Dominion says seismic vibrations tripped reactors

Sep 28 - McClatchy-Tribune Regional News - Michael Martz Richmond Times-Dispatch, Va.

Seismic vibrations from a magnitude-5.8 earthquake shut down both reactors at the North Anna nuclear-power plant, Dominion Virginia Power has concluded, not an off-site electrical power loss.

Dominion officials informed the U.S. Nuclear Regulatory Commission on Tuesday night that the reactors tripped because seismic vibrations caused internal power levels to drop and control rods to automatically interrupt the nuclear fission process, shutting the units down, just after the quake struck at 1:51 p.m. approximately 11 miles away.

The "negative flux rate," as the power drop is called, preceded the loss of electricity from an off-site transformer by a second and occurred just before plant operators could manually shut down the reactors. Dominion said the reactors tripped because of the vibrations and sensors within the reactors picking up the flux.

"This was an expected and desired plant response considering the magnitude of the earthquake," Dominion concluded in its root-cause evaluation of the shutdown.

The company had initially thought that the loss of off-site electrical power to the station knocked the two 980-megawatt reactors off line.

Dominion submitted the analysis less than a week before the NRC is scheduled to hold a public meeting in Mineral to present findings from the agency's inspections of the plant since the shutdown, the first caused by an earthquake in the United States.

The meeting, scheduled at 1 p.m. at the North Anna Nuclear Information Center, also is likely to draw many of the same Louisa residents and anti-nuclear activists who staged a protest on Tuesday at Dominion Virginia Power's offices on East Cary Street in downtown Richmond.

Dominion is trying to move from understanding what happened during the earthquake to restarting one of the idled reactors over the objection of Paxus Calta and other participants in Tuesday's protest who want the plant retrofitted to a higher seismic standard before operating again.

"This is not a faraway problem," said Calta, representing Not on Our Fault Line, a Louisa-based opposition group. "This is at Richmond's back door."

The group based its name on the discovery of a fault beneath the plant site during construction more than 40 years ago. The power company was fined by federal regulators in 1975 for not disclosing the existence of the fault, which Dominion says has been inactive for more than 250 million years.

However, utility officials acknowledge that central Virginia is subject to more frequent and stronger seismic events than previously thought, though not of the type experienced on the West Coast and in Japan, which was devastated by an earthquake and tsunami that crippled a nuclear plant in March.

Dominion officials say the North Anna plant suffered no significant damage from the Aug. 23 quake and have asked for permission to restart Unit 1. North Anna's Unit 2 is undergoing a refueling.

"The NRC has yet to come to any conclusions regarding either the root cause of North Anna's shutdown, or Dominion's analysis of the root cause," spokesman Scott Burnell said Tuesday. "The NRC continues to examine whether Dominion can meet the restart standard -- ensuring North Anna's safety equipment is fully functional.

Not on Our Fault Line doesn't want the units to be allowed to restart until Dominion retrofits the two units to meet higher seismic safety standards, such as those applied to a proposed third unit at the plant, as well as criteria still being developed by the NRC to apply to existing commercial reactors east of the Rocky Mountains.

"We could easily have another quake between now and the time the standards are released," Calta said after the half-hour protest by about 25 people.

Dominion spokesman Rick Zuercher said the plant underwent a rigorous inspection at a higher design standard about 15 years ago. All but about 50 of its 1,800 components would withstand a much larger quake than the plant was designed for, and some of those components were hardened, he said.

The protests also focused on underground piping that the groups said had not been inspected for leaks since the earthquake, despite a history of leaks that were reported by Dominion to the NRC several years ago.

Zuercher said the utility had voluntarily reported finding an elevated level of tritium, a radioactive isotope, in a water sample on the site about five years ago. "None of the spill got into groundwater and none made it" to Lake Anna, he said.

Dominion said it had inspected underground pipes considered most vulnerable to seismic vibrations, unearthed them, and found no evidence of damage. Most of the piping systems do not carry radioactive water.

Burnell, at the NRC, said Dominion's safety analysis would have to include "any underground or buried pipes that have a safety function."

mmartz@timesdispatch.com (804) 649-6964