LOS ANGELES (AP) -- A report commissioned by an environmental group warned Tuesday that running California's San Onofre nuclear plant at reduced power would not resolve problems with badly worn tubing that have kept its twin reactors offline for more than three months -- and might make it worse.
The 13-page report issued by Friends of the Earth also expanded an earlier allegation that Southern California Edison misled federal regulators about extensive modifications to its troubled steam generators, a charge disputed by the Nuclear Regulatory Commission and the company.
Edison said in a statement that "the NRC was fully informed that the replacement would be conducted under the same regulations as had been previously applied at other plants."
Earlier this month the company announced a tentative plan to restart and run the seaside reactors at lower power, at least for several months, because engineers believe that will ease vibration that could be causing unusual wear to tubes inside the huge generators, which have nearly 40,000 tubes that carry radioactive water.
The seaside reactors between Los Angeles and San Diego have been idle since January, following a tube break in the Unit 3 reactor. 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 in both units.
The NRC has said there is no timetable to restart the reactors, as investigators continue to look for the cause of the unusual tube wear.
The report, written by Vermont-based nuclear consultants Fairewinds Associates, suggested the best alternatives might be scrapping and replacing the costly equipment, or spending as much as $400 million on an extensive repair. The generators were replaced in 2009 and 2010 in a $670 million overhaul.
"Power reductions do not solve underlying and serious degradation problems," said the report, a copy of which was obtained by The Associated Press in advance of its public release Tuesday. It warned of possible tube breaks that could endanger public health.
It said running at lower power would not prevent damaged tubes and tube supports from vibrating and damaging others nearby. "It will worsen the existing damage," it concluded.
Gradual wear is common in such tubing, but the rate of erosion at San Onofre alarmed officials since the equipment is relatively new. The company last week said more than 1,300 tubes will be taken out of service, a far higher number than previously disclosed, but that number is well within the margin to allow the generators to keep operating.
Edison has been facing pressure from some nearby communities and nuclear activists who have raised safety concerns, while the company looks for a solution to the tube problem. State officials have warned of possible power shortages in the region this summer while the plant remains dark.
The generators were designed to meet a federal test to qualify as "in-kind," or essentially identical, replacements, which allowed them to be installed without prior approval from federal regulators.
The report said a string of complex modifications involving tubes and supports should have triggered a more extensive review by the government, including public hearings, but did not. The report speculated that the NRC would have identified problems with the design if a thorough review was conducted.
In a statement, NRC spokesman Victor Dricks said the company informed the agency of the steam generator design changes "in accordance with NRC requirements," and that portions of the design were reviewed by federal inspectors during the installation. He added that a team of investigators is reviewing those changes as part of its probe into tube wear at the plant.
The four generators at San Onofre -- two per reactor, each with 9,727 alloy tubes -- function something like a car radiator, which controls heat in the vehicle's engine. The generator tubes circulate hot, radioactive water from the reactors, which then heats a bath of non-radioactive water surrounding them. That makes steam, which is used to turn turbines to make electricity.
The tubes represent a critical safety barrier -- if a tube breaks, there is the potential that radioactivity can escape into the atmosphere. Also, serious leaks can drain protective cooling water from a reactor.
Edison did not immediately respond to questions about the report.