On the use of a monitoring system for fatigue usage calculations

Fatigue is a limiting factor in structural component life. Design calculations are performed according to conservative methodologies that shorten component life. Fatigue Monitoring Systems are an alternate that register the component service conditions. This work studies the application of one of those systems to determine the fatigue usage of the safe end of the feedwater system nozzle of the Santa María de Garoña nuclear power plant. The transients recorded are compared to the ones evaluated in the design specifications and the fatigue usage is compared with the one provided in the stress report. The main conclusion is that the fatigue monitoring system calculates a fatigue usage that is significantly lower than the design one, this is due to two main reasons: actual transients are less severe than design ones, and the number of occurrences of actual transients is lower than specified.