REFERENCES

Academic Projzram for Nuclear Power Plant Personnel,Volume III, Columbia, MD, General Physics Corporation, Library of Congress Card #A 326517, 1982.

Foster and Wright, Basic Nuclear Engineering,Fourth Edition, Allyn and Bacon, Inc., 1983.

Glasstone and Sesonske, Nuclear Reactor Engineering,Third Edition, Van Nostrand Reinhold Company, 1981.

Reactor Plant Materials,General Physics Corporation, Columbia Maryland, 1982.

Savannah River Site, Material Science Course,CS-CRO-IT-FUND-10, Rev. 0, 1991.

Tweeddale, J.G., The Mechanical Properties of Metals Assessment and Significance,American Elsevier Publishing Company, 1964.

Weisman, Elements of Nuclear Reactor Desijzn,Elsevier Scientific Publishing Company, 1983.

TERMINAL OBJECTIVE

1.0 Without references, DESCRIBE the importance of minimizing thermal shock (stress).

ENABLING OBJECTIVES

1.1 IDENTIFY the two stresses that are the result of thermal shock (stress) to plant materials.

1.2 STATE the two causes of thermal shock.

1.3 Given the material's coefficient of Linear Thermal Expansion, CALCULATE the thermal shock (stress) on a material using Hooke's Law.

1.4 DESCRIBE why thermal shock is a major concern in reactor systems when rapidly heating or cooling a thick-walled vessel.

1.5 LIST the three operational limits that are specifically intended to reduce the severity of thermal shock.

1.6 DEFINE the term pressurized thermal shock.

1.7 STATE how the pressure in a closed system effects the severity of thermal shock.

1.8 LIST the four plant transients that have the greatest potential for causing thermal shock.

1.9 STATE the three locations in a reactor system that are of primary concern for thermal shock.

THERMAL STRESS

Thermal stresses arise in materials when they are heated or cooled. Thermal stresses effect the operation of facilities, both because of the large components subject to stress and because they are effected by the way in which the plant is operated. This chapter describes the concerns associated with thermal stress.

EO 1.1IDENTIFY the two stresses that are the result of thermal shock (stress) to plant materials.

EO 1.2STATE the two causes of thermal stresses.

EO 1.3Given the material's coefficient of Linear Thermal Expansion, CALCULATE the thermal stress on a material using Hooke's Law.

EO 1.4DESCRIBE why thermal stress is a major concern in reactor systems when rapidly heating or cooling a thick-walled vessel.

EO 1.5LIST the three operational limits that are specifically intended to reduce the severity of thermal shock.

Thermal Shock

Thermal shock (stress) can lead to excessive thermal gradients on materials, which lead to excessive stresses. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials.

Thermal shock is caused by nonuniform heating or cooling of a uniform material, or uniform heating of nonuniform materials. Suppose a body is heated and constrained so that it cannot expand. When the temperature of the material increases, the increased activity of the molecules causes them to press against the constraining boundaries, thus setting up thermal stresses.

If the material is not constrained, it expands, and one or more of its dimensions increases. The

thermal expansion coefficient () relates the fractional change in length called thermal strain, to the change in temperature per degree .

where:

Table 1 lists the coefficients of linear thermal expansion for several commonly-encountered materials.

In the simple case where two ends of a material are strictly constrained, the thermal stress can be calculated using Hooke's Law by equating values of from Equations (3-1), (3-2), and (3-3).

where:

Example: Given a carbon steel bar constrained at both ends, what is the thermal stress when heated from 60F to 540F?

Solution:

Thermal stresses are a major concern in 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.

Figure 1 Stress on Reactor Vessel Wall

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.

Summary

The important information in this chapter is summarized below.

Thermal Stress Summary

Two types of stress that can be caused by thermal shock are:

Tensile stress Compressive stress

Causes of thermal shock include:

Nonuniform heating (or cooling) of a uniform material

Uniform heating (or cooling) of a nonuniform material

Thermal shock (stress) on a material, can be calculated using Hooke's Law from the following equation. It can lead to the failure of a vessel.

Thermal stress is a major concern due to the magnitude of the stresses involved with rapid heating (or cooling).

Operational limits to reduce the severity of thermal shock include: Heatup and cooldown rate limits

Temperature limits for placing systems into operation

Specific temperatures for specific pressures for system operation