Usually, exceeding heatup or Cooldown limits or other potential operational thermal transient limitations is not an immediate hazard to continued operation and only requires an assessment of the impact on the future fatigue life of the plant. However, this may depend upon the individual plant and its limiting components.

Individual components, such as the pressurizer, may have specific heatup and Cooldown limitations that, in most cases, are less restrictive than for the PCS.

Because of the Cooldown transient limitations of the PCS, the reactor should be shut down in an orderly manner. Cooldown of the PCS from full operating temperature to 200F or less requires approximately 24 hours (depending upon Cooldown limit rates) as a minimum. Requirements may vary from plant to plant.

Soak Times

Soak times may be required when heating up the PCS, especially when large limiting components are involved in the heatup. Soak times are used so that heating can be carefully controlled. In this manner thermal stresses are minimized. An example of a soak time is to heat the reactor coolant to a specified temperature and to stay at that temperature for a specific time period. This allows the metal in a large component, such as the reactor pressure vessel head, to heat more evenly from the hot side to the cold side, thus limiting the thermal stress across the head. Soak time becomes very significant when the PCS is at room temperature or below and very close to its RTNDT temperature limitations.

Summary

The important information in this chapter is summarized below.

Heatup-Cooldown Rate Limits Summary

Heatup and cooldown rate limits are based upon impact on the future fatigue life of the plant. The heatup and cooldown rate limits ensure that the plant's fatigue life is equal to or greater than the plant's operational life.

Large components such as flanges, reactor vessel head, and the vessel itself are the limiting components.

Usually exceeding the heatup or cooldown rate limits requires only an assessment of the impact on the future fatigue life of the plant.

Soak times:

May be required when heating large components

Used to minimize thermal stresses by controlling the heating rate

Become very significant if system is at room temperature or below and very close to RT_NDT temperature limitations