Coal is fine with steam. Make it supercritical and do fancy stuff. You're simply piping water through furnace with extra steps. It is a material science questio

Coal is fine with steam. Make it supercritical and do fancy stuff. You're simply piping water through furnace with extra steps. It is a material science questions. For all traditional water reactors you are using the water to slow down the neutrons. Enter the Pressurized Water Reactor(PWR). Now the entire reactor is pressurized with all the complications that bring and through Carnot's Theorem you gain some efficiency. This is nuclear's problem with trying to become more efficient.