Damage to the reactor head of the Davis-Besse power plant in Ohio ranks among the top five most serious nuclear plant accidents or near-accidents since Three Mile Island, the Nuclear Regulatory Commission said Monday.

Davis-Besse, along Lake Erie in northwest Ohio, was closed for two years after inspectors found corrosion on the reactor in March 2002. Leaking boric acid almost had eaten through a 6-inch-thick steel cap; repairs cost $600 million.

While the plant was shut down, engineers found that its undersized sump could have become clogged with debris during an accident, which choked off the flow of water to cooling pumps, said an NRC analysis released Monday.

Federal regulators estimated there were six chances in a 1,000 that the plant could have experienced a meltdown during the year before it was shut down for routine maintenance in February 2002.

Normally, the risk of an accident happening at Davis-Besse is about six in 100,000, NRC spokesman Scott Burnell said. The NRC considers the risk "significant" when circumstances at a plant bring the possibility of core damage within one chance in 1,000.

Richard Wilkins, a spokesman for plant operator FirstEnergy Corp., said the agency's analysis assumes that all pumps and safety systems would fail, which is highly unlikely. Still, he said the conditions that led to the NRC's risk estimate was unacceptable.

Since the 1979 accident at Pennsylvania's Three Mile Island, only three other events at nuclear plants had a higher probability of causing a meltdown than the corrosion at Davis-Besse, the report said. These events were:

-The 1985 breakdown of feedwater pumps necessary to cool the nuclear core at the Davis-Besse plant, a seven out of 100 chance of core damage;

-The 1981 failure to close quickly of three main steam isolation valves at the Brunswick plant near Southport, N.C., which had a nine out of 1,000 risk factor; and

-The 1991 unavailability of a high-pressure injection pump at the Shearon Harris plant southwest of Raleigh, N.C., which had a risk rating of a little more than six out of 1,000.

Two other events in the last decade have had about the same risk factor as the reactor head damage at Davis-Besse: the draining of reactor coolant at the Wolf Creek plant near Burlington, Kan., during a 1994 maintenance outage; and the loss of offsite power at the Catawba plant near Rock Hill, S.C., in 1996.

Even if Davis-Besse's core had been damaged, its containment vessel and other safety systems would have protected the public from a radioactivity release, the NRC report said.

Paul Gunter, a nuclear expert at the watchdog group Nuclear Information and Resource Service, said the analysis shows how close the plant came to a serious accident.

"You don't have to be a rocket scientist to look at this and realize how fast and loose and reckless both the industry and regulator played with public safety here," Gunter said.

The NRC report also corrects previous estimates that the Davis-Besse plant's reactor head could have continued to operate safely for two to 13 months after it was shut down in 2002. Regulators now say the plant would have been safe to operate for two to 22 months.