Cesium (Gamma Emitter)
Two weeks ago, a lost medical gauge containing cesium was discovered in North Carolina. Imagine that the cesium in this device was exploded in Washington, DC in a bomb using ten pounds of TNT. The initial passing of the radioactive cloud would be relatively harmless, and no one would have to evacuate immediately. However, residents of an area of about five city blocks, if they remained, would have a one-in-a-thousand chance of getting cancer. A swath about one mile long covering an area of forty city blocks would exceed EPA contamination limits, with remaining residents having a one-in-ten thousand chance of getting cancer. If decontamination were not possible, these areas would have to be abandoned for decades. If the device was detonated at the National Gallery of Art, the contaminated area might include the Capitol, Supreme Court, and Library of Congress, as seen if Figure 1.
Figure 1. Long-term Contamination Due to Cesium Bomb in Washington, DC.
Inner Ring: One cancer death per 100 people due to remaining radiation (5% increase)
Middle Ring: One cancer death per 1,000 people due to remaining radiation (.5% increase)
Outer Ring: One cancer death per 10,000 people due to remaining radiation (.05% increase): EPA recommends decontamination or destruction
Example 2:
Cobalt (Gamma Emitter)
Now imagine if a single piece of radioactive cobalt from a food irradiation plant were dispersed by an explosion at the lower tip of Manhattan. Typically, each of these cobalt "pencils" is about one inch in diameter and one foot long, with hundreds of such pieces often being found in the same facility. Admittedly, acquisition of such material is less likely than in the previous scenario, but we still consider the results, depicted in Figure 2. 
Fig. 2 Long-term Contamination Due to Cobalt Bomb in NYC - EPA Standards.
Inner Ring: One cancer death per 100 people due to remaining radiation (5% increase)
Middle Ring: One cancer death per 1,000 people due to remaining radiation (.5% increase)
Outer Ring: One cancer death per 10,000 people due to remaining radiation (.05% increase): EPA recommends decontamination or destruction
Again, no immediate evacuation would be necessary, but in this case, an area of approximately one-thousand square kilometers, extending over three states, would be contaminated. Over an area of about three hundred typical city blocks, there would be a one-in-ten risk of death from cancer for residents living in the contaminated area for forty years. The entire borough of Manhattan would be so contaminated that anyone living there would have a one-in-a-hundred chance of dying from cancer caused by the residual radiation. It would be decades before the city was inhabitable again, and demolition might be necessary.
For comparison, consider the 1986 Chernobyl disaster, in which a Soviet nuclear power plant went through a meltdown. Radiation was spread over a vast area, and the region surrounding the plant was permanently closed. In our current example, the area contaminated to the same level of radiation as that region would cover much of Manhattan, as shown in Figure 3. Furthermore, near Chernobyl, a larger area has been subject to periodic controls on human use such as restrictions on food, clothing, and time spent outdoors. In the current example, the equivalent area extends fifteen miles.
Fig. 3 Contamination Due to Cobalt Bomb in NYC - Chernobyl Comparison. More...
Inner Ring: Same radiation level as permanently closed zone around Chernobyl
Middle Ring: Same radiation level as permanently controlled zone around Chernobyl
Outer Ring: Same radiation level as periodically controlled zone around Chernobyl