If you've got good enough nanobots to remove unstable isotopes you almost certainly have the tech to do full out repair. I don't know if the radiation less than what you get from solar incidence. I suspect that it is but I also suspect that in a general underground environment much more radiation will be due to one's own body than the sun.

Cracking can include information theoretic damage if it mangles up the interface at synapses badly enough. We don't actually have a good enough understanding of how the brain stores information to really make more than very rough estimates. And cracking is also a problem for the cryonics proponents who don't self-identify with a computerized instantiation of their brain.