US and the british had steam catapults without nuclear power plant because at the time they had steam turbines.
Gas turbines, exactly for their high efficiency, simply do not leave enough energy in form of heat to generate enough high pressure steam.
Steam turbines, and nuclear power plants rely on steam turbines too, transform most of the energy into heat and then in high pressure steam. After the stages transforming that energy in mechanical propulsion, most of the energy is still there in form of high pressure steam.
A typical nuclear power plant generating X MW of thermal energy, usually transform far less than 50% of that energy in mechanical propulsion.
Energy dispersion apart, more than 50% of energy is still there, as high pressure steam, to be used for electric generation or, sometimes, to power steam catapults.
Something almost impossible to do with gas turbines, anyway so difficult and tricky nobody ever accomplished to operate steam catapults powered through gas turbines in the real world.
Last edited by Peŕrier on Thu Jan 04, 2018 7:12 pm; edited 1 time in total