Material problems in a nuclear reactor plant can be grouped into two categories, one concerning the nuclear reactor core and one that will apply to all plant materials. This chapter discusses specific material problems associated with fatigue failure, work hardening, mechanical forces applied to materials, stress, and strain.

EO 1.14 DEFINE the following terms:

a. Fatigue failure b. Work hardening c. Creep

EO 1.15 STATE measures taken to counteract or minimize the effects of the following:

a. Fatigue failure b. Work hardening c. Creep

Fatigue Failure

The majority of engineering failures are caused by fatigue. Fatigue failure is defined as the tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength. Although the fracture is of a brittle type, it may take some time to propagate, depending on both the intensity and frequency of the stress cycles. Nevertheless, there is very little, if any, warning before failure if the crack is not noticed. The number of cycles required to cause fatigue failure at a particular peak stress is generally quite large, but it decreases as the stress is increased. For some mild steels, cyclical stresses can be continued indefinitely provided the peak stress (sometimes called fatigue strength) is below the endurance limit value.

A good example of fatigue failure is breaking a thin steel rod or wire with your hands after bending it back and forth several times in the same place. Another example is an unbalanced pump impeller resulting in vibrations that can cause fatigue failure.

The type of fatigue of most concern in nuclear power plants is thermal fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature. Large components like the pressurizer, reactor vessel, and reactor system piping are subject to cyclic stresses caused by temperature variations during reactor startup, change in power level, and shutdown.

Fundamental requirements during design and manufacturing for avoiding fatigue failure are different for different cases. For a pressurizer, the load variations are fairly low, but the cycle frequency is high; therefore, a steel of high fatigue strength and of high ultimate tensile strength is desirable. The reactor pressure vessel and piping, by contrast, are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, to minimize thermal stresses. Although the primary cause of the phenomenon of fatigue failure is not well known, it apparently arises from the initial formation of a small crack resulting from a defect or microscopic slip in the metal grains. The crack propagates slowly at first and then more rapidly when the local stress is increased due to a decrease in the load-bearing cross section. The metal then fractures. Fatigue failure can be initiated by microscopic cracks and notches, and even by grinding and machining marks on the surface; therefore, such defects must be avoided in materials subjected to cyclic stresses (or strains). These defects also favor brittle fracture, which is discussed in detail in Module 4, Brittle Fracture.

Plant operations are performed in a controlled manner to mitigate the effects of cyclic stress. Heatup and cooldown limitations, pressure limitations, and pump operating curves are all used to minimize cyclic stress. In some cases, cycle logs may be kept on various pieces of equipment. This allows that piece of equipment to be replaced before fatigue failure can take place.