Take radioactive cesium, stolen from a common medical device. Blow it up with 10 pounds of dynamite at the National Gallery of Art in Washington, D.C., and you could force the abandonment of the U.S. Capitol, Library of Congress and Supreme Court for decades.
That scenario, from a study by the Federation of American Scientists, is just one of many possibilities for wreaking havoc with "dirty bombs."
The radioactive raw materials are too easy to come by, experts say.
The Nuclear Regulatory Commission says that it receives an average of 300 reports a year of small amounts of radioactive materials missing from construction sites, hospitals and other users.
Lost or stolen
Sometime on the weekend of March 15-16, for instance, a device that uses cesium-137 for testing construction materials was stolen from a locked trailer in Columbia.
Another went missing after it fell from a pickup truck in Adamstown two years ago.
Both were recovered.
But a terrorist who obtained such a device and some conventional explosives would have a dirty bomb capable of spreading radioactive debris, fear and economic disruption.
A dirty bomb is by definition too crude to produce nuclear fission - a nuclear blast.
Immediate fatalities - from the explosion or from radiation exposure - would likely be few compared with crashing an airliner into a building.
Rather, "it is an 'area denial' strategy," said David C. Wright, a physicist with the Union of Concerned Scientists. The terrorist "can disrupt things by forcing people to be evacuated from an area, and forcing time-consuming and expensive decontamination."
Easy and effective
Its genius, according to Henry Kelly, president of the Federation of American Scientists, is that the terrorist doesn't need weapons-grade uranium or plutonium nor the know-how to build an atomic bomb.
Instead, he said, "materials that could easily be lost or stolen could contaminate tens of city blocks at a level that would require prompt evacuation and create terror in large communities, even if radiation levels were low."
In testimony March 6 before the Senate Foreign Relations Committee, Kelly said that "since there are often no effective ways to decontaminate buildings that have been exposed at these levels, demolition may be the only practical solution."
No terrorist is known to have exploded such a device. But accidental releases of common radioactive materials have hinted at what might ensue.
Deadly scavenging
In 1987, a metal scavenger stole a medical device from a defunct cancer clinic in Goiania, Brazil, stripped the device of its cesium-137 source - a capsule weighing less than an ounce - and took it home.
The scavenger and three other people subsequently died, and 249 were contaminated.
"You can see this is not a weapon of mass destruction," said Mark Gwozdecky, spokesman for the United Nations' International Atomic Energy Agency in Vienna. Even so, "a fair amount of disruption can take place."
The radioactive debris can lodge in the cracks and crevices of buildings or bond chemically with soils and pavements, posing a long-term health threat.
In Goiania, more than 125,000 drums of contaminated material had to be collected and 85 houses were demolished and hauled away.
About 110,000 people are being monitored for medical problems.
Enormous disruption
In busy, modern cities, the disruption could be enormous.
Federation scientists studied how the cesium-137 from a common medical device, if dispersed from the National Gallery of Art in Washington by 10 pounds of TNT, would affect the capital.
They found that the radioactivity would contaminate 40 city blocks stretching a mile downwind from the blast.
Residents who stayed there would have a one-in-10,000 risk of getting cancer from the fallout. That's enough under EPA guidelines to prompt the abandonment of the area, including Capitol Hill, the scientists' group said.
In another scenario, the scientists predicted that exploding a single piece of cobalt-60 from a food irradiation device in lower Manhattan would spread contamination downwind as far as Connecticut.
"The entire borough of Manhattan would be so contaminated that anyone living there would have a one-in-a hundred chance of dying from cancer caused by the residual radiation," Kelly testified.
"It would be decades before the city was inhabitable again, and demolition might be necessary."
The fear and panic sparked by such a blast likely would extend far beyond the actual danger zone, because of the public's lack of information about radioactivity, said Gwozdecky. Natural radiation from sunlight and other natural sources bombards us daily, he added, "at far higher levels than you would imagine."
"A lot of this residue [from a dirty-bomb blast] would not rise to the level where it would require cleanup," he said.
The health risks from a "dirty bomb" would depend on wind direction, distance from the blast and the size of the radioactive particles created by the explosion, experts say.
Losses catalogued
The danger posed by loss of radiological materials has gotten increased scrutiny since Sept. 11, but worrisome incidents of theft and smuggling have been going on for years.
The International Atomic Energy Agency has catalogued nearly 250 incidents of lost or stolen radiological materials and devices, including about a dozen each year involving weapons-grade materials.
Just yesterday, the IAEA began searching in Georgia for two more missing nuclear batteries containing strontium-90 once used by the Soviet Union to power remote navigation stations.
Six had previously been recovered.