MINERAL, Va. -- Dennis Anderson almost immediately thought of the nearby North Anna nuclear power plant when a magnitude-5.8 earthquake began rattling the boat shop on Lake Anna where he works as a mechanic on Aug. 23.

Clad in a worn black tank top, the Virginia native with a penchant for storytelling rubbed dark stains from his strong hands the other day and recalled how confused he was when the quake hit.

Anderson was repairing a Malibu ski boat at the Anna Point Marina here in Mineral, about 2 miles from the nuclear plant, when the earthquake started rattling boat parts and tools hanging on the walls of his shop.

At first, he thought a fellow mechanic had rammed the side of the building with a forklift, but he soon realized the trembling was an earthquake and ran outside to look for smoke wafting up from Dominion Resources' plant, which is about 40 miles northwest of Richmond.

"The first thing we did was go out and look for the mushroom cloud," Anderson said to resounding chuckles in the mechanic shop.

Stories like Anderson's are easy to find in the restaurants, stores, homes and shops surrounding the serene waters of the 13,000-acre Lake Anna. There, tight-knit communities are struggling to make sense of a historic earthquake that federal seismologists say may have exceeded what the 1,806-megawatt plant was designed to withstand.

A haven for vacationers, boaters and fishermen, the man-made 17-mile-long lake was created to provide cooling water for Dominion Resources' North Anna plant, which sits at the midpoint of the lake. The lake is fed by more than 60 miles of the North Anna River and continues southeast to link up to the Pamunkey River. Federal officials have said the dam is secure.

On the northern shores of the lake, swimmers, picnickers and hikers flock to Lake Anna State Park in hot summer months.

But that haven was shaken when the largest earthquake in Virginia in more than a century jostled states from Georgia to Maine and knocked out off-site power feeding the North Anna plant. The plant's two reactors shut down immediately, and four diesel generators started automatically to provide cooling.

So far, Dominion has reported no severe damage at the plant, but the Nuclear Regulatory Commission confirmed last week that the temblor shifted 25 115-ton dry storage casks -- massive canisters holding spent nuclear fuel -- up to 4.5 inches.

"We have some indications that show we were close to or perhaps above our design basis for earthquakes at some locations at some frequencies," David Heacock, Dominion Nuclear's president and chief nuclear officer, said during an interview last week. "We're waiting for some additional data to figure out what that means."

The NRC said evidence suggests the quake was within the plant's safety margin, and only hairline cracks in cinder blocks away from the reactors, damaged transformers and loose insulation have been found. Heacock said it was all "very minor stuff," but that the company is conducting in-depth inspections of the plant and equipment and analyzing data from monitors at the site. Dominion must ensure that the two pressurized water reactors are safe before restarting them, the NRC said.

As aftershocks wane throughout the area, residents living near the plant say they don't question Dominion's safety culture but are concerned the quake may have shifted or damaged equipment and piping at the plant.

Harry Ruth, founder of the local environmental group Friends of Lake Anna, who lives less than 2 miles from the plant, said he shares those concerns. Looking out the high windows of his waterside home in Bumpass, on the shores of Lake Anna, Ruth said he's worried about the plant, especially with Japan's nuclear disaster still fresh in everyone's minds.

Sitting at his kitchen table with maps of the lake and residential zones spread before him, Ruth said he fears that equipment at the plant may have shifted and that the quake, strong enough to move 115-ton casks, may have jostled underground pipelines carrying radioactive water.

Ruth said he also wants reassurance that the 90-foot-tall earthen dam built in the 1970s holding Lake Anna intact and providing cooling water for the reactors is secure.

"If the dam breaks, there's no water to cool the reactor," Ruth said.

Reassessing risk

Federal seismologists say the earthquake may have caused the ground to shake at 0.26 g at the site, a measurement of how intensely the ground shook on Aug. 23. Those preliminary measurements exceed what the plant was designed for, namely the ability to withstand 0.12 g on rock and 0.18 g on soil.

Heacock explained that different parts of the plants can withstand different amounts of shaking, and that preliminary data from monitors at the site show lose levels were possibly exceeded.

Data from electric monitors that continued to operate during the quake have been sent to California to be analyzed, which Heacock said will provide more accurate information about the quake. "What it really does is give us a lot more data," Heacock said. "If we determine later that we didn't exceed design basis earthquake, we'll retract that report."

The plant was designed and built during the 1970s, with Unit 1 coming online in 1978 and Unit 2 coming online in 1980.

Seismic protections within the design basis were based on historical information about the strongest earthquakes in the region, known as the Central Virginia Seismic Zone. But the plant, which was designed to withstand a magnitude-5.5 earthquake, was built at a time when ground motion equations were "pretty primitive," said Clifford Munson, a senior-level technical adviser for structural mechanics in the NRC's Office of Nuclear Reactors.

Heacock shared that assessment.

"The only data available at the time was West Coast data," he said. "So they said, 'OK, let's take a 5.7-, 5.9-magnitude earthquake and draw the curve with the data that we have,' and that's how the plant was designed."

But federal officials and Dominion say margins of error that differ for each component throughout the site may have protected the facility from severe damage. Heacock said the company would have expected to see concrete walls cracking or windows breaking, but Dominion hasn't endured any such damage. "The plant has a lot of design margin over and above" protections in the design basis, he said.

Even so, a preliminary review the NRC conducted has found that many nuclear plants may need seismic upgrades, particularly in the eastern and central United States, where plants are more likely to get hit with a quake that exceeds their design basis. The NRC plans to unveil the new seismic risk model later this year, which operators will be required to use and comply with (Greenwire, Feb. 24).

Arthur Frankel, a U.S. Geological Survey research seismologist, said Virginia's seismic zone -- where the North Anna plant is located -- is an area of known higher seismic risk. Unlike the easily identifiable faults of the western United States, Eastern faults are not exposed or are covered with vegetation, and are much rarer and harder to identify.

The Aug. 23 earthquake could help seismologists understand which ancient fault ruptured under Virginia and provide data about the properties of such faults, Frankel said.

The NRC also sent an augmented inspection team to the plant, which will stay on-site for about two weeks and then have an exit meeting in about a month, said Scott Burnell, an NRC spokesman.

"The degree to which their work supports the agency's finding and whether or not the plant is safe remains to be seen," he said. "Dominion is also going to continue its investigations, so at this point we don't have a target date for whether or not they can restart."

Expanding North Anna

Dominion has incorporated updated seismic risk into its application for a third unit it proposed to build at the site, Burnell said.

The company submitted information for that proposed unit in 2003 that includes an "updated understanding of ground motion hazards, so they know what the current hazards are for that site," Burnell said.

Dominion hopes to build an advanced pressurized water reactor as a third unit there, which would also use water from Lake Anna as coolant. The company says the unit is needed to fill a projected energy gap of 4,500 MW by 2020, which will only be exacerbated as demand grows in the region.

But environmentalists opposing the proposed reactor are asking the NRC to halt licensing decisions until the agency has finished pulling lessons from the March 11 earthquake and tsunami in Japan, which crippled the six-unit Fukushima Daiichi nuclear plant on the country's northeastern coast.

The NRC's six-member task force -- assembled in March -- released a dozen safety recommendations for nuclear plants and called for operators to re-evaluate the facilities to ensure they can withstand seismic events without losing safety functions. The task force said such reviews should be conducted at "appropriate intervals" as new seismic data and models emerge.

Edwin Lyman, a senior scientist with the Union of Concerned Scientists, said he doesn't believe the NRC will shut down any plant based on seismic analyses because the agency has already decided that the seismic risks remain "acceptably small in absolute terms," even with the increased hazards that are being revealed in current seismic analyses under way.

"The NRC should take a hard look at the highest-risk plants," he said. "If the uncertainties are high enough, it may be prudent to shut down these plants for seismic upgrades, as Japan is doing at its Onagawa Nuclear Power Plant."

The three-unit plant has been in cold shutdown since March 11.

"Hopefully, this event should be a wake-up call to the NRC to promptly approve the recommendations of the Task Force and to start answering these questions," Lyman said.