FILE - A March 1, 2010 file photo, shows the San Onofre nuclear power plant in north San Diego County, Calif. Federal regulators say design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the San Onofre nuclear power plant. (AP Photo/Lenny Ignelzi, File)
FILE - A March 1, 2010 file photo, shows the San Onofre nuclear power plant in north San Diego County, Calif. Federal regulators say design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the San Onofre nuclear power plant. (AP Photo/Lenny Ignelzi, File)
In this Sunday, June 17, 2012 photo, Elmo Collins, regional administrator of the Nuclear Regulatory Commission, poses for a photo in San Juan Capistrano, Calif. The NRC is holding a public meeting to discuss the state of the San Onofre nuclear power plant on Monday in San Juan Capistrano. After months of investigation, federal regulators have determined that design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the power plant. (AP Photo/Grant Hindsley)
In this Sunday, June 17, 2012 photo, Elmo Collins, regional administrator of the Nuclear Regulatory Commission, speaks during an interview in San Juan Capistrano, Calif. The NRC is holding a public meeting to discuss the state of the San Onofre nuclear power plant on Monday in San Juan Capistrano. After months of investigation, federal regulators have determined that design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the power plant. (AP Photo/Grant Hindsley)
In this Sunday, June 17, 2012 photo, Elmo Collins, regional administrator of the Nuclear Regulatory Commission, speaks during an interview in San Juan Capistrano, Calif. The NRC is holding a public meeting to discuss the state of the San Onofre nuclear power plant on Monday in San Juan Capistrano. After months of investigation, federal regulators have determined that design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the power plant. (AP Photo/Grant Hindsley)
In this Sunday, June 17, 2012 photo, Elmo Collins, regional administrator of the Nuclear Regulatory Commission, appears during an interview in San Juan Capistrano, Calif. The NRC is holding a public meeting to discuss the state of the San Onofre nuclear power plant on Monday in San Juan Capistrano. After months of investigation, federal regulators have determined that design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through the power plant. (AP Photo/Grant Hindsley)
CAPISTRANO BEACH, Calif. (AP) ? After months of investigation, federal regulators have determined that design flaws appear to be 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 was dispatched to the plant in March after the discovery that some tubes were so badly corroded that they could fail and possibly release radiation, a stunning finding inside the virtually new equipment.
Flaws in fabrication or installation were considered as possible sources of the rapid tube decay but "it looks primarily we are pointed toward the design" of the heavily modified generators, Nuclear Regulatory Commission Regional Administrator Elmo Collins told The Associated Press in an interview Sunday.
Collins couldn't rule out that one or more of the generators, installed in a $670 million overhaul in 2009 and 2010, might have to be replaced.
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 added. "The answers are very difficult, or they already would have emerged."
The disclosure will rivet new attention on a series of alterations to the equipment design, including the decision to add 400 tubes to each generator and installing V-shaped supports that were intended to minimize tube wear and vibration.
It's possible operator Southern California Edison could face penalties stemming from the federal investigation, Collins said.
The generators were designed to meet a federal test to qualify as "in-kind," or essentially identical, replacements for the original generators, which would allow them to be installed without prior approval from federal regulators.
An environmental group, Friends of the Earth, has claimed Edison misled the NRC about the changes that it has identified as the likely culprit in excessive tube wear. The federal agency previously disputed that charge, but Collins said that's under review as part of the investigation.
Inside the guts of the machinery, the original steam generators and the replacements "look substantially different," Collins added.
The NRC is scheduled to discuss its findings Monday evening at a meeting near the plant.
Collins said safety would remain the first consideration at San Onofre. About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes.
"These are significant technical issues. They are not resolved yet," Collins 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.
Edison has been facing pressure from some nearby communities and anti-nuclear activists that have raised safety concerns, while the company looks for a solution to the tube problem and a path to restarting the plant, an important source of power in Southern California. The design of the generators is also under congressional scrutiny.
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.
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