Acoustic Emission Characteristics of Thermal Fatigue Crack to Examine the Safety of Structural Components in NPPs

摘要

In the nuclear power plants, early detection of fatigue crack using non-destructive testing is the essential procedure due to the social importance of nuclear structure and the possibility of operating incidents. Usually, the damages in the nuclear facilities are caused by cyclic loadings due to the mechanical or thermal fatigue on the reactor coolant system. Thermal fatigue is the one of the life-limiting mechanisms in nuclear power plant conditions during operation and it can initiate and propagate cracks in various parts of reactor coolant system. In this study, stainless steel 304 pipe was used to fabricate the thermal fatigue crack, which is commonly used as parts of reactor coolant system in the nuclear power plants. An experimental apparatus was built to perform accelerated test to initiate the crack, and the experimental signals were continuously monitored by acoustic emission sensors. To fabricate thermal fatigue crack, the thermal cycle that comprised of 60 seconds heating and 30 seconds cooling was applied to identify and monitor the characteristic of thermal fatigue crack propagation. Through the data analyses, the characteristics of the signals to detect the crack initiation are investigated and the result shows different signal characteristic between the effective signals and noise signals. Once a crack initiating on the specimen, significantly high level of signal strength was occurred and continued to the end of experiment. The results provide preliminary information for the non-destructive technique to apply to the on-line monitoring system of the structure failure detection.