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The visit to San Onofre on Friday by NRC Chairman Gregory Jaczko comes about two months after the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped, but officials said there was no danger to workers or neighbors.

The agency announced last week that the plant will remain offline while investigators determine why tubing in its massive steam generators is eroding quickly _ and repair the problems.

The company has found that the tube wear is being caused by vibration and friction with adjacent tubes and bracing, however investigators don’t know why that’s happening.

Operator Southern California Edison has said 321 tubes with excessive wear will be plugged and taken out of service at the two reactors, well within the margin to allow them to keep operating.

“The agency is very concerned,” NRC spokesman Victor Dricks said. “The steam generators are a vital and very important piece of plant equipment, so ensuring their integrity is important” to the company and the NRC.

Jaczko will be joined by U.S. Sen. Dianne Feinstein, D-Calif.

The plant’s four steam generators each contain nearly 10,000 alloy tubes that carry hot, pressurized water from the reactors. The Unit 2 reactor was shut down for maintenance when workers discovered extensive wear on its tubing.

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.

Last week, an environmental group claimed the utility misled the NRC about design changes that it said are the likely culprit in excessive tube wear. The report by nuclear consultants Fairewinds Associates, and produced for nuclear watchdog Friends of the Earth, warned that a more detailed study is needed on the tubing before the reactors are restarted.

Friends of the Earth issued a statement Wednesday with another environmental group, San Clemente Green, urging the chairman to make a “full determination” of problems at the plant. Meanwhile, some officials in nearby communities have been calling for the plant to shut down permanently.

The equipment is relatively new _ the generators were installed in a multimillion-dollar makeover in 2009 and 2010.

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.