More unusual wear is in the tubing that carries radioactive water of an idled nuclear plant on the California coast. Investigators found the latest disclosure in a mystery involving the plant’s steam generators.

Investigators at the San Onofre Nuclear Generating Station found additional damage on tubes in its Unit 2 generators similar to degradation in its sister reactor, Unit 3, though at a lower level, said plant operator Southern California Edison (SCE).

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At first investigators remained confused as to why tube damage appeared different in the side-by-side units, even though the equipment is essentially identical.

That mystery seems resolved because the new findings show “all of the generators are exhibiting the same kind of wear, though the wear in unit 3 is more excessive than in Unit 2,” said Nuclear Regulatory Commission (NRC) spokesman Victor Dricks.

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The seaside plant has been down for more than two months while federal regulators and company officials try to find out why tubing designed to withstand many years of use under high pressure has eroded at an unusual rate, in some cases rapidly.

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

Unit 2 was offline earlier that month for routine maintenance and refueling, but investigators later found unusual wear on tubing in both units.

The excessive tube wear has raised questions about the integrity and safety of replacement generators the company installed in a multimillion-dollar makeover in 2009 and 2010.

There are nearly 10,000 alloy tubes in each of the plant’s four steam generators. The latest 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.

Previously, the heavy wear on Unit 2 tubing ended up limited to areas around bracing and supports.

Officials do not know, however, what that is happening. Now that similar damage is in both units, it could help investigators pinpoint a cause.

The NRC said the plant, located between Los Angeles and San Diego, will remain down until the company determines the cause of the wear and fixes it. The company has said they will plug 321 heavily damaged tubes and take them out of service at the two reactors, well within the margin to allow them to keep operating.

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