Although it seems strange, the Massachusetts Institute of Technology- in the city of Cambridge has on its campus a small nuclear reactor core surrounded on the outside by concrete. It was built in 1958, then renovated in 1975. This reactor runs on enriched Uranium 235 and is used to generate neutrons. It does not generate enough high pressure or hot temperatures to make heat energy.
Instead, the neutrons are created for use in experiments, mainly for cancer treatment research. In fact there have been some amazing breakthroughs using this reactor to deliver therapies to cells by the high speed reactor. Still it is a very basic level of science and has not resulted in cancer curing therapy.