PORT CLINTON — More cracks were found in the concrete “shield building” at the Davis-Besse Nuclear Generating Station, including two areas of subsurface cracks “not associated” with cracks in the structure’s architectural features, FirstEnergy said Monday in a letter to investors.

The newly discovered “indications” of cracks were identified during electronic testing and concrete sampling stemming from an initial discovery of a 30-foot hairline crack in the shield building that appeared after utility contractors cut a hole through its concrete for access to install a new reactor head. Davis-Besse has been shut down since Oct. 1 for that procedure.

The testing has revealed “similar subsurface hairline cracks in most of the building’s architectural elements,” which protrude up to 18 inches beyond the main structure of 2½-foot-thick reinforced concrete, according to the letter. But the two areas of sub-surface cracking deemed “not associated” with that cracking are being investigated “as a separate issue,” the letter said.

“Our overall investigation and analysis continues. We currently expect Davis-Besse to return to service around the end of November,” wrote Ronald Seeholzer, FirstEnergy’s vice president of investor relations. Nuclear Regulatory Commission inspectors remain at the plant and are observing the cracks’ assessment, he added.

First Published November 1, 2011, 4:15 a.m.