Are they feasible/possible? I saw an article (forgotten where) a while ago claiming that they are not actually dangerous, as the dust is not concentrated enough for long enough to cause radiation sickness.... etc etc. Did anyone else see this/does anyone else know anything about this?
From what I have gathered, dirty bombs are more psychological than anything else. Thats not to say that people will get killed in the explosion etc but the fact that a "nuclear bomb" will have gone off makes things far worse.
A "dirty bomb" would typically use enriched uranium or plutonium with half lives of up to 90 years. This means that the dust from the explosion could continue to be dangerous for this time. Only very little dust needs to remain (around 11 in ten millionths) for danger to be present.
A dirty bomb used (say) in Westminster, Heathrow or at Clapham Junction would probably not take many lives but would have catastrophic and long lasting economic and strategic consequences.