OAK HARBOR - FirstEnergy Corp.'s Davis-Besse nuclear plant became the focus of a national investigation yesterday after officials learned that acid from the plant's reactor ate a half-foot chunk out of a steel cap that covers the vessel.

Described by the U.S. Nuclear Regulatory Commission and the utility as the most extensive corrosion ever found on top of an American nuclear plant reactor, the radioactive boric acid came within a half-inch of burning a hole through the huge vessel head - a domelike structure that is 17 feet wide and made primarily of two types of steel.

The corrosion, missed in previous inspections, prompted the U.S. Nuclear Regulatory to immediately notify the nation's 102 other commercially operated nuclear plants to be on the lookout for a similar problem.

"It's a very significant degradation of the vessel head," Jan Strasma, NRC spokesman, said. "It's certainly very unusual. ... Certainly, it's a deterioration of a very important safety feature."

None of the cracks caused an immediate threat to the public, because Davis-Besse is shut down for normal refueling and maintenance since Feb. 16.

The utility had hoped to restart the plant by the end of March, but said the corrosion problem - to be addressed by a team of 50 scientists, technicians, and nuclear experts from all parts of the country - will keep the plant idle until at least late April.

Mr. Strasma gave no assurances that the utility will be able to stick to that schedule.

"They'll develop a repair procedure and we'll see if that is sufficient to deal with the problem," he said.

Trace amounts of boric acid, which the reactor creates during the nuclear fission process, are believed to have dribbled for a long time - possibly years - from at least one of 69 extended, vertical tubes called control rod drive mechanism nozzles.

The nozzles, which operators use to maneuver control rods and keep the plant running safely, are permanently implanted into the reactor head and are supposed to be welded airtight.

Somehow, acid escaped.

It burned through all six inches of carbon steel that forms the outer layer of the reactor head and made contact with the stainless steel on the cap's innermost side. The stainless steel layer is only about three-eighths of an inch thick- about the size of an eraser head - but the corrosion did not penetrate it because that type of metal is impervious to boric acid, said Richard Wilkins, FirstEnergy spokesman.

The corrosion created a cavity four inches wide and five inches long around the most troublesome nozzle, he said.

"We were not expecting to see that extent of corrosion," he said. "This has not been seen in the industry before."

FirstEnergy is repairing five of the 69 reactor nozzles which are believed to have cracked sometime during the plant's 25-year history.

Two are now thought to be leaking - something which wasn't previously known.

The reactor head's corrosion appears linked to at least one of those two leaking nozzles or to aging weld seams surrounding them, Mr. Wilkins said.

Even if it had been operating, the reactor does not make direct contact with the environment. It is sealed off in a separate building designed to trap radiation that might escape from the vessel.

The utility also believes operators would have had plenty of time to shut down the plant if an emergency had arisen. There was never any hint of a problem: The plant ran at full power for months, without any noticeable loss in reactor pressure, Mr. Wilkins said.

The NRC wants the cracks repaired so that they do not ever impede efforts to shut down the plant.

FirstEnergy plans to install a new reactor head during the plant's next refueling outage in 2004, complete with 69 new nozzles, Mr. Wilkins said.

The reactor head cannot be installed now, because it will take months to build it and transport it to Davis-Besse, he said.

Projects of that magnitude can cost as much as $20 million, according to the Nuclear Energy Institute in Washington. The utility has declined to say how much it expects to spend on that or on its immediate repairs.

The plant's 40-year license expires in 2017, but company officials notified the government several months ago that they intend to seek a 10-year extension.

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