“It must be a burst steam pipe in the turbine room.”

That thought flitted through the mind of North Anna nuclear power plant senior operator Jason Russell when the Aug. 23 earthquake began shaking the facility’s control room.

Within seconds, indicators showed a loss of electricity needed to cool the plant’s two reactors.

“That’s when I knew it was something else,” said Russell, 30, who was supervising Unit 1 when the largest Virginia earthquake in more than a century struck.

Russell gave the order to manually squelch the reactor. “Perform immediate operator action E Zero,” he said. As his two charges reached for the switch — a single flip will shut down the reactor — automatic systems raced ahead and dropped control rods into the core, stopping the fission chain reaction. Seconds later, the plant’s backup diesel generators kicked in, restoring cooling power.

“There was no panic,” Russell said during an interview at the facility arranged by the plant’s owner, Dominion Virginia Power. “We just went through our procedures.”

During a tour Friday clearly designed to reassure the public, company officials repeated this message: The “defense in depth” built into all U.S. nuclear plants worked as designed.

Yet nearly two weeks after the quake, Dominion officials were unable to say whether the quake shook the facility more than it was designed to handle.

“I don’t have those numbers,” Daniel Stoddard, Dominion’s senior vice president for nuclear operations, said repeatedly.

It will be another week before final analysis of the “shake plates,” which recorded ground motion at the site, is finished, he said, although a Dominion spokesman had promised that analysis by Friday.

In the control room, a 1970s-era seismic detector failed to record data for a critical eight seconds when primary power went down, slowing the company’s analysis. The company has added a battery backup to the unit to prevent a recurrence.

Stoddard did say that “preliminary analysis” showed jolting “at or slightly above” the intensity that could damage non-critical facilities such as the administration building. Such shaking would not damage the two critical nuclear containment structures, he said.

Stoddard emphasized that the plant will not restart until the company and the Nuclear Regulatory Commission, which is conducting its own investigation, declare the site safe.

On Thursday, the company said the quake jolted 25 giant concrete silos that store spent nuclear fuel, moving some of the 115-ton structures 41 / 2 inches. But no leaks were sprung, and the company does not expect to reposition the containers.

Despite the upbeat picture painted of a nuclear plant that withstood the first earthquake-triggered shutdown in the 53-year history of commercial nuclear power in the United States, environmental groups have long questioned the wisdom of locating the plant in the Central Virginia Seismic Zone.

Regulators are also weighing in. After the Fukushima Daiichi disaster in Japan, the NRC ordered renewed study, now underway, of seismic risks to 27 U.S. nuclear plants, including North Anna.

“The power plant shouldn’t have been built there in first place,” said Lou Zeller of the Blue Ridge Environmental Defense League, which is seeking to stop construction of a proposed third North Anna reactor. “Mother Nature has weighed in here.”

The group won a minor victory Thursday when the NRC’s Atomic Safety and Licensing Board Panel agreed to hear its objections to the third reactor.

But Dominion expects to build the new plant. On Friday, bulldozers moved dirt in early preparations for the reactor’s containment building, designed to be sturdier than the existing structures. Company official Page Kemp said Dominion could receive a license for the reactor in 2013 or 2014.