Radioactive gasses disburse too fast to really be effective, and have to be contained in a pressure container (heavy, big, shield the gasses from teh explosion!, and don't have enough "material" (atoms) decaying fast enough to matter.) (Other than simple fright weapons.)
rescue and nearby areas are affected when the radioactive powder spreads around and contaminates people, things, buildings...
Terror weapon ONLY .... It isn't enough radiation to do any real damage or harm. (More than 25 Rem is needed to even make most people "sick" - and 250 - 400 REM is needed to ensure death of HALF those exposed.) Spreading powder won't kill people.
A simple face mask (dust filter) is enough to prevent breathing in the dust. The rest can readily washed off.
Radiological Dispersal Device (RDD): combination of conventional explosives or other dispersal agent to scatter radioactive debris (e.g. spent fuel rods). Depending on concentration, radiation damage might take weeks if not months or years to produce large numbers of fatalities. According to one estimate (Krieger, 1977), the release of 4.4 pounds of plutonium oxide in powder form in a population centre could produce a 100 per cent probability of bone and lung cancer for every person within a distance of 1,800 feet downwind of the release point and 1 per cent risk as far as 40 miles downwind. Amount and type of radioactive material used also determine damage and fatality. The effect appears to be mainly psychological, making it a potentially useful terrorist tool rather than a militarily useful weapon. However, the radiation overdose hazard for those constructing and delivering the weapon seem to weigh against its use, except in symbolic quantities as in the November 1995 incident in Moscow.([my emphasis] that's from the link I put up above)
Doesn't sound like something that can destroy a city, but it sounds bad -- worse than these silly anthrax letters...
Mark W.