Tube integrity is an important aspect for safe and reliable operation of nuclear power plant steam generators. As a U.S. industry and licensing requirement, all in-service steam generator tubes shall retain structural integrity over the full range of normal operating conditions and design basis accidents by meeting the structural integrity performance criterion (SIPC) as given in NEI 97-06. The SIPC margin shall be maintained during plant operation between tube examinations.
The burst strength of tubes subjected to wall thinning will depend on the extent and mode of degradation, and the magnitude of design loads to include pressure differential across the tube wall during normal operation and postulated accident conditions. In addition, non-pressure loads that can occur during postulated accident events shall be evaluated and included in the assessment of tube integrity if determined to significantly reduce the tube burst strength. The EPRI Flaw Handbook provides burst pressure relationships for flaws which include a reduction factor that accounts for the effect of applied bending stress on circumferential degradation. However, this previous industry work was only for planar crack-like flaws and did not directly address uniform volumetric wall loss which can have both axial and circumferential extent.
This paper describes a test program to determine the effect of bending loads on the burst pressure of a tube with uniform thinning over a given axial length. The uniform thinning geometry was selected since it represented a bounding case of general wall loss and is conservative for calculating a tube repair limit for volumetric degradation for a given steam generator design. Tube repair limits are required for defining an upper limit on in-service degradation for which a tube is to be removed from service. Tube repair limits are cited in the Plant Technical Specifications, which is an important part of the licensing basis.