LOS ANGELES (AP) -- Federal regulators Thursday concluded that the operator of the San Onofre nuclear power plant in California did not mislead the government about modifications to its troubled steam generators, where damage has been found on scores of tubes that carry radioactive water.
Environmental activists had accused Southern California Edison of duping the Nuclear Regulatory Commission about a series of changes to the massive machines before they began operating in 2010 and 2011, including boosting the number of tubes and redesigning internal supports that hold them in place.
The seaside plant between San Diego and Los Angeles has been idle since January, after a tube break in one of the generators released traces of radiation and set in motion a series of events that led to a federal probe.
The agency staff investigation concluded that Edison "provided the NRC with all the information required under existing regulations about proposed design changes to its steam generators," according to a statement.
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But a key question remains under study: Does the agency need to change the process that was used to approve the replacement generators? At issue is whether tubing problems that eventually sidelined the reactors might have been identified by changing rules under which utilities swap equipment at nuclear power plants.
For example, the report concluded that there were "major design changes" between the original and replacement generators at San Onofre, yet they qualified as essentially identical replacements that did not require an exhaustive review by the NRC.
The rules, as written, do not require Edison "to presume deficiencies in the design or fabrication" of the generators, the report said.
Daniel Hirsch, a lecturer on nuclear policy at the University of California, Santa Cruz, who is a critic of the nuclear power industry, said the agency was attempting to exonerate itself from blame for the plant's ongoing troubles.
The "NRC said they did nothing wrong" even though the faulty generators caused unprecedented tube damage and raised the risk of a serious accident, Hirsch said in a statement.
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.
After a three-month investigation, the NRC announced last month that a botched computer analysis resulted in design flaws that caused excessive vibration and resulted in unprecedented wear in tubes. Edison has been trying to determine how, or if, the problems can be fixed.
"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 the nuclear 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 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.