[Andrew_nenakhov]

That's clearly wrong. Debris from a crater is just what it is: harmless ash, with no dangerous isotopes. See [1] for the correct answer.

[1]: https://news.ycombinator.com/item?id=28479051

[user-the-name]

Neutron flux from the explosion will turn the atoms of surrounding harmless materials into heavier isotopes that are, in fact, dangerous.

[pfdietz]

And more importantly, the fission products from the bomb condense/agglomerate onto debris from the ground, allowing them to fall out more quickly.