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OAK HARBOR, Ohio — Nobody knows why, but there apparently was a problem sealing up Davis-Besse nuclear power plant’s shield building after the plant’s worn-out reactor head was replaced in fall 2011.
FirstEnergy Corp. notified the Nuclear Regulatory Commission at 11:14 a.m. Friday that the utility discovered an extensive air pocket or gap of concrete in the shield building's inner wall late Thursday night. The discovery was made while the nuclear plant was offline and in the early stages of a $600 million project to replace the plant’s two original steam generators — major pieces of equipment that create steam so the plant’s turbine generator can spin and, thus, make electricity.
After cutting a hole through the shield building to move the new generators in and take the old ones out, workers noticed a large void on the building’s inner wall.
The flaw runs the 25-foot length of a cut made in fall 2011 when the new reactor head was brought in and the old one was removed, said Jennifer Young, a spokesman for FirstEnergy Nuclear Operating Co.
The void varies in width from six to 12 inches. The depth of it is something less than the 2.5-foot thickness of that mostly concrete-and-steel structure, because there is no evidence of the flaw on the structure’s exterior, Ms. Young said.
“It’s probably an air pocket that got in there when the concrete was [last] poured,” Ms. Young said.
In its notification to the NRC — scheduled to be made public on the agency’s Web site (nrc.gov) on Tuesday — the utility characterized the structural defect as an “unfilled area” that “is likely due to not completely repouring the shield building wall opening in 2011.”
The gap did not affect plant operations.
FirstEnergy engineers are of the belief the gap was not big enough to have compromised the structural integrity of the shield building, which is supposed to protect Davis-Besse — especially from outside threats ranging from tornadoes to plane crashes.
“We believe it did not impact the building’s ability to meet its function,” Ms. Young said.
The NRC is not ready to go that far.
“This is something we have certainly not formed any conclusions about, the ‘as-is’ conditions of the shield building,” Viktoria Mitlyng, NRC spokesman, said of the building’s strength. “That is the question we expect the company to provide us an answer with, and we will assess it before making a determination.”
The federal agency now plans to send more inspectors to the site, to augment its team already assembled there for the steam generator replacement project. Additional manpower will include engineers who specialize in nuclear materials and structures, Ms. Mitlyng said.
She said the investigation is more than anecdotal: FirstEnergy will be sealing up the same structure again once the new steam generators are installed.
FirstEnergy does not want a repeat of what happened, Ms. Young agreed.
The NRC “will be reviewing how the plant responds to this,” Ms. Mitlyng said.
“They will have to address this void before they start up. We will be evaluating how they handle the concrete pouring and the conditions to make sure these conditions do not recur,” she said. “We need to understand what happened and why and what possible implications there might have been.”
Ms. Mitlyng said she was not sure if NRC inspectors were on hand when the concrete was poured in 2011.
Davis-Besse’s planned restart in fall 2011 was delayed until early 2012 because of cracks in the same structure.
FirstEnergy believes those are unrelated to the large void in concrete that was just discovered. Utility engineers previously attributed them to weather impacts from the Blizzard of ’78.
The NRC has no reason to believe they’re related.
“It doesn’t appear to have anything to do with it,” Ms. Mitlyng said.
FirstEnergy is having engineers pore over data to see how it occurred to avoid a repeat when the structure is sealed back, Ms. Young said.
The utility will move forward with replacing the plant’s two original steam generators in tandem with its investigation into the missing concrete.
Steam generators are like heat exchangers, and are among the biggest — and among the most important — pieces of plant equipment. They create super-intense steam that spins the turbine generator that makes electricity.
The original steam generators still work, but needed to be replaced to keep the plant viable. The utility hopes to get a 20-year extension and keep Davis-Besse operating through April, 2037.
FirstEnergy cut within the same footprint to begin the process of replacing the steam generators, Ms. Young said.
Some of the shield building’s rebar needs to be replaced. It appears to have been damaged by the cut made through the wall, she said.
“There’s a high level of confidence [the rebar damage] was a direct result of the hydro cut,” Ms. Young said.
The concrete and steel are cut with a high-pressure water drill to minimize damage. A quarter-inch steel form was left in place as backing to further help minimize damage from the cut, Ms. Young said.
There is a 4-foot gap of air space between the reactor's primary containment — a solid, 1.5-inch-thick canister of carbon steel — and the outer shell.
Davis-Besse is one of two Ohio nuclear plants. It is located in Ottawa County, 35 miles east of Toledo and along the Lake Erie shoreline. The other plant is the Perry station east of Cleveland, also along Lake Erie.
Both are owned and operated by Akron-based First-Energy.
Contact Tom Henry at: firstname.lastname@example.org or 419-724-6079.