That makes more sense to me. It’s not like these waste canisters are going to somehow float to the surface. The biggest problem would have to be sealing the entrance and descending tunnels in such a way to keep the site hidden. Is that possible? If you could do that then the only danger would be a sufficiently advanced culture for some reason trying to drill or dig in that area. And why would they do that if the geology of the area isn’t one which would lead them to expect minerals. Fast forward any further and they’re only going to discover it with some kind of ground penetrating radar which indicates a pretty sophisticated society.
>The Onkalo repository is expected to be large enough to accept canisters of spent fuel for around one hundred years, i.e. until around 2120.[10] At this point, the final encapsulation and burial will take place, and the access tunnel will be backfilled and sealed.
So they're already planning to fill the tunnels completely. Considering the depth of the main facility, I would think this will make it very difficult for a primitive civilization to reach the facility even if they found the former entrance and wanted to investigate.
I don't know about the one in Finland, but the one in New Mexico is in the Permian Basin[1]. There are oil and gas resources all over the place. There are also Potash mines right next to the New Mexico facility (and it itself is a salt mine). Leaving it unmarked poses a real risk that someone someone will drill or dig into it accidentally looking for resources.
”And why would they do that if the geology of the area isn’t one which would lead them to expect minerals.”
Any significant structure would introduce a gravitational and/or magnetic anomaly. Given that, keeping the structures as small as possible seems the way to go, but of course, you also want to contain the waste for thousands of years.
On the other hand, one could hope that anybody who can detect such anomalies knows about radiation, too.
