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Running California’s San Onofre nuclear power plant at reduced power would not pose a significant safety risk, federal regulators said.

That was the preliminary ruling Wednesday from Nuclear Regulatory Commission (NRC) staff representing an important step for operator Southern California Edison, which wants to restart the Unit 2 reactor by summer.

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The plant between Los Angeles and San Diego has been idle since January 2012, after a small radiation leak led to the discovery of unusual damage to tubes that carry radioactive water.

Unit 2 at San Onofre could safely restart and work at full power, but only for 11 months, a SCE consultant said.

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The utility, operator and majority owner of the San Onofre plant, said a technical evaluation by Intertek APTECH of Sunnyvale — which came in response to Nuclear Regulatory Commission (NRC) questions — found Unit 2 could operate at 100 percent with full integrity of steam generator tubes. Intertek used simulations and models to predict the time line of how long the reactor could operate without risking more tubes rupturing. Under one model, the unit could run for a little under a year, under the other model, a little more than a year.

Unit 2 was undergoing scheduled maintenance at the time and a subsequent investigation found steam generator tubes were degrading faster than expected, and many of them ended up plugged. Unit 2 never restarted.

SCE filed a plan with the NRC to restart Unit 2 at 70 percent power. The company has not fixed the problem, but has determined the cause, and believes operating at lower power will remove the steam conditions that caused the tubes to rattle, rub against each other, and wear out prematurely.

In the tentative ruling, the NRC staff agrees that running the reactor at no more than 70 percent power would not significantly increase the risk of any accident.

The issue represents one of a series of steps Edison must meet with the NRC, which oversees safety.

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