Southern California Edison (SCE) on Friday released a new technical evaluation that concludes steam generators at its San Onofre Nuclear Generating Station (SONGS) Unit 2 can safely be operated at 100% power. The evaluation reinforces a more conservative plan proposed by the company to federal regulators to operate the unit, which has been shut down since January 2012, at 70% for five months.
The operational assessment, prepared in response to questions from the Nuclear Regulatory Commission (NRC), determined Unit 2 "could be at full power for 11 months with full tube integrity," SCE said. The assessment was performed by Sunnyvale, Calif.-based Intertek APTECH.
"While we have no intent to restart Unit 2 at full power, this demonstrates the amount of safety margin we have built into our analyses. We welcome this additional safety analysis but remain steadfast in our commitment to restart Unit 2 at only 70 percent power,” said Pete Dietrich, SCE senior vice president and chief nuclear officer. SCE has not yet presented a restart plan for Unit 3, which has more damage than Unit 2.
The twin-reactor plant’s Unit 2 has remained shut down since Jan. 9, 2012, when it was taken out of service for a planned outage. SCE operators performed a rapid shutdown of Unit 3 on Jan. 31, 2012, after a leak was detected at one of that unit’s steam generator tubes. Later investigations revealed premature tube wear in Unit 3 and indications of wear in Unit 2.
For both units, this was the first cycle of operation with new replacement steam generators from Mitsubishi Heavy Industries (MHI) that cost $671 million: Unit 2 replaced its steam generators in January 2010 and Unit 3 in January 2011. Each steam generator has 9,727 steam generator tubes, and the new equipment was warrantied to last until the plant’s license expires in 2022.
SCE has previously pointed to a faulty computer model used by MHI as failing to predict conditions in the steam generators, along with other design defects. The NRC publicly disclosed the computer code flaws three months before MHI completed its evaluation.
Earlier last week, the NRC released a redacted 135-page version of the "root cause analysis" from MHI. The report claims that minimizing tube wear resulting from vibration was a high priority in the design of the SONGS replacement steam generators.
"As part of the design process, [SCE] and MHI formed a special AVB Design Team to develop an effective anti-vibration design, focusing on the design of the anti-vibration bars (AVBs) that provide support for the tubes in the U-bend region of the replacement steam generators. The AVB Design Team conducted numerous technical meetings and included third-party experts in the design and construction of large U-bend steam generators," Edmund Baumgartner, MHI corporate counsel, said in a letter to the NRC.
SCE hailed the letter as confirmation that the utility believed the reactors’ steam generators were safe when installed and that safety measures were not sacrificed for licensing reasons.
"The anti-nuclear activists have called the MHI report a ‘bombshell’ which couldn’t be further from the truth," said Dietrich. "In fact, the MHI letter explains that SCE and MHI rejected the proposed design changes referenced in the evaluation because those changes were either unnecessary, didn’t achieve objectives or would have had adverse safety consequences. Our decisions were grounded in our commitment to safety. SCE did not, and would never install steam generators that it believed would impact public safety or impair reliability."
SCE owns 78.21% of the nuclear plant, along with San Diego Gas & Electric (20%) and the city of Riverside (1.79%). The plant is the largest source of base-load generation and voltage support in the region and is a critical asset in meeting California’s summer electricity needs.
Sources: POWERnews, SCE, NRC