THERMAL STRESSDOE-HDBK-1017/2-93Thermal ShockThermal stresses are a major concern inFigure 1 Stress on Reactor Vessel Wallreactor systems due to the magnitude of thestresses involved. With rapid heating (orcooling) of a thick-walled vessel such asthe reactor pressure vessel, one part of thewall may try to expand (or contract) whilethe adjacent section, which has not yet beenexposed to the temperature change, tries torestrain it. Thus, both sections are understress. Figure 1 illustrates what takes place.A vessel is considered to be thick-walled orthin-walled based on comparing thethickness of the vessel wall to the radius ofthe vessel. If the thickness of the vesselwall is less than about 1 percent of thevessel's radius, it is usually considered athin-walled vessel. If the thickness of thevessel wall is more than 5 percent to 10percent of the vessel's radius, it isconsidered a thick-walled vessel. Whethera vessel with wall thickness between 1percent and 5 percent of radius isconsidered thin-walled or thick-walleddepends on the exact design, construction,and application of the vessel.When cold water enters the vessel, the cold water causes the metal on the inside wall (left sideof Figure 1) to cool before the metal on the outside. When the metal on the inside wall cools,it contracts, while the hot metal on the outside wall is still expanded. This sets up a thermalstress, placing the cold side in tensile stress and the hot side in compressive stress, which cancause cracks in the cold side of the wall. These stresses are illustrated in Figure 2 and Figure 3in the next chapter.The heatup and cooldown of the reactor vessel and the addition of makeup water to the reactorcoolant system can cause significant temperature changes and thereby induce sizable thermalstresses. Slow controlled heating and cooling of the reactor system and controlled makeupwater addition rates are necessary to minimize cyclic thermal stress, thus decreasing thepotential for fatigue failure of reactor system components.Operating procedures are designed to reduce both the magnitude and the frequency of thesestresses. Operational limitations include heatup and cooldown rate limits for components,temperature limits for placing systems in operation, and specific temperatures for specificpressures for system operations. These limitations permit material structures to changetemperature at a more even rate, minimizing thermal stresses.MS-03Page 4Rev. 0
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