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UPDATE 3-NRC needs months to review San Onofre reactor restart

* NRC must approve of San Onofre Unit 2 restart

* SCE wants to run Unit 2 at 70 pct for about 5 months

* Unit 3 to remain shut for additional tests

(Adds NRC comment about months before making restart decision)

Oct 4 (Reuters) - U.S. nuclear regulators said Thursday they would take months to review Southern California Edison's (SCE) plan to restart a reactor at the long idled San Onofre nuclear power plant in California.

"Our primary focus now must be on analyzing SCE's response ... before addressing the restart question. The agency will not permit a restart unless and until we can conclude the reactor can be operated safely," U.S. Nuclear Regulatory Commission (NRC) Chairman Allison Macfarlane said in a release.

"This could take a number of months. Our inspections and review will be painstaking, thorough and will not be rushed," she said.

SCE said in its response to the NRC's Confirmatory Action Letter on Thursday that it wants to restart the 1,070-megawatt (MW) Unit 2 at San Onofre and operate it at up to 70 percent for about five months before shutting it again for inspections.

Both San Onofre units have been out of service since January following a small radioactive steam leak at one unit that pointed to a problem with the tubes in both units' steam generators.

SCE also said it will keep the 1,080-MW Unit 3 offline while the utility continues to study potential solutions that are unique to that unit.

The NRC had determined that the unprecedented damage to thousands of tubes inside the generators was so serious that the utility must identify the root cause and find ways to manage tube vibration before seeking approval to restart the units.

"We have concluded that Unit 2 at San Onofre can be operated safely and within industry norms," Ron Litzinger, president of SCE, said in SCE's release. "When implemented, this plan will get San Onofre Unit 2 back to providing reliable and clean energy to Southern Californians."

SCE is a unit of California power company Edison International

. Edison International stock was up about 1.5 percent, or about 70 cents, Thursday morning to about $47.30, which was well over the rise in other utility stocks.

GRID CHALLENGES

The prolonged shutdown of San Onofre, located in San Clemente about half way between Los Angeles and San Diego, has been felt by the California grid agency, other power producers and the state's young carbon market set to hold its first allowance auction in November.

With San Onofre - the biggest power plant in Southern California - out of service for the summer months, California's power grid operator had to make contingency plans to be sure there was enough power available on the hottest days when consumers crank up their air conditioners to escape the heat.

Those plans included returning old natural gas plants back into service. The California ISO, which operates the grid, has also started making plans for next summer in case San Onofre was still down.

The two reactors at San Onofre can generate enough power to meet the needs of about 1.4 million Southern California homes.

SCE said its response to the NRC covers the causes of the tube wear, repairs and corrective actions required for the Unit 2 steam generators, actions to prevent the extensive tube-to-tube wear observed in Unit 3, and inspection and safe operation protocols.

SCE said the tube-to-tube wear in the Unit 3 steam generators was 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.

The company said the conditions also existed in Unit 2, meaning Unit 2 was susceptible to the same vibration-causing environment. However, of the nearly 20,000 tubes in Unit 2, all but two were known to have been effectively supported throughout its 21-month operating period, the company said.

NEW STEAM GENERATORS

The steam generators built by Japanese multinational engineering firm Mitsubishi Heavy Industries

were newly installed over the past few years.

SCE said it will operate Unit 2 at 70 percent power, which will prevent the vibration-causing environment by decreasing steam velocity and increasing moisture content.

The company said it would shut Unit 2 after 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 it plugged six tubes in Unit 2 indicating wear with greater than 35 percent through wall depth and preventively plugged more than 500 other tubes. 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.

SCE shut Unit 2 on Jan. 9, 2012, for a planned outage. Unit 3 was safely taken offline Jan. 31, 2012, after station operators detected a small leak in a steam generator tube.

SCE operates San Onofre for its owners - SCE (78.21 percent), Sempra Energy's

San Diego Gas & Electric (20 percent) and the city of Riverside (1.79 percent).

(Reporting By Scott DiSavino in New York and Eileen O'Grady in Houston; Editing by Maureen Bavdek)

((scott.disavino@thomsonreuters.com)(+1 646 223-6072)(Reuters Messaging: scott.disavino.thomsonreuters.com@reuters.net))

Keywords: ENERGY EDISON/SANONOFRE