work, creating 1,800 tons of HEU that is housed at research facilities, weapons depots and other storage sites in as many as 24 countries, according to William Potter, director of nonproliferation studies at the Monterey Institute of International Studies. Of greatest concern is the more than 300 tons of HEU in the former Soviet Union. Some of the material may have already gone missing: since 1991, there have been seven attempted thefts reported of small amounts of bomb-grade material and more than 700 reported thefts of unrefined nuclear material. In Chelyabinsk, Russia, in 1998, Russian intelligence uncovered a plot by employees at a nuclear facility in the region to smuggle out 40 lbs. of HEU for sale on the black market.
With sufficient fissile material in hand, a trained engineer could build a crude device without too much difficulty. The most basic design is that of the Hiroshima bomb, which fired two pieces of HEU at each other from opposite ends of an artillery tube. The bomb could be assembled at a basic machine shop and would fit in the back of a truck. If smuggled into the U.S. and detonated in a major metropolitan area, such a weapon could kill hundreds of thousands.