OAK HARBOR, Ohio - The stainless-steel liner under the lid of the now-closed Davis-Besse nuclear reactor was thinner than officials originally believed and had begun to crack in several places, a laboratory analysis shows.

The strength of the liner was critical because when corrosion created a large hole in the reactor's lid, the liner became the only barrier between the reactor's high-pressure coolant and the building that houses the reactor, The (Cleveland) Plain Dealer reported Wednesday.

The liner was supposedly three-eighths of an inch thick. But testing determined that the liner was significantly thinner in places - in one location nearly 50 percent thinner - than plant owner FirstEnergy Corp. believed.

Officials of Davis-Besse and the Nuclear Regulatory Commission don't know how or when the cracks formed, or even how deep they go, and said Tuesday that they are still reviewing the findings.

But the presence of cracks raise questions about whether the barrier was as strong as FirstEnergy Corp. has said and how much longer it would have been able to withstand the reactor's operating pressures.

The answers to those questions will affect the outcome of NRC and Davis-Besse studies to determine how close the Toledo-area plant came to a major nuclear accident. That finding, in turn, will dictate what penalties and continuing scrutiny FirstEnergy faces from the NRC.

"We want to know more, obviously," said William Beecher, an NRC spokesman.

"It's important to know what happened . . . every detail," agreed FirstEnergy spokesman Todd Schneider.

Davis-Besse officials learned of the cracks Monday night, Schneider said.

The reactor has been idle since February, when the plant shut down so workers could inspect for cracks in the 69 nozzles that penetrate its massive steel lid. Nozzle cracking has been a recognized issue industrywide for years in older reactors. The cracks were thought to be relatively benign, but the NRC became alarmed when more extensive, possibly dangerous cracks were found at a South Carolina reactor in 2001.

At Davis-Besse, workers found not only serious cracking, but also the 6-by-8-inch hole formed by the reactor's corrosive coolant leaking from the cracks and pooling undetected on the lid for years.

FirstEnergy has used the liner's strength as part of its argument to the NRC that Davis-Besse wasn't close to an accident, and thus it doesn't deserve a harsh punishment. Although the company will have to revise its safety calculations based on the new crack findings, its argument still holds, Schneider said.

"We have to remember that the liner held normal operating pressure and that the plant shut down safely," he said.

Information from: The Plain Dealer