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

lubricant film thickness during start-up is considerably less than the dimensions of gear surface irregularities
harmonic vibrations associated with critical speed can easily be reached during start-up
the danger of blade erosion damage from dry steam impingement is greater during start-up
differential expansion can result from the temperature difference between the rotor and rotor casing

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