Fundamental needs:

Fundamental needs during design and manufacturing for preventing fatigue failure are variant for variant cases. To a pressurizer, the load variations are fairly low, other than the cycle frequency is high; thus, a steel of high fatigue strength and of high ultimate tensile strength is desirable. The reactor pressure vessel and piping, through contrast, are subjected to huge load difference, but the cycle frequency is low; thus, high ductility is the main needed for the steel. Thermal sleeves are used in a few cases, like as spray nozzles and surge lines, to minimize thermal stresses. While the main cause of the phenomenon of fatigue failure is not well known, it apparently arises from the initial beginning of a small crack resulting from a defect or microscopic slip in the metal grains. A crack propagates slowly at first and then more rapidly while the local stress is increased because of a decrease in the load-bearing cross section. A metal then fractures. It could be initiated through microscopic cracks and notches, and even through grinding and machining marks on the surface; thus, such defects must be prevented in materials subjected to cyclic stresses (or strains).

The Plant operations are performed in a controlled manner to mitigate the effects of cyclic stress. Cooldown and Heatup 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 permits which piece of equipment to be exchanged before fatigue failure could take place.