Federal regulators Monday released a key presentation on generator design changes at San Onofre Nuclear Generating Station that had been missing from records of the public meeting where it was presented.

The 2006 presentation by plant operator Southern California Edison to the U.S. Nuclear Regulatory Commission was billed as an overview of its plans to modify and install replacement generators.

The presentation, however, didn’t include major design alterations that an outside nuclear expert has concluded were responsible for a minor radiation leak in late January. And several of the changes that were not in the presentation should have triggered a much more thorough review by the government, according to the expert, Arnie Gundersen.

San Onofre remains shut down after the Jan. 31 radiation leak, which officials say was caused by extremely rapid wear to generator tubes. Edison now believes the rapid wear, in turn, was likely caused by a “design defect.”

The presentation’s release came the next business day after Voice of OC reported that a month after regulators told Gundersen that they couldn’t find the document there still was no sign of it.

Voice of OC’s calls and emails in recent days to both Edison and NRC officials on the issue have not been returned.

The plant’s replacement steam generators, which were installed in 2010 and 2011, were supposed to last decades. But tubes vibrated and rubbed against each other, wearing them down much more quickly than expected, according to Edison and the NRC.

In just a year, tubes in Unit 3 had worn down so much that a break occurred, causing the small leak of radiation into the nonradioactive side of the plant. The leak posed no risk to workers or the public, officials say.

The same type of wear was found in the other unit’s generators, and more than 1,300 tubes have been taken out of service since the leak investigation began.

Since March, Gundersen, who was hired by the environmental group Friends of the Earth to conduct an independent investigation, has charged that the wear was caused by significant design changes Edison made to the replacement generators, stemming from the addition of nearly 400 tubes in each of the four generators.

Gundersen also alleges the company mislead the government about the changes in order to avoid a thorough independent analysis of their safety risks.

Edison, meanwhile, insists it was fully transparent with regulators about the alterations. The company and the NRC also say it’s too early to draw conclusions about the ultimate cause of the wear.

In a third report issued Tuesday, Gundersen states the changes met nearly 40 triggers, each of which, on their own, requires a thorough review of safety risks by the government.

In its 2006 presentation to the NRC, however, Edison declares that the generator replacement “will be implemented under 10CFR 50.59,” a process without government analysis of safety risks. The federal nuclear safety agency ultimately accepted that path, though it’s unclear at this point whether regulators were provided with complete information.

Edison’s presentation covers six design changes but neglects to mention key modifications that Gundersen says were responsible for the excessive tube wear.

One of the changes was the removal of the stay cylinder, which Gundersen describes as “the main structural pillar” in the generator. That alteration, combined with other changes, created the vibrations and rubbing that caused excessive wear, Gundersen wrote.

After saying in early April it would provide details on when it informed the NRC of the specific changes mentioned by Gundersen, Edison hasn’t returned several follow-up messages on the topic.

Neither Edison nor the NRC had a response to Gundersen’s latest report by mid-morning on Tuesday.

Gundersen noticed the missing presentation and asked the NRC to provide it on April 12, emails show. The next day, the agency replied that it couldn’t find it and would seek a copy from Edison.

Contacted on Thursday for information about why the record was missing, representatives for Edison and the NRC told Voice of OC they needed time to research the issue. They had nothing further to add on Friday, and did not return messages Monday afternoon seeking more information.

However, the LA Times reported Monday evening that the presentation was released earlier in the day.

The NRC recently said it is reviewing the generator approval process to “ensure that [Edison] followed proper procedures.”

You can reach Nick Gerda at ngerda@gmail.com, and follow him on Twitter: twitter.com/nicholasgerda.

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