ETHESDA, Md., April 10 — Officials from an Ohio nuclear power plant assured federal regulators today that they could repair corrosion that had eaten nearly all the way through a reactor lid, but faced a barrage of questions from the Nuclear Regulatory Commission staff.

Executives of the Davis-Besse nuclear plant near Toledo met with commission officials to convince them that they could repair the hole by filling it with a 13-inch stainless steel disk, welded into place.

After a three-hour meeting, the executives left with a long list of questions to answer, including how they would make sure that the heat of a welder's torch would not further damage the metal.

Sixty-eight other reactors around the nation have a design similar to Davis-Besse's, and the commission is trying to determine if any of them have incurred the same kind of corrosion. All 68 have said they did not, but some did not provide enough of a basis for their assurances, said Ken Karwoski, a corrosion specialist with the commission.

At Davis-Besse, which is owned by the FirstEnergy Corporation of Akron, Ohio, cooling water from the reactor leaked from nozzles on the reactor head; boric acid, which is mixed into the water to control the nuclear reaction, ate away about 70 pounds of metal, going through six inches of exterior steel.

When the 25-year-old reactor was shut for refueling and repair of the nozzles this year, all that was left was a thin layer of steel meant to control corrosion inside the vessel.

The regulators were shocked by the extent of the corrosion. Leaks were well known, but government and industry officials believed that when they occurred, the temperature at the vessel head, more than 600 degrees, would boil the water away and leave nothing but a harmless boron powder.

After investigating, the commission staff concluded that the Davis-Besse operators had missed many opportunities to find the problem before it became so serious.

Critics of nuclear power agreed.

"When you're using a crowbar to knock the stuff off the reactor head, it's a sign you've gone too far," said David Lochbaum, a nuclear engineer with the Union of Concerned Scientists. Workers had pried boric acid off the head during a refueling shutdown in 2000.

At the meeting today, about a dozen commission staff members asked about the "repair concept" that company officials presented.

"It's a first-of-a-kind repair," said Brian W. Sheron, associate director for project licensing and technology assessment at the commission. "The staff is very concerned that whatever we approve, they are confident it is going to hold up."

One issue, Mr. Sheron said, was "just the sheer size of the weld" — to hold in place a piece 13 inches in diameter and about 6 inches thick.

FirstEnergy officials said the session had given them a clear indication of what information their plan would need to include to satisfy the commission. The company had hoped to submit that plan next week but company executives said after the session that it might take longer.

If contractors cannot repair the vessel head, the company plans to replace it with the head from a reactor in Midland, Mich., that was abandoned during construction, or the head of a retired plant in Sacramento. They have also ordered a new reactor head, but do not expect delivery before February 2004.

Delays are expensive because the plant employs 780 people, whether or not it generates electricity; property taxes alone run $500,000 a month. Officials hope to have the reactor running by summer.

Opponents say that would be too soon. Christine Patronik-Holder, a spokeswoman for the Safe Energy Communication Council, said that until everyone agreed on exactly how the corrosion occurred, "plans to place patches amount to little more than Russian roulette with the lives of northern Ohioans."

But the company is proceeding to figure out repair details, including how it will check for leaks when the work is completed.

Radiation dosage in the repair area is so high that a welder would absorb in two hours as much radiation as the industry usually allows workers to incur in a year. In two and a half hours, the welder would reach the annual limit the commission sets. So the plan will rely on robot welders.

Indeed, radiation in the affected area is so high that it will be a challenge just to X-ray the completed repairs to look for any flaws. Framatome, the French reactor company that will do much of the work, said it could compensate for the high background radiation.