Basis:

Heatup and cooldown rate limits, as displays in Figure, are based upon the impact on the future fatigue life of the plant. A heatup and cooldown limits  ensure  in which  the  plant's fatigue life is equal to or greater than the plant's  operational life. Huge parts such as flanges, the reactor vessel head and even the reactor vessel itself are the limiting components. Commonly the most limiting component will set the cooldown and heatup rates.

The thermal stress imposed by a rapid temperature change (a fast ramp or even a step change) of approximately 20º F  (depending upon the plant) is insignificant (10⁶ cycles permitted depending upon parts) and has no effect on the design life of the plant.

Figure:  Heatup and Cooldown Rate Limits