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The metallic components of a high-temperature reactor with operating temperatures above 880°C have to be ensured against the, in comparison to primary stresses, respectively higher secondary stresses due to loading transients. For the calculation of the damage accumulation due to primary, secondary and peak stresses, elastic methods are used in the temperature region up to 800°C. These elastic methods are too conservative at temperatures above 900°C. Therefore a simplified inelastic method is shown for calculation of the creep damage of a reformer tube with operating temperatures at about 900°C under start-up and shut-down loadings. With this method a design is possible for the calculated start-up and shut-down transients during the lifetime of the component.

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 Pipes and components previously affected by material, design and manufacturing problems have been replaced by components built to the new high standard of quality.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.

This paper describes the procedure for the elevated-temperature structural analysis of the SNR-300. The preliminary analysis based on elastic calculations will be followed by detailed inelastic analysis, if required to provide assurance against all failure modes. In coherence with the progress of the project, experimental work is being done as a first phase of preparation to the elevated temperature design. On the one hand material data will be established and on the other hand analytical work for computer codes and design criteria will be done. The first inelastic investigations are running. Changes in the geometry of the components and/or the planned operating conditions can be considered in case the structural adequacy cannot be demonstrated even through detailed inealstic analysis.