After months of investigation, it appears design flaws are the cause of excessive wear in tubing that carries radioactive water through California’s troubled San Onofre nuclear power plant, a top federal regulator said.

The twin-reactor plant between Los Angeles and San Diego has been idle since January, after a tube break in one of four, massive steam generators released traces of radiation. A team of federal investigators went to the plant in March after the discovery that some tubes were so badly corroded they could fail and possibly release radiation.

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Flaws in fabrication or installation were a possible source of the rapid tube decay but “it looks primarily we are pointed toward the design” of the heavily modified generators, said Nuclear Regulatory Commission (NRC) Regional Administrator Elmo Collins.

Collins could not rule out one or more of the generators, installed in a $670 million overhaul in 2009 and 2010, might have to undergo replacement.

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Eight tubes failed during earlier pressure tests in the Unit 3 reactor and “we have not seen that in the industry before,” Collins said.

“It’s these four steam generators that either have, or are susceptible to, this type of problem,” Collins said, referring to the unusual damage caused when alloy tubes vibrate and rattle against each other or brackets that hold them in place.

So far, a fix has remained elusive.

“It’s not too hard to frame up the problem,” he said. “The answers are very difficult, or they already would have emerged.”

The disclosure will bring attention to a series of alterations to the equipment design, including the decision to add 400 tubes to each generator and installing V-shaped supports intended to minimize tube wear and vibration.

Operator Southern California Edison could face penalties stemming from the federal investigation, Collins said.

The generators were to meet a federal test to qualify as “in-kind,” or essentially identical, replacements for the original generators, which would allow for installation without prior approval from federal regulators.

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