Steam flowed much too quickly through the generators installed at the San Onofre Nuclear Generating Station, which is why the plant had to shut down and have kept it from restarting, federal regulators said during a public meeting in San Juan Capistrano, CA.

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

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“We detected that the computer simulation used by 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.

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“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 San Onofre’s brand-new generators since utility Southern California Edison shut down the plant Jan. 31 after detecting a leak in one of the components.

Edison has already 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 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 they could repair or need to replace the generators, which cost nearly $700 million to replace in 2009 and 2010, Collins said he did not know.

Steam generators cool the plant’s nuclear reactors and produce steam that turns the plant’s electrical generators.

Each of the massive capsules utilizes 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 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 shipped from Japan where Mitsubishi Heavy built them.

Werner, the NRC chief, said tubes inside one of the Unit 3 generators may not have received the proper support during transport and added 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 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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