The most critical period of main turbine operation is during cold start-up, rather than hot shutdown because __________.

Correct Answer: Option D — differential expansion can result from the temperature difference between the rotor and rotor casing

Differential expansion between the hot rotor and relatively cool casing during cold start-ups poses severe risks of blade rubs and distortion, making it the most critical operational phase governed by strict warm-up procedures under 46 CFR.