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Overview of Steam Generator Tube Degradation and Integrity Issues


Quiz Questions

1.Stress corrosion cracking (SCC), both on the primary and secondary sides of the steam generator tubes, has become the principal degradation mode leading to tube plugging in the U.S. and worldwide.
True
False
2. Primary Water Stress Corrosion Cracking (PWSCC) most commonly occurs in the tube expansion transition region, or roll transition zone (FSZ), where the tube is expanded into the tube sheet. The cracks are usually axial, but circumferential cracking has also been observed and can occur near the end of the expanded region above the tube sheet or farther down in the expanded portion of the tube.
True
False
3.PWSCC can also occur in the plugs used to remove steam generator tubes from service.
True
False
4. ODSCC/IGA is a leading cause of PWR steam generator tube plugging in the U.S. and worldwide, and the tube support plate crevice is the predominant location for this process.
True
False
5.The U.S. Nuclear Regulatory Commission (NRC) is presently developing a performance-based rule and regulatory guide for steam generator tube integrity.
True
False
6.The OD cracking observed in the tube support plate crevice is generally intergranular in nature and commonly consists of short axial cracks of varying length and depth within the crevice region. Adjacent cracks are often separated by ligaments of sound tubing material, and crack linkage can occur as ligament sections are corroded.
True
False
7. ODSCC and IGA also commonly occur in the tube sheet region, either in the built-in crevice between the tube sheet and the tubes that was present in earlier Westinghouse designs or in the sludge pile atop the tube sheet. At either location, the process is similar to that operative in the tube support plate crevices, with the creation of highly caustic or acidic conditions in the crevice because of local boiling and concentration effects.
True
False
8.ODSCC/IGA can also occur in the sludge-pile region above the RTZ. The situation here is very similar to that present at the tube support plate crevices filled with corrosion product and sludge. These cracks are commonly axial.
True
False
9.Conventional eddy-current bobbin coils are currently the principal tool utilized for detecting flaws in steam generator tubing. Multiple frequencies are used to acquire data, with the frequencies selected dependent on the tube material, geometry and inspection objectives.
True
False
10.The principal concern under a severe accident scenario is containment bypass. The steam generator tubes make up more than 50% of the surface area of the primary pressure boundary, and the failure of a significant number of tubes during a severe accident would result in an unacceptable release of radionuclides to secondary side and eventually to the environment.
True
False