[Steven W.]

this ("dirty bombs") is reportedly one of the fears that led to the alert being issued on Monday; that terrorists in Canada and US are now actively preparing to detonate bombs with radioactive materials that could dessimate major population areas and spread major doses of radiation to thousands of Americans.

[pgkdan]

I don't know...maybe Al-Qaeda knows we're listening and their just issuing disinformation to keep us off balance.

[Thud]

It depends on your definition of major. It takes energy to make energy. Unless the device is capable of creating a significant out of control chain reaction to flood the fissionable material with the right sort & pattern of neutrons (i.e., a fizzle), it would rely on the already existing radioactive energy of whatever stuff the weapon is packed with. And that just isn't much unless the device is either unshieled and promptly detectable, or so shielded as to be prohibitively massive save when detonated on a ship, in which case it would be a self-solving problem.