Ohio Sen. George Voinovich, head of a Senate subcommittee on nuclear safety, asked NRC Chairman Nils Diaz to explain why the agency didn't do more to address safety standards at the 103 U.S. commercial nuclear plants it regulates.

Diaz called the incident at the Davis-Besse plant an "unacceptable failure" by the NRC and FirstEnergy. But imposing specific "safety culture" rules is the responsibility of plant owners, not the NRC, Diaz said.

"We do not believe that's the role of the commission," Diaz said at a hearing of the Senate subcommittee. FirstEnergy "did not meet its own definition of safety culture," Diaz said.

Voinovich rebuked Diaz. "If you won't do it, I'll get legislation passed to get it done," the senator said.

The General Accounting Office study found the NRC "should have, but did not, identify or prevent the corrosion at Davis-Besse because its oversight did not generate accurate information on plant conditions." Other problems could occur because the NRC hasn't done enough to monitor safety, it said.

Leaking boric acid, used as a coolant, ate a cantaloupe-sized hole in the outer hull of the reactor in Oak Harbor, Ohio, about 35 miles east of Toledo.

No radiation was released into the air, but it was a serious safety violation. The NRC ordered the plant shut in early 2002.

FirstEnergy returned the plant to full power last month after it spent $600 million to repair the damage.