Thanks Jim, for the links. I've added a few comments on each, or excerpted some relevant quotes from them.
A more mild assessment than the others below, "quickly leave the immediate area, or go inside until being further advised. Subsequent decontamination of the affected area could involve considerable time and expense."
"In the past eight years, 175 cases have been recorded worldwide of nuclear materials (not bombs) being smuggled out of former Soviet territories and other countries."
"the area struck would be off-limits for at least several months ? possibly years ? during cleanup efforts, which could paralyze a local economy and reinforce public fears about being near a radioactive area."
Message: get-the-hell-outta-there. They don't say for how long.
"A test explosion and subsequent calculations done by the DoE found that assuming nothing is done to clean up the affected area and everyone stays in the affected area for 1 year, the radiation exposure would be "fairly high".
"Because a terrorist dirty bomb is likely to cause few deaths, many do not consider one to be a weapon of mass destruction. Its purpose would presumably be to create psychological, not physical, harm through mass panic and terror. Additionally, decontamination of the affected area might require considerable time and expense, rendering affected areas unusable, and causing extensive economic damage."
NOTE: "Rendering affected areas unusable." It would appear my phrase "required semi-permanent evacuation of a large city" may not be as exaggerated as Jim claims, but I'd be very happy to be corrected with a more detailed analysis, provided it's believable.
Maybe Jim objects to my term, "large city," but consider. If a portion of the city is declared off limits, who wants to live or work in the block immediately next to the off-limit line? And in the block next to that? In fact, who would want to live in the city at all, given a choice? Wouldn't that be a common reaction?
Maybe I'd be tempted to go run my business there, because of the cheap yet perhaps high-quality buildings, but wouldn't I have more trouble getting and keeping good employees? We constantly live now with new cases of cancer discovered in our friends and neighbors, and a steady stream of cancer deaths. So, consider, what would be my eventual psychological reaction and fear level to a steady stream of new cases of cancer if I was living and working near an off-limits contaminated area? What after several of my employees get cancer? Wouldn't I eventually come down with a bad case of the hebbie-jebbies and leave, taking my company with me? Wouldn't I tell all my acquaintances of my choice? I'm sure we'd see a stream of news reports of one case after another like that. Being told we had statistically-normal cancer levels wouldn't work.
The possibility of a dirty-bomb attack is not an issue to be taken lightly; it could be very painful to us as a society. In addition to the added fear, I'd expect to see a considerable further loss of personal freedom and privacy in its aftermath. I'd also expect to see further politicizing of various new issues and severe division in the country. We'd just be better off skipping the whole mess.
Of course, skipping the whole mess is only to a limited extent our choice. But what we can choose is to be truly ready, with tested plans that minimize the societal bad experience, and thus minimize the damaging psychological reaction. That's why I was surprised that FEMA's three top most likely major disasters were 9/11, the New Orleans hurricane, and a massive California earthquake. I'd be more comfortable if they had a top four list, and were trained and ready to deal effectively with a small dirty nuclear explosion.