Goiânia accident comes to mind. Very nasty incident indeed but not very efficient. After two weeks of exposure in different locations it managed to kill only 4 people in Ferreira scrap yard. If dispersed around several city blocks it would cause evacuation, chaos and slow and costly clean up but I doubt any radiation poisonings would occur.
Yes, that's the one I had in mind. But in Goiânia the Cs-137 got out through ignorance; a dirty bomb would get it out by intent, and probably much more efficiently.
Preparedness plans are not public but I'd guess fire brigades would measure radioactivity after any bomb blast and start evacuation if necessary.
Yes. Cs-137 has a 30-year half life. That's too long to simply wait for it to decay but still short enough to be very radioactive. What makes it especially dangerous is its water solubility, like all the alkali metals. But that also allows it to wash away and/or sink into the soil. Over decades Cs-137
is the most problematic isotope from both nuclear weapons tests (e.g., Bikini, Eniwetak) and nuclear reactor accidents (Chernobyl, Fukushima) so we now have some experience with its behavior in the environment.
Fortunately, there seems to be a trend away from radioisotopes such as Cs-137 in medical and industrial applications to less mobile and/or shorter-lived isotopes and to particle accelerators, so hopefully there will be less of it around to steal and/or misplace. Unfortunately, Cs-137 exists in large amounts as an otherwise useless fission product, and that made it attractive to those who didn't fully consider the risks. There are (or were) quite a few abandoned Soviet-era radioisotope generators using it, and some of it also got out when found and scavenged by people who, as in Goiânia, didn't know what it was.
