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Federal regulators say the San Onofre nuclear plant on the California coast will remain shut down until investigators determine the cause of unusual tube wear and make sure there is no safety risk.
Operator Southern California Edison says that tubes in massive steam generators are vibrating and rubbing against adjacent tubes and bracing, but investigators don't know why.
Unit 3 was shut down as a precaution in January after a leak sprung in a tube carrying radioactive water, and extensive wear was found on similar tubing in its twin, Unit 2, which has been shut down for maintenance.
Confirming an agreement with SCE, Nuclear Regulatory Commission regional Administrator Elmo E. Collins says in a letter Tuesday the reactors will not restart until the cause is determined and the problem repaired.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
The utility that runs California's San Onofre nuclear plant misled federal regulators about equipment and design changes that are the likely cause of extensive wear on tubing that carries radioactive water, a report commissioned by an environmental group claimed Tuesday.
The report by nuclear consultants Fairewinds Associates warned that a more detailed study is needed on the alloy tubing in the plant's steam generators before the twin reactors at San Onofre are restarted.
The study was produced for nuclear watchdog Friends of the Earth and was authored by engineer Arnie Gundersen, a former nuclear industry executive who was a licensed reactor operator.
A series of untested equipment and design changes to the massive generators "created a large risk of tube failure at the San Onofre reactors," the report found, citing a review of publicly available records.
Since the alterations, the plant has "experienced extraordinarily rapid degradation of their steam generator tubes," it said, adding that such rapid wear can raise the potential for an accident that could release radioactivity.
Jennifer Manfre, a spokeswoman for operator Southern California Edison, said in a statement the company provided "open and transparent information" to the Nuclear Regulatory Commission about the generators. She said the reactors would not be restarted "until we are satisfied it is safe to do so."
Among the modifications, the report said the tube alloy was changed, bracing was redesigned and more tubes were added. It said the company never disclosed that such extensive changes were made, instead describing it as an exchange of similar equipment that allowed SCE "to avoid the requisite NRC oversight of a steam generator replacement process."
NRC spokesman Scott Burnell said in a statement that the agency was aware on the design changes.
The company "had to show by analysis that their design was acceptable. All the information available at that time showed the replacement steam generators would meet our requirements for safe operation," Burnell said.
Gundersen said he believed the additional tubes were a way to allow SCE to set the stage to generate more power at San Onofre while avoiding more scrutiny from regulators.
"They made too many changes. The only thing I can conclude is the ultimate goal was a power upgrade, to squeeze more power out of the plant," he said.
A team of federal investigators is trying to determine the cause of unusually heavy wear on hundreds of tubes at the seaside plant, located about 45 miles north of San Diego. The federal agency has said the plant will remain shut down until the cause of the wear is found and corrected.
The problems have raised questions about the integrity of replacement generators the company installed at the two reactors in a multimillion-dollar makeover in 2009 and 2010.
Unit 3 was shut down as a precaution in January after a leak sprung in a tube carrying radioactive water, and extensive wear was found on tubing in its twin, Unit 2, which has been shut down for maintenance and refueling.
Traces of radiation escaped during the January leak, but officials said there was no danger to workers or neighbors.
Nearly 20,000 tubes are in each of the two reactors. Following the leak, tests found that seven tubes that carry radioactive water from the reactor were in danger of rupturing under high pressure in Unit 3. The company has said a total of 321 tubes will be plugged and taken out of service at the two reactors, well within the margin to allow them to keep operating.
Inside a steam generator, hot pressurized water flowing through bundles of tubes heats a bath of non-radioactive water surrounding them. The resulting steam is used to turn turbines to make electricity.
The tubes are one of the vital barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.
Serious leaks also can drain cooling water from a reactor.
The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries, according to company officials.
Fairewinds is a Vermont-based consultant that has worked with groups critical of nuclear power.