Media CoverageEnter the 'FLEX' dome


In September construction was completed on a steel-reinforced concrete dome at Dominion's North Anna power station in Virginia, USA. The building, which will be used to store emergency equipment and supplies, is the first of its kind in the United States.

The dome was designed by Engineering System Solutions (ES2) and built by American Business Continuity Domes (ABC Domes) and Dome Technology. Peter Fedele CEO of American Business Continuity Domes told Nuclear Engineering International that the companies partnered several years ago to work with communities in Texas, Oklahoma and other Tornado Alley states to develop emergency shelters for schools and local community centres. After Fukushima they worked with the nuclear industry to engineer so-called Fukushima Flex domes that meet NEI-12-06 requirements.

Domes are among several types of facilities that are being built in response to US Nuclear Regulatory Commission order EA-12-049 issued in March 2012, which requires nuclear facilities to implement mitigation strategies for a beyond-design-basis external event. These strategies include additional portable equipment at every US nuclear facility to provide backup electrical power and cooling capability. The new equipment is to be stored at 'diverse locations' and protected to ensure that it can be used if other systems are compromised. An additional layer of protection will rely on offsite resources stored at regional response centres in Phoenix, Arizona and Memphis, Tennessee. Utilities were given two refuelling cycles or until the end of 2016 to implement the requirements.

Dome design

The dome at the North Anna site is 36.6 m in diameter and is 11.6 m high. The structure is 600 mm thick and made of multiple layers of insulating foam, rebar, and pneumatically blasted concrete or shotcrete. The outside of the dome is completely waterproof and the interior surface is non-combustible.

The dome has been engineered to withstand winds of up to 360 mph, missiles and seismic forces.

Thanks to its aerodynamic characteristics, the pressures that develop on the dome during high winds are significantly lower than those that would develop on a rectangular structure, explains Jason South, VP of engineering at ES2. Loading creates axial forces inside the dome, rather than the shear forces that would occur in a rectangular structure. Axial forces are much more efficient at carrying load, and as a result the dome is able to withstand larger seismic or wind loading at less cost and less design than a rectangular building.

The dome has two hardened equipment doors made from 3-inch-thick solid steel and located on opposite sides of the structure. The doors are electric powered, but are able to run on a battery backup. In the unlikely event that the door-close mechanism is damaged, pins can be pulled to decouple the hydraulic rams and doors can be opened manually.

Personnel access is via a tortuous path, which means that it is impossible for an airborne missile to make a direct hit on the man doors, notes South.

The dome's free span construction makes the emergency equipment inside easily accessible.

“There is enough equipment for each of the reactors and then we have installed a third set in case some of this equipment didn't work, so that if you need something you can make a repair and it's all inside this dome," said Dominion's director of nuclear engineering Eric Hendrixson, introducing the North Anna dome.

Equipment includes "anything and everything you can imagine" ranging from large 340 V generators to flashlight batteries and food and water provisions, he added.

The nuclear industry dome has been designed for at least a 75-year life, with minimal maintnenace requirements. The exterior may need recoating after 30 years.

Future projects

In addition to North Anna, the ES2/ABC Domes team is working on two other dome projects for Dominion. The first dome at its Millstone power station in Connecticut was completed in October, and another at the utility's Surry power station in Virginia was due for completion in December.

Duke Energy has purchased four domes for its nuclear fleet and ABC Domes is working with SGT, the prime contractor on those projects. It is also working on three domes for Southern Company, which operates a nuclear plant at Farley in Alabama, as well as the Hatch and Vogtle plants in Georgia.