Analysis of JNES Seismic Tests on Degraded Piping

Nuclear power plant safety under seismic conditions is an important consideration. The piping systems may have some defects caused by fatigue, stress corrosion cracking, etc., in aged plants. These cracks may not only affect the seismic response, but may also grow and break through causing loss of coolant. Therefore, an evaluation method needs to be developed to predict crack growth behavior under seismic excitation. This paper describes efforts conducted to analyze and better understand a series of degraded pipe tests under seismic loading that was conducted by Japan Nuclear Energy Safety Organization (JNES). A special “cracked-pipe element” (CPE) concept, where the element represented the global moment-rotation response due to the crack, was developed. This approach was developed to simplify the dynamic finite element analysis. In this paper, model validation was conducted by comparisons with a series of pipe tests with circumferential through-wall and surface cracks under different excitation conditions. These analyses showed that reasonably accurate predictions could be made using the ABAQUS connector element to model the complete transition of a circumferential surface crack to a through-wall crack under cyclic dynamic loading. The JNES combined-component test was analyzed in detail. The combined-component test had three crack locations and multiple applied simulated-seismic block loadings. Comparisons were also made between the ABAQUS FE analyses results to the measured displacements in the experiment. Good agreement was obtained and it was confirmed that the simplified modeling is applicable to a seismic analysis for a cracked pipe on the basis of fracture mechanics. Pipe system leakage did occur in the JNES tests. The analytical predictions using the CPE approach did not predict leakage, suggesting that cyclic ductile tearing with large-scale plasticity was not the crack growth mode for the acceleration excitations considered here. Hence, the leakage was caused by low-cycle fatigue with small-scale yielding. The procedure used to make predictions of low-cycle fatigue crack growth with small-scale yielding was based on the Dowling ΔJ procedure, which is an extension of linear-elastic fatigue crack growth methodology into the nonlinear plasticity regime. The predicted moments from the CPE approach were used using a cycle-by-cycle crack growth procedure. The predictions compare quite well with the experimental measurements.