The generators were designed specifically to meet a federal test to qualify as "in-kind," or essentially identical, replacements, which allowed them to be installed without prior approval from federal regulators.
The report said a string of complex modifications involving tubes and supports should have triggered a more extensive review by the government, including public hearings, but did not.
In a statement, NRC spokesman Victor Dricks said the company informed the agency of the steam generator design changes "in accordance with NRC requirements," and that portions of the design were reviewed by federal inspectors during the installation. He added that a team of investigators is reviewing those changes as part of its probe into tube wear at the plant.
The agency announced Tuesday it was launching a routine review of analyses that Edison performed during the past three years for changes in plant equipment or procedures that could affect safety. The review was not prompted by the problems with the generators.
State officials have warned of possible power shortages in the region this summer while the plant remains dark.
Edison said in a statement that "the health and safety of the public and employees is the number one priority for San Onofre."
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