In the case of man-made disasters, who knows. During the Gulf War I attended a lecture essentially on what to do as a health care worker in the event of a nuclear attack. What I remember was: [and I have no idea if my memory is correct, or if this info is current; take it with a large grain of salt]
1) the basic immediate problem is burns, not rads. Anyone with >60% body surface area burns is going to die [assuming the nations' burn units are completely overwhelmed]. Anyone 30-60% is likely to die, to the point that you shouldn't waste antibiotics on them. Someone with <30% BSA burns try to treat, mostly requiring large amounts of oral fluids.
2) the fluids you want should have sodium and potassium chloride [ie, gatorade or a mix of salt and light salt].
3) radioactive iodine is a problem with certain types of bombs, but it's biologic halflife is short if you can get the person some non-radioactive iodine orally.
After 9/11 my family made some basic plans, to get together at my parents' place. We haven't really mentioned it since then.