The fallout from the Hiroshima & Nagasaki bombs, both of which detonated in the air, was very different to that from a ground level explosion and was negligible soon after detonation. The Hiroshima bomb exploded 1890 feet above ground and the majority of the radiation, emanating from the fireball, was lethal within about 3.5 km of ground zero, along with the heat, blast and flying debris. Most of the radiation was contained in the mushroom cloud and remained in the atmosphere for several weeks, decaying to safe levels before falling to the ground. In the following weeks many people who were seemingly unhurt died from radiation sickness delivered at the time.
Chernobyl and Fukushima were ground explosions and produced caesium 137 and iodine 131 fallout with half lives of 30 years and 8 days respectively mixed with large quantities of dust and steam blown into into the atmosphere and spread over many thousands of square kilometres.