HEATUP AND COOLDOWN RATE LIMITS
Exceeding Heatup and Cooldown Rates
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 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.