LOS ANGELES -- The utility that runs the troubled San Onofre nuclear power plant on the California coast sharply denied Thursday that it was aware of equipment problems linked to a 2012 tube break that released a trace of radiation.
On Wednesday, Sen. Barbara Boxer pressed federal regulators to open an investigation at the plant after uncovering documents that she said suggest that Southern California Edison took engineering shortcuts and compromised safety.
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The Democratic senator said in a letter to Nuclear Regulatory Commission Chair Allison Macfarlane that a confidential report obtained by her office shows Edison and Mitsubishi Heavy Industries, the Japan-based company that built the plant's steam generators, were aware of design problems before the equipment was installed in 2009 and 2010.
But Edison said in a statement "it is simply not accurate" to suggest the company was aware of design problems, and pointed out the equipment carried a 20-year warranty against defects.
"SCE would never, and did not, install steam generators that it believed would not perform safely," the company said. Edison "sought to purchase replacement steam generators that would meet or improve upon the safety standards and performance of the original steam generators."
The seaside plant located between San Diego and Los Angeles hasn't produced electricity in more than a year, after a tiny radiation leak in January 2012 led to the discovery of damage to hundreds of steam generator tubes that carry radioactive water.
Boxer, who chairs the Environment and Public Works Committee, said the report written by Mitsubishi raises concerns that Edison and its contractor rejected safety modifications and sidestepped a more rigorous safety review.
"Safety, not regulatory short cuts, must be the driving factor in the design of nuclear facilities, as well as NRC's determination on whether (San Onofre) can be restarted," Boxer said in a letter cosigned by Rep. Edward Markey, D-Mass.
In a statement, the NRC said it received the letter and "will review all available information in making a judgment as to whether the plant would meet our safety standards if restart were permitted."
Mitsubishi said design decisions were made "in accordance with well-established and accepted industry standards" along with a wealth of operating experience.
"Nothing is more important to us than the safe design and manufacturing of nuclear-energy facilities," a company statement said. "A thorough investigation has been ongoing and will continue. We will continue cooperating fully."
Boxer's disclosure further clouds the future of the twin-domed plant, which is seeking NRC permission to restart the Unit 2 reactor and run it at reduced power in hopes of slowing or halting tube damage.
The future of heavily damaged Unit 3 is not clear.
Last year, federal officials blamed a botched computer analysis for design flaws that are largely to blame for unprecedented wear in tubes at the plant. They found a Mitsubishi analysis vastly misjudged how water and steam would flow in the reactors.
Gradual wear is common in steam generator tubing, but the rate of erosion at San Onofre stunned officials because the equipment, installed in a $670 million overhaul, is relatively new.
Boxer's letter adds new weight to a long-standing -- and unresolved -- question at San Onofre. Did Edison modify the generators so extensively before they were installed that the company should have sought an amendment to its operating license, a process that can take months or even years?
Edison has long argued such an amendment was unnecessary. However, environmentalists and other critics of the nuclear power industry have claimed the company deceived the NRC about the extent of the changes and want the agency to find the company acted improperly.
Those alterations included adding 400 tubes to each generator, compared to the originals, and installing V-shaped supports that were intended to minimize tube wear and vibration. According to company documents, each of the replacement generators weighed nearly 24 tons more than the original generators.
Engineers for Edison and Mitsubishi, writing last year in a trade magazine, said they needed to design generators that would require only minor modifications within the rest of the plant, but also meet a federal test to qualify as "in-kind," or essentially identical, replacements, which would allow them to be installed without prior approval from federal regulators.
Boxer said the report documented that Edison and Mitsubishi rejected some safety modifications, apparently because they believed they could force the company to seek a lengthy license amendment. It also indicates the decision to reject additional safety modifications contributed to the faulty design, she said.
Boxer's office did not release the report.
The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate something like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds, with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter.
Overall, NRC records show investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the plant's four generators, two in each reactor.
The plant is owned by SCE, San Diego Gas & Electric and the city of Riverside.