LOS ANGELES (AP) — The chairman of the Nuclear Regulatory Commission said Monday there is no timetable for restarting the sidelined San Onofre nuclear plant on the Southern California coast, where investigators are trying to determine the cause of unusual wear on hundreds of tubes that carry radioactive water.

The statement from Chairman Gregory Jaczko came just days after a senior executive for Southern California Edison disclosed that the company was hoping for a possible June restart. The twin-reactor plant, located between Los Angeles and San Diego, has been offline for more than three months.

The integrity of complex machinery inside the seaside plant has come under close scrutiny since investigators found that tubing that snakes through massive steam generators eroded at an unexpected rate, in some cases rapidly.

Jaczko said the federal agency is waiting for documentation on repairs and other work at the plant and "any discussion of a date for the restart ... is clearly premature."

"We will take whatever time is necessary to conduct a thorough safety review," he added.

A restart would require federal approval.

Last week, Edison executive vice president Stephen Pickett said the company was looking at the possible June restart for at least one of the ailing reactors. The company is drafting a plan under which the reactors would run at reduced power, at least for several months, because engineers believe that will solve a problem with vibration that the company believes has been causing unusual wear in the alloy tubing.

When Jaczko visited the plant in April, he said there must be a clear understanding of the cause of the excessive tube wear before either reactor returns to service.

Ted Craver, chairman of SCE parent Edison International, told investors in a phone call last week that unusual wear was found in about 1 percent of nearly 39,000 tubes in the steam generators.

Costs related to the long-running shutdown could climb over $100 million, company officials say, and state officials have warned about possible rotating blackouts in Southern California while the reactors are offline.

About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes.

The trouble at San Onofre began to unfold in late January, when the Unit 3 reactor was shut down as a precaution after a break in a steam generator tube carrying radioactive water. Traces of radiation escaped, but officials said there was no danger to workers or neighbors.

Unit 2 had been taken offline earlier that month for routine maintenance. But investigators later confirmed accelerated wear on tubing in both units. Hundreds of tubes that were heavily damaged will be taken out of service at the two reactors.

That number is well within the margin to allow them to keep operating, Edison officials say.

SCE projects that the bill for repairs and tests could run as high as $65 million, and $30 million was spent on replacement power through March 31 — a bill that keeps mounting.

Gradual wear is common in such tubing, but the rate of decay at San Onofre was startling because the equipment is relatively new. The generators were installed in a multimillion-dollar makeover in 2009 and 2010.

The plant's four steam generators each contain nearly 10,000 tubes that carry hot, pressurized water from the reactors. The tubes are a critical safety barrier — if one or more break, there is the potential that radioactivity could escape into the atmosphere. Also, serious leaks can drain cooling water from a reactor.

Test results show that two types of wear have occurred at both units. Tubes are rubbing and vibrating against adjacent tubes, as well as against support structures inside the generators.

Activists have suggested that the threat of blackouts is creating unneeded pressure to restart a plant that they see as unsafe.

The company asserts the reactors can run safely at lower power but "where is the basis for that?" asked Shaun Burnie of the environmental group Friends of the Earth.

The plant 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.