By Rory Carroll
SAN FRANCISCO (Reuters) - A U.S. nuclear inspection team has identified a number of issues that require further study before a damaged California nuclear plant can restart, according to a report issued on Thursday.
The two-reactor San Onofre Nuclear Station, located in Orange County, has been shut for more than five months due to the discovery of premature wear of steam generator tubes.
Located halfway between Los Angeles and San Diego, the nuclear station is critical to bolster the state grid and allow power to be imported to Southern California from outside the state. The California grid operator has warned that a prolonged shutdown of the plant increases the possibility of rolling outages over the summer months when power demand rises.
Unit 2 shut in early January for scheduled maintenance and Unit 3 was shut January 31 after operators discovered a small radiation leak.
The 2,150-megawatt nuclear plant is operated by Edison International's Southern California Edison utility. SCE owns a 78-percent stake in the plant while Sempra Energy's San Diego Gas & Electric owns 20 percent.
The U.S. Nuclear Regulatory Commission inspection report released on Thursday detailed what regulators told the public at a meeting in Orange County in June - that inadequate computer testing of the Unit 3 steam generator led to excessive vibration between tubes carrying radioactive water inside the generator, resulting in the leak.
Tube degradation was found in both units, but was greater in Unit 3 than Unit 2, the report said.
The inspection team identified 10 issues that require additional scrutiny by the NRC, mostly related to the design of components in the steam generators that allowed the excessive vibration.
After a number of tests, workers have plugged more than 1,300 tubes, or about 3 percent, of the tubes inside the giant steam generators.
An SCE official said the company will continue to use "conservative decision making" as it works on repairs and planning for a future restart.
SCE must seek NRC approval before restarting the units. The utility has not given a specific timeline for doing so.
Environmental group Friends of the Earth was critical of the inspection team's report, calling it "an effort to validate the NRC's prior failure to require that Edison file for a license amendment" when it submitted plans to replace the original steam generators with new generators manufactured by Mitsubishi Heavy Industries.
Since the new generators were significantly different from the ones they replaced in 2010 and 2011, the longer, more public process of amending the license was warranted, said Damon Moglen, climate and energy campaign director for the group.
California Senator Dianne Feinstein said Thursday she will continue to monitor NRC efforts to ensure that the problems are resolved, noting that more than 7 million Californians live within 50 miles of San Onofre.
"Today's report indicates there is more work to be done before we can fully understand the problem and determine a possible solution," she said.
(Additional reporting by Eileen O'Grady in Houston; Editing by Ed Davies)