The cracks were discovered by utility subsidiary Exelon Generation Co. on June 12 at the twin-reactor Quad Cities power plant in Cordova, Illinois, the NRC said in a statement.

The cracks were found in part of the Unit 2 steam dryer, which is inside the reactor vessel above another piece of equipment called a steam separator and the nuclear fuel.

The NRC, which regulates the nation's 103 atomic power units, said there was no release of radioactivity from the steam dryer and no hazard to plant workers or the public.

Quad Cities Unit 2 had been shut to investigate the cause of moisture in the steam that drives the plant's turbine generator. The steam dryer is designed to remove moisture from the steam before it leaves the reactor.

The NRC said the adjacent Quad Cities 1 unit continues to operate and there is no evidence of cracks in its steam dryer.

The agency's inspection team, which includes metallurgical experts, will review the causes of the cracks and evaluate Exelon's repair plans. It will also weigh the potential for the cracking problems to occur at similar nuclear plants, the NRC said.

There was no word on how long the inspection could take.

The cracks were found in a cover plate on the steam dryer and repairs are under way, said Pete Resler, a spokesman for Exelon.

The Quad Cities station can generate more than 1,700 megawatts of electricity, or power for about 1.7 million homes.

The nuclear power industry has been wrestling with cracking problems in reactor vessel heads after severe corrosion was discovered last year at a FirstEnergy Corp. FE.N power plant in Ohio.