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

The materials used previously for steam generator 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 steam generator 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 to 850C and stresses in the range of 30 to 350 MPa, with failure times to creep rupture ranging from 3 to 870 hours. Based on the creep test results, creep life predictions were then made using the well-known Larson Miller Parameter method. Steam generator tube rupture tests were also conducted under the conditions of a constant temperature and pressure ramp using steam generator tube specimens. 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 steam generator tubes was predicted using a creep rupture model and a flow stress model. A modified creep rupture model for Alloy 690 steam generator 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.

Measurement of Steam-Generator Tube Damping Caused by Anti-Vibration-Bar Supports

In 2013, Atomic Energy of Canada Limited (now Canadian Nuclear Laboratories) took vibration measurements to examine low-frequency damping associated with tube vibration parallel to anti-vibration bar supports. These tests were performed to better understand and control in-plane fluidelastic instability of tubes in the U-bend region of recirculating steam generators. In the tests, the damping ratios of a single steam generator tube vibrating parallel to a single pair of anti-vibration bars were measured using both a log-decrement and a power-based method. Non-contacting excitation and position-sensing techniques were employed to improve accuracy. The tests examined the effects of fluid (water or air), natural frequency, gap width, preload, and vibration normal to the bars. Additional tests were also done using a drilled-hole support and without supports to allow validation against previously published work. Most of the tests were performed in water. This paper describes the tests including the test apparatus, test methods, and analysis techniques. A summary of the results is presented. These results show that the damping ratios measured without any supports and with a drilled-hole support are consistent with previously published data. Contrary to an existing design guideline, the anti-vibration bars resulted in no significant additional viscous or squeeze-film damping for vibration parallel to the bars. The results also showed that anti-vibration bars can introduce significant in-plane Coulomb-type damping because of friction and impacting.