The extremely rapid wear on generator tubes at the San Onofre Nuclear Generating Station, which led to a minor radiation leak in January, was the result of design changes at the plant that operator Southern California Edison did not properly disclose to federal regulators, a nuclear expert said this week.
“Edison’s decision to cram an additional 377 tubes into the replacement steam generators was the root cause” of the leak, wrote nuclear engineer Arnie Gundersen in a recent report commissioned by the environmental group Friends of the Earth.
The additional tubes, which were part of generators installed in early 2010 and 2011, "created unanalyzed flow and stress design changes” that “severely compromised” the reactor, he added.
By asserting that the new generators were a “like-for-like” replacement despite key changes, Gundersen added, Edison avoided a thorough review by regulators.
Edison, meanwhile, insists it “provided open and transparent information” to federal regulators “at all times during the steam generator replacement process.” The company didn't return phone calls seeking comment.
Officials have shut down San Onofre’s two nuclear reactors while they investigate the ultimate cause of the leak, which officials say posed no risk to workers or the public. The plant lies less than three miles south of San Clemente.
Edison determined that the rapid wear was caused by tubes “vibrating and rubbing against adjacent tubes and against support structures.” Federal regulators have accepted that explanation, though officials from both the utility and the U.S. Nuclear Regulatory Commission have yet to elaborate on why they believe the rubbing took place.
Gundersen, however, concluded it was the result of Edison’s “numerous unreviewed fabrication and design changes to the replacement steam generators.”
Gundersen, who often casts a critical eye on the industry’s safety oversight, has a long history as a nuclear engineer, including serving as chairman of a state panel overseeing the Vermont Yankee Nuclear Power Plant in Vernon, Vt.
To make room for the additional tubes, Gundersen says, Edison removed a key component known as a stay cylinder from the generator’s “tubesheet,” which he described as a “critical safety barrier.”
The tubesheet is “the key thing that prevents radiation going from the primary side directly out into the environment,” Gundersen said in an interview.
The company also added a ring to restrict the flow of cooling water back to the reactor, leading to a different water flow within the generator and likely playing a role in the rapid wear on the tubes, he added.
Edison declared in a statement issued Thursday that “no reliable conclusions can be drawn” about the leak’s cause until its investigation is completed. The NRC declined to comment on whether any of the changes cited by Gundersen contributed to the rapid tube wear.
Gundersen also claims Edison misled the federal government by asserting that the new generators wouldn’t result in any additional safety risk.
“By misleading the NRC on the true nature of the replacement, Edison fooled the NRC into giving a rubber stamp and not conducting a thorough NRC review and approval process,” Gundersen wrote in his first report last month.
To support his charge that Edison misled regulators, Gundersen points to a recent article co-authored by an Edison engineer.
“At SONGS [San Onofre Nuclear Generating Station], the major premise of the steam generator replacement project was that it would be implemented under the 10CFR50.59 rule, that is, without prior approval by the US Nuclear Regulatory Commission,” wrote Boguslaw Olech of Edison and Tomoyuki Inoue of Mitsubishi Heavy Industries, which built the generators, in the January 2012 issue of Nuclear Engineering International.
NRC and Edison officials, however, say all the design changes were approved by the agency before the generators were installed. “The NRC was aware of the design change,” said NRC spokeswoman Lara Uselding. “Anything that has been installed … was reviewed by the NRC prior to install.”
“We informed them of all the changes,” Edison spokeswoman Jennifer Manfre said last week.
The company also said a week ago that it would provide details of when it notified the NRC of the changes, though it hasn’t yet released that information.
Gundersen says that while Edison may have told the NRC about the changes, it used a process in which it affirmed the changes were not significant enough to increase the likelihood of a safety accident.
Under the process Edison used, the NRC doesn’t “dig their teeth in and really take a look at what’s going on” before approving the changes, Gundersen said.