The top U.S. nuclear official said on Friday his agency has not set any timetable for restarting the troubled San Onofre nuclear station in Southern California and that it would only do so if safety was assured.

Nuclear Regulatory Commission Chair Gregory Jaczko made his comments after touring the facility where both reactors have been shut since January due to the discovery of premature wear on tubes in giant steam generators installed in 2010 and 2011.

Senator Dianne Feinstein, a Democrat, and U.S. Representative Darrell Issa, a Republican, also visited the plant in a sign of mounting concern over the shutdown of the 2,150-megawatt facility and the problems with the tubes.

San Onofre Nuclear Generating Station, which is known by the acronym SONGS and is nestled on the Pacific Coast between Los Angeles and San Diego, is one of two nuclear plants in California. Its two units which date from the early 1980s generate enough power for 1.4 million homes.

I came to the San Onofre plant today because the NRC wants to get to the bottom of why SONGS is having trouble with relatively new steam generators, Jaczko told reporters.

The bottom line for us is we have to have assurances of safety before we allow the plant to restart, he said, adding that Southern California Edison (SCE), the utility that runs the plant, would have to show what had caused the problem with the steam generator tubes and provide a remediation plan.

Those tubes provide an additional barrier inside a containment building to prevent a release of radioactive steam. In January, a small leak from a generator tube in one unit released a small amount of radioactive gas.

SCE officials have said the wear is due to tubes vibrating and rubbing against each other and against support structures. The reason that is occurring is still unknown.

When asked by a reporter how long the plant could be out of operation, Jaczko said, I don't want to put a timetable on how long it could take.

Local residents and environmentalists have expressed distrust of the utility, which is bracing for a surge in power usage in the summer, as well as fears of a nuclear disaster such as last year's meltdowns at a plant in Fukushima, Japan.

CONCERNED RESIDENTS

Issa, whose district includes the plant, said he hopes at least one reactor will be running over the summer, when more air conditioning units are in use. SCE has said blackouts or brownouts could occur in California without San Onofre.

There's a balancing act. We want to make sure we have 100 percent safety, but we also want to make sure the rate payers are getting what they're paying for, Issa said.

In a statement Feinstein said: Nearly 7.4 million Californians live within 50 miles of San Onofre, so it is clear the generator must remain offline until it is carefully inspected and any safety issue is definitively diagnosed and fixed.

Edison said it installed the tubes at a cost of about $680 million, which will be passed on to rate payers through higher electricity rates. The tubes came from Mitsubishi Heavy Industries.

The troubles at San Onofre come about a year after a massive earthquake and tsunami wrecked the Fukushima nuclear power complex north of Tokyo, leading to meltdowns that sent radiation into the air and forced 80,000 people to evacuate the area.

Anti-nuclear activists living near San Onofre have seized on the disaster to argue that nuclear plants are unsafe.

We saw what happened in Japan and we're terrified that it could happen right here in Southern California, said Patti Davis, a member of the local community groups San Clemente Green and San Onofre Safety.

Concerned residents in the area around the plant have bought Geiger counters that plug into smartphones and iPads to monitor for any potential radiation emissions from the plant.

Former nuclear executive Arnie Gundersen of Fairewinds Associates, a consulting company, prepared a report released last month that argued that design modifications in the newly installed steam generators, such as different alloy for the tubes, led to problems at the plant.

Jaczko on Friday declined to comment on the report.

In one unit at the plant, there are 192 tubes that had to be plugged because of high levels of wear, said SCE spokeswoman Jennifer Manfre. In the other unit, stress testing of 129 tubes resulted in the failure of eight tubes, she said.

Each unit at the plant has about 10,000 tubes.

We welcome the Chairman (Jaczko) and the legislators here, Manfre said. Safety has been ... where our priority is, and that's where the chairman's priority is and we're all aligned on that.

(Reporting By Alex Dobuzinskis; Editing by Greg McCune and Paul Simao)

California