Study on Cycle-Based Fatigue Monitoring Technique

According to NUREG/CR-6909, the coolant environments have great influence on the fatigue resistance of the nuclear piping and equipment. As there are many conservatives in the cycle number and temperature/pressure scope of design transient, the fatigue usage factor of some main equipment may exceed the design requirement if the environmentally-assisted fatigue (EAF) is considered in the design phase. In order to solve this problem, the fatigue usage factor may be calculated by using real transient cycle and sequence based on cycle-based fatigue monitoring technique in nuclear power plant. Using this method the conservatives of design transients will be deleted and the life of main equipments should not be influenced by considering EAF. The method flow and key technologies for cycle-based fatigue monitoring technique were studied. Four steps were needed to get fatigue usage factor: transient statistics, stress reconstruction, fatigue calculation and EAF correction. Plant signals needed for the fatigue monitoring system were provided based on in service nuclear power plant. Transient statistics (include transient detection, classification and statistics) was the most important and difficulty of the four steps and it could not only benefit the fatigue life, but also can benefit the aging management, design and operating improvement. Transient statistics was done according to reactor power level, the initial state and causes of the transient, the protection actions and some of other important plant signals. According to reactor power which is the most important signal, the transient can be divided into four types: transients during reactor at no-load, full-power, power varying and reactor trip. The initial and final state of the transient and rate of change were used to classify the no-load transients. At full power state, only two transients were included. Power varying transients can be classified by initial and final power level. Reactor trip signals, pump state, pressure, temperature and other operating signals were used to classify reactor trip transients. A stress sequence was built according to transient sequence. The extremal stress of the sequence was selected and fatigue usage factor was calculated. The environmental correction factor Fen mentioned in NUREG/CR-6909 was used to modify the fatigue usage factor in order to consider the influence of EAF. The method was verified by assumption events and the result shows that the method could detect and classify operating transients accurately. The cycle-based fatigue monitoring technique should be improved by using the real plant parameters and can be programmed and used in plant fatigue monitor.