Michael May, a former director of the Lawrence Livermore National Laboratory, where U.S. nuclear weapons are designed, and now a professor emeritus at the Center for International Security and Cooperation at Stanford, said the technological hurdles to a terrorist bomb remain, realistically, quite high.
He discounted the possibility terrorists could make use of a stolen warhead because of all the sophisticated security devices built into them. He also said it would be all but impossible for a non-state terrorist group to develop the capability of making its own weapons-grade uranium, because of the industrial infrastructure required.
The real fear, he said, is that terrorists could steal or buy from corrupt officials weapons-grade uranium, either from Russia or perhaps a country like Pakistan, where many government and military officials are sympathetic to radical Islamists. Getting that material is far more difficult than actually creating a workable weapon, he said.
"Scientists have been pointing to this possibility for years," May said. "What higher priority can there be? It's not a high-likelihood event, but the results are so catastrophic you have to pay attention."
He said that a relatively small, 1 kiloton bomb -- about one-fifteenth the size of the weapon that destroyed Hiroshima -- would kill most of the people within an 800-meter radius, or about a half-mile. Depending on the direction and speed of the winds, the fallout could spread for miles and poison huge numbers of people.
Bunn helped prepare a study at Harvard that estimated a 10-kiloton weapon detonated in Manhattan could kill 500,000 people and cause $1 trillion in immediate economic disruption.
One problem with a nuclear attack is that, unlike other kinds of attacks, there is no way of mitigating the devastation. "Once it happens, there's nothing to do except pick up the wounded and care for them," said May.
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