RPT-UPDATE 2-SCE Calif. San Onofre 2 reactor shut

(Repeats to widen distribution; no changes to headline or text) (Recasts, updates with company comment, changes dateline, previously NEW YORK)

LOS ANGELES, June 6 (Reuters) - Southern California Edison's 1,070-megawatt Unit 2 at San Onofre nuclear power station in California shut late Thursday night while returning from a brief maintenance outage, the utility said on Friday morning.

The unit was at 97 percent of production capacity after shutting over the weekend for a "tune up" before high summer electricity demand, said an SCE spokesman.

At 10:57 PDT Thursday, workers "observed a minor irregularity in the electric generator's cooling water system, a nonnuclear part of the plant," SCE said.

"Plant personnel are conducting tests to determine and resolve the cause," SCE said in a statement on Friday. "At this time it appears the unplanned outage will be brief."

The 2,150 MW San Onofre station is in San Clemente in San Diego County about 60 miles north of San Diego. There are two units at San Onofre, the 1,070 MW Unit 2 and 1,080 MW Unit 3, which entered service in 1983 and 1984.

Unit 3, meanwhile, ramped up to full power from 99 percent Thursday.

One MW powers about 700 homes in California.

So Cal Ed, a subsidiary of Edison International (EIX.N), operates the station for its owners: So Cal Ed (78.21 percent), Sempra Energy's (SRE.N) San Diego Gas & Electric subsidiary (20 percent), and the California city of Riverside (1.79 percent).

Edison International, of Rosemead, California, owns and operates about 14,000 MW of generating capacity, markets energy commodities in North America, and transmits and distributes electricity to about 4.8 million customers in central and Southern California. (Reporting by Bernie Woodall and Scott DiSavino; Editing by David Gregorio)