Not really. The blast itself not only turns a portion of the material into energy, but basically throws the rest of it all over the place, as particles. This would definitely have an effect on the biosphere, but remember that the core of a fission weapon is rather small. It's not that much hazardous material, spread out over thousands (maybe millions) of square miles. Higher cancer rates near ground zero, perhaps slight increases in outlying areas. We wouldn't have hordes of mutants, though. Chernobyl was entirely different. That sucker was irradiating steam and weather like crazy (IIRC).
Either way, a nuclear blast of any size would cause the relocation of millions.
A trivial amount -- around 1%.
but basically throws the rest of it all over the place, as particles. This would definitely have an effect on the biosphere, but remember that the core of a fission weapon is rather small. It's not that much hazardous material, spread out over thousands (maybe millions) of square miles.
True, but you seem to be overlooking the fact that the energy from the blast itself, especially in a groundburst, converts a *huge* amount of surrounding material (dirt, stone, steel, etc.) into radiactive isotopes. The great majority of fallout comes not from the original core of the bomb itself, but from the secondary conversions. This is why groundbursts are much "dirtier", fallout-wise, than airbursts. They also stir up more of the irradiated material and send it up into the atmosphere, to fall... somewhere.
Although today, people would know to hop in their cars and get out from the probable path of fallout, unlike the unsuspecting Japanese in 1945.