'Dirty bomb:' low-tech, but high fear

Most bombs are weapons of brute force. The bigger the explosion, the more effective the bomb.

A "dirty bomb" would be an exception -- an insidious weapon of fear and disruption far worse than the destruction it might cause.

For that reason, a dirty bomb is considered attractive to terrorists. It is a low-tech project that can be assembled from a wide range of ingredients: Find some radioactive material, wrap it around a core of ordinary high explosives and blow it up.

There is no nuclear explosion with a dirty bomb. There is, however, the dispersal of nuclear contamination and all the fears that go with it.

"A dirty bomb can be an effective bomb whether large or small," said physicist David Wright of the Union of Concerned Scientists. "If a terrorist can obtain spent fuel rods from a power plant, for example, he could make a large dirty bomb and cause widespread contamination and some deaths."

No dirty bomb has been used, and lethality estimates vary widely. Under one scenario in a Pentagon study, a backpack-sized 100-pound bomb with weak radioactive particles, such as those used in cancer treatment, detonated near the Washington Monument would kill no one with radiation.

At the other extreme, a truck-mounted device with the same amount of explosives, but with more than 100 pounds of bundled spent nuclear fuel rods, would produce potentially lethal doses of radiation for up to about half a mile.

But even with weaker radioactive material and a smaller explosion, Wright said, "there would be tremendous disruption." A large area of a city would have to be evacuated and decontaminated, "with tremendous economic impact."

Contamination is hard to clean up, he said. "It gets in the cracks and crevices of the sidewalks and buildings, and some buildings might have to be demolished. If this took place in the middle of, say, the financial district in Manhattan, you can imagine the disruption."

And the fear. While deaths might be light, a dirty bomb could cause a higher incidence of cancer in residents even decades after the attack.

Some experts offered reassurance on that point Monday. In most scenarios, the radiation exposure would be of such a small dose that it would cause no immediate health concerns -- and maybe no long-term adverse health effects, they said.

"The impact is more psychological," said Phil Anderson, a security expert at the Center for Strategic and International Studies. "It's the fear factor we've got to focus our energies on."

On that score, he said, "we failed miserably" last year in trying to explain the threat posed to the average American from anthrax-contaminated mail.

Making a dirty bomb could prove difficult, too.

The most dangerous material is from a nuclear reactor, but it is highly radioactive and can kill those who come in contact with it. Transportation and handling of a really "hot" material would be a major problem. It's also held under tight security.

A dirty bomb, therefore, might use a less guarded isotope such as cesium, cobalt-60 or strontium-90 found in waste material or used in medicine and research.

Last month, the Nuclear Regulatory Commission disclosed that it received an average of 300 reports a year of small amounts of radioactive materials missing from construction sites, hospitals and other users of these radioisotopes.

NRC officials said they have no evidence of anyone collecting this material to have enough for a dirty bomb, but critics argue no one knows why it was taken.

According to the UN's International Atomic Energy Agency, there have been 175 cases of trafficking in nuclear material and 201 additional cases of trafficking in radioactive material from medical or industrial sources since 1993.

-- Information from Times wires was used in this report.

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