The reactors are used to make superheated steam just like the oil powered ships.
How did you think the reactors turned the propellers?
I know that the reactors are used to boil water to create steam to turn the screws and power the catapult system.But that’s still different than a regular oil fired boiler system that requires insulation on the inside to keep it from melting.
The steam is created indirectly by the reactors so the temperature of the water is not great enough to melt the steam system even when it is pressurized.