Unit 2 had been taken offline earlier that month for maintenance, but investigators later found unexpected — in some cases extensive — wear on hundreds of tubes inside steam generators in both units.
Tests found some tubes were so badly corroded that they could fail and possibly release radiation, a stunning finding inside the nearly new equipment.
Edison's proposal calls for operating Unit 2 at up to 70 percent power for five months then shutting it down for inspections. Company officials expressed confidence in the proposal, which followed more than 170,000 tube inspections over more than eight months.
The future of the heavily damaged Unit 3 reactor is not clear.
Collins promised a thorough review, whether or not the agency requires Edison to seek an amendment to its operating license.
"We don't experiment with safety," he said.
In June, a team of federal investigators announced that a botched computer analysis resulted in design flaws that are largely to blame for unprecedented wear in the tubes.
The agency is still considering penalties against the company for issues related to the generator problems that have left the plant dark for more than eight months, Collins said.
The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate something like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds, with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter. They were manufactured by Japan-based Mitsubishi Heavy Industries.
Cracked and corroded generator tubing has vexed the nation's nuclear industry for years.
Decaying generator tubes helped push San Onofre's Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004. The following year, the Trojan nuclear plant, near Portland, Ore., was shuttered because of microscopic cracks in steam generator tubes, cutting years off its expected lifespan.
San Onofre is owned by SCE, San Diego Gas & Electric and the city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.
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