North Anna Reveals equipment may have contributed to unnatural shutdown

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Parts of the North Anna Power Station in Mineral, Va., 11 miles from its epicenter, endured jolts equal to 26% of the force of gravity (0.26g) from some of the higher-frequency vibrations unleashed by the quake, said Scott Burnell, spokesman of the Nuclear Regulatory Commission.

Initially, managers said that the earthquake, whose epicenter was only about 10 miles from North Anna’s reactors, caused a circuit breaker on a transformer to open, cutting off outside power to the plant and causing the shutdown. This last part now turns out to be incorrect.

Nuclear plants usually rely on power from the grid, as opposed to power that they generate, to run some vital pumps and other systems so that those systems keep working even if a plant “trips” and shuts down.

But that means that if there is a grid problem like a blackout, this will cause the reactors to turn off.

In North Anna’s case, a flaw was found: a sensor that measured the oil level in a transformer apparently mistook the sloshing of oil inside for a serious transformer problem, said Eugene S. Grecheck, Dominion’s vice president for nuclear development and a former manager of the North Anna plant.

Some engineers suggest that an older sensor that used analog equipment instead of microchips might not have recognized the change so quickly and could have allowed the operators to shut the reactors down under usual procedures.

So while an interruption of power from the transformer would have shut down the plant, this now appears not to have been the first event in the sequence.

Rather, the reactors themselves seem to have undergone changes that caused automatic systems to shut them, Mr. Grecheck said.

Because the earthquake moved the water in the reactors, the sensors indicated that the neutron population there was no longer distributed properly, and the system called for the automatic shutdown, Dominion officials say.

In an automatic shutdown, control rods slide between the fuel rods and choke off the flow of neutrons.

Inspections Still Undergoing

While one group of engineers continues to review data from the plant’s systems, another group is looking for damage to structures.

They are finding various cracks, including one in a wall inside a reactor’s containment dome, but that one is in the grout covering a joint where two concrete sections join.

Though the Mineral quake produced high-frequency shaking, plant personnel so far have not found any pumps that started or stopped, or motor-operated valves that changed positions, without operator commands.

Quake Doubled Plants Design Despite Utilities Statements

The magnitude-5.8 earthquake last month in Virginia caused about twice as much ground shaking as a nearby nuclear power plant was designed to withstand, according to a preliminary federal analysis.

“It’s the things inside the buildings that may have been shaken more than the design called for,” Burnell said, adding that the buildings themselves appear to have been less affected. He said the analysis is based on a seismograph reading taken about 30 miles away by theU.S. Geological Survey.

Parts of the North Anna Power Station in Mineral, Va., 11 miles from its epicenter, endured jolts equal to 26% of the force of gravity (0.26g) from some of the higher-frequency vibrations unleashed by the quake, said Scott Burnell, spokesman of the Nuclear Regulatory Commission.

Parts of the North Anna Power Station in Mineral, Va., 11 miles from its epicenter, endured jolts equal to 26% of the force of gravity (0.26g) from some of the higher-frequency vibrations unleashed by the quake, said Scott Burnell, spokesman of the Nuclear Regulatory Commission.

Source: USA Today, via Enformable

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