Thermal fatigue striping damage assessment from simple screening criterion to spectrum loading approach

Thermal fatigue is an important degradation mechanism for the life time assessment of nuclear reactor components. A reliable life-assessment of components is difficult because usually only the nominal temperature differences are known and the thermal surface loadings are not known. This paper outlines a multi-level procedure for assessment of pipe components subjected to thermal fatigue. The different levels are: (a) simple screening criterion, (b) the thermal spectrum replaced by a sinusoidal load (SIN-method) with constant amplitude and frequency and assessment of crack initiation and crack propagation in relation to a critical frequency, and (c) spectrum loading applied to crack initiation and propagation. The different levels are described together with the underlying assumptions. The different levels in the procedure are applied to assess the life of the Civaux case, where a pipe failed due to thermal fatigue. The different levels of the procedure give conservative estimates of the thermal fatigue life but where the conservatism is reduced with the more complex higher level assessments. The influence of important factors such as boundary conditions and primary loads are illustrated. It is also shown that the SIN-method can be used to determine a threshold below which there is no thermal fatigue failure.