Protection against fatigue damage with respect to the environmental influence of LWR operating conditions

Abstract

As nuclear power plant operation proceeds, a certain number of isolated leakages and cracks are found in pipes and components. Considering the extensive design and analysis efforts made, it is at least the early occurrence of the incidents in relation to the design lifetime of the material that is surprising. In the Federal Republic of Germany, a tremendous effort has been directed towards the development of optimized steels with respect to fracture toughness and weldability, the manufacturing of integral forged products (forged-on nozzles, seamless pipes and elbows), and the reduction of peak stresses by an advanced design (basic safety concept). The design of the areas of cold water injection has been improved to prevent temperature stratification. The safety of pipes and components has been considerably improved by these developments. Pipes and components previously affected by material, design and manufacturing problems have been replaced by components built to the new high standard of quality. An area which still deserves attention with respect to the lifetime of a plant is the protection against fatigue damage. The present design fatigue curve (ASME Section III or KTA 3201.2) only allows a generic assessment of the safety against cyclic loadings. In most cases, this concept has proved its worth and is sufficiently safe. In some cases, however, and especially in the presence of a corrosive environment, a detailed investigation is necessary. The design fatigue curve can no longer be applied if cracks form in the protective magnetite layer and, owing to this, strain-induced corrosion cracking occurs. The concept developed in this paper defines a number of correction factors which take into account the effects of a high-temperature water environment as a function of the duration of the strain cycle. It is based on work presented by General Electric. The concept developed allows a more detailed assessment of the local conditions and, consequently, serves as a useful tool for both the designer and the operator of the plant.