Feds: California nuclear plant didn't mislead regulators

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 on scores of tubes in both units.

"We are committed to continuing to work with the NRC on the steam generator issues and will continue to use conservative decision-making as we work on repairs and planning for the future," said Edison Senior Vice President and Chief Nuclear Officer Pete Dietrich. "The number one priority is the safety of the public and our employees."

The problems have raised questions about the integrity and safety of replacement generators the company installed at the twin reactors in a multimillion-dollar makeover.

Earlier this year, a report issued by environmental group Friends of the Earth asserted that equipment and design changes to the generators "created a large risk of tube failure." The group, which is critical of thenuclear power industry, 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."

A spokesman for the group, Damon Moglen, said in a statement that "the bottom line is that the NRC was asleep at the wheel then, and now we've left it up to the same people to justify their mistake."

The generators, which resemble massive steel fire hydrants, are one of the central pieces of equipment in anuclear plant. 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.

The tubes are one of the barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity 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.

The design of the generators also is 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.

Follow Michael R. Blood at http://twitter.com/MichaelRBloodAP

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