CLEVELAND, Ohio -- FirstEnergy Corp. has hired contractors to remove and replace outer layers of cracked concrete from two areas of the protective walls at the Davis-Besse nuclear reactor.

The reactor will continue to operate while the crews do the repairs.

One of the sections being repaired, at an estimated cost of $670,000, measures 10 by 20 feet. The other measures 12.5 feet by 50 feet. Thickness of the new concrete will vary.

The repairs are being made to the walls of the huge steel-reinforced "shield building" which is designed to protect the nuclear reactor from storms, tornadoes and terrorists. The round building is 296 feet tall, about 144 feet in diameter and has a circumference of 452 feet.

A separate smaller "containment building" made of welded heavy plate-steel sits inside the shield building. It contains the reactor and related equipment. And it is what keeps radiation out of the environment.

The repairs come as FirstEnergy warns it will close or sell its nuclear power plants unless it wins approval by the state to charge customers additional fees. The plants cannot compete well against new gas turbine plants, the company said.

The sub-surface cracks are located in the concrete of unique architectural "shoulders" that were designed and incorporated into the outside surface of the shield building's walls.

There are 16 of these shoulders, flat slab-like structures, around the circumference of the building, jutting out 2-18 inches from the true wall and giving a "flute-like" appearance to the building.

The shoulders are not load-carrying structures and just add dead weight to the outside of the building, according to the company.

The first crack was discovered in 2011 by contractors cutting a large doorway in the building in order to move a new reactor lid inside. Similar renovations in 2003 had not found cracking in the concrete.

Initial measurements determined that the first crack discovered, running about 30 feet vertically, was 0.005 inches wide, or five-thousandths of an inch wide.

Later ultrasonic inspections of the building's walls found cracks as wide as 0.05 inch, or five-hundredths of an inch. They also found a few as wide as 0.125 inch, or one-eighth of an inch, said Jennifer Young, a FirstEnergy spokeswoman.

The cracks do not travel from the exterior surface of the building to the rebar buried in the concrete, she stressed. But some cracks are near the rebar, though none have been found to have moved past the rebar and deeper into the concrete.

The issue has been whether the cracks would grow and lead to the deterioration of the concrete and corrosion of the heavy steel rebar, or whether they have already compromised the integrity of the building.

National concrete experts retained by FirstEnergy determined that the cracks did not appreciably weaken the building. And the Nuclear Regulatory Commission, which hired its own concrete consultants to bolster its in-house staff, agreed with the company.

Anti-nuclear opponents made the cracks an issue in the company's now concluded and successful application for re-licensing, allowing the company to operate Davis-Besse another 20 years, to 2037.

As part of the new license, the Nuclear Regulatory Commission has required the company to conduct annual inspections of the building's concrete walls.

The company has also sought to learn what caused the cracking in the first place. National concrete experts looking at the power plant's construction records discovered that the building's surface had never been waterproofed.

They also theorized that a 1978 storm that started with rain before turning into a snow storm had driven water deep into the then-new concrete. That's what created the cracking.

The Nuclear Regulatory Commission eventually agreed with that supposition and conclusion, but insisted on the regular ultrasonic examinations of the building's walls.

The company had the building waterproofed in 2012. Its consultants believe there is still water within the concrete causing the growth of the cracks.

The removal and replacement of the concrete in the shoulders could take up to seven weeks, depending upon the weather, Young said.

Viktoria Mitlyng, NRC spokeswoman, said the agency's own concrete expert will be at the plant during the demolition of the old concrete and the application of the new concrete.

The Pullman Co., a national concrete contractor, is doing the work. And it may be back in future years. Young said the company performs on-going ultrasonic crack inspections of the huge building and annually submits its inspection reports to the NRC.