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Federal regulators say manufacturer’s computer models were dated and flawed

San Onofre Nuclear Generating Station. Photo by Andrea Swayne
San Onofre Nuclear Generating Station. Photo by Andrea Swayne

By Jim Shilander

Federal regulators issued a ruling Monday citing both the manufacturer of nuclear generators at the San Onofre nuclear power plant and its operator for issues leading to the plant’s retirement in June.

The Nuclear Regulatory Commission issued notice to Southern California Edison, the majority owner and operator of the San Onofre Nuclear Generating Station, stating the design of replacement steam generators at the plant constituted a violation of “low to moderate safety significance.”

Mitsubishi Heavy Industries, the manufacturer of the generators, was issued a “notice of nonconformance” from the federal agency for problems associated with their design, specifically citing the computer modeling system used as “inadequate.” The agency does not regulate Mitsubishi.

In 2009, Edison replaced two steam generators in the Unit 2 and 3 reactors of the plant for $680 million. Less than three years later, in January 2012, tubes that carry radioactive water in Unit 3 ruptured. A subsequent investigation discovered similar wear in Unit 2, though not to the same level found in the other generator. Investigators concluded the wear was a result of “fluid elastic instability,” a vibration of the tubes against one another caused by the velocity of steam traveling through them. Edison ultimately removed nuclear fuel from Unit 3, but proposed restarting Unit 2 at 70 percent power for five months. However, delays in the federal approval process led the utility to retire the plant.

The NRC’s finding against Mitsubishi was for the generator in Unit 3, which was labeled a “white” on a four-color safety violation scale (green is the lowest, followed by white, yellow and red, the highest level of violation). Mitsubishi was also faulted for the design of Unit 2 at the green level. Two other violations were identified during a review of the proposal to operate the plant at 70 percent.

As part of the NRC’s investigation into the generators’ failure, regulators discovered a dated computer code—nearly 35 years old—assumed an incorrect gap needed to account for the velocity of steam traveling through the tubes. The code was created in 1978 for Westinghouse Electric Company generators, and later modified in 1992 for Mitsubishi use. A similar design issue was identified at four other nuclear power plants, although only generators at the San Onofre plant showed the instability created by the design flaw.

In a release from the utility, Peter Dietrich, Edison’s chief nuclear officer, said while Edison was cited for failure to monitor the manufacturer’s modeling, the agency’s findings reflected the utility’s belief that Mitsubishi’s faulty design was the major issue.

“Mitsubishi designed the system. Mitsubishi built the system. Mitsubishi’s system failed,” Dietrich was quoted as saying. “They are the experts. SCE was the customer.”

Edison filed a “notice of dispute” with Mitsubishi in July, in an attempt to resolve how the utility could recoup damages and the cost of replacement power. Partial owner of the nuclear plant, San Diego Gas & Electric, has sued Mitsubishi for costs associated with the replacement of steam generators and power.

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