Waterford — Better known as “the dome,” Building 717 is the newest structure with the newest equipment at the Millstone Power Station, yet it is the least likely to be occupied by workers during anything but training and maintenance exercises.

“It’s all loaded up and ready to be used in the unlikely event that we need it,” Ken Holt, spokesman for the nuclear power plant, said Thursday.

The light gray, 10,000-square-foot concrete structure, designed in a semi-spherical shape for added strength, was built and furnished over the last seven months on a former parking lot at one of the highest elevations at the 520-acre Millstone site, 33 feet above the expected maximum height of storm surge tides, said Tom Paulantonio, implementation project manager.

“This gives us a very robust strategy for dealing with the unanticipated,” he said, standing amid the array of showroom-shiny heavy machinery. “There is a primary strategy and an alternate strategy and a backup strategy” with all the equipment needed to support all three.

Millstone owner Dominion spent $10 million to build the dome and equip it with new tractors, diesel-powered water pumps, generators, portable lights and a mobile communication station, along with miles of firehose and cable, protective clothing and dozens of other items. All of it is labeled with “BDB” stickers for “beyond design basis,” the term used to connote equipment reserved for extreme, worst-case scenario events that would disable the two working nuclear reactors at the site.

“These are tractors that can do many things. They can clear debris, haul equipment,” Paulantonio said, referring to two hulking green John Deere vehicles parked at the center of the dome. “In this building we have three sets of equipment. One for each unit and a spare. It’s all situated so it can be pulled out quickly.”

The dome is a major component of the projects undertaken at Millstone and the nation’s other nuclear power plants to fulfill requirements made by the Nuclear Regulatory Commission as part of its post-Fukushima orders issued in 2012, one year after the Japanese nuclear power plant disaster. The orders were written to ensure that the nation’s nuclear plants would not face the kind of situation that occurred at Fukushima, when a major earthquake followed by a tsunami disabled the power supply that cooled three nuclear reactors, leading to meltdowns of the three cores.

“The issue there was that the plant was left without any source of power,” said Neil Sheehan, spokesman for the NRC. “This gives additional assurance that plants can recover. The key thing is to make sure equipment would be able to withstand extreme events.”

An essential element of the order, he said, is for each plant to have a disaster-proof facility, like Millstone’s dome, that would store all the equipment needed to restore plant safety functions, such as reactor cooling, after a hurricane, earthquake, flood or other extreme event. In addition to the on-site equipment, the nuclear industry also has created warehouses in Memphis and Phoenix that store emergency equipment that can be airlifted to a site in 24 hours.

Tom Kauffman, spokesman for the Nuclear Energy Institute, an industry group, said all the post-Fukushima projects must be completed by December 2016, and that thus far, eight power plants have completed the work and about two-thirds are expected to be done next year.

The dome and other structures like it, he said, are part of the “diverse and flexible coping capability,” or FLEX strategy, the industry developed in cooperation with the NRC. Each structure is designed for the unique conditions of each plant site, depending on the risk of flooding, hurricanes, earthquakes, extreme cold and other rare and severe weather events, Kauffman said.

“All associated staffing, procedures, maintenance and training must be in place as well,” he added.

According to NEI’s website, the FLEX strategy “provides an additional layer of backup power after an extreme event by stationing vital emergency equipment — generators, battery packs, pumps, air compressors and battery chargers — in multiple locations. Implementing FLEX will help maintain cooling if normal systems and other backup systems fail by stationing additional pumps and power sources ... to provide cooling water to the reactors.”

Holt said at Millstone, all the modifications required for the post-Fukushima orders have been completed at Unit 3, and projects at Unit 2 are near completion.

The dome, Paulantonio said, was engineered to remain usable in five “perils” — hurricanes, tornados, earthquakes, tsunamis and severe snow and ice storms. With two eight-ton concrete doors at each end, steel-reinforced 2-foot thick concrete walls inside and an exterior coating of mylar, the dome is capable of withstanding the force of 300mph winds and an earthquake of more than magnitude 3.9 on the Richter scale, he said.

“It’s for those things that happen at the tick of the clock, things you can’t really foresee,” he said. “At Fukushima, they suffered from the inability to be flexible.”

j.benson@theday.com

Twitter: @BensonJudy