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“These are significant technical issues. They are not resolved yet,” Collins said.

Edison did not immediately comment on the NRC findings.

The company said in a statement the Unit 2 reactor likely would remain offline at least through August, pending NRC approval for a restart. It did not project a restart date for Unit 3, where tube damage has been more severe. The company is expected to submit a plan to the NRC later this summer to restart one, or both, reactors, which would have to outline how the company can control the tube damage.

“We know that the outage and the tube wear issue have generated concern in our community,” Edison president Ron Litzinger said.

Cracked and corroded generator tubing has vexed the nation’s nuclear industry for years.

Decaying generator tubes helped push San Onofre’s Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004. The following year, the Trojan nuclear plant, near Portland, Ore., was shuttered because of microscopic cracks in steam generator tubes, cutting years off its expected lifespan. Westinghouse Electric Corp. weathered a legal battle with five utilities in the 1990s that wanted the company to replace steam generators it manufactured for the Beaver Valley nuclear power plant in Pennsylvania after tubing corroded.

But the troubled San Onofre generators, manufactured by Mitsubishi Heavy Industries, might be a unique case because of the extensive modifications. Only one other U.S. nuclear plant uses Mitsubishi generators, the Fort Calhoun Nuclear Station, about 20 miles north of Omaha, but its generators are smaller than those at San Onofre and have not displayed excessive tube decay, federal officials say.

The cause of the unusual wear has been eagerly anticipated, as Edison prepares to submit a proposal to the NRC to restart one or both of the reactors. The company has suggested the reactors would run for a test period under reduced power to reduce vibration.

“The phenomenon that we think causes this tube-to-tube interaction is definitely proportional to the power,” Collins said. “At least in some theoretical sense, that might be part of the answer.”

The company has announced that 510 tubes have been plugged, or retired from use, in the Unit 2 reactor, and 807 tubes in its sister, Unit 3. Each of the generators has nearly 10,000 tubes, and the number retired is well within the limit allowed to continue operation.

What’s at issue is why so many tubes degraded so quickly, when the design changes were intended to improve the plant’s performance and longevity.

The steam generators _ two in each reactor _ function something like a car radiator, which controls heat in the vehicle’s engine. The generator tubes circulate hot, radioactive water from the reactors, which then heats non-radioactive water surrounding them. That makes steam, which is used to turn turbines to make electricity.

The tubes have to be thin enough to transfer heat, but thick enough to hold up under heavy pressure. They represent a critical safety barrier _ if a tube breaks, there is the potential that radioactivity can escape into the atmosphere. Also, serious leaks can drain protective cooling water from a reactor.

The trouble began to unfold in January, when the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped at the time, but officials said there was no danger to workers or neighbors. Unit 2 had been taken offline earlier that month for maintenance, but investigators later found unexpected wear in tubes in both units.

The NRC has said there is no timetable to restart the reactors.

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