The first new nuclear reactor ordered in the U.S. in roughly three decades is beginning to take shape near Augusta, Ga. Southern Company and its partners have dug 27.5 meters down to reach bedrock and are now refilling the hole to provide a stable, anchored foundation for what is likely to be the first of a new generation of reactors in the U.S.: two new AP1000 models at the Vogtle Electric Generating Plant that stand next to two older pressurized water reactors, which came online in the 1980s—the first of some 14 AP1000s and 20 new reactors in total that may be built in the U.S. in the next 15 years.

"The nuclear revival is underway in Georgia," said Jim Miller, chief executive officer of Southern Nuclear Operating Co., the subsidiary charged with administering the corporation's nuclear power plants in February. "It will provide safe, clean, reliable, low-cost electric energy to our customers for generations to come."

Of course, that was before the accident at the Fukushima Daiichi nuclear power plant in Japan, following the 9.0-magnitude earthquake and subsequent tsunami. That power plant boasted six boiling-water reactors built in the 1970s by General Electric, Toshiba and Hitachi, and capable of pumping out more than 4 gigawatts of electricity. It also proved incapable of withstanding the twin perils of an earthquake that disconnected it from the electrical grid and a tsunami that wiped out back-up diesel generators and flooded electrical equipment.

"First you rely on the grid," explains Scott Burnell, a spokesman for the U.S. Nuclear Regulatory Commission, which oversees safety at the 23 such boiling water reactors in operation in this country. "If the grid is no longer available, you use diesel generators. If there is an issue with the diesels, you have a battery backup. And the batteries usually last long enough for you to get the diesels going."

That did not prove to be the case at Fukushima Daiichi. But new reactor designs—including the Economic Simplifed Boiling Water Reactor from GE-Hitachi that passed its safety rating from the NRC on March 9, two days before the quake—are meant to provide cooling even in the absence of power.

For example, the AP1000s being built in Georgia boast "passive" safety features—safety technology that kicks in with or without human intervention or electricity. In the case of the Westinghouse AP1000 design that means cooling water sits above the reactor core and, in the event of a potential meltdown like at Fukushima Daiichi or Three Mile Island in Pa., will, with the opening of a heat-sensitive valve, simply flow water into the reactor, dousing the meltdown. "Never has so much money been spent to prove that water runs downhill," Westinghouse spokesman Vaughn Gilbert told Scientific American in 2009.

Further, although the thick steel vessel containing the nuclear reactor is encased in a further shell of 1.2-meter-thick concrete, that shell is surrounded by a building that is open to the sky. Should the concrete containment vessel begin to heat up during a meltdown, natural convection would pull in cooling air.

But that open-air building was initially rejected by the NRC for a lack of structural strength. The U.S. regulator argued that it would not withstand a severe shock such as an earthquake or airplane impact, because it was initially planned to be built from pre-fabricated concrete and steel modules in order to save money.

The modified design now under review by the NRC employs more steel reinforcement as well as improved venting (maintaining such venting has proved critical at Fukushima Daiichi). But some critics, such as engineer Arnie Gundersen of Fairewinds Associates, have further concerns. For instance, if the containment building housing the reactor core were to spring a leak—as appears to have happened at Fukushima Daiichi— radioactive material would be wafted up and out of the AP1000 thanks to that same natural convection.

In the end, all nuclear power plants suffer from a balancing act between absolute safety and acceptable cost. "With earthquakes, there are limits to what you can do," says nuclear engineer Michael Golay of the Massachusetts Institute of Technology. "What risk are you willing to tolerate?"

Source: scientificamerican.com