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.
IDENTIFY the two stresses that are the result of thermal shock
(stress) to plant materials.
STATE the two causes of thermal stresses.
Given the material's coefficient of Linear Thermal Expansion,
CALCULATE the thermal stress on a material using
DESCRIBE why thermal stress is a major concern in reactor
systems when rapidly heating or cooling a thick-walled vessel.
LIST the three operational limits that are specifically intended
to reduce the severity of 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.