A new era for nuclear power is taking shape as third-generation reactors, designed to be simpler and safer, inch through the U.S. Nuclear Regulatory Commission's (NRC) design certification process. Much of nuclear's revival hinges on the ability of new reactors to outshine those of yore in terms of safety, economics, construction time and life span.

Of the 26 new reactor applications under current NRC scrutiny, 14 are for Westinghouse Electric Co.'s AP1000 pressurized water reactor. What sets the reactor apart is its modular design and passive safety system: Instead of relying on an operator or electronic feedback to shut down the reactor should it overheat, it employs the natural forces of gravity, convection and air circulation.

In the case of an emergency (a one-in-two-million chance according to a probabilistic risk assessment) the plant itself responds to changes in pressure, temperature and coolant level, says Westinghouse's Bruce Bevilacqua, vice president of new plants engineering.

If everything goes according to plan the first of two AP1000 reactors will go online in 2016 at Plant Vogtle in Burke County, Ga., marking the first U.S. nuclear construction project to break ground since the 1970s; the second is set to go online in 2017. Yet it remains to be seen whether the AP1000 will herald nuclear's next generation, especially because the reactor is on its 18th design revision and a couple of key safety questions remain unresolved.

The first safety concern arose four years ago over the durability of the reactor's shield building. The structure is the outermost layer of defense in a nuclear reactor and provides protection against severe external events such as earthquakes, hurricanes, tornado-generated projectiles and airplane collisions. In the AP1000's case it also supports a very large water tank (one of the passive safety measures).

"Westinghouse first submitted the AP1000 design in the early 2002, which was approved and certified in 2006," explains Scott Burnell, spokesperson for the NRC, adding that one year later the company modified the shield building's design to meet aircraft-impact design standards.* "While the original design used the same reinforced concrete used for decades, the altered design used a more prefab approach, where concrete would be sandwiched between steel plates." After long talks, the NRC said the sandwich module approach would not stand up to severe external events.

Westinghouse returned to the drawing board. In May the company submitted a more robust shield building design that adds steel reinforcement between the walls, fortifies the connection joints between the steel composite wall and the reinforced concrete mat, and improves the venting.

The NRC anticipates completing the overall design certification review around September 2011.

A second safety issue recently came to fore. Arnie Gundersen, a former nuclear industry executive and chief engineer of Fairewinds Associates, an energy consulting company, stepped forward to spotlight what he sees as a possible fatal flaw in the AP1000's design: the separation of the concrete shield building from the steel containment vessel.

Typically, pressurized water reactors' containment systems consist of a concrete dome reinforced with steel to prevent radioactive fumes from escaping during a nuclear accident. With the AP1000, there is a space between the steel containment vessel and the shield building, the latter of which has a hole in its roof. Because the AP1000 design uses a chimney effect to draw air outside of the containment vessel upward (a design Westinghouse envisioned for cooling the containment and preventing rust using natural circulation), if radioactive air were to seep from the steel vessel, it would be ushered up into the outside air through the hole in the shield building roof.

"Do I think this is a one-in-10 event? No," Gundersen says. "But I don't think it's a zero-probability event either. Perhaps it's one in a thousand, and that's significant when you're dealing with consequences this big."

Gundersen points to dozens of containment breaches as proof. One hole was discovered by visual inspection in 2009 at the Beaver Valley Nuclear Generating Station in Pennsylvania. It bore through the containment's metal lining. Another was an 18-meter-long delamination (fissure caused by layer separation), found at the Crystal River Nuclear Power Plant in Florida the same year.

The solution, Gundersen says, is to install filters in the hole of the shield building's roof to capture any vapors that could leak out of the containment vessel, if breached. This modification would probably delay certification.

So why choose the AP1000 reactor when four other next-gen reactors—the economic simplified boiling water reactor, the EPR (for European pressurized reactor), the U.S. advanced pressurized water reactor, and the advanced boiling-water reactor—are going through the same design certification process? "There's a number of reasons," says Doug McComb, engineering manager of Southern Co., the utility that runs Plant Vogtle. "Probably the biggest is we were looking at the schedule for when we would need additional generating capacity and at the schedule for various designs, and the AP1000 was the only one that would meet our need." Meaning that the AP1000 was the only reactor scheduled to be certified in time.

On June 18, Southern Co. accepted a $3.4-billion conditional loan guarantee from the U.S. Department of Energy for the twin Plant Vogtle reactors and continues early site construction. The next probable step? AP1000 certification.

*Correction (7/30/10): This sentence was edited after posting. It originally stated that the Nuclear Regulatory Commission is part of the U.S. Department of Energy. The NRC is an independent commission appointed by the president.