San Onofre’s future hinges on finding cause of abnormal tube wear

On Jan. 31, alarms alerted the control room at the San Onofre Nuclear Generating Station that a radiation leak was occurring in one of the nearly 39,000 tubes that carry radioactive water in the steam generators.

That failure led to an unparalleled shutdown of one of California’s two nuclear power plants and triggered more than three months of detective work by Southern California Edison officials and federal nuclear regulators that has yet to determine the problem’s root cause or when San Onofre will reopen.

Since then, the outlook for the plant has gotten worse.

Now, about 1,300 tubes ? more than 3% of all the plant’s tubes ? have been taken out of service because of unexpected wear. And Edison has said that even when San Onofre reopens, it probably won’t run at full capacity.

Figuring out the exact cause of the wear is crucial.

Edison finished installation of the $671-million steam generators less than two years ago, promising customers they would create major energy savings. Now, officials estimate it will cost as much as $65 million to fix the problems and tens of millions more to replace the lost power.

Officials said the investigation has yielded a better understanding about why the tubes are wearing down. When the plant is running at full power, the rate of steam flow is causing the tubes to vibrate, much like a guitar string being plucked. Some tubes vibrated enough that they rubbed against each other, causing the tube walls to wear down much faster than expected.

This is a potential danger because tube ruptures could release radiation and in extreme cases compromise the reactors’ cooling system.

But officials have not yet pinpointed what missteps led to the problem.

Investigators set out to determine whether the problem was in the design, the way the equipment was built or installed, or the way it was operated.

Tube wear is expected to happen over time, and other plants also have had issues in new steam generators. But the rate and way in which it has happened at San Onofre is an anomaly, according to Nuclear Regulatory Commission officials and industry experts. The NRC has forbidden Edison from firing the plant back up until it submits a plan to fix the problems.

Several experts said the problems suggest a design flaw.

“This is very unusual behavior in steam generators, and I think the most likely scenario was that there was some error in the design,” said Per Peterson, chairman of UC Berkeley’s Department of Nuclear Engineering.

Peterson suspects the problem involves the design of support structures meant to prevent excessive vibration. He added that the solution suggested by Edison ? running the plant at a lower power level ? would probably address the vibration problem but would result in less power, cutting into Edison’s revenues.

Sen. Barbara Boxer (D-Calif.) this week demanded documentation from the NRC and Edison about design changes for the new generators. Boxer is trying to determine whether Edison fully informed federal regulators about the design changes.

Determining what caused the flaw leads to the question of who will end up paying for the debacle: Edison and its shareholders, steam generator manufacturer Mitsubishi Heavy Industries or Southern California ratepayers.

Edison’s contract with Mitsubishi includes a $137-million warranty that covers repairs but not the cost of buying energy on the market to replace the lost power.

Replacement power costs had already reached $30 million by the end of March, Edison reported, and will continue to rise if the plant stays out of service, or even if it runs at a lower power level for an extended period.

When California Public Utility Commission officials approved the price tag for replacing the steam generators, they assumed that the plant would run at an average of 88% capacity until its license expires in 2022.

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