Southern California Edison (SCE) last week outlined measures it had completed to correct issues identified in the steam generator tubes of its beleaguered San Onofre Nuclear Generating Station (SONGS) Units 2 and 3, as requested by the Nuclear Regulatory Commission (NRC). As part of a restart plan also submitted to regulators last week, the company proposed to restart Unit 2 at 70% power for a five-month trial period.
The 2,150-MW plant’s Unit 2 has been shut down since Jan. 9 for a planned refueling and maintenance outage; SCE operators performed a rapid shutdown of Unit 3 on Jan. 31 after a leak was detected at one of that unit’s steam generator tubes. For both units, this was the first cycle of operation with new replacement steam generators 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 tubes, and the new equipment is intended to last until the plant’s license expires in 2022.
SCE said last week in a statement that safety was its "top priority" and that more than 170,000 inspections had been conducted since the units had been shutdown to understand and prevent the problem from recurring. Responding to a March 27 confirmatory action letter that described actions that the NRC and SCE agreed would be needed to ensure safe operations, SCE confirmed a prior determination that the tube-to-tube wear in the Unit 3 steam generators was caused by a "fluid elastic instability," a combination of high–steam velocity and low-moisture conditions in specific locations of the tube bundles and ineffective tube supports in the same locations. The same high–steam velocity and low-moisture conditions existed in Unit 2, making that unit’s steam generator "susceptible to the same vibration-causing environment," the company said. "However, of the almost 20,000 tubes in Unit 2, all except two are known to have been effectively supported throughout its 21-month operating period."
If it could secure NRC approval, the company proposed to operate Unit 2 at 70% power to prevent the vibration-causing environment by decreasing steam velocity and increasing moisture content. "The 70 percent power level will result in steam velocities and moisture content consistent with those that the industry has successfully operated under for many years," the company said. After five months, SCE said it would shut down the reactor to inspect its steam generator tubes to ensure continued structural integrity of the tubes and measure tube wear. No date was proposed for the potential restart.
According to the utility, six tubes in Unit 2 showing wear with greater than 35% through-wall depth had been plugged. More than 500 others had also been preventatively plugged. "Steam generators are built with an allowance of extra tubes so that tubes may be taken out of service for a variety of reasons, including wear, and only 2.6 percent of the total tubes in Unit 2 have been plugged," it said.
The restart plan proposes additional monitoring, detection, and response activities to include installation of early-warning monitors that can detect extremely small tube leaks faster; enhanced sensitivity of vibration monitors; additional monitoring and analysis systems; and better operator training to respond to small tube leaks.
On Monday, the NRC’s Regional Administrator Elmo Collins told reporters that the federal body was considering whether to require an amendment to the plant’s operating license, a process that could last between six months and two years.
Meanwhile, the federal regulator hosted a webcast panel discussion on Tuesday in Dana Point, Calif., to allow representatives from the California Energy Commission, the California Public Utilities Commission, and local community organizations to answer questions and debate merits of restarting the plant’s Unit 2. At the event, some panelists and attendees called on the NRC to hold evidentiary hearings before it made a decision on whether to approve restart of the reactor.
More than 850 people reportedly attended the public meeting. Before the meeting began, anti-nuclear activists staged a rally outside the venue, some calling for extensive review of SCE’s proposal, others, for closure of the plant. The panel discussion was at times chaotic, marked by cheering or jeering as the meeting went on.
Sources: POWERnews, SCE, NRC
—Sonal Patel, Senior Writer (@POWERmagazine)