SSM perspective
Background
As Swedish NPPs reach their original technical design life, re-assessment of the fatigue life of structural components is needed for safe long term operation (LTO). In these re-assessments one must consider that new knowledge has shown that environmental effects on fatigue life must be considered.
One common method to verify the fatigue life of structural components subjected to an LWR coolant environment is to use so-called attention values. This simplistic method is beneficial as re-assessments can be based on existing fatigue analyses. The basic idea behind the method is that if the accumulated fatigue utilization factor in an existing fatigue analysis (not considering environmental effects) is lower than the attention value, the fatigue criterion is fulfilled also for environmental conditions. That is, the fatigue life margin is large enough to to deal with the negative effects of the LWR coolant environment.
The present study aims to investigate how attention values can be established and used. Furthermore, the study investigates the possibility to apply a general attention value for Swedish NPPs.
Results
The report presents fatigue evaluations of different locations in BWR systems and one location in a PWR system. Environmental conditions are included in the analyses with the purpose to get an enhanced under-standing of how attention values could be established and used.
For austenitic stainless steel in BWR environments, an attention value of 0.4 is suggested. For carbon and low-alloy steel in BWR environments, an indicative value of 0.2 is presented. For PWRs, no attention values are suggested as data for only one component was available.
Relevance
The work has increased the understanding for how attention values can be established and used. Furthermore, attention values for BWRs and PWRs are presented. These attention values may be used in re-assessments of fatigue life when environmental effects must be considered.
Need for further research
The work is based upon a relatively few number of locations in class 1 piping components. Hence, more locations, especially for PWRs but also in the region of 0.2<Uacc<0.4 for BWRs, would increase the significance of the attention values presented.