Southern California Edison brought out a chief engineer from the San Onofre Nuclear Generating Station Friday night to give an intensely technical presentation in a bid to convince regulators they should be able to restart Unit 2 at the plant.
A January radioactive steam leak in Unit 3 revealed plant generators were riddled with that had to be taken out of service. The plant has been shuttered since, as inspectors comb data related to the leak and unprecedented wear of steam tubes.
Meanwhile, local and national anti-nuclear activists continue to call for the plant's shutdown, using the latest reported potential sabotage to safety backup generators to further justify their protests. The latest protest includes Buddhist monks' starting a six-day fast in front of the plant.
Protesters crowded the Laguna Hills meeting Friday, booing and chanting during certain intervals.
Edison officials said they well understood the factors that caused the leak and could safely address them in the restart of the less-damaged Unit 2. They described to Nuclear Regulatory Commission inspectors in deeper detail than ever before what their testing and assessments revealed about the problems at the plant.
Design flaws by Mitsubishi Heavy Industries were responsible for the tube wear, caused by vibrations of tubes against one another and against support structures, they said.
Tom Palmisano, Edison's vice president of engineering at San Onofre, explained the complex science behind the tube failures, the four central reasons the tubes wore down, how Mitsubishi made their mistakes, and why Edison's plan to operate Unit 2 at 70-percent power would solve the tube-wear problems.
Most importantly, said Chief Nuclear Officer Pete Dietrich, Unit 2 would have been able to restart under current guidelines (it had been down for routine maintenance when the Unit 3 leak happened), but the plant "conservatively" chose to treat the generators there as if they could have the same problems.
"For a unit that has not seen the same type of wear ... we're treating it as if it did," he said. "We took a unit that operated successfully at 100 percent power for 21 months and we're operating at 70 percent for five."
The NRC still has weeks of work to determine if Edison's plan is adequate.
Tube-to-Tube Wear
Inside each of the four steam generators at the plant – two per unit – are about 10,000 heat-transfer tubes containing radioactive steam. The tubes boil pure water in a separate system to make steam that turns turbines.
Palmisano said the cause of the wear was "fluid elastic instability," the phenomenon in which intense heat and hydraulic pressure causes the tubes to vibrate. Engineers take this into account when designing steam generators, Palmisano said, using support structures and special alloys.
But at San Onofre, a new type of vibration that nuclear operators hadn't seen before caused the worst wear, Palmisano said.
The U-shaped tubes are arranged in a nesting pattern; each U-tube smaller than the next and stacked inside the last and laying flat (see the attached photo). There is a space between each U-tube. Then, each of those "columns" of U-tubes are stacked on top of one another inside the generator and separated by anti-vibration bars and retainer bars, Palmisano said.
In the Unit 3 generators at San Onofre, the tubes experienced "in-plane vibration," meaning the tubes vibrated back and forth against their fellow tubes in their column, rather than tubes in adjacent columns, something scientists had never seen in a nuclear plant steam generator, Palmisano said.
According to multiple independent assessments, Palmisano said operating Unit 2 (which was far less damaged than Unit 3 and could have been restarted after the leak) at 70 percent wouldn't produce the levels of heat and pressure that caused the tubes to vibrate back and forth.
Retainer Bar Wear
Also implicated in tube wear were components situated among the steam generator tubes' U-bends, called retainer bars. Palmisano said these bars serve no function during normal operations but are necessary during the manufacturing process.
In San Onofre's Mitsubishi generators, newly installed in the last few years, half the retainer bars were thinner than the manufacturers had used in the past, leading to increased vibration, or "fluttering," Palmisano said.
As a precaution in both Unit 2 and Unit 3, operators removed from service the 94 tubes in each generator that came into contact with retainer bars.
Squeeze Film Damping
Another factor in the tube wear that caused the shutdown was a phenomenon called "squeeze film damping," specifically the lack of it.
Squeeze film damping refers to the tendency of the steam inside the generators to create a film of water between the heat exchange tubes and the support structures, Palmisano said.
That film is enough to help keep the tubes from vibrating against the support structures, he said.
In the San Onofre generators – at least, in Unit 3 – some patches of steam that had boiled up into the turbine had way less moisture than other patches, meaning the crucial water film was non-existent, leaving the tubes to vibrate against support structures freely, Palmisano said.
Palmisano said that many independent calculations showed that operating Unit 2 at 70 percent would keep the steam more even and ensure that the squeeze film remains intact during the operation.
Improved Mathematical Models
The real crux of the problem with the San Onofre Generators, Palmisano said, was the outdated mathematical model Mitsubishi used. There were mistakes in the parameters concerning the arrangement of the heat transfer tubes, Palmisano said.
Mitsubishi typically makes generators with tube arrangements in a different shape than the tube arrangements in the San Onofre generators. Because of this, Mitsubishi engineers had to plug in a different batch of numbers to their equation – numbers that proved to be wrong, Palmisano said.
The older formula also included mistaken assumptions about the nature of fluid elastic instability, Palmisano said, leading to a drastic underestimation of the amount of heat and pressure that would run through the tubes, and in turn, leading to the unprecedented vibration.
To figure out how it would be safe to restart Unit 2, Edison, Mitsubishi and other independent consultants used a better formula developed by the Electric Power Research Institute.
Independent consultants – including Mitsubishi competitors Areva, Westinghouse and B&W Canada – also used the better formula and found that it would have predicted the kind of problems San Onofre experienced if Mitsubishi had used it during the manufacturing.
It follows, Palmisano said, that this formula can predict accurately safe operating parameters for Unit 2, which include operating it at 70 percent power for 5 months with certain tubes plugged and taken out of service, as has been done.
