OAK HARBOR, Ohio — Workers will spend several weeks repairing cracks first discovered six years ago in the concrete walls that protect the reactor at a nuclear plant along Lake Erie.

FirstEnergy Corp. said that the work is needed because some of the cracks have grown and that it believes there is still water within the concrete at the Davis-Besse nuclear plant near Toledo.

The Akron-based utility and the Nuclear Regulatory Commission both say consultants have determined that the cracks have not weakened the concrete shield building.

Crews this fall will remove and replace concrete in the areas where the cracks are located. The plant will continue to produce electricity during the work.

The first cracks were found in the plant’s concrete shell in 2011. FirstEnergy later said the problem traces back to a 1978 blizzard that likely pushed moisture into the concrete.

The first crack was about 30 feet long and about five-thousandths of an inch wide.

Ultrasonic inspections of the building’s walls found cracks that were as wide as one-eighth of an inch, said Jennifer Young, a FirstEnergy spokeswoman.

Independent experts determined the cracks had not compromised the building.

FirstEnergy waterproofed the building and stepped up inspections, but the cracks have spread slightly within the shell that protects the reactor from storms, tornadoes or a terrorist attack, the company said.

“Even though the building continues to perform its function safely as is with the cracks, this just gives us some added assurance for the future,” Young said.

Federal inspectors will have their own concrete expert at the plant during the repairs, said Viktoria Mitlyng, spokeswoman for the NRC.