> Little Boy had around 140 pounds of uranium, Fat Man contained about 14 pounds of plutonium and reactor number four had about 180 tons of nuclear fuel.
Also, nuclear bombs are not that efficient. Little Boy was 1.5% efficient which means only 2 kg of fission products. Fat Man was probably about the same fallout. The two bombs were detonated at altitude so the fallout spread over large area.
There was also radiation from explosion but that degraded quickly and was limited to destroyed area.
> Little Boy had around 140 pounds of uranium, Fat Man contained about 14 pounds of plutonium and reactor number four had about 180 tons of nuclear fuel.