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Severed Tubes in Operating Nuclear Steam Generators: Case Analysis — I

[+] Author Affiliations
M. K. Au-Yang

Framatome ANP

Stephen Leshnoff

Exelon Corporation

Paper No. PVP2003-2076, pp. 71-80; 10 pages
doi:10.1115/PVP2003-2076
From:
  • ASME 2003 Pressure Vessels and Piping Conference
  • Flow-Induced Vibration
  • Cleveland, Ohio, USA, July 20–24, 2003
  • Conference Sponsors: Pressure Vessels and Piping Division
  • ISBN: 0-7918-4156-1
  • Copyright © 2003 by ASME

abstract

During a routine inspection in October 2001, it was discovered that a plugged tube in one of the steam generators at a nuclear station was completely severed at the upper tubesheet. The severed surface appeared to be fresh, clean, 360-deg. and with no evidence of necking down, ductile failure or visible flaws. Eddy current inspection revealed that the four neighboring tubes downstream of the severed tube each had wedge-shaped wear marks starting from the tubesheet end. However, there was no visible mid-span impact wear marks. Furthermore, the tube was visibly swollen due to over-pressurization. This tube was plugged in 1986 with no indications of flaw or wear marks. It was replugged later. The tube was not stabilized because there was no apparent potential for a circumferential sever to occur anywhere in the tube. Flow-induced vibration based on classic linear theory showed that this tube should have wide margin against fluid-elastic instability, turbulence or vortex-induced fatigue failure. However, it also was found that if for any reason the tube was laterally restrained (clamped) at the support plates, the margin against fluid-elastic instability would be reduced significantly. Still, this tube should have been stable and turbulence or vortex-induced vibration alone, as predicted by the classic linear theory without fluid-structure interaction, should not have caused fatigue failure. This study points out a deficiency in the classic linear turbulence-induced vibration analysis without taking into account the effect of fluid-structure coupling. When the cross-flow gap velocity is near the critical velocity, fluid-structure coupling would greatly increase the vibration amplitude over what is predicted by the classic acceptance integral approach. It is this increase in turbulence-induced vibration that caused this particular tube to fail by fatigue.

Copyright © 2003 by ASME

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