Officials rejected some changes to crippled San Onofre generators


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A report on the root causes of problems at the San Onofre nuclear plant shows that officials considered making design changes to the plant’s new steam generators before they were installed but rejected some fixes in part because they would require further regulatory approvals.

Some of the generators began malfunctioning a year after they were installed, and the nuclear power plant has been shuttered for 14 months. The closure has already cost San Onofre’s operators, Southern California Edison and San Diego Gas & Electric, $470 million.


Ratepayers across the region are already shouldering some of those costs and could be on the hook for hefty future repair bills.

The report was released Friday by the Nuclear Regulatory Commission. It was written by Mitsubishi Heavy Industries, which built the generators.

The report provides the most detailed picture to date of how the flawed system was designed.

But both companies insisted Friday said that they were not aware of the problem that crippled San Onofre. Mitsubishi argued that the changes officials contemplated before installation would not have made a major difference.

Mitsubishi, however, acknowledged that it had made an incorrect input into a computer code that resulted in underestimating the velocity of steam flow in the plant’s replacement steam generators. Again, the company said that that error did not cause the failure.

The report comes amid a furious debate over who is to blame for defects that led to the shutdown.


Sen. Barbara Boxer (D-Calif.) and U.S. Rep. Ed Markey (D-Mass.) obtained a leaked version of the proprietary Mitsubishi report. They wrote to the head of the U.S. Nuclear Regulatory Commission alleging that the report showed Edison and Mitsubishi were aware of design defects in the generators before they were installed and chose not to make fixes.

On Friday, after weeks of back and forth with Mitsubishi, the NRC released a redacted version of the report.

San Onofre was shuttered after a tube in the plant’s replacement steam generator system leaked a small amount of radioactive steam on Jan. 31, 2012. Eight other tubes in the same reactor unit later failed pressure tests, an unprecedented number in the industry, and thousands more tubes in both the plant’s units showed signs of wear.

The wear was blamed on tube vibration caused by excessively dry and high-velocity steam and inadequate support structures, particularly in one of the plant’s two units. Tube vibration and wear has been a problem at other plants, but the specific type of vibration seen at San Onofre had never been experienced before in the industry.

According to the Mitsubishi report, a design team recognized that the steam could be dryer than in previous designs. The team considered various possible changes to address the condition, but eventually decided the final design was “optimal, based on the overall…design requirements and constraints.”

One factor they considered, Mitsubishi said, was that major changes would require a license amendment. This could be a lengthy process that might require new public hearings.


Boxer released a statement Friday expressing “serious concerns” about Edison and Mitsubishi’s actions, and Markey said the firms “cut safety corners to avoid a new NRC licensing process.”

Friends of the Earth, an environmental group that has been pushing for the NRC to require a license amendment before the unit can be fired up again, said the report showed that Edison “clearly knew about design problems.”

The two companies, however, strong disagreed.

Mitsubishi said it was impossible to predict the problems at San Onofre because that type of wear has never been seen at any other nuclear power plant. The changes considered in the design process would not have made a difference, the company added.

“None of these alternatives had a large enough effect…to justify such a significant change,” Mitsubishi wrote in the supplemental report.

As for the code error, the report said Mitsubishi had used an “inappropriate definition of the gap between tubes” in its calculations. But the company said that even using the correct code would not have predicted the type of wear that occurred.

Edison officials denied any suggestion that the company rejected design changes simply to avoid a license amendment.


“At no time was SCE informed that the maximum void fraction or flow velocities estimated by MHI could contribute to the failure of steam generator tubes. At the time, the design was considered sound,” Pete Dietrich, Edison’s senior vice president and chief nuclear officer, said in a statement.

Edison contracted with Mitsubishi to build replacement steam generators in hopes of lengthening the life of the plant, which provided power to about 1.4 million Southern California homes.

The steam generator replacement cost Edison and co-owner San Diego Gas & Electric at least $780 million, according to regulatory filings, which ratepayers are currently repaying. The California Public Utilities Commission is investigating whether those costs and others relating to the plant should be refunded or removed from rates, in light of the extended outage.

Edison has proposed restarting one of the plant’s two reactors – which showed less wear overall and less of an unusual type of wear caused by tubes knocking against adjacent tubes---at 70% power for five months before taking it offline again for inspections. The company argues that reducing the power would alleviate the conditions that led to the excessive wear. The NRC is still weighing the proposal.

NRC Chairwoman Allison Macfarlane said previously that the agency is looking at the Mitsubishi report as part of an “expansive investigation” launched in September into information Edison gave the agency about the plant’s replacement steam generators. It is unusual for the NRC to speak publicly about such investigations, which can lead to a referral to the Department of Justice for criminal charges.

The NRC’s investigation into the issues largely blamed Mitsubishi’s computer modeling errors for the problems.


Meanwhile, Edison and Mitsubishi are also wrangling over the $138-million warranty cap on the steam generators, which does not cover replacement power. Edison argues the cap should not apply in this case – Mitsubishi disagrees. The dispute may go to international arbitration. Edison has also said it will seek to recover costs through insurance.

--Abby Sewell