The two nuclear power reactors in North Anna, Va., were cooling off Wednesday after being shut down by Tuesday’s earthquake, but the debate about what lessons should be learned for nuclear safety was still simmering.

Dominion Resources, owner of the reactors, said that its safety procedures worked. When three shaken transformers tripped like circuit breakers, four locomotive-size diesel generators kicked in. When one of those generators sprang a radiator water leak, another backup generator was turned on.

Company spokesman Jim Norvelle said that as of noon Wednesday, the nuclear plant dropped from “alert” status to “unusual event,” and that by 1:30 p.m. it dropped that warning, too. He said inspections so far showed no damage to the plant or the nearby mile-long dam on Lake Anna.

But some nuclear power experts said that the size of the earthquake — big for a relatively tame seismic zone — combined with the disaster at Japan’s Fukushima Daiichi nuclear complex, showed that the Nuclear Regulatory Commission should raise the bar for earthquake protection.

“[What] I would say in terms of lessons learned from Fukushima and now yesterday’s quake is that setting reactor design . . . hazard limits just slightly above recorded human experience is turning out to be really shortsighted,” said Allison Macfarlane, an environmental policy professor at George Mason University.

The North Anna plants — which came online in 1978 and 1980 — were designed to withstand a 6.2-magnitude quake, and Tuesday’s temblor weighed in at 5.8. That’s just slightly less than the Virginia record of 5.9 set more than a century ago. Fukushima Daiichi was designed to withstand a magnitude-7.9 earthquake because the largest ever in that region was 7.8 — until March, when a bigger one, along with a tsunami, knocked out grid power and all the backup generators.

“We were really close here,” Macfarlane said. “Some would say that’s evidence of us getting it right, but I would say that we’re really pushing the envelope. With something like a nuclear reactor, I would like a large safety margin.”

Of course, she added, “a larger safety margin entails higher costs,” and the nuclear power industry doesn’t like that.

“There’s a heck of a lot of alarmists out there,” said Steve Kerekes, spokesman for the Nuclear Energy Institute, an industry group. He said “the response of U.S. nuclear energy facilities in [the] eastern half of [the] country is evidence that our facilities . . . were well designed and that they are sturdily built.”

But how sturdy? On May 13, the NRC issued a post-Fukushima report on U.S. nuclear plants that found flaws. At the North Anna plants, the NRC said, “portions of the water and gaseous suppression systems and hose stations are not seismically designed.” It said that the fire pump storage area was not strong enough to withstand a severe earthquake and that a flood wall was in a turbine building that was not designed to handle a severe quake.

The report’s critique of North Anna’s suppression systems was “a big red flag,” said Damon Moglen, director of climate and energy at Friends of the Earth, foes of nuclear power.

No matter how sturdy U.S. nuclear plants were when they were built, most of them are aging and standards may have changed. In the wake of Fukushima, the NRC has proposed reviewing standards every 10 years.

Unlike the General Electric reactors at Fukushima, the North Anna plants were designed by Westinghouse. Located northwest of Richmond, the North Anna reactors use cooling water from Lake Anna, a 9,600-acre reservoir created to serve the power plant. Together, the units produce 1,806 megawatts, enough to power 450,000 homes.

“These are both old machines, and old machines break down under stress,” Moglen said.

The NRC report also looked at whether U.S. nuclear plants could withstand natural disasters whose likelihood was about the same as the likelihood of the tsunami that hit Fukushima. Five reactors — including PG&E’s Diablo Canyon units in California and Duke Energy’s Oconee units in South Carolina — could face a crippling seismic event more often than Fukushima’s projected frequency of once in a thousand years.

“Thus, NRC knows that several U.S. reactors have vulnerable designs against earthquakes of equal or greater vulnerability than Fukushima’s design against tsunamis,” said David Lochbaum, a nuclear expert at the Union of Concerned Scientists.

The North Anna plants could face an earthquake exceeding safe shutdown levels every 4,762 years.

But when that quake could arrive isn’t known. “The bottom line,” Macfarlane said, “is that geology is a retrodictive science: It explains the past and doesn’t predict the future, except in very general terms.”

Staff writer Spencer S. Hsu contributed to this report.