In the fifties and sixties, some underground nuclear power plants, UNPP, have been built and operated in Europe and Siberia. It is clear that underground siting of any sensible and/or dangerous plant and depot will improve safety against external menaces (plane crash, bombing, etc.) as well as against consequences of internal accidents. The technical and economic feasibility of UNPP depends on various criteria, which are tentatively proposed herein, under main headings: i) Physiography (relief and water); ii) Geology (nature and structure of rocks, distance to geological hazards such as active faults, volcanism, glaciers, slope movements, tsunamis, etc.); iii) Population and Human activities (distance to large towns, industrial plants, agriculture, etc.); iv) existing service networks (harbors, roads and railways, transmission lines, etc.).
All along the nuclear era, many advisers have called for siting reactors underground as had been often done for explosives and now, more and more, for sensible and nuclear wastes. Long before the accidents at Three Mile Island, Chernobyl and Fukushima, a partial core melt occurred in the underground Lucens plant, Switzerland, without any harm to people and environment. The practice of underground hydropower plants together with some other uses of man-made caverns, from hydrocarbon storage to the Norwegian ice rink and caverns needed for neutrino research are shortly recalled as they provide the bases for the design of caverns. The senior author has followed the cavern construction of the French plant Chooz (only second to Siberian plants as an underground nuclear plant with a significant output) and has attended the only international conference on the topic, organized by the government of Germany in Hannover in 1981, soon after the TMI accident.