The utility operating a troubled nuclear plant in California didn't hide relevant facts when it sought to install the replacement steam generators now blamed for a radiation leak, federal regulators said yesterday.

Southern California Edison Co. adequately reported the differences between new and older model equipment at the San Onofre Nuclear Generating Station, the Nuclear Regulatory Commission said.

Environmental groups have charged that Edison purposely minimized design changes when it switched generators because it wanted to avoid going through the license amendment process, which would have meant public hearings. But NRC found no fault with the utility's approach.

"Southern California Edison provided the NRC with all the information required under existing regulations about proposed design changes to its steam generators prior to replacing them in 2010 and 2011," NRC said in a statement.

The NRC judgment came as the agency issued a 95-page report on its investigation into the Jan. 31 leak at the seaside plant north of San Diego.

The probe blamed design flaws in the generators as well as problematic computer modeling. Those combined to create a situation where tubes in the generators vibrated and knocked against each other, causing rapid degradation. That allowed reactor coolant to leak into a generator.

"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," Pete Dietrich, Edison's senior vice president and chief nuclear officer, said in a statement. "The number one priority is the safety of the public and our employees."

NRC's ruling on Edison's actions when it replaced the generators failed to quiet activists, who called it convenient.

The NRC report indicates that there were "significant design changes" between generators, said Dan Hirsch, president of Committee to Bridge the Gap, a nuclear safety advocacy group. That should have warranted a closer look before installation, he said, but "NRC gives itself a pass, and Edison, for not having done a thorough review and for bypassing the license amendment process."

The utility was not required to apply for a license amendment for most of the design changes because they did not affect plant operations and safety, said NRC spokesman Victor Dricks.

San Onofre has been shut down since the leak. NRC says that there is no timetable for reopening the plant, which supplied power to about 1.4 million homes.

Edison cannot restart the plant until it submits a report outlining how to run the facility in a manner that does not allow continued tube degradation, NRC said. Federal regulators will have to agree that the plan will ensure safe operations.

Edison is preparing that report, which will seek to restart the plant. But the utility will not pursue the action until it is safe to do so, said spokeswoman Jennifer Manfre.

"We are not in a rush to turn on either one of these units," Manfre said, adding that it is a "very high bar" to show that San Onofre will not have future problems.

The NRC report makes clear that there are problems in both the Unit 2 and Unit 3 generators at San Onofre. Both generators came from Mitsubishi Heavy Industries, which when it made the equipment used a computer model to approximate water and steam levels. That analysis was faulty, underestimating velocities of steam and water inside the steam generators by factors of three to four times.

Both units suffered tube to tube wear with greater damage in Unit 3. Eight steam generator tubes there failed "to maintain structural integrity," the NRC report says.

Unit 3 was older than Unit 2 and did not have some of the structural updates of the newer component. The NRC report details that while there were far more extensive problems with tube wear in Unit 3, Unit 2 has some similar design flaws that put it at risk.

"Unless changes are made to the operation or configuration of the steam generators," the NRC report says, there will continue to be "excessive tube wear and accelerated wear that could result in tube leakage and/or tube rupture."

Hirsch said that he suspects Edison initially will ask to boot up just the Unit 2 generator. Restarting either unit is dangerous, he said.

The report "makes clear that there are fundamental design problems in Unit 2 and 3 [generators] and without substantial design changes they can't run safely," Hirsch said.

Sen. Dianne Feinstein (D-Calif.) said in a statement that she is tracking NRC's investigation.

"I recently met with NRC inspectors and believe the team is carefully and thoroughly investigating the cause of tube degradation at San Onofre," Feinstein said. The report "indicates there is more work to be done before we can fully understand the problem and determine a possible solution."

"More than 7 million Californians live within 50 miles of San Onofre," Feinstein added, "so I will continue to closely monitor NRC's efforts to ensure that problems with the San Onofre steam generators are safely and completely resolved."

NRC, meanwhile, sent out an advisory to all operators of nuclear plants to be watchful for the kind of tube-to-tube wear seen at San Onofre and other plants.