LOS ANGELES -- The operator of California's troubled San Onofre nuclear power plant on Thursday proposed to restart one of the facility's twin reactors after concluding it could be run safely despite damage to scores of tubes that carry radioactive water.
A plan to return even one reactor to service is a milestone for Southern California Edison, which has spent months unraveling what caused excessive tube vibration and friction inside the plant's virtually new steam generators, then determining how to fix it.
The proposal was immediately denounced by environmentalists who have argued for months the seaside plant between San Diego and Los Angeles is too damaged to restart safely. About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes.
The plan "is a reckless gamble that flies in the face of the utility's claim that it puts safety ahead of profits," the advocacy group Friends of the Earth, which is critical of the nuclear power industry, said in a statement.
Edison filed the 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."
Edison wants to operate Unit 2 at 70 percent power, which it predicted will prevent tube vibration. The company said it will shut down the reactor after five months for inspections.
After conducting more than 170,000 inspections "we have concluded that Unit 2 at San Onofre can be operated safely," Edison President Ron Litzinger said in a statement. "This plan will get San Onofre Unit 2 back to providing reliable and clean energy to Southern Californians."
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 scores 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.
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.
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.
With the plant dark for months, the bills have been piling up.
Company officials said earlier this year that SCE had accumulated $48 million in inspection and repair costs through June 30 -- a bill that is growing steadily. And $117 million has been needed to buy power to replace the electricity that the plant would have generated. It would cost $25 million more to get one of the damaged reactors running at reduced power.
The company did not update those figures Thursday. Utility regulators are expected to debate later this year who gets stuck with the growing bill, customers or shareholders.
As far back as early May, Edison officials talked optimistically about restarting Unit 2, where tube damage has been less extensive.
The prospects for its twin reactor, Unit 3, look bleaker. Company officials have left open the possibility that the heavily damaged generators in Unit 3 might be scrapped, and it's also possible the plant will never return to its full output of electricity.
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.