AK HARBOR, Ohio, April 5 — The Nuclear Regulatory Commission today placed the blame for extensive corrosion discovered in the lid of a nuclear reactor squarely on the power company that operates the reactor.

A commission report cites several missed opportunities for officials at the company, the FirstEnergy Corporation, and its Davis-Besse plant, east of Toledo, to have detected the corrosion as much as four years ago.

The Davis-Besse staff had information that could have resulted in identification of the problem before it became a significant issue, the report said. In addition, had the Davis-Besse staff properly carried out programs required by the commission, "this problem would have been prevented," said John A. Grobe, director of the commission's reactor safety division.

The report, presented today to an audience of 300 area residents assembled in the local high school, detailed a lavalike boric acid buildup that had accumulated for so long that it had to be pried off with crowbars.

FirstEnergy officials did not dispute the findings and took full responsibility for the corrosion, the worst reported case at a nuclear power plant in United States history.

"We are clearly responsible for this condition of the reactor head," Robert F. Saunders, president of FirstEnergy's nuclear division, said.

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Mr. Saunders qualified this admission with an explanation that FirstEnergy was a "learning organization" and would inevitably encounter occasional problems. His remarks were interrupted occasionally by agitated members of the audience who shouted, "Shut it down!"

Fred Cohn, 74, of Curtis, Ohio, asked officials of the nuclear agency and FirstEnergy, "How can you people tell us you're learning, you're learning, you're learning? Don't you think this is a pretty expensive way to learn at our expense?"

The corrosion was detected in a routine shutdown for refueling in February. The plant, which has not reopened since, will not reopen for several more months at least while plant officials determine whether to repair or replace the reactor head.

Both the nuclear agency and FirstEnergy maintained that the public was never in danger as a result of the corrosion. The worst-case scenario, the agency said, would have been a "radiological mess" contained within the reactor building.