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Terrorists might attack the U.S. homeland again this summer, the Justice Department and the FBI warned last month. The same day, the Department of Energy announced a $450 million plan to counter terrorist nuclear weapons and dirty bombs. And shortly afterwards, the Justice Department released some details about Jose Padilla, the one-time street thug who had received extensive al Qaeda training and had hoped to explode a dirty bomb in the United States.

But according to the Justice Department announcement, al Qaeda had doubted that Padillas proposal to build a dirty bomb was practical. They directed him instead to blow up two apartment buildings using natural gas. They apparently felt that such an action would have a greater chance of spreading death and destruction than would a radiological weapon.

Al Qaeda was right. Perhaps that should scare you. Al Qaeda appears to understand the limitations of these devices better than do many government leaders, newspapers, and even many scientists.

Our experience with radiological weaponsthe fancier name for dirty bombsis limited. They do not require a chain reaction like fission or fusion weapons, but instead use ordinary explosives to spread pre-existing radioactive material. Saddam Hussein reportedly tested such a weapon in 1987, but abandoned the effort when he saw how poorly it worked. In 1995, Chechen rebels buried dynamite and a small amount of the radioactive isotope cesium-137 in Moscows Ismailovsky park. They then told a TV station where to dig it up. Perhaps they recognized the truth: that the bombs news value could be greater if it were discovered before it went off. For such weapons, the psychological impact can be greater than the limited harm they are likely to cause.

I dont mean to suggest that radioactive materials are harmless. Indeed, consider the story of scavengers in Goiania, Brazil, who found and dismantled an abandoned radiotherapy machine in 1987. The machine contained 1,400 curies of cesium-137. (A curie is the radioactivity of one gram of radium.) Two men, one woman, and one child died from acute radiation poisoning; 250 additional people were contaminated. Several of the 41 houses evacuated could not be cleaned adequately and were demolished.

Imagine now if that radiation werent confined to a few houses, but were spread over the city by an explosion. Wouldnt fatalities be higher? The surprising answer is: No. If the radioactivity were dispersed in that way, larger area would have to be evacuated, yet in all probability no specific deaths could be attributed to the event.

To understand the details, lets walk through the design of a dirty bomb similar to what Padilla wanted to build. Ill assume the same amount of radioactive material as was in Goiania:  1,400 curies of cesium-137. Radiation damage is measured in units called rem, and if you stand one meter from that source, youll absorb 450 rems in less than an hour. Thats called LD50, for lethal dose 50 percent. Untreated, youll have a 50 percent chance of dying in the next few months from that exposure.

To try do enhance the damage, lets use explosives to spread our 1,400 curies over a larger area, say a neighborhood one kilometer square. That will result in a radioactivity of 1.4 millicuries per square meter, and a careful calculation shows that residents will get a dose of 140 rems per year. But radiation illness is nonlinear. For extended exposures, the lethal dose increases by the fourth root of time, to approximately 1,250 rems for a one-year exposure and 2,500 rems for a 16-year exposure. So 140 rems per year is not enough to trigger radiation illness, even if you stayed there 24/7 for a decade. Radioactive contamination may be the one case for which the solution to pollution really is dilution.

There will be no dead bodies at the scene, unless someone is killed by the explosion itself. I suspect thats why al Qaeda instructed Jose Padilla to abandon the dirty bomb concept and try to plan a natural gas explosion instead.

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Tagged: Energy

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