Michael Crowley in Popular Science:
…but of more than a dozen nuclear-arms experts I interviewed, almost all agreed that assembling a crude nuclear bomb, though extremely difficult, is by no means impossible.
Just ask Graham Allison. In his recent book Nuclear Terrorism: The Ultimate Preventable Catastrophe, he concludes that a terrorist nuke attack is “inevitable” unless the U.S. works much harder and faster to safeguard nuclear material. A former assistant secretary of defense who served under President Bill Clinton and now teaches government at Harvard University, Allison is actually taking small bets from colleagues that terrorists will detonate a crude nuclear bomb in a U.S. city within a decade. “If this happened tomorrow,” he says, “I could almost explain it more easily than I could explain why it hasn’t happened.”
Not everyone is as alarmist as Allison. Most experts with whom I spoke said that a nuclear terror attack is plausible but not inevitable, and that there’s no way to precisely gauge the odds. “I don’t think the public ought to lose a lot of sleep over the issue,” says nuclear physicist Tom Cochran of the Natural Resources Defense Council.
There is a consensus, though, about how such a nightmare would unfold. What follows is an examination of each step a terrorist organization would need to take to pull off a nuclear attack, and what is being done to raise the hurdles.