Your guess was too high. More like 25%. Did you know the U-235 fissile loses 10% of its energy in the form of neutrinos ? If you had some method of detecting those ghostly particles our submarines would shine like mega spotlights.
Modern PWR do routinely hit 34-36% even CANDU can hit 30% with feed water reheat.
https://world-nuclear.org/our-association/publications/technical-positions/cooling-of-power-plants
“Nuclear plants currently being built have about 34-36% thermal efficiency, while one of the new reactor designs boasts 39%.”
Sodium fast reactors operate much hotter than PWR in the 500C range they are approaching 44% typical 40-43% running at 500-550C much hotter than the 330C of a PWR.
Supercritical CO2 turbines at 550C cross 50% gross efficiency it’s why SCO2 was developed in the first place for advanced high temp reactors the gas cooled kind first then sodium metal cooled.