The plant on the California coast has been offline for more than two months, and NRC Chairman Gregory Jaczko said investigators have yet to pinpoint why hundreds of tubes that carry radioactive water have worn away at an unexpected rate.

He told reporters after touring the plant that the wear identified in Unit 2 is not as severe as in its sister, Unit 3, but there will have to be a “clear understanding” of the cause before either of the twin reactors is restarted.

He called the tube wear in the plant’s steam generators a “very unique phenomenon,” especially in Unit 3, which was shut down as a precaution after a tube break in January. He credited operator Southern California Edison with conducting a cautious investigation.

While the problems at Unit 2 are less severe, the company must show its tube degradation “is not related” to its twin before a restart would be approved, Jaczko said.

The NRC “wants to get to the bottom” of the plant’s problems, he said. “The issue of the steam generators is a very serious issue.”  “We have to have assurance of safety before we will allow the plant to restart,” he added.

Democratic Sen. Dianne Feinstein and Rep. Darrell Issa, R-California, toured the seaside plant between San Diego and Los Angeles with Jaczko.

With 7.4 million Californians living within 50 miles (80 kilometers) of San Onofre, Feinstein said in a statement that the plant must remain offline “until it is carefully inspected and any safety issue is definitively diagnosed and fixed.”  No timetable has been set for a restart.

The visit to the seaside plant comes 10 days after the NRC announced that San Onofre will remain dark until Southern California Edison fixes the widespread problem with the tubing.

The high-profile trip also underscored the concern inside the federal agency tasked with ensuring the safe operation of the nation’s commercial nuclear industry.

Southern California Edison provides electricity to nearly 14 million residents in Central and Southern California and has assured them that the reactors will not be restarted until it is safe to do so.

San Onofre can power 1.4 million homes, and state energy officials have warned of rotating blackouts in the power-hungry region if a heat wave hits while the plants are offline.

The troubles began to unfold in late January, when the Unit 3 reactor was shut down. 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 and refueling. But investigators later found unusual wear on tubing in both units. The company has said 321 tubes that were heavily damaged will be plugged and taken out of service at the two reactors, well within the margin to allow them to keep operating.

Gradual wear is common in such tubing, but the rate of degradation at San Onofre has been unsettling to officials since 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 alloy 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.

Several environmental groups issued statements urging the chairman to take the strongest possible action and determine the root of the problem before even considering a restart.

“Something has gone seriously wrong at the nuclear plant and the way in which Edison conducted itself,” Gene Stone, of Residents for a Safe Environment, said in a statement. “If the Nuclear Regulatory Commission wants any credibility in this town, they need to know they can’t sweep shoddy equipment and radiation leaks under the rug.”  Last week, a report commissioned by another environmental group, Friends of the Earth, claimed Southern California Edison misled the NRC about design changes that it said are the likely culprit in excessive tube wear.

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.