Nuclear Terrorism: 5 Scary Scenarios

(April 12) -- Security experts have long warned of the danger posed by terrorists determined to obtain a nuclear weapon. But now nuclear terrorism has moved to the top of the international agenda, with President Barack Obama deeming it "the greatest threat to U.S. and global security."

This week's nuclear security summit in Washington focuses on securing fissile material that could be used in a nuclear weapon, but experts have described many different ways in which terrorists could use nukes to nefarious ends. Here are five of the leading scenarios:

1) Terrorists steal a nuclear weapon -- or buy a stolen one.

Among the more harrowing scenarios would be if terrorists obtained a complete nuclear weapon, possibly from Russia, where lax controls, especially in the 1990s, made such a prospect plausible. Even today, experts wonder whether insiders in the Russian system could have slipped across the porous borders to ex-Soviet neighboring states with a nuclear weapon destined for the black market. Nuclear terrorism expert Graham Allison has been warning about such a scenario for decades, including in his 2004 book, "Nuclear Terrorism: The Ultimate Preventable Catastrophe."

If terrorists did obtain a nuclear weapon, how easy would it be for them to actually detonate it? The U.S. equips its nuclear weapons with permissive action links, or PALs, which are integrated into the warhead and make it difficult for an unauthorized user to set if off. But some other countries, including Pakistan, notes David Albright of the Institute for Science and International Security, don't use PALs on their weapons.

2) A terrorist group builds its own nuke.

An al-Qaida "Los Alamos" in the middle of the United States? It sounds preposterous, but some nuclear experts say a group of rogue terrorists working on a nuclear bomb in a remote U.S. location is in fact a plausible scenario. Peter Zimmerman, a nuclear physicist, and Jeffrey Lewis, an arms control expert, said in a Foreign Policy article that a technically competent terrorist could build a crude nuclear bomb using highly enriched uranium -- presumably bought on the black market -- on a budget of less than $10 million. A gun-assembled weapon, which, like the Hiroshima bomb, uses an initial charge to bring two fissionable masses together rapidly, could be transported in the back of a van or small truck for use in a terrorist attack on a major city. "In fact, it is perhaps easier to make a gun-assembled nuclear bomb than it is to develop biological or chemical weapons," they wrote.

That's not to say such a scenario is likely. "A terrorist group would have to do many of the same things (though technological progress would make some steps easier) all while attempting to hide from law enforcement and intelligence," Michael Levi, a fellow at the Council for Foreign Relations, wrote of the terrorist-constructed nuke scenario. "This doesn't mean that terrorists couldn't build a gun-type bomb, but it suggests that their chances of failure aren't negligible."

3) A rogue state provides terrorists with a nuclear weapon.

Would a desperate North Korean regime on the brink of collapse sell or give a nuclear weapon to a terrorist group? What about Iran? Improbable, say most analysts, who point out that countries that have gone through the trouble of developing nuclear weapons tend to keep a close eye on them. "It seems unlikely that terrorists could obtain a usable nuclear weapon from any of the nine countries that currently possess them, although there is some concern that a possible source could be the Pakistani stockpile, should that unstable country implode," wrote retired Lt. Gen. Robert G. Gard Jr., chairman of the Center for Arms Control and Non-Proliferation, in 2008. That said, even the unlikely prospect of terrorists being given a nuclear weapon clearly worries security experts.

4) Terrorists construct a 'dirty bomb.'

Even those who believe a terrorist group could build or obtain a nuclear weapon admit that constructing a radiological dispersal device, better known as a "dirty bomb," would be significantly easier for terrorists to master. All it takes is radioactive materials -- such as medical or industrial isotopes -- and some conventional explosives. The International Atomic Energy Agency, which tracks trafficking of nuclear material, has registered 336 cases of unauthorized possession of nuclear or radiological material between 1992 and 2008. There is only some solace in the fact that just 15 of those cases involved highly enriched uranium or plutonium -- material suitable for a nuclear weapon. A dirty bomb wouldn't be nearly as destructive as even the smallest nuclear weapon, but by spreading radioactive material, it could sow mass panic and disruption. Set off in New York or Washington, such a device could have a devastating impact, making parts of a city uninhabitable for the long term.

5) Terrorists attack a nuclear facility.

After the 9/11 attacks in New York and Washington, U.S. officials admitted they had never considered the prospect of an attack using commercial airliners on nuclear facilities. Now the Nuclear Regulatory Commission says the nation's nuclear power plants are secure from attack, but at least one former intelligence official claims that's not the case. Charles Faddis, a retired CIA official who reviewed nuclear plant security around the country, recently concluded that the facilities were woefully unprepared for a terrorist attack. "A full-scale meltdown of a major reactor would be catastrophic," he warned. "Such an incident at the Indian Point Plant in New York state, for instance, would likely render large parts of the metropolitan New York City area uninhabitable for decades and likely kill tens of thousands."