NRC says steam flow too great in San Onofre generators

Jun 19 - McClatchy-Tribune Regional News - Paul Sisson North County Times, Escondido, Calif.

 

Steam flowed much too quickly through the generators installed at the San Onofre Nuclear Generating Station, leading to the problems that shut the plant down and have kept it from restarting, federal regulators said during a public meeting in San Juan Capistrano on Monday night.

Greg Werner, a branch chief with the U.S. Nuclear Regulatory Commission, said that Mitsubishi Heavy Industries, the company that built the 65-foot-tall heat exchangers, used a flawed computer model to design the components. Werner said the NRC ran its own computer model that found a major design discrepancy.

"We detected that the computer simulation used my Mitsubishi to design the steam generators had under-predicted the velocities of steam water inside the steam generators by factors of two to three times," Werner said.

He added that fast-moving steam worked in conjunction with anti-vibration bars that were supposed to hold the thousands of thin tubes inside the generators steady. Those bars, he said, are looser than they should be in the Unit 3 generators.

 

"Essentially, the tubes are not held in place securely enough, so it allows them to slide or vibrate," Werner said.

It was the first public explanation of what's going on inside the San Onofre's brand-new generators since utility Southern California Edison shut down the plantafter detecting a leak in one of the components.

Edison has already announced that it has blocked more than 1,300 tubes inside the steam generators attached to the plant's Unit 2 and Unit 3 reactors.

But while the NRC officials said Monday night that they believe they understand the cause of wear, they did not announce a fix that would allow the plant to restart.

Elmo Collins, regional administrator for the NRC, said additional inspections will be necessary to delve deeper into what went wrong at San Onofre. When asked whether the generators, which cost nearly $700 million to replace in 2009 and 2010, could be repaired or would need to be replaced, Collins said he did not know.

Steam generators are used to cool the plant's nuclear reactors and produce steam that turns the plant's electrical generators.

Each of the massive capsules is filled with nearly 10,000 thin alloy tubes. Hot and radioactive reactor coolant circulates inside these tubes, heating a water bath of clean water that turns into steam.

A crack in one or more of the thin tubes can result in radioactive coolant mixing with steam water and exiting the plant's protective containment domes.

The shutdown occurred after monitors detected a very slight increase in radiation on the plant's turbine deck. While regulators said the release was not enough to harm plant workers, the public or the environment, it was enough to tell operators they had a leak in one of the unit's brand-new steam generators.

The NRC shared additional information about the steam generators Monday night that the regulatory agency will follow up on in the coming weeks and months.

Some of that information involves how the steam generators for Unit 3 were shipped from Japan where they were built.

Werner, the NRC chief, said tubes inside one of the Unit 3 generators may not have been properly supported during transport and added that the shipper failed to control environmental conditions such as the oxygen level when the shipments moved across the ocean. He also said that several motion sensors that were attached to the one of the generators showed a shock.

For one generator, "all three accelerometers registered an excessive force, which could indicate mishandling during the transportation of the steam generators," Werner said.

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