Southern California Edison (SCE) has outlined its proposed action plan for the restart of San Onofre Nuclear Generating Station unit 2. The reactor has been offline along with sister unit SONGS 3 since January, following the discovery of unexpected wear in steam generator tubes. SCE is seeking to operate SONGS 2 at 70% power for around five months before shutting down to inspect the steam generators.

The Nuclear Regulatory Commission confirmed, 4 October, that it has received a letter from Southern California Edison, but said that: “months of NRC inspection and analysis will precede any decision on whether to restart the reactor.”

“Our primary focus now must be on analyzing SCE’s response to the [Confirmatory Action Letter] before addressing the restart question. The agency will not permit a restart unless and until we can conclude the reactor can be operated safely,” NRC Chairman Allison Macfarlane said. “This could take a number of months. Our inspections and review will be painstaking, thorough and will not be rushed.”

SCE said in a statement that the response to the CAL and the restart plan had been submitted simultaneously ‘to provide the NRC with all the relevant information needed to evaluate the full spectrum of repairs, corrective actions and additional safety measures proposed for restart and safe operations at the plant.’

The utility, which owns over 78% of the SONGS plant, concluded that tube-to-tube wear in steam generators at both SONGS 2 & 3 were caused by a phenomenon called 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.

It attributed the ‘significantly lower level’ of tube-to-tube wear in SONGs 2 to manufacturing differences.

“The high-steam velocity and low-moisture conditions existed in unit 2 and hence unit 2 was susceptible to the same vibration-causing environment. 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.”

SCE said that it plans to operate unit 2 at 70% power, which will prevent the vibration-causing environment by decreasing steam velocity and increasing moisture content.

The reactor will also be shut down after a ‘conservative’ operating period of five months for inspection of the steam generator tubes to: ensure the continued structural integrity of the tubes, to measure tube wear and to confirm that the solutions are working.

SCE said that it has plugged six tubes in unit 2 indicating wear with greater than 35 percent through wall depth and preventively plugged more than 500 other tubes. Overall, only 2.6% of the total SG tubes in SONGS 2 have been plugged, it said.

The SONGS 2 restart plan also includes proposals for additional monitoring activities. These 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 enhanced operator training to respond to extremely small tube leaks.

Unit 2 cannot be restarted until all plans are approved by the NRC. Unit 3 remains shutdown for further evaluation.

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