Rosemead, Calif., July 20, 2012 — Southern California Edison received the Nuclear Regulatory Commission (NRC) Augmented Inspection Team report regarding the unusual degradation of steam generator tubes in the San Onofre Nuclear Generating Station (SONGS) Units 2 and 3.
The report provides comprehensive information on the issue, including the response actions by the SCE plant operators to the tube leak in Unit 3, the causes of the wear and the outstanding issues that the inspection team is still reviewing. SCE understands that the tube-to-tube contact arises from excessive vibration of the tubes in certain areas of the steam generators.
The excessive vibration is believed to be caused by a phenomenon called fluid elastic instability, which results from the interaction of a combination of factors, including steam velocity, moisture content of the steam and effectiveness of the supports in the areas where the vibration occurs.
The report notes that SCE plant operators responded in a manner that protected public health and safety and that the commission’s onsite inspectors observed conservative decision-making and the appropriate questioning attitude displayed by the operators during the event.
Additionally, the commission report states that the steam generator design changes were appropriately reviewed in accordance with the 10 CFR 50.59 requirements, which govern design changes between original and replacement steam generators, noting that the changes at SONGS are common in replacement steam generators today.
The report also stated that, with the exception of the wear found at the tube retainer bar locations, the wear related to support structures is similar to wear found at other replacement steam generators after one cycle of operation.
SCE continues to work on its response to the commission’s Confirmatory Action Letter, the formal step in the process of restarting Units 2 and 3. The company will not restart the units until the utility and the commission are satisfied it is safe to do so.
Both SONGS units are currently safely shut down for inspections, analysis and testing. Unit 2 was taken out of service Jan. 9 for a planned outage. Unit 3 was safely taken offline Jan. 31 after station operators detected a leak in a steam generator tube.