Edison's plan, which must be approved by federal regulators, calls for operating Unit 2 at reduced power for five months, then shutting it down for inspections. The outlook for the more heavily damaged Unit 3 is bleaker no decision is expected on its future until at least next summer.
Meanwhile, the company is facing a state review of costs related to the long-running outage that could leave customers or shareholders with a huge bill for repairs and replacement power a figure that had reached $165 million at midyear. The company did not update those figures Thursday.
Edison, a subsidiary of Edison International, filed its proposal with the Nuclear Regulatory Commission, which is expected to take months to review the details. The NRC has said there is no timetable to restart the plant.
"The agency will not permit a restart unless and until we can conclude the reactor can be operated safely," NRC Chairman Allison Macfarlane said. "Our inspections and review will be painstaking, thorough and will not be rushed."
The proposal was immediately denounced by environmentalists and anti-nuclear activists who have argued for months that restarting the plant between San Diego and Los Angeles would set the stage for a catastrophe. About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes.
"Both these reactors are alike and neither is safe to operate," said S. David Freeman, a former head of the Los Angeles Department of Water and Power who advises Friends of the Earth. "While Edison may be under financial pressure to get one up and running, operating this badly damaged reactor at reduced power without fixing or replacing these leaky generators is like driving a car with worn-out brakes."
Edison wants to operate Unit 2 at 70 percent power, which company officials predicted would prevent vibration that has caused excessive wear to tubing. Company officials expressed confidence in the proposal, which followed more than 170,000 tube inspections over more than eight months.
"This is not an experiment," Pete Dietrich, senior vice president and chief nuclear officer at SCE, told reporters in a conference call.
The problems center on four steam generators that were installed at San Onofre during a $670 million overhaul in 2009 and 2010. Tests found some tubes were so badly corroded that they could fail and possibly release radiation, a stunning finding inside the nearly new equipment.
The trouble began Jan. 31, when the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped at the time, but officials said there was no danger to workers or neighbors. Unit 2 had been taken offline earlier that month for maintenance, but investigators later found unexpected wear on hundreds of tubes inside both units.
In a March letter, federal regulators outlined a series of benchmarks Edison must reach to restart the plant, including determining the cause of vibration and friction that damaged tubes, and how it would be fixed and then monitored during operation.
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.
Overall, investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the four generators. And in about 280 spots virtually all in the Unit 3 reactor more than 50 percent of the tube wall was worn away.
In Unit 2, investigators found that the wall thickness had been worn away by at least 20 percent in 147 tubes. When about a third of the wall thickness wears away, a tube is deemed too risky to keep in service. Edison has retired, or plugged, more than 500 tubes in Unit 2 because of damage or as a precaution, a number within the margin to continue operating the plant.
Dietrich said Unit 2 was susceptible to the same problems that ravaged Unit 3, but engineers believe that the extent of damage was different because of manufacturing and assembly differences that resulted in looser tubes in Unit 3. Running at lower power should correct the trouble, at least in Unit 2, he 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.
If a tube breaks there is the potential that radioactivity could escape into the atmosphere, and serious leaks also can drain cooling water from a reactor.
Company executives have left open the possibility that the heavily damaged generators in Unit 3 might be scrapped.
The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries. The design of the generators also is under congressional scrutiny.
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.