Assuming civilization collapses or regress to pre-industrial levels where the concept of radiation is lost, any future group trying to extract this mythical energy source would have to first discover the location of something buried so deep underground, and then have to develop the logistics to try and access it with manpower. If the repository is placed in the middle of a desert, the challenge of providing water to the workers would be substantial. If they placed it in Antarctica, simply getting there and keeping the workers from freezing to death would be so monumentally expensive that it would bankrupt any civilization attempting to do so. Even if industrialized capacity does return, it would still be economically monumental to try and get to the waste repository. There is also a decent chance that a future civilization with said capacity would rediscover the concept and dangers of radiation.
When I talk about expense, I also like to highlight this: https://en.wikipedia.org/wiki/Pyramid_of_Menkaure Here is a project to dismantle a pyramid that was canceled because of how expensive it was and because of how little of a return on investment the endeavor appeared to be.