Flaw Growth Prediction and Fitness-for-Service Assessment of a PWR Nuclear Plant Steam Generator U-Tube

Primary water stress corrosion cracking (PWSCC) continues to be a dominant degradation mechanism affecting the service life of steam generators in several operating pressurized water reactor (PWR) nuclear plants. Recently, in one operating nuclear plant, a steam generator U-tube ruptured catastrophically while the unit was on-line. Although the plant operators were able to shutdown the reactor without significant release of radiation, the Nuclear Regulatory Commission (NRC) and the owner utility launched a full-scale investigation of the incident. The owner utility requested that a crack growth analysis and engineering evaluation of the tube rupture be performed, as well as assess the fitness-for-service of the generator for continued operation. This paper presents a summary of elastic-plastic finite element and fracture mechanics analyses performed for a steam generator U-tube, subjected to crack initiation at the inner diameter of the tube in the apex region. Residual stresses were computed from a finite element model of the tube simulating the mechanical bending process with the use of an anvil. Fracture mechanics and crack growth evaluations were performed to predict the time required for a pre-existing flaw at the inside diameter of the tube to propagate through-wall. Additionally, a fitness-for service assessment was performed in order to permit a degraded tube to remain in service, given an initial flaw size as determined by nondestructive examination.