The Mechanics Of A 'Dirty Bomb'

If terrorists wanted to build a radiological -- or so-called "dirty bomb" -- in America, they might very well start their search for materials in our own scrap metal yards, reports CBS News Correspondent Jim Stewart.

Forty-five times a year, on average, radioactive elements are mistakenly thrown away in this country. One of the most recent incidents was at the Nucor Steel recycling plant in Winton, N.C. On its way to being dumped in the furnace, Geiger counters sniffed out a radioactive industrial gauge. It was still loaded, and still hot.

Cesium-137 is one of the most common radioactive elements used in heavy industry. Osama Bin Laden knows that. That's why he had it on a shopping list recently recovered in Afghanistan, along with a description of a dirty bomb, which is not a nuclear bomb like the type seen on ICBM missiles, but a conventional bomb surrounded by nuclear material.

"A nuclear weapon and a dirty bomb have very little in common," said Gary Milhollin, an expert on nuclear weaponry.

"If you're making a nuclear weapon you have to achieve a chain reaction -- and you have to have material that's very hard to get your hands on. To make a dirty bomb, the materials are easier to get and there's no science that's very difficult in how you blow it up."

And it is the ease of obtaining those materials that makes a dirty bomb so worrisome. From medical devices, to mechanical gauges using cesium, to food irradiation facilities using cobalt, there are more than 18,000 sources of industrial radiation in America. The old Soviet Union has even more. Woodcutters there stumbled on a lost core of it last December and almost died from the experience. It became a volunteers-only job to recover the stuff.

And therein lies the biggest problem in building a dirty bomb. Even if you find all the parts, putting them together can kill you. To make an effective one you need a lot of radioactive material -- which either means making a shield so heavy the bomb becomes impossible to move -- or building a bomb without a shield, which would mean almost instant death for the bomb maker from radiation poisoning.

Which is why no military on earth -- save Iraq's, which failed -- has tried to make a dirty bomb.

On Monday, CBS News National Security Correspondent David Martin reported that a key lieutenant of bin Laden arrested in Pakistan last month has provided interrogators with alarming information pertaining to al Qaeda's ability to build a dirty bomb and smuggle it into the United States.

But as Milhollin said, "The Pentagon has decided that radiation bombs are not militarily effective because no one's been able to figure out a way to take radioactivity intense enough to hurt people and transport it somewhere and make a bomb out of it. It's that simple."