DOE-HDBK-1017/2-93HEATUP AND COOLDOWN RATE LIMITSBrittle FractureExceeding Heatup and Cooldown RatesUsually, exceeding heatup or cooldown limits or other potential operational thermal transientlimitations is not an immediate hazard to continued operation and only requires an assessmentof the impact on the future fatigue life of the plant. However, this may depend upon theindividual plant and its limiting components.Individual components, such as the pressurizer, may have specific heatup and cooldownlimitations 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 anorderly manner. Cooldown of the PCS from full operating temperature to 200F or less requiresapproximately 24 hours (depending upon cooldown limit rates) as a minimum. Requirementsmay vary from plant to plant.SoakTimesSoak times may be required when heating up the PCS, especially when large limiting componentsare involved in the heatup. Soak times are used so that heating can be carefully controlled. Inthis manner thermal stresses are minimized. An example of a soak time is to heat the reactorcoolant to a specified temperature and to stay at that temperature for a specific time period. Thisallows the metal in a large component, such as the reactor pressure vessel head, to heat moreevenly from the hot side to the cold side, thus limiting the thermal stress across the head. Soaktime becomes very significant when the PCS is at room temperature or below and very close toits RTNDT temperature limitations.MS-04Page 12Rev. 0
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business