U.S. Operating Experience with Thermally Treated Alloy 690 Steam Generator Tubes (NUREG-1841)

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Publication Information

Manuscript Completed: March 2006
Date Published: August 2007

Prepared by:
K.J. Karwoski, G.L. Makar, and M.G. Yoder

Division of Component Integrity
Office of Nuclear Reactor Regulation
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

This report documents the background and performance of thermally treated Alloy 690 steam generator tubing in U.S. commercial pressurized-water reactors (PWRs). The industry has used this material extensively for replacement steam generators since 1989. As of December 31, 2004, it was being used in 30 units, or about 43 percent of the operating PWRs in the United States. Of the 577,070 thermally treated Alloy 690 tubes placed in service, only 333 tubes (0.06 percent) have been plugged after approximately 173 calendar years of operation. The majority of these tubes (65 percent) were plugged prior to placing the steam generators in service. The dominant inservice degradation mode, responsible for about 24 percent of the plugged tubes, has been wear caused by a support structure or loose part. No corrosion or cracking had been detected as of the time this report was prepared. The superior performance experienced to date with thermally treated Alloy 690 tubes compared to earlier tube materials is attributed to the alloy chemistry (principally the higher chromium content), the corrosion-resistant microstructure developed by the combination of alloy chemistry and thermal processing, and design improvements in replacement steam generators.