As for Reactors, they will only do HU.
Yeah I know, "thermal" and "fast" refer to the speed of the neutrons used in the reaction.
Thermal (slow) neutrons need a moderator to slow them down, only work with specific isotopes (e.g. U235) and produce radioactive waste.
Fast neutrons can cause fission in basically anything (including the radioactive waste of thermal reactors), but require exotic coolants because water is a moderator. Some designs use molten sodium, molten lead or pressurized helium. My personal favourite is the Molten Salt reactor, where Uranium salts are mixed with the coolant directly and circulate with it around the reactor