A lot of fallout comes from detonating over the ground so that the force smacks into earth and spins up a ton of debris which is breathed in and causes all those nasty diseases. How long a place is irradiated is pretty much tied to how long those particulates float around causing mayhem. After that, sweeping up all the debris and containing it is a solid way of preparing the area for humans again.

"After that, sweeping up all the debris and containing it is a solid way of preparing the area for humans again."

Sounds like lots of effort. And would still leave me wondering at every rock I encounter, of whether I should avoid this one, because it was overlooked.

Underground detonation.