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OAK HARBOR, Ohio — FirstEnergy Corp. said today it discovered an extensive air pocket or gap of concrete in the inner wall of the shield building that is designed to protect its Davis-Besse nuclear reactor in Ottawa County.
The flaw runs the 25-foot length of a cut made into the inner wall of that structure in the fall of 2011, Jennifer Young, FirstEnergy Nuclear Operating Co. spokesman, said.
It 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.
She said engineers are assessing the extent and cause of it.
"It's probably an air pocket that got in there when the concrete was [last] poured," Ms. Young said.
The structure was last sealed in November, 2011, after FirstEnergy finished replacing a worn-out reactor head.
The Nuclear Regulatory Commission was notified shortly before noon about the latest problem as a procedural measure, Ms. Young said.
The NRC is not expected to post the company's event report on its Web site, www.nrc.gov, until Tuesday morning.
Events reported after 7 a.m. on Fridays usually are not posted until the following Monday.
Davis-Besse went back into service in 2012 and continued operation until it began a scheduled refueling-and-maintenance outage this month.
Its restart was delayed because of cracks in that same structure. FirstEnergy believes they are unrelated.
FirstEnergy has no reason to believe the flaw compromised the integrity of the outer shield building while the plant was online the last two years, Ms. Young said.
She said the utility is having engineers pore over data to see how it occurred to avoid a repeat when the structure is sealed back.
The plant was taken offline earlier this month for one of the biggest projects in its history, a $600 million swap-out of the plant's two original steam generators with two new ones.
Steam generators are like heat exchangers, and are among the biggest - and among the most important - pieces of plant equipment. They created 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.
The utility noticed the flaw late Thursday night while inspecting the inside of the shield building.
There is a four-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.
The outer shell is mostly for exterior forces, such as tornadoes and other types of major storms.
It is also is the reactor's first line of defense against plane crashes.