Nuclear Reactor's Heat Engine's Thermal Efficiency

Most nuclear reactor-driven power plants use water as the working medium. They boil the water to steam at some temperature, spin a turbine with the steam to extract work from it, then condense the spent steam back to water and repeat the cycle. The difference between the temperature of the hot steam versus the spent steam sets the maximum thermal efficiency of the overall cycle and to make that efficiency as large as possible, you want that difference to be as big as possible. For the steam turbine cycle, the turbine efficiency can reach 50% in practice.

This does not mean that a steam turbine power plant can be 50% efficient overall, because the electric generator being spun by the turbine will be 90% to 95% efficient, the transformers that boost the generator output to transmission levels will be 90% to 95% efficient, and the reactor-driven steam boiler will not be 100% efficient either because of heat leakage and heat transfer limitations.

The overall efficiency start-to-finish of a nuclear power plant is the product of all the individual efficiencies of the processes going on inside it, which is why the turbine by itself can be 50% efficient but the whole plant will be only 33% efficient.