Nuclear Energy for Propulsion of Ships

With three heavy fissile elements U233, U235 and Pu239, and moderators including heavy water, light water, beryllium and its oxides, carbon, organic materials such as phenyls and hydrides such as zirconium hydride, there are already many variants possible and if to this is added coolants such as gas (hydrogen, helium, carbon dioxide, etc.), liquid metal, light and heavy water and organic liquids, it would appear that the choice of reactors is bewildering in the possible combinations of these variants. Yet by following some guiding principles the choice is rapidly narrowed.