And you'd better believe that terrorists took notice, nuclear arms expert Gary Milhollin told CBS News Correspondent Jim Stewart.
"You can't see radioactivity," he said. "You can't smell it. You can't hear it, but you know it's there because people say it's there. So we have a formula for fear."
Osama bin Ladin's formula for fear is called a "dirty bomb." He left plans for one behind in Afghanistan. It's a simple scheme that doesn't need a nuclear reactor like Chernobyl -- just any lost or stolen nuclear material, like cobalt pellets from a medical machine. Pack them around a regular explosive, like the one that blew up U.S. embassies in Africa, and scientists tell Congress you're looking at a mass evacuation.
"If it were spread over a square mile, that would make the area uninhabitable,'' said Dr. Steven Koonin, with the California Institute of Technology.
The scientists paint a scary scenario of a dirty bomb set off in lower Manhattan. Contamination would spread over 300 city blocks, up to Central Park. Demolition might be necessary. Property losses would near $2 trillion, estimates Dr. Henry Kelly with the Federation of American Scientists.
"The biggest impact of a dirty bomb is not killing people. This is a very poor way to kill people. But the problem that you're gonna face is contaminating a large area,'' said Kelly.
An area considered most likely for such an attack is Washington itself. A likely target might be somewhere along Pennsylvania Avenue, with the White House at one end and the Capitol at the other. For maximum impact, experts say, the bomb would be placed in the middle.
A bomb set off at the corner of 10th street and Pennsylvania could contaminate the FBI headquarters, the Justice Department, and possibly the Commerce and Treasury Departments -- depending on the weather that day.
"If you're going down Pennsylvania Avenue you'll have wind swirls. There'll be hot spots. There'll be places where it's carried a long way down the avenue and up to Capitol Hill,'' said Kelly.
Only a handful of people would die from the actual blast, said Kelly. And the risk from radiation would likely be limited. But that's not the greatest worry.
"One of our concerns is that more people would be injured in the evacuation than would be injured in the actual event. You could cause real terror in people not realizing the level of danger they're exposed to."
And causing real terror is what terrorism is all about. Whether that means taking down whole buildings -- or just making them too risky to live in.
This is Part Two of Jim Stewart's series on dirty bombs. Read Part One on how the materials for dirty bombs can be found in U.S. scrap yards.