The U.S. Nuclear Regulatory Commission (NRC) said tubes at the 2,150 MW San Onofre Nuclear Generating Station (SONGS) in California were not properly tested by the manufacturer prior to installation.
SONGS has been shut down since January 31, when a small radiation leak was discovered in Unit 3. Unit 2 was taken offline January 9 for routine maintenance but remained shutdown as plant operators continue inspections of the tubes.
NRC said during a public meeting on June 18 that Mitsubishi Heavy Industries (MHI) was to blame for the tube leaks. MHI's thermal/hydraulic computer simulation model did not accurately predict the behavior of the steam and water in the steam generators, according to NRC. Additionally, eight tubes failed pressure testing, and the tubes were not held together tightly enough inside Unit 3, allowing them to rub against each other and causing premature wear. The difference in tube degradation between both units was caused by the differences between manufacturing of the steam generators, plant operators said. More than 1,300 tubes were plugged in both units in May.
NRC said there are 10 items that the Augmented Inspection Team (AIT) must follow-up on, including the evaluation of and disposition of numerous Unit 3 loose parts monitor alarms, steam generator shipping requirements, a lack of tube bundle support for steam generators during shipment and the adequacy of MHI's computer simulation model.
Plant owner Southern California Edison, a subsidiary of Edison International (NYSE: EIX), said the plant would likely remain offline through August due to safety inspections and testing the utility is performing. SCE said it will deliver a plan to the NRC by July regarding Unit 2, and the NRC will need time to respond. A separate plan for Unit 3 is expected to take longer to develop.
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