Fission reactor concept being developed for small reactors in deep underground boreholes, with claimed cost and safety benefits:
https://deepfission.com/
This is similar in concept to Sakharov's ideas expressed in 1990 after the Chernobyl disaster:
https://time.com/archive/6714880/sakharov-mankind-cannot-do-without-nuclear-power/
"Plainly, mankind cannot renounce nuclear power, so we must find technical means to guarantee its absolute safety and exclude the possibility of another Chernobyl. The best way is international legislation requiring that all new nuclear reactors be sited deep enough underground so that even a worst-case accident would not discharge radioactive substances into the atmosphere."
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