The company made choices in designing support structures at San Onofre that were intended to prevent one type of vibration, but ended up creating another type of vibration that ultimately led to the plant's closure, said Frank Gillespie, senior vice president with Mitsubishi Nuclear Energy Systems.
The problematic vibration, he said, had not been seen at any other plant before, although it had been observed in experimental conditions.
That vibration led to excessive wear on the tubes, particularly in the plant's Unit 3, where one tube sprang a leak and released a small amount of radioactive steam on Jan. 31, 2012, and eight tubes failed pressure tests.
The nuclear facility has been closed for more than a year.
Mitsubushi discussed the design process in a proprietary report that was made public in a redacted form earlier this month.
Gillespie said designers working on the new system in 2005 put "paramount focus" on controlling vibration and reducing wear. In the process, they added more anti-vibration bars, but made other changes that led to less contact between the bars and tubes.
In Unit 3 in particular, the bars were flatter, leading to about half the amount of pressure between bars and tubes as in Unit 2, the plant's other working reactor unit, which also saw an unusual but less severe amount of wear.
“What they didn’t understand at the time is, some of the steps ... actually made in plane [vibration] worse,” Gillespie said. "...There was an underappreciation for the fact that the pressure of the bars against the tubes actually performed a very important function."
Anti-nuclear activists and some lawmakers -- notably, Sen. Barbara Boxer (D-Calif.) and U.S. Rep. Ed Markey (D-Mass.) -- have accused Mitsubishi and plant operator Southern California Edison of being aware of defects in the equipment's design prior to installation and failing to make modifications that might have prevented the problem in order to avoid going through a potentially lengthy license amendment process.
Mitsubishi's root cause report did show that some changes were rejected in part because they would have required a license amendment. The changes were intended to reduce the dryness of the steam flowing around the tubes, which ended up being a factor in the problematic vibration.