Under pressure: San Onofre nuclear plant's tube troubles expand

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File: The San Onofre nuclear power plant

More problems at San Onofre: Four more tubes that carry radioactive water at the nuclear power plant failed pressure tests, bringing the total number to seven.

The four tubes failed Thursday, SoCal Edison announced. Three had failed Wednesday.

"This is a significant issue," said NRC spokeswoman Lara Uselding. "A tube rupture is really the concern."

The utility shut down the plant's Unit 3 reactor and began testing samples from thousands of tubes in its steam generators on Jan. 31 after a leak was found. Traces of radiation escaped during the leak, but officials said there was no danger to workers or neighbors.

The Nuclear Regulatory Commission said Thursday it was sending a special team of inspectors to try to determine why the metal tubes, which were installed only a few years ago, have become frail enough to pose a risk of leaks.