Tangentially related: the Swiss built a research-focused nuclear reactor in a cavern. It had a meltdown in 1969, and they were able to keep the decontamination/decommissioning efforts to a minimum by just entombing the thing in concrete.

Honestly, so long as the water table isn't too close, that's not a terrible place to put a nuclear reactor.

[0] https://en.wikipedia.org/wiki/Lucens_reactor