The hole, which was stopped by a layer impervious to the acid, does not pose a safety threat, said Nuclear Regulatory Commission spokesman Jan Strasma.
If the acid had penetrated the massive cap and allowed steam to escape, safety systems would have immediately cooled the reactor, he said. And while the steam would contain some radioactive material, it would have been confined by the reactor containment building. Even if steam had escaped from the building, there would have been no danger to the public, Strasma said.
"It's only when you get into the what-ifs that you would have had any leakage from the reactor cooling system," Strasma said Tuesday.
"There was no hazard," he said earlier. "It's certainly very unusual. It's a deterioration of a very important safety feature."
The regulatory commission has alerted the nation's 102 other commercial nuclear plants to watch for similar problems. It said this was the most extensive corrosion ever found on top of a U.S. nuclear plant reactor.
The hole was discovered last week while the Davis-Besse nuclear plant was shut down for normal refueling and maintenance.
It could have been slowly leaking for years, Strasma said.
Trace amounts of boric acid, a byproduct of the nuclear fission process inside the reactor, are believed to have dribbled onto the cap from at least one of the reactor's 69 control rods.
The acid did not penetrate an inner layer of the cap, only about three-eighths of an inch thick, because that layer of steel is impervious to boric acid, said Richard Wilkins, a spokesman for FirstEnergy Corp., the plant's operator.
"We were not expecting to see that extent of corrosion," he told The Blade of Toledo. "This has not been seen in the industry before."
The corrosion problem will keep the plant closed an extra month, Wilkins said. The plant, located along Lake Erie and about 25 miles east of Toledo, has been shut down since mid-February. The utility said it would remain shut down until at least late April.
Plant officials discovered the corrosion during repairs to five control rod nozzles after cracks were found earlier during the shutdown.
The corrosion appears linked to at least one of those two leaking nozzles or to aging weld seams surrounding them, Wilkins said.
FirstEnergy plans to install a new reactor head during the plant's next refueling shutdown in 2004, Wilkins said.
By John Seewer