Fatigue Damage Assessment Method of Crack-like Discontinuous Zone in Nuclear Reactor Structure

There are a large number of local structural discontinuous zones in the form of cracklike defects in the reactor structure, due to the existence of the stress concentration at weld toes, the fatigue performance of the structure will be remarkably reduced. Therefore, the fatigue damage assessment in the cracklike discontinuous zone becomes an important issue in the nuclear reactor structure analysis. In this paper, based on the cracklike discontinuous zone caused by the welds between the closure head on the reactor pressure vessel and a penetrated pipe, two types of fatigue assessment methods were studied which are the general fatigue assessment methods considering the correction coefficient for welds and the fatigue reduction factor, and the fatigue assessment methods for the cracklike discontinuous zone in RCCMRx rules and RCCM rules. In RCCM rules, there are two approaches involved: the dedicated fatigue curve algorithm and the cyclic stress strain curve algorithm. For the cracklike fatigue assessment methods, singular elements were first used to calculate the stresses in the cracklike discontinuous zone, then the fatigue damage factors were calculated with the considerations of both the main cycles and subcycles of stresses, and the conservatism of the methods involved was compared. The results show that, compared with the general fatigue assessment methods, the stress concentration of the cracklike discontinuous zone can be better simulated by the cracklike fatigue assessment methods, and it is found that the stress level in the characteristic distance range is too high, and the stress curve is flat outside the characteristic distance, so the problem that the fatigue damage factors cannot be calculated caused by the stress singularity in the cracklike region tip, is effectively avoided. Compared with RCCM rules, the fatigue damage factors calculated by the RCCMRx method are higher due to the more conservative plastic correction. When the methods based on the RCCM rules were used, the fatigue damage factors calculated by the cyclic stress strain curve algorithm are slightly higher than that of the dedicated fatigue curve algorithm, indicating that the results are more conservative. Compared with the fatigue damage factors calculated by the cracklike assessment methods, the fatigue damage factors calculated by the correction coefficient for welds are lower. When the fatigue reduction factor gradually increases, the fatigue damage factors change from being less conservative to much more conservative. Therefore, when the fatigue reduction factor and the correction coefficient for welds cannot be accurately determined, there appear certain limitations of the engineering fatigue damage assessment methods based on the means of correction coefficients, and the cracklike fatigue assessment methods are more suitable for fatigue damage assessment in the cracklike discontinuous zone.