We may eventually need to burry something nasty and difficult to destroy, but I would assume that long before nuclear waste storage becomes a cultural problem, we will have figured out how to build an MSR that can burn most of that stuff. And, when that happens, whatever's left can be safely kept for a much shorter time.

Instead of thinking about the interesting problem of keeping the next civilization safe, could we direct some more effort to the boring problem of spent fuel reprocessing?