Studying ‘Dirty’ Bomb Scenario

In the heart of a major American city, terrorists set off an explosive encased in radioactive waste that scatters metal debris for hundreds of yards and coats several blocks with radioactive dust.

A handful of people suffer radioactive burns, and a small number of others will develop cancer as a result. The attack stirs a panic that prompts a major evacuation and forces local officials to consider razing scores of buildings that they fear no one will ever willingly use again.

This scenario has been troubling federal officials, and especially so this week, when the imprisoned Al Qaeda leader Abu Zubeida warned that his group knows how to build a device that would spread radioactivity over a wide area.

While these “dirty” bombs are likely to kill relatively few directly, they exploit fears of radioactivity in a way that makes them a devastating terrorist weapon.

“The sheer psychological impact is enormous,” said Nikolai Sokov, an analyst at the Monterey Institute’s Center for Non-Proliferation Studies. “More people may be killed from the stampede that follows than from the bomb.”

U.S. officials and outside experts acknowledge that some of their knowledge of the subject is based on guesswork.

So far no group has set off a dirty bomb.

Most experts reason that the chances of such an attack in the United States remain low because terrorists could be reluctant to use such a complex and risky device when conventional explosives are easy to use and readily available.

Nevertheless, the materials and know-how needed to build such devices are widely available, and even apart from Zubeida’s statements there have been indications that militant groups have been seeking to build such devices.

Radioactive waste materials are widely available in the United States and elsewhere, from civilian and military sources. Radioactive products are used in medicine and industry, including those from nuclear power generation.

Governments have sought to control the handling of such materials, but officials have acknowledged since Sept. 11 that rules haven’t been tight enough.

Officials are adding detection devices at key border crossings, yet they acknowledge that smugglers could cross the border in unguarded areas.

Overseas, highly radioactive nuclear materials are scattered widely, including in the former Soviet Union. In December in the former Soviet republic of Georgia, three hunters gathering firewood died of exposure to three abandoned canisters of radioactive material.

U.S. officials are not sure how seriously to take the threats of Zubeida, who may have been trying to frighten Americans. But Al Qaeda’s interest in dirty bombs has been well established, including through the 1998 trial of suspects in the bombings of U.S. embassies in Kenya and Tanzania.

In 1995, Chechen rebels left an unexploded dirty bomb in a Moscow park to frighten Russians, noted John Parachini, an analyst at the Rand Corp. think tank in Washington. The event received wide publicity, he said, though the radioactive level of the materials was so low that Russian authorities “laughed it off.”

No conventional military is believed to keep dirty bombs because of their ineffectiveness against troops.

Yet few doubt that they are within the technical capabilities of major terrorist groups.

A senior U.S. official said such devices are “fairly simple to make; anyone who can come up with the material can build it. . . . The range of entities that can do this is probably pretty broad.”

It is more complicated to use the devices effectively, experts say.

The user would need to figure out how powerful an explosive it would take to spread enough radioactivity to contaminate an area, based on how radioactive their materials were. Wind conditions and other factors also would affect the blast.

“There are a lot of environmental variables that are hard to gauge,” said Rand’s Parachini.

People near a dirty-bomb explosion could protect themselves from low-level radioactivity by taking some simple precautions, such as staying inside and taking showers, for example. Gas masks offer protection as well.

The difficult question for the authorities in such an attack would be how much radioactivity exposure to accept.

At a Senate hearing in March, Steven E. Koonin, provost of Caltech, testified that under U.S. government guidelines the dispersion of a fraction of a gram of a certain isotope over a square-mile area would make the area uninhabitable.

Yet, exposing the population to that quantity of the material would only add four more cases of cancer per 100,000 people--in addition to the 20,000 cases that are statistically likely to result from all other causes, Koonin testified.

Koonin said he also could imagine an attack that exposed people in 100 square blocks of a business district to three times the acceptable level of radioactivity. At that level, the attack would probably produce no fatalities.

Nonetheless, the area could be sealed off for months of decontamination, and hundreds of thousands of people could be expected to show up at hospitals for screening. Dozens of buildings might be razed because of the difficulty of decontaminating them.

“There would be billions of dollars of economic damage,” Koonin said.