I watched a documentary about that: Into Eternity

What I thought was interesting about the film was the balance between entertaining a fantastical vision of some future explorer stumbling across the radioactive site, and the mundanity of most of the actual work.

One of the engineers said something like: "When we seal this up with so much concrete, there's no way you're getting in here without machinery. We should be more concerned about a future civilization that comes back here for radioactive materials when they've exhausted all other natural sources"

And then there's a whole section of the film about rules-lawyering the storage site. The dump was chartered by the Finish government to seal waste "for all time", and the engineers were mad that nothing is truly permanent.