The North Anna nuclear plant took a step toward normal today as Dominion Resources canceled the emergency alert declared yesterday when a 5.8 magnitude earthquake struck just a few miles from the site.

But the quake is giving the Nuclear Regulatory Commission new reasons, and new data, to reassess whether older US nuclear plants have enough safety margins to withstand expected earthquakes.

Walkdowns and inspections for any damage at North Anna from yesterday’s quake continue, but no damage had been found to any major equipment or buildings as of midday on August 24, the day after the earthquake.

Dominion spokesman Jim Norvelle said there’s no scheduled restart date, and the company is considering whether to keep the units off-line for some routine maintenance before restarting them, especially as the August weather continues to be mild and electricity demand is down.

The only known damage from the earthquake was in an on-site substation. A failure there cut power to the main station when the earthquake hit at 1:51 p.m. yesterday. The two reactors automatically shut down, as designed.

Norvelle said four emergency diesel generators started up as expected and were supplying power to the station, but operators checking them found one leaking coolant. They shut that generator and substituted a fifth backup diesel generator. Normal power from the grid was restored at 8:51 p.m., and the reactors are now in cold shutdown.

Understanding what went wrong in the substation is a prerequisite for restart, Norvelle said, as is a better understanding of the earthquake effects generally. He said Dominion is working with the vendor of “scratch plates,” tapes in ground monitors that recorded actual ground acceleration around the plant site, to get accurate readings. Those readings should enable both Dominion and regulators to analyze the earthquake.

NRC spokeswoman Elizabeth Hayden said the agency has been told that analysis will take several days.

North Anna was designed and built in the 1970s. In 2005, NRC staff recommended that the agency take a new look at the seismic design basis of all older US plants, given new findings about seismic hazards in the last several decades.

Last year, the staff ranked North Anna as the tenth riskiest for seismic damage, due to new earthquake data as well as old design. Risk was much lower for California plants, built to tighter seismic standards because much more was known about earthquakes there.

The staff report admitted, however, that a great deal of data on how plants were actually built was missing, and NRC is in the process of gathering that data for eastern and Midwestern plants, a process slated to go into 2012. Only then does NRC plan to decide whether action is needed.

North Anna Cited For Quake Shortfall

But after an earthquake and tsunami hit the Fukushima Daiichi plant in Japan in March, NRC performed special inspections to assess US plants’ capabilities to withstand similar assaults. At North Anna and other stations, that inspection found some equipment important to fighting fires and withstanding floods that was not designed as earthquake-resistant. All operators involved, including Dominion, have been evaluating how to respond to the NRC findings.

Yesterday’s earthquake has been assigned a preliminary magnitude of 5.8. North Anna is designed to withstand specific ground accelerations, which translate roughly to magnitude 5.9 for rock portions of the site and 6.2 for the soil portions, Norvelle said.

The Science

Much of the damage potential in an earthquake depends on ground acceleration – how fast the earthquake makes the ground move. Magnitude is a measure of total energy released, but acceleration can vary among quakes of the same magnitude. So determining actual acceleration from the scratch plates will be key to discovering how close North Anna came to its design margin.

If NRC finds any older plants need seismic hardening, there are numerous ways to achieve it, but they are usually plant-specific. Improved supports can be put on vulnerable piping, for instance, and insulating bumpers placed between equipment that might collide. But decisions on such changes require detailed engineering studies.

No other nuclear plant stopped operating in yesterday’s quake, though 12 plants housing 19 reactors in six states did declare “unusual events,” the lowest of NRC’s four emergency stages, when the tremor was detected. That declaration signals heightened alert in the face of some threat, and is commonly declared in advance of things like hurricanes.

All exited that status last night, but coastal plants are preparing for Hurricane Irene, and new “unusual events” could be coming in the next few days.

For more on the US reaction to Fukushima, and the industry’s readiness for another earthquake, read Breaking Energy’s nuclear coverage on the issue here.

Photo Caption: The North Anna, Virgina, #1 and #2 nuclear power generation stations operated by Dominion Virginia Power are seen March 24, 2011, at Lake Anna, Virginia, in this aerial photo. The Lake Anna Reactor is ranked 7th most at-risk for earthquake damage. According to the Nuclear Regulatory Commission, North Anna #1 and # 2 face an annual 1 in 22,727 chance of the core being damaged by an earthquake and exposing the public to radiation. The national average for US nuclear plants is a 1 in 74,000 chance. The top five most at-risk plants are all on the east coast: Indian Point, north of New York City; the Pilgrim Plant south of Boston, Limerick outside of Philadelphia, the Sequoyah plants near Chattanooga Tennessee and Beaver Valley near Pittsburgh. These five plants are at a higher statistical risk than those along fault lines in California, for example, because they were not designed for and built in presumed strong quake danger areas. Since they were constructed the US federal government has revised upwards the quake risks where they are. According to Jim Norvelle with Dominion Power, North Anna was designed to withstand a magnitude 5.9-6.1 earthquake.