The Richmond Times-Dispatch reported yesterday:
Nine days after an earthquake shut down the North Anna nuclear plant in August, federal regulators launched a public reappraisal of the seismological risks posed to commercial reactors east of the Rocky Mountains.
The timing might have been coincidental, but the magnitude-5.8 quake is reshaping a national public debate over safety standards for nuclear reactors in central Virginia and other areas more vulnerable to seismic upheaval than once thought.
“What we have seen at North Anna has underscored that the reassessment is right on target,” said Joey Ledford, a spokesman for the U.S. Nuclear Regulatory Commission.
The Times-Dispatch article carried the most detailed analysis yet published of the sequence of events at North Anna when the quake hit:
Eleven seconds after the earthquake began in Mineral — 11 miles southwest of the North Anna plant — both nuclear reactors shut down.
The reactors had been tripped by what is called “negative flux rate trip,” a sign that control rods had dropped into place to interrupt the nuclear fission process.
Dominion officials say the fail-safe system was triggered by a loss of off-site electrical power to the reactors, but they aren’t sure exactly what happened first.
“We don’t know for certain just what tripped the reactors,” Heacock said last week.
A second after the reactors tripped, electrical breakers opened in plant transformers, causing those systems to shut down.
In the control room, plant operators had felt the ground vibration and reached for the levers to shut down the reactors manually. By then, however, the shutdown already had occurred.
Not everything at the plant functioned as designed during the event:
The quake had measured a ground vibration double of the plant’s design, according to the NRC, but an electronic sensor that is supposed to sound when vibration reaches two-thirds of the plant’s design lost power for eight seconds before the plant’s backup diesel generators started.