[scohesc]

Why don't we build these reactors deep underground? Doesn't the United States use an old abandoned mine to store spent nuclear rods while they decay?

Is it extremely cost prohibitive to dig down into existing abandoned mines (or even quarries)? and retrofit them with nuclear reactors?

Even if something catastrophic happens, if it's geologically separated by thousands of tons of rocks/dirt/sediment and is away from a water table, it wouldn't be a problem, no?

[moftz]

Cooling can be a problem. Reactors usually have access to large amounts of water (cheap) to expel excess heat so you would need to pump the heat over a long distance to somewhere cool enough to dump the heat to. Or you would need to surround everything in TEGs to convert the excess heat (expensive).