Federal regulators Friday said the shuttered Davis-Besse nuclear power plant in northern Ohio is safe to restart even though the plant's operator is still investigating cracks in the plant's protective outer concrete wall.

The Nuclear Regulatory Commission said in a "confirmatory action letter" that FirstEnergy Nuclear Operating Corp. has provided "reasonable assurance" that cracks in the outer shield building do not compromise the safety of the single reactor at the site, about 20 miles east of Toledo, Ohio.

The 900-megawatt plant sits on the shores of Lake Erie and has been shut down since Oct. 1 for workers to replace a corroding lid or "head" for the reactor.

Workers cut a hole through the plant's 2-foot concrete shield building on Oct. 10 to begin replacing the lid and found a 30-foot crack in the shield building.

The concrete structure is surrounded by a steel containment vessel that protects the reactor from natural disasters, terrorist attacks and serves as a secondary containment system. Different types of cracks in the structure were discovered through later investigations (Greenwire, Nov. 18).

Prema Chandrathil, a spokeswoman for NRC, said the commission reviewed FirstEnergy's analysis of the cracks and found the shield building can withstand the force of flying objects, tornadoes and earthquakes.

But NRC also required FirstEnergy to evaluate why the cracks occurred and ordered the utility to monitor the fissures before and during scheduled fuel outages next year to see if they have changed or expanded. The company must also determine the "root cause" of the cracks and corrective actions by Feb. 28, 2012, and then provide a long-term monitoring program, Chandrathil said.

NRC has the authority to shut the plant down if it is not operating safely and can take additional regulatory action if FirstEnergy does not comply with the commission's orders, she said.

"We won't hesitate to take action if need be," Chandrathil said.

NRC will conduct its own review and is planning a public meeting for FirstEnergy to discuss its technical analysis and explain why the plant is safe to operate with cracks in the shield building. A date and place have yet to be determined.

Jennifer Young, a spokeswoman for FirstEnergy, said the company is restarting the plant but would not say when the plant will be fully operational.

But the cracks have raised safety concerns and fueled arguments that the plant should not be relicensed after its current permit expires in 2017. Young said the plant could be relicensed next year.

Rep. Dennis Kucinich (D-Ohio), who opposes relicensing the plant, asked NRC Chairman Gregory Jaczko in a Nov. 21 letter whether carbon dioxide was seeping through the concrete, corroding steel rebar in the shield building and expanding cracks.

"Cracking has been detected in 15 out of 16 of the 'wings' or 'shoulders' of the building, and discovered in other areas of the wall, all suggesting a delamination of the concrete, at the outermost rebar, caused by concrete carbonation," he said.

Delamination of concrete occurs when the metal corrodes and causes stress on the concrete, which results in a separation of the concrete from the metal.

Kucinich had also asked NRC to hold hearing on the cracks before allowing the plant to restart.