THERMAL STRESS
DOE-HDBK-1017/2-93
Thermal Shock
Thermal stresses are a major concern in
Figure 1 Stress on Reactor Vessel Wall
reactor systems due to the magnitude of the
stresses involved. With rapid heating (or
cooling) of a thick-walled vessel such as
the reactor pressure vessel, one part of the
wall may try to expand (or contract) while
the adjacent section, which has not yet been
exposed to the temperature change, tries to
restrain it. Thus, both sections are under
stress. Figure 1 illustrates what takes place.
A vessel is considered to be thick-walled or
thin-walled based on comparing the
thickness of the vessel wall to the radius of
the vessel. If the thickness of the vessel
wall is less than about 1 percent of the
vessel's radius, it is usually considered a
thin-walled vessel. If the thickness of the
vessel wall is more than 5 percent to 10
percent of the vessel's radius, it is
considered a thick-walled vessel. Whether
a vessel with wall thickness between 1
percent and 5 percent of radius is
considered thin-walled or thick-walled
depends 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 side
of 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 thermal
stress, placing the cold side in tensile stress and the hot side in compressive stress, which can
cause cracks in the cold side of the wall. These stresses are illustrated in Figure 2 and Figure 3
in the next chapter.
The heatup and cooldown of the reactor vessel and the addition of makeup water to the reactor
coolant system can cause significant temperature changes and thereby induce sizable thermal
stresses. Slow controlled heating and cooling of the reactor system and controlled makeup
water addition rates are necessary to minimize cyclic thermal stress, thus decreasing the
potential for fatigue failure of reactor system components.
Operating procedures are designed to reduce both the magnitude and the frequency of these
stresses. Operational limitations include heatup and cooldown rate limits for components,
temperature limits for placing systems in operation, and specific temperatures for specific
pressures for system operations. These limitations permit material structures to change
temperature at a more even rate, minimizing thermal stresses.
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