The Nuclear Regulatory Commission (NRC) issued a Confirmatory Action Letter documenting actions that Southern California Edison Co. (SCE) officials have agreed to take related to unusual wear on steam generator tubes prior to restarting both units of the 2,200 MW San Onofre Nuclear Generating Station near San Clemente, Calif.
On Jan. 31, 2012, the NRC said operators performed a rapid shutdown of the Unit 3 reactor after indications of a steam generator tube leak. Unit 2 has been shut down since Jan. 9 for a planned refueling and maintenance outage. Subsequent inspections at both units have identified unusual wear in many tubes of the steam generators, which were replaced in January 2010 at Unit 2 and January 2011 in Unit 3.
SCE has identified two causes of the unusual wear: tubes are vibrating and rubbing against adjacent tubes and against support structures inside the steam generators. They are still working to determine why this is occurring.
Only one tube required pressure testing on Unit 2. However, six other tubes required plugging, and 186 additional tubes were plugged as a precautionary measure. Eight tubes failed pressure testing at Unit 3, indicating that these tubes could have failed under some accident conditions. Evaluation for additional plugging or other corrective actions are continuing for Unit 2, based on ongoing evaluations of Unit 3 test results.
As a result, SCE has committed to take the following actions prior to restart of each unit:
• SCE will plug all tubes in Units 2 and 3 for which testing indicated wear in excess of industry guidelines, as well as all tubes susceptible to this wear because of their location.
• SCE will determine the causes of the tube-to-tube degradation and establish a protocol of inspections and or operational limits for Unit 2 and 3, including plans for mid-cycle shut down for further inspections. The protocol is intended to minimize the progression of tube wear, and ensure that any tube wear does not progress to the point where it compromises tube integrity.
The NRC said the plant will not be permitted to restart until they are satisfied that the cause of the tube wear in both steam generators is understood and appropriately addressed.
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