Do we actually have that much control over what the fission products are? My impression was that you hit U-235 with a neutron, and you get the same kinds of fission products out whether it's a bomb or a reactor. It's just determined by physics what kinds of isotopes are likely to come out, and my impression was that we don't know how to influence it so that only the shorter-lived kinds are created. Bombs may be designed to consume a very high fraction of the fuel, but that would tend to make them worse, since the fission products are more radioactive than the starting Uranium, and there will be more of them. The mushroom cloud thing does make sense as an explanation for why Hiroshima is still inhabited, though. And it's clear that Corium wouldn't be able form in the middle of a nuclear bomb explosion.