U2: Did you ever get a chance to read the studies on the bombings of Hiroshima & Nagasaki? Some of them talked about the amount of time it took for people within certain distances from the epicenter to begin to show the results of radiation exposure. I can't remember exact distance and timeframes from the study (read it about 7 or so yrs ago), but the cloud looked kind of small from Jack's position (IMO). Was that kid's house in Valencia (where the explosion happened) or LA? I don't remember if that was made clear.
Found this link just now:
http://www.atomicarchive.com/Docs/MED/med_chp10.shtml