The US Nuclear Regulatory Commission missed an opportunity during a 2009 baseline inspection of the San Onofre Nuclear Generating Facility to identify "weaknesses" in the plant's screening and evaluation of replacement steam generators, or RSGs, the NRC Office of the Inspector General said in a report.
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Those problems later triggered Southern California Edison's decision in 2013 to permanently close the plant.
The OIG report, released Tuesday, evaluated NRC's oversight of the Title 10 Code of Federal Regulation 50.59 process that SCE used to replace steam generators at San Onofre-2 and -3 starting in 2009 and 2010, respectively. The SCE station is in San Clemente, California.
The OIG report noted a former NRC deputy administrator in Region IV, where San Onofre is located, said that "in hindsight, he believes that [San Onofre] should have requested a license amendment from NRC" and that "[h]e also believes the steam generator design was fundamentally flawed and would not have been approved as designed." The report did not name the official.
A like-for-like replacement of components at power reactors can be made under 10 CFR 50.59 and does not require an NRC license amendment. Both SCE and NRC have said the 50.59 process is not limited to like-for-like replacements and that the regulation sets criteria for similar designs.
A key change in the RSGs involved the use of alloy 690, a material more resistant to corrosion than the alloy 600 used in the original steam generators. But alloy 690 is about 10% less effective in transferring heat than alloy 600, which meant the RSGs were larger and had more tubes in order to maintain the same thermal performance.
"Former senior NRC officials told NRC's IG that if Southern California Edison had followed NRC rules, the flawed steam generator design would have been rejected," according to a statement issued Tuesday by Senator Barbara Boxer, a California Democrat and chairwoman of the Senate Environment and Public Works Committee, which has jurisdiction over NRC. Boxer initiated an investigation into the safety of San Onofre following the plant's closure.