Federal regulators and FirstEnergy Corp. are inspecting indications of a hairline crack in the protective concrete outer shell of the Davis-Besse nuclear plant in Ohio.

Contractors working at the plant on Monday discovered what could be a 30-foot-long crack as they were cutting a hole into the shield building to install a new reactor vessel head, said Jennifer Young, a spokeswoman for FirstEnergy.

The "crack-like indications" were found 6 inches inside the wall, which is more than 2 feet thick and made of concrete and reinforcing steel to protect the plant from tornadoes and other external hazards, she said.

The 900-megawatt plant sits on the shores of Lake Erie about 20 miles east of Toledo, Ohio, and has been shut down since Oct. 1 to put in the new reactor head, Young said. FirstEnergy hired engineering firms Bechtel and Sargent & Lundy to determine whether a crack exists, and the companies are expected to release the results of their review today, Young said.

"They would let us know if there was a crack that we need to be concerned about, if there's something that impacts structural integrity, the cause of the crack and actions we might need to take to resolve it," she said. Young said it's too soon to tell how long the plant will stay shuttered but said the structure does not pose a safety hazard to the plant or to the public.

The Nuclear Regulatory Commission sent concrete experts to the plant this week to conduct a separate, independent inspection and to oversee FirstEnergy's review, said Prema Chandrathil, a spokeswoman for the agency. There is no significant safety concern right now because the plant is shut down and contains no fuel, she said.

"But we want to understand the genesis of this and what the impact is on the structure," Chandrathil said.

She could not say what the utility would be required to do if a crack was confirmed.

FirstEnergy is in the process of replacing the nuclear plant's reactor head because it was found to be corroding, Chandrathil said.

The company was scheduled to replace the vessel head in 2014 but decided to expedite the process after some deterioration of the current head was found last year during a fueling outage, Young said.

The new reactor head will be less susceptible to corrosion and will be better suited to handle the high temperatures and pressures at which the plant operates, Young added.

Safety concerns surrounding the reactor vessel head have surfaced at the Davis-Besse plant in past years.

In 2002, investigators discovered that acid in cooling water ate a hole into the plant's 6-inch-thick nuclear reactor lid (Greenwire, Jan. 20). The NRC later said the plant was allowed to continue operating even though the agency had ordered the company to shut the plant down.

The plant was shut down from 2002 to 2004, and FirstEnergy later agreed to pay $28 million to settle charges that it covered up the safety violations.