Nuclear Executive Update   
An EPRI Progress Report, September 2010
TECHNICAL HIGHLIGHTS
Guidelines and Software Enhance Steam Generator Tube Integrity Assessments

Technical guidelines, a flaw handbook, and corresponding flaw characterization software enable steam generator engineers to ensure tubes meet relevant performance criteria and structural limits.

Recent guidance and flaw characterization software enable nuclear power plants to more effectively demonstrate that flawed steam generator tubes meet structural and leakage integrity performance criteria established by technical specifications. Such tools also enable plants to conduct “what-if” analyses when planning inspections.

At the end of each operating cycle, U.S. nuclear plants and some non-U.S. plants must demonstrate that the steam generators met performance criteria and will continue to meet the criteria during the next operating cycle. EPRI’s Steam Generator Integrity Assessment Guidelines provide the technical bases and implementing procedures for meeting the objectives of steam generator tube integrity assessments, including the capabilities of eddy current systems for detecting degradation, how to characterize degraded steam generator tubes, appropriate structural and leakage limits, and how to apply uncertainties.

Revision 3 of the guidelines (Product No. 1019038), issued in 2009, incorporates recent operating experience such as cracking in thermally treated tubes, fatigue failures, cracking in higher-row U-bends, and excessive wear in replacement steam generators. Revision 3 also includes a new methodology for leakage assessments that factors in bending loads where applicable.

By documenting the latest industry experience with tube degradation, the guidelines can help steam generator engineers conduct more thorough degradation assessments. For example, Revision 3 highlights EDF plant experience in France, where excessive sludge deposits in broached support plates increased the local flow conditions in a manner that resulted in high-cycle fatigue. The broader PWR community should be aware of this experience to ensure this type of failure mechanism is not applicable to their steam generators.

The Steam Generator Degradation Specific Management Flaw Handbook, Revision 1 (Product No. 1019037), issued in December 2009, compiles burst test data, mathematical degradation models, and defined burst pressure equations to guide users in assessing specific modes of degradation relative to current industry experience. Degradation mechanisms addressed include fatigue, stress corrosion cracking, denting, fretting, wear, and intergranular attack. The handbook is used for developing flawed tubing structural limits, such as those shown in the accompanying figure, which are essential in informing integrity assessments.

The Flaw Handbook Calculator software, also released in 2009, is an automated Microsoft Excel® spreadsheet that calculates steam generator tube structural limits using the Flaw Handbook degradation models and the probabilistic methodologies defined in the Integrity Assessment Guidelines. The software produces limit curves (see figure) that define maximum degradation dimensions (e.g., throughwall depth, axial length) which will provide the margins of safety necessary to meet regulatory performance criteria for tube integrity. For example, degradation identified during an inspection with an axial length of 2 inches would fail condition monitoring performance criteria when the throughwall depth reaches about 50%. When projecting the size of degradation at the end of the operating cycle, the degradation could not exceed about 60% throughwall to stay within margins. The best estimate curve is an illustration of the conservatism in the required margins.

For more information, contact Helen Cothron at 865.773.4033, hcothron@epri.com, or James Benson at 704.595.2550, jbenson@epri.com.