Evaluation of the Creep Rupture Behavior of Alloy 690 Steam Generator Tubes Considering the Pressure Ramp Rate

Abstract

Steam generator tubes are among the main components that form the pressure boundary of a nuclear power plant and studies of ruptures of steam generator (SG) tubes are important because they can ensure the safety of a nuclear power plant in case of a severe accident. The materials used previously for SG tubes around the world have been replaced and will be replaced by Alloy 690 given its improved corrosion resistance relative to that of Alloy 600. However, studies of the high-temperature creep and creep-rupture characteristics of SG tubes made of Alloy 690 are insufficient compared to those focusing on Alloy 600. In this study, several creep tests were conducted using half-tube shape specimens of the Alloy 690 material at temperatures ranging from 650 °C to 850 °C and stresses in the range of 30–350 MPa, with failure times to creep rupture ranging from 3 h to 870 h. Based on the creep test results, creep life predictions were then made using the well-known Larson–Miller parameter (LMP) method. Steam generator tube rupture tests were also conducted under the conditions of a constant temperature and pressure ramp using SG tube specimens. As the pressure ramp rate increases, the failure behavior changes due to the rapid change in strain near the crack tip. The rupture test equipment was designed and manufactured to simulate the transient state (rapid temperature and pressure changes) in the event of a severe accident condition. After the rupture test, the damage to the SG tubes was predicted using a creep rupture model and a flow stress model. A modified creep rupture model for Alloy 690 SG tube material is proposed based on the experimental results. A correction factor of 1.7 in the modified creep rupture model was derived for the Alloy 690 material. The predicted failure pressure was in good agreement with the experimental failure pressure.