Your very welcome. You are probably correct that downtown buildings will be a more likely target than the outskirts of town. If I may ease your fears of high rad exposure, the number of scenarios involving acute high doses are pretty much reduced to an actual nuclear device, or a criticality device. Both of which are probably the least likely of all scenarios. A dirty bomb will not be a high rad exposure device, as by it's very nature it disperses the concentration of radioactive material.

If you are distant from the incident (of course that is relative depending on the incident) and upwind then you probably wouldn't even need roofing or walls.

Take care!

Greg