3) if a waste barrel explodes, somehow, underground how does the waste make it's way through a mile of bedrock?
4) Again, how does a fire bring the wast up through a mile of bedrock?
5) This is just a vague statement.
6) So the concern is that future society will forget that this is a waste site, mine a mile deep and retrieve waste, and never figure out that the waste is bad for them? This is rather specific hypothetical that IMO demonstrates just how hard it is for a nuclear waste site to result in contamination.