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The technical answer is that the threat is still considered to be remote; there is no hard evidence that any terrorist group, including bin Laden's, has a finished nuclear weapon in its arsenal. But not long ago, anthrax seemed a distant threat. And it is possible for the bad guys to assemble an atom bomb with contraband uranium and off-the-shelf parts. "It's not particularly probable, but it's possible,'" says Anthony Cordesman, a senior fellow at the Center for Strategic and International Studies in Washington. "The difficulty is that we are dealing with a wide range of low-probability cases. We can't be afraid of any one, but we have to be concerned about all of them." Among those probabilities: "dirty" conventional bombs loaded with radioactive garbage and attacks on nuclear plants that cause massive radiation leaks.
For years, cloak-and-dagger stories have circulated that Soviet suitcase nukes (also known as atomic demolition munitions, or ADMs) had gone unaccounted for and presumably ended up on the Russian black market. The Russians have offered confusing and conflicting statements about the disposition of their ADMs, leading some to suspect the worst. The ADMs weigh from 60 lbs. to 100 lbs., according to Bruce Blair, a former U.S. Air Force officer and expert on Soviet nuclear weapons. They could be carried in a case 8 in. by 16 in. by 24 in. The fissile material inside the mini-nukes degrades over time, though, and it's unlikely that the Russians maintained them or that their new owners could. "There's no good evidence that any rebel group or terrorist has these," says John Lepingwell, a nuclear expert with the Monterey Institute of International Studies.
If terrorists can't buy portable nukes, they would have to make them. And in a frightening study done by the Nuclear Control Institute, a nonproliferation group in Washington, a panel of nuclear-explosives experts concluded that a group of dedicated terrorists without nuclear backgrounds could assemble a bomb if it had the right materials (such as plutonium 239, uranium 235, plutonium oxide and uranium oxide). It would take about a year to complete the job. "There's little question that the only remaining obstacle is the acquisition of the material," says Paul Leventhal, the institute's president. Less than 110 kg of active ingredients could yield 10 kilotons of explosive power--a Hiroshima-size weapon. Even if the terrorists didn't get the recipe quite right, a 1-kiloton yield could still devastate a city. And forget the suitcase: a truck will do, or a container ship to float the bomb into an American port.
Where would bin Laden get the material? Again, the most common answer is Russia, with its reputation as a fissile flea market. And a bin Laden associate has told authorities that the mastermind is shopping for nuclear ingredients. Adds Leventhal: "My feeling is that the prudent assumption is that bin Laden is nuclear capable in some fashion." Other experts are less certain that any terrorist group could pull off a nuke. A 1999 Rand study on terrorism noted somewhat reassuringly that "building a nuclear device capable of producing mass destruction presents Herculean challenges for terrorists and indeed even for states with well-funded and sophisticated programs."
Which is why the greater danger may lie in dirty bombs, conventional weapons used to spray radioactive material--anything from used reactor rods to contaminated clothing--over wide areas. Although the death toll wouldn't be great, the contamination and the public panic could be widespread. "The ultimate dirty bomb is a nuclear power reactor," says NCI's Leventhal. That someone will run a jet into a cooling tower isn't the only risk. Periodically the Nuclear Regulatory Commission has staged mock attacks against facilities, and the faux intruders won half the time--meaning they were in a position to cause severe damage. It's a target-rich environment: not only is the core vulnerable, but one NRC study also concluded that if terrorists blew up the cooling pool that holds the spent fuel, the radiation could kill 6% of the people living within 10 miles of the plant.