I would like to place a character in a situation where they are in a forest outside a city. An H-bomb hits the city, and they are knocked out by the shock wave. This is a big weapon as in ICBM missile, not a terrorist attack which almost certainly would be a very small weapon. I would like him to regain consciousness, and see that the fallout cloud has spread to such a point that he has no reasonable prospect of escaping lethal radiation poisoning. Plausible?