A dirty bomb has a spread effect -- as an explosive it may not take out more than a few yards -- but the contamination and rad effect can be over many square miles. And while the half-lifes are short for most things, some are not. In even in that short span -- minutes/hours -- tens of thousands of people can be directly effected.
For example: Cobalt. I took a whole-body dosimeter one day, right behind a turbine engineer who -- very bad for him -- had a spec of Cobalt-60 in his lungs. That's as bad as plutonium. Probably picked it up from some of the alloyed metals used in the boiling water nuke turbines he also worked on.
I'll never forget his "Aw Sh&t!".
Hopefuly he had surgery to have it removed before it started a cancer Radioactivity tags itself. It does make comprehensive clean-up possible and not that difficult.
http://www.wired.com/news/conflict/0,2100,53110,00.html How Bad Can a 'Dirty Bomb' Be?
The most likely radioactive element in a dirty bomb is cesium-137, according to Phil Anderson, a senior fellow at the Center for Strategic and International Studies. And the "consensus government view," according to a March report in The Washington Post, is that al-Qaida "has probably acquired" the isotope, which has a half-life of 30 years.
What's worse, cesium is the most "reactive" metal there is -- in nature, cesium's always found combined with another element. So the isotope becomes easily attached to roofing materials, concrete, and soil, said Fritz Steinhausler, who led the International Atomic Energy Agency's environmental assessment of the disaster at Chernobyl.
"The Russians tried to clean it up for years, and they eventually gave up. It just wasn't economically viable," said Steinhausler, who's currently a physics professor and visiting scholar at Stanford University's Center for International Security and Cooperation.
In Goiania, Brazil, four people died and more than 34,000 people had to be individually screened for contamination after a man in 1987 found an abandoned medical device filled with cesium-137 in a junkyard.
That's because cesium interacts disturbingly well with muscle tissue because of its chemical similarity to potassium, which muscles need to flex.
Fortunately, the body is used to processing these kind of chemicals, and excretes half of the cesium it absorbs within 100 days. (In contrast, radioactive strontium-90, similar to calcium, is absorbed into bone, and can take 30 years for the body to get rid of half.) But the absorbed cesium "would nevertheless cause a radiation dose, potentially increasing the risk for cancer," Steinhausler said.
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Is this is set up in a major city, it could make it uninhabitable for a long time!
http://hps.org/publicinformation/ate/q379.html The following question was answered by an expert in the appropriate field:
Q: What is the biological half-life for 54Mn, 58Co, and 60Co? Thanks.
A: The biological half life for cobalt is complex. Half of the material entering the blood stream is assumed to go directly to excretion, 5% goes to the liver and the remaining 45% goes to other tissues where it assumed to be uniformly distributed. Of the cobalt that goes to any tissue fractions 0.6, 0.2, and 0.2 are assumed to be retained with half-lives of 6, 60, and 800 days respectively. The half life for material in the blood stream is 0.5 days.
Manganese: 35% of the material entering the blood stream goes to bone, where it is retained with a half-life of 40 days. Of the remainder, fractions 0.1 and 0.15 go to the liver and are retained with half-lives of 4 and 40 days, respectively, while fractions 0.2 and 0.2 are assumed to become uniformly distributed among other organs and tissues with half-lives of 4 and 40 days, respectively.
The biological half-life is independent of the isotope. The effective half-life includes the radioactive decay but this will have little effect on the shorter half-lives given above.
Gary Kramer, PhD