There are designs that allow that. With liquid salts in which the fuel is in the reactor is said to self-regulate with minimal problems, the problem with this i

There are designs that allow that. With liquid salts in which the fuel is in the reactor is said to self-regulate with minimal problems, the problem with this is the trash. Recent studies say that they generate more nuclear waste than "normal" reactors. The reason we mostly have water based Reactors is that an army general responsible for nuclear reactors in submarines was also part of the comitee for the civilian usage of nuclear energy and they had water based reactors because the scientists thought the marine would know water.