if they can't read our ancient languages or have lost the records what are the chances of them digging far enough safely...
oh wait you're not storing them safely underground.. not that it matters since if you're still running an active power plant at the location you're going to need security anyways.
for finnish aspect, maybe check this out
http://www.intoeternitythemovi...
furthermore, if they have "lost" all civilization(capable of detecting the threat) then it's going to be a quite localized threat, like asbestos landfills and what have you... since if they have lost all that they're not going to be moving on about long distances.
also a rant about descending into chaos in the past 6000 years - GLOBALLY there has not been any descent into such chaos, it's all been unifying and forward going progress and the more there has been trade and communications the faster and more the advancement has been.
we have quite well known already for quite some time how the romans technology worked, how the greeks did their thing and of the things we don't know exactly we know several explanations how to have done those. we know better how they built the pyramids than the fucking romans knew and todays chinese know how the romans built their aquaducts...