German KTA consider the intensively discussed topic of environmentally assisted fatigue (EAF) by introducing so called attention thresholds for the cumulative fatigue usage factors (CUF). Threshold values for both austenitic steels and ferritic steels are defined. If the threshold value is exceeded, further measures like NDT, in-service inspections or detailed assessment procedures have to be performed. One way to handle those measures is to apply sophisticated procedures and to show that the numerically calculated CUF is below the defined attention thresholds. The article presents some examples to reduce calculated CUFs.
Fatigue analyses performed during design phase of NPP are based on specified loads consisting of conservatively determined reference transients, covering the whole lifetime including events with significantly fewer cycles than assumed during design phase as well as events with very low probability of occurrence at all. But there might be also operational loads which are not included in the load specifications. In general, there is a lot of potential to reduce computed CUFs in terms of considering realistic loading scenarios.
In German NPPs various temperature measurement planes are applied at fatigue relevant positions among others to calculate actual CUFs based on real loads. Detailed finite element (FE) calculations considering these loads can reduce computed CUFs considerably. Therefore detailed evaluation of temperature measurements in combination with FE calculations is carried out, reducing calculated CUFs below the threshold values given in German KTA.