Southern Co. announced on June 16 that construction has been completed on a new FLEX dome—a structure built to house emergency equipment needed to respond to an extreme external event—at its Vogtle nuclear plant.

The Nuclear Regulatory Commission (NRC) is requiring U.S. nuclear plants to build protected structures like the FLEX dome in response to lessons learned from the Fukushima Daiichi nuclear accident. FLEX is a strategy developed by the nuclear energy industry to implement NRC task force recommendations. The portable emergency equipment housed in the dome adds another layer of protection to facilities that are already protected with redundant safety systems, including backup power resources such as DC battery banks and diesel generators.

“The dome is the most visible part of the FLEX strategy, but we’ve also made modifications throughout the plant that further strengthen our ability to protect the health and safety of our workers and the public,” Keith Taber, site vice president for Plant Vogtle Units 1 and 2, said in a press release.

The Vogtle FLEX Dome (Figure 1) is 39 feet tall, nearly 50 yards wide, and features steel-reinforced concrete walls that are 24 inches thick. Southern Co. is also constructing similar domes at its Hatch and Farley nuclear facilities.

1. The Vogtle FLEX Dome.
Built to withstand earthquakes, tornados, and airborne flying objects, the structure houses emergency equipment, which could potentially be needed during an extreme external event. Courtesy: Southern Co.

In a video about the project, Ron Tyler, building project lead at the Farley plant, and Derrick Caldwell, site project lead for Vogtle 1 and 2, explained the steps in the construction process. They said a reinforced concrete circular ring beam was constructed first. Following installation of a series of anchor bolts around the circumference of the ring beam, an air form was rolled out and inflated using large fans, creating the shape of the dome. A layer of polyurethane foam was applied to hold the profile, followed by the installation of multiple layers of shotcrete reinforced with rebar.

After the main dome was in place, missile doors were installed for equipment access, along with smaller doors for personnel. The floor slab was poured and a mezzanine level was installed, as was lighting and other electrical equipment needed to complete the facility.

“The dome is built to withstand the design basis earthquake for the plant, 360-mph tornadic winds, as well as any windborne missiles, such as trees, light poles, or vehicles,” Tyler said.

The building houses three sets of equipment—one set for each operational unit at Vogtle and a spare set—adding an extra layer of defense for the site. The dome will store portable diesel-driven generators and pumps, communication equipment, refueling equipment, cables, hoses, and other miscellaneous resources that might be needed if the power supply to the plant were interrupted for an extended period of time. The portable equipment would be put into service only in the event other redundant resources at the plant were depleted or damaged.

Although Vogtle is the most recent nuclear plant to complete its FLEX building, it is not the first. TVA’s Watts Bar facility holds claim to that designation.

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)