PORT CLINTON - The Davis-Besse nuclear power plant for decades had a cooling system flaw that, under a worst-case scenario, could have led to a meltdown of the fuel core, plant owner FirstEnergy has told the Nuclear Regulatory Commission.

The flaw, a wider-than-allowed gap in a sump strainer system, was discovered last September during an inspection by FirstEnergy and reported to the NRC in December. That gap could have allowed debris created during what is called a loss-of-coolant accident to clog an emergency cooling system, rendering it inoperable, the Akron utility's report to the NRC said.

FirstEnergy is spending $2.3 million to replace the old sump system with a larger, state-of-the-art system.

Because of the upgrade, the discovery of the flaw, while not trivial, is not an issue for the plant's restart, said Jack Grobe, an NRC official who heads an oversight panel looking into the troubled Oak Harbor power plant. He and other NRC officials met with FirstEnergy managers Tuesday at Camp Perry as part of a series of regular monthly meetings to get updates on progress being made to repair and refurbish Davis-Besse. The plant has been closed for nearly a year after the discovery that boric acid ate unprecedented cavities on top of the reactor's former vessel head.

The flaw in the old sump system, a ¾-inch-wide-by-6-inch- long gap, was apparently part of the plant's original construction in the 1970s, the FirstEnergy report said. Subsequent inspections of the sump system focused on the grating and did not require close looks at the area where the flaw was, the report said.

``This procedure did not explicitly require inspection to ensure all design bases functions were met,'' the report said.

``For some reason it was not noticed,'' said Jim Powers, director of nuclear engineering at Davis-Besse. Powers, who pushed to have the Davis-Besse sump system redesigned so that it is state of the art for the industry, previously put through sump system improvements at FirstEnergy's Perry nuclear plant.

Grobe said the NRC's inspections focus more on a nuclear reactor's moving parts rather than static safety components such as the sump grating. The flaw in the old Davis-Besse sump does not indicate there is also a flaw in the NRC's inspections, he said.

Sump system safety has been an issue for years in the nuclear power industry, Grobe and others said. The NRC plans to send out a draft regulatory guide in a month about improving sump system requirements. The sumps are intended to recirculate coolant back to the fuel core to prevent a meltdown. If debris caused by the explosive release of steam and coolant from the reactor clogs the sump, the system cannot recirculate the coolant. That in turn could cause the fuel to overheat and begin melting.

The NRC's analysis of the sump system flaw indicates a core meltdown would have been unlikely if the boric acid cavities on top of the old reactor head had burst and released coolant, Grobe said. But if there had been an accident that released a larger amount of coolant, the old sump design may not have been sufficient to protect the reactor, he said.

The 68 other reactors in the United States that are similar in design to Davis-Besse must at least evaluate the sump problem, said David Lochbaum, nuclear expert with the Union of Concerned Scientists.

In related news, FirstEnergy executives said they may begin reloading nuclear fuel as soon as today into Davis-Besse as part of preparations to test the bottom of the reactor for coolant leaks in March.