UPDATE 2-SCE sets plan to restart Calif. San Onofre Unit 2 reactor

* 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 details, background)

Oct 4 (Reuters) - Southern California Edison (SCE) on Thursday released a plan to restart Unit 2 at the San Onofre nuclear power plant and operate the unit at up to 70 percent for about five months before shutting it again for inspections.

SCE could not say when the 1,070-megawatt (MW) Unit 2 would restart since the plan must first be approved by U.S. nuclear regulators.

The company also said in its response to the U.S. Nuclear Regulatory Commission's Confirmatory Action Letter that the 1,080-MW Unit 3 will remain offline while the utility continues to study potential solutions that are unique to that unit.

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

SCE is a unit of California power company Edison International .

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

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 increase air conditioner use.

Those plans included returning old idled units to service. The California ISO, which operates the grid, has also started making plans for next summer in case San Onofre was still down.

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.

The steam generators at San Onofre 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 down 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))