Among the catapult users, only the US and France have enough spare power from their nuclear/steam engines to launch ‘heavy’ aircraft with weights of 20 or more tonnes. So that’s one reason why the Russians and Chinese have stuck with ski-jumps. I believe that the new electro-magnetic capapult system being developed for the USN would negate the need for nuclear power.
Not for US carriers. The EM cats are going to use a lot of power themselves. And the US FORD class has nuclear plants that are 200% more powerful than the earlier reactors. The US Navy has plans for that power...think Laser CIWS, railgun, etc.
You can't change Physics. Force through a distance is still work energy.