For plant lifetime extension programs, several products provide a customized solution for thermal load monitoring and fatigue calculation. Requirements like the consideration of Environmentally Assisted Fatigue (EAF) effects or flexible plant (e.g. load follow) operation have a crucial impact on the engineering practice of a detailed fatigue assessment considering the realistically occurring loads.

Several fatigue monitoring products are available on the market, some with local instrumentation added and some with transfer function approaches using existing plant instrumentation. The regulator in Germany requires thermal loading to be evaluated as realistically as practical, and thus additional instrumentation is typically added to measure the thermal response close to the monitored location. Elsewhere, and specifically in the U.S., the regulator has not required the addition of instrumentation, and thus utilities typically do not choose to add instruments.

It is one purpose of the paper to explain in detail the different engineering approaches and the methodology of fatigue assessment, also under consideration of Environmentally Assisted Fatigue (EAF) and the calculation of the complete six terms stress tensor for given load-time histories. Considering the fatigue analysis of given input data (stress-time-histories) the comparability of fatigue calculations constitutes a challenging item. Recommendations will be given how to harmonize the approaches based on the technical and regulatory requirements.

Finally, the issue of design code based fatigue monitoring guidelines and approaches is discussed. In this context, the German Nuclear Safety Standard KTA 3201.4 is addressed.

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