Investigators had been puzzled why tube damage appeared different in the side-by-side units, even though the equipment is essentially identical.

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,” Nuclear Regulatory Commission spokesman Victor Dricks said.

The seaside plant has been shut 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 was 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 had been taken 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 was limited to areas around bracing and supports.

It’s not known, however, why that is happening. Now that similar damage has been witnessed in both units, it could help investigators pinpoint a cause.

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

The company has said safety is its first priority.

The new findings come just days after NRC Chairman Gregory Jaczko signaled that Unit 2 might be able to go back into service more quickly than its twin. Meanwhile, nuclear watchdog Friends of the Earth said the disclosure buttressed its case that more study is needed before a restart is considered at either reactor.

The group last month claimed the company misled the NRC about design changes that it fingered as the likely culprit in excessive tube wear in the generators. On Thursday, it released research that said nearly 400 additional tubes were added to each generator as part of those changes, compromising reliability and safety.

“Southern California Edison continues to try to downplay the issue, even as they finally admit the truth – there is no difference between reactors 2 and 3 and they have the same problems” said Shaun Burnie, a nuclear specialist with the group.

SCE issued a statement that said the company has provided “open and transparent” information to the federal agency.

“No reliable conclusions can be drawn until the inspection, analysis and testing process is completed,” the statement said.

Inside a steam generator, hot pressurized water from the reactor flows through bundles of tubes that heat non-radioactive water surrounding them. The resulting steam is used to turn turbines to make electricity.

The tubes are one of the primary barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity to escape into the atmosphere. Serious leaks also can drain cooling water from a reactor.

Gradual wear is common in such tubing, but the rate of wear at San Onofre has alarmed officials since the equipment is relatively new.

The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries, according to company officials.

The Friends of the Earth report, written by Vermont-based nuclear consultants Fairewinds Associates, said generator changes included removing a key support pillar to make way for more holes to accommodate the increase in tubes.

The additional tubes created “unanalyzed flow and stress” inside the generators that compromised the reactor’s operation, the report said.

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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The twin reactors located between San Diego and Los Angeles have been idled while investigators determine why tubing carrying radioactive water is eroding at an unusual rate, and the Nuclear Regulatory Commission chairman will visit the plant Friday to highlight the agency’s concern over the ailing equipment.

San Onofre can generate enough electricity for 1.4 million homes, but it could take up to two months to restart two retired power plants in Huntington Beach that have been pinpointed as an important source of replacement power, officials said Wednesday.

The twin, natural gas-fired plants in Huntington Beach, in northern Orange County, were retired earlier this year. The gas line feeding the plants was severed and 3-foot holes were cut in the boilers.

The California Energy Commission has not received a request to restart, though state officials have identified it as a source of replacement power. Commission Deputy Director Roger Johnson said it might take up to two months for the operator, AES Corp., to work through steps to rekindle Huntington Beach.

Repairs to the boilers and other equipment could be completed within 30 days, predicted Barry Wallerstein, executive officer at the South Coast Air Quality Management District.

However, AES does not have a contract with the state to operate the units, and state agencies are still piecing together steps it would take to get the Huntington Beach units on line, Wallerstein added.

Stephanie McCorkle, a spokeswoman for the agency that operates the state’s wholesale power system, the California Independent System Operator, did not dispute the possible two-month timeline.

“We are optimistic the units can be back in service in time for hot summer weather,” she said in a statement.

Eric Pendergraft, president of AES Southland, which operates the plants, said the company had not received a formal request from the California Independent System Operator to restart them. He said repairs and negotiating a contract could proceed simultaneously, allowing a restart in about a month.

“We are sort of on hold,” he said.

San Diego Gas & Electric counts on power from San Onofre to help the utility bring in electricity from elsewhere — it takes power to move power. Spokeswoman Jennifer Ramp said the loss of the plant can restrict power imports into San Diego area by up to 30 percent.

Without the nuclear plant “we are going to be reduced,” she said, adding that the utility hopes a new, $1.8 billion transmission line will be completed by summer, which would help fill any shortages.

The visit to San Onofre on Friday by NRC Chairman Gregory Jaczko comes about two months after the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped, but officials said there was no danger to workers or neighbors.

The agency announced last week that the plant will remain offline while investigators determine why tubing in its massive steam generators is eroding quickly — and repair the problems.

The company has found that the tube wear is being caused by vibration and friction with adjacent tubes and bracing, however investigators don’t know why that’s happening.

Operator Southern California Edison has said 321 tubes with excessive wear will be plugged and taken out of service at the two reactors, well within the margin to allow them to keep operating.

“The agency is very concerned,” NRC spokesman Victor Dricks said. “The steam generators are a vital and very important piece of plant equipment, so ensuring their integrity is important” to the company and the NRC.

Jaczko will be joined by U.S. Sen. Dianne Feinstein, D-Calif.

The plant’s four steam generators each contain nearly 10,000 alloy tubes that carry hot, pressurized water from the reactors. The Unit 2 reactor was shut down for maintenance when workers discovered extensive wear on its tubing.

The tubes are a critical safety barrier — if one or more break, there is the potential that radioactivity could escape into the atmosphere. Also, serious leaks can drain cooling water from a reactor.

Last week, an environmental group claimed the utility misled the NRC about design changes that it said are the likely culprit in excessive tube wear. The report by nuclear consultants Fairewinds Associates, and produced for nuclear watchdog Friends of the Earth, warned that a more detailed study is needed on the tubing before the reactors are restarted.

Friends of the Earth issued a statement Wednesday with another environmental group, San Clemente Green, urging the chairman to make a “full determination” of problems at the plant. Meanwhile, some officials in nearby communities have been calling for the plant to shut down permanently.

The equipment is relatively new — the generators were installed in a multimillion-dollar makeover in 2009 and 2010.

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.

In a four-page letter to Edison last week, NRC Regional Administrator Elmo E. Collins scrolled out a series of steps the utility must take before restarting the seaside reactors, underscoring concern about the unusual degradation in the tubes.

The plant’s four steam generators each contain nearly 10,000 alloy tubes that carry hot, pressurized water from the reactors. The Unit 3 reactor was shut down as a precaution in January after a tube break, and extensive wear was found on similar tubing in its twin, Unit 2, which has been shut down for maintenance.

Traces of radiation escaped during the January leak, but officials said there was no danger to workers or neighbors.

Authorities in California have been scrambling to find additional power in case the reactors remain idled through summer, when energy use typically peaks. That could include restarting retired plants in Huntington Beach in northern Orange County.

Unusual wear has been found on hundreds of similar tubes that carry radioactive water at its twin, Unit 2, which was shut down earlier this year for routine maintenance, leaving questions about the integrity of equipment the company installed at the two reactors in a multimillion-dollar makeover in 2009 and 2010.

A special team of federal investigators was dispatched to the seaside site, about 45 miles north of San Diego, on Thursday to focus on Unit 3. While gradual tube wear is common in steam generators over time, no one knows why so many state-of-the-art tubes in relatively new equipment have degraded so quickly.

There are nearly 20,000 steam generator tubes in each of the two reactors. After tests, the company 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 continue to operate.

No date has been set to restart the reactors.

The testing “is designed to help us understand the potential safety implications and significance of this situation,” Ron Litzinger, president of plant operator Southern California Edison, said in a statement.

The company has finished surveying the wall thickness of the tubes but has yet to determine the cause of the unusual wear, which could range from debris circulating within the generators to a manufacturing or design defect.

The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries, according to company officials.

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, according to the NRC. 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, said David Lochbaum, director of the nuclear safety project for Union of Concerned Scientists.

“If there is a tube leak, the water that is leaking out is more likely to reach the environment. If there is any radioactivity in that water, it’s more likely to get places it shouldn’t be,” he said.

Nineteen percent of all power used by SCE customers comes from nuclear generation.

A spokeswoman for the agency that operates the state’s wholesale power system, the California Independent System Operator, said the San Diego and Los Angeles areas could see rotating power outages this summer if both reactors remain off line. The agency is taking steps to prevent those shortages.

“It’s all about balancing supply and demand,” said ISO spokeswoman Stephanie McCorkle. “You have to have a certain amount of plant (power) generation where the heavily populated areas of California are.”

An Edison statement said the utility welcomes the NRC inspection team, which is expected to begin work Monday.

The plant is owned by Edison, San Diego Gas & Electric and the city of Riverside. Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.

The ponytailed Burkhart was arrested Monday near the Sunset Strip in a van with Canadian license plates loaded with fire-starting materials, and he has stonewalled investigators while being placed on a suicide watch. His mother, Dorothee Burkhart, appeared disoriented in federal court Tuesday after being arrested on a fraud warrant from Germany, where she referred to Nazis and questioned if her son had died.

Their family history remains murky, but documents reveal both mother and son struggled with mental illness. They had a vagabond lifestyle, with addresses at various times in Germany, the U.S. and Canada, and the son holds a German passport but authorities say he was born in Chechnya.

Medical records dated March 2010 and submitted in a lengthy dispute over commercial space the mother rented in Vancouver say she suffered from depression, anxiety, severe post traumatic stress disorder and panic attacks. A separate note, also dated March 2010, says Harry Burkhart suffered from autistic spectrum disorder since his childhood, and he has severe anxiety, post traumatic stress disorder, depression and “is not stable mentally because of increase stress due to fear.”

His mother said in court Tuesday that he is mentally ill.

Harry Burkhart was charged Wednesday with 37 counts of arson as part of a rash of fires that caused more than $3 million in damage, while his mother was being held without bail after being detained on 19 counts of fraud from Germany, including failing to pay for a 2004 breast-augmentation surgery and pilfering security deposits from renters and landlords.

In requesting Harry Burkhart be held on no bail, investigators said in court documents that a search of Burkhart’s Hollywood apartment turned up news articles about the Los Angeles fires as well as a series of car fires in Frankfurt last September. Authorities couldn’t comment on whether Burkhart is a suspect in the German fires. His bail was set at $2.85 million.

The fire at the German house that belonged to the Burkhart family has been ruled an arson, Marburg prosecutors’ spokeswoman Annemarie Wied told The Associated Press Wednesday.

Burkhart did not live in the area, but his name surfaced as a suspect after he filed an insurance claim shortly after the fire, Wied said.

“When one files an insurance claim on a house the same day it burns down, it raises eyebrows,” she said.

Burkhart, whom Wied identified only as “Harry B.” in keeping with German privacy laws, has not yet been questioned in the case and no arrest warrant has been issued for him. She said she did not know how long ago he had been identified as a suspect in the arson investigation.

Burkhart was in Los Angeles by Oct. 26 — 12 days after the Marburg area fire — according to U.S. court papers, which say that he went with his mother on that day to the German consulate to renew his passport.

Neighbors say they kept mostly to themselves in a second-floor apartment across from a supermarket in Hollywood, where doors in the apartments are shielded by steel gates.

Their lives in the U.S. began unspooling last week.

Harry Burkhart watched as his mother was arrested on fraud charges from Germany, and a day later he exploded in a profanity-laced rant against the U.S. at her court hearing, saying “F— the United States!” or “F— all Americans,” authorities said.

The next day, police say, he began setting car fires at night, many in the Hollywood area near his apartment. Authorities believe he began the rampage after being outraged by his mother’s legal troubles.

Court documents give “a sense that this particular individual was set off by the incarceration of his mother, with whom he appears to be quite close, and he had latent anti-American views. That combination apparently set him off on this binge,” said Los Angeles County District Attorney Steve Cooley.

Harry Burkhart was taken into custody after authorities received a tip from federal officials who recognized him in a security video that showed a ponytailed man emerging from a garage where a car was set ablaze.

Burkhart’s nonimmigrant visa is set to expire Jan. 18, authorities said. His mother last entered the country lawfully in January 2007 and she left four months later, officials said.

A website offering appointment-only sensual massage is registered to Dorothee Burkhart, though her name is not mentioned on the site.

Frankfurt court spokesman Guenther Meilinger told the AP that Dorothee Burkhart will go on trial for the fraud charges once she is extradited back to Germany.

“We expect and hope that the U.S. authorities will look into the request for extradition … so that the proceedings against her can continue,” he said.

The extradition request has yet to be drawn up and sent to the German Justice Ministry for relay to U.S. authorities, said Doris Moeller-Scheu, a spokeswoman for Frankfurt prosecutors.

Asked about the discovery of news articles in Harry Burkhart’s Los Angeles apartment about a series of car fires in Frankfurt last September, Moeller-Scheu said that there was no active investigation of him in Frankfurt.

Dorothee Burkhart faces only the fraud charges, but that it was not unusual for an international arrest warrant to be issued in such a case, Meilinger said.

She was originally scheduled to go on trial in September 2007, but she fled before the proceedings opened, prompting the international arrest warrant, Meilinger said.

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Associated Press writers Dorothee Thiesing in Frankfurt and Bradley Klapper and Pete Yost in Washington contributed to this report. Rising reported from Berlin.