It started routinely enough for the midnight shift at the Davis-Besse Nuclear Power Plant that early morning last June. One reactor operator was snacking in the kitchen, the other reading manuals at his desk as the giant turbines spun out electricity for the farmhouses and other homes of northwest Ohio.

But the tranquility of the control room was pierced at 1:35 a.m. by the whine of a dying turbine, then the thud of a reactor shutdown.

For the next 29 minutes, experts say, Davis-Besse came as close to a meltdown as any U.S. nuclear plant since the Three Mile Island accident of 1979. Faced with a loss of water to cool the reactor and the improbable breakdown of 14 separate components, operators performed a rescue mission noted both for skill and human foible: They pushed wrong buttons, leaped down steep stairs, wended their way through a maze of locked chambers and finally saved the day last June 9 by muscling free the valves and plugging fuses into a small, manually operated pump not designed for emergency use. It resupplied water to the overheating system temporarily, allowing technicians to restore the plant's normal safety mechanisms.

Federal investigators later described the mission as a "high stress situation," which, had it not been checked, could have led to core damage in as little as another hour. No radiation was released, and no major damage was done to the facility 21 miles east of Toledo.

Officials at Toledo Edison Co., which operates Davis-Besse, said the plant was never in danger of a core meltdown. Even though the facility has been shut down by the Nuclear Regulatory Commission for the past 11 months, they noted, it has never been branded unsafe.

But, in the wake of the deadly Chernobyl accident in the Soviet Union and amid the refrain of some U.S. officials that "it could never happen here," the close call at Davis-Besse has raised questions about the safety of American plants and the effectiveness of the NRC in enforcing safety standards.

Six years earlier, the NRC had identified weaknesses in Davis-Besse's cooling water system and urged Toledo Edison to bolster plant safety with an extra emergency pump. The commission had calculated before the mishap that failure of the system posed a risk of core meltdown five times greater than what was considered acceptable, the NRC told Congress last year.

The 860-megawatt reactor at Davis-Besse was similar to that at Three Mile Island, and the NRC had concluded that defects in the emergency cooling water system contributed to the partial core meltdown there.

On June 9, 1985, however, Davis-Besse still had not installed the additional emergency pump.

It is not certain whether an extra pump would have made a difference, given the conclusion by a special NRC task force that the "underlying cause" of the accident was the utility's "lack of attention to detail in the care of plant equipment."

Davis-Besse had long been known as a poorly managed plant, receiving the lowest ranking in 5 of 11 categories surveyed by NRC inspectors in a 1984 report. In the six months before the June 9 mishap, the plant suffered three temporary losses of cooling water, six times the average for such failures at U.S. facilities. A week before the accident, the two main pumps shut off unexpectedly, and some operators were "uneasy" about running the plant at full power before the problem was diagnosed and resolved, according to the task force.

Yet Davis-Besse, with NRC consent, was running close to full power when the main cooling pumps jammed.

"Many people in the industry and the NRC knew of the weaknesses in that plant for years," said NRC member James K. Asselstine. "You had a breakdown of the regulatory process for Davis-Besse. When we began to see poor performance, the NRC should have insisted upon rapid improvement, and failing that improvement, should have shut down that plant until corrective actions were taken.

"There's no question that plant posed an undue risk. Anytime you have widespread failures of safety systems and equipment, you have the potential for much more serious accidents."

Toledo Edison was fined $900,000 by the NRC and was ordered to upgrade its plant. In the past year, it has spent $100 million in safety improvements, senior vice president Joe Williams Jr. said.

Williams said Toledo Edison balked at NRC recommendations for another emergency pump -- its cost was estimated at $1.2 million in 1981 -- because the utility had modified equipment at the recommendation of its consultants.

But, at the time of the mishap, he said, "all the forward-looking management things came a cropper. There's no question the extra pump would have been a handy device to have had."

Davis-Besse is equipped with a pressurized water reactor. Inside, uranium atoms split and create intense heat transferred to water covering the reactor core. This "primary" reactor water flows through thousands of narrow metal tubes that form the steam generator.

Cool water is simultaneously pumped, or fed over the hot tubes, producing steam that turns the turbines to generate electricity.

The "feedwater" is important not only to create steam but to cool the primary water, which flows back to the core. Unless heat is removed in this fashion, the reactor will overheat and eventually melt down. Even after the plant shuts down, residual heat must be removed to prevent core damage.

On June 9, Davis-Besse's two main feedwater systems malfunctioned, automatically shutting down the reactor. An operator, trying to turn on one of the two emergency feedwater pumps, hit the wrong buttons. His mistake prevented the backup system from working.

"At this point," the NRC report said, "things in the control room were hectic. The plant had lost all feedwater; reactor pressure and temperature were increasing, and a number of unexpected equipment problems had occurred.

"The seriousness of the situation was fully appreciated."

Without feedwater, the reactor core temperature rose. If no corrective actions had been taken, the reactor core would have lost its protective covering of water in 41 minutes, exposing it to danger. Certain remedial actions could extend the time 30 minutes.

The Davis-Besse crew managed to recover 12 minutes after the feedwater stopped. Two workers dashed to the emergency pump room in hope of manually restarting the system. The pumps were three floors below the control room in a locked chamber. The slower operator, lagging 10 feet behind his colleague, tossed him the padlock key to speed his entry, the report said.

Inside the pump room, they were unable to get the equipment started because they had never performed the task, the report said.

The assistant shift supervisor, meanwhile, decided to try another backup system. A small, motor-driven pump, used to restart the plant after shutdowns, could be put into service to cool the steam generators temporarily while operators worked on the emergency feedwater system.

But the startup pump had to be assembled. That required turning four valves in different rooms and inserting fuses, a job that normally took 15 to 20 minutes.

On June 9, the pump was started in four minutes.

By 1:51 a.m., the startup pump began sending water to the generators. One of the emergency pumps was restarted two minutes later. At 2:04 a.m., the plant was "essentially stable," the report said, despite failure of eight valves, two pumps, nuclear instrumentation and other equipment.

"It wasn't even close to a meltdown," Williams said. "If the operators had sat on their duffs and done nothing for 30 minutes or so, you'd have had some serious problems. But except for the wrong buttons, the operators did exactly what they were supposed to do."

Nevertheless, the feedwater breakdown jarred members of Congress. Toledo Edison had failed to install the extra emergency pump that many lawmakers until corrective actions were taken.

"There's no question that plant posed an undue risk. Anytime you have widespread failures of safety systems and equipment, you have the potential for much more serious accidents."

Toledo Edison was fined $900,000 by the NRC and was ordered to upgrade its plant. In the past year, it has spent $100 million in safety improvements, senior vice president Joe Williams Jr. said.

Williams said Toledo Edison balked at NRC recommendations for another emergency pump -- its cost was estimated at $1.2 million in 1981 -- because the utility had modified equipment at the recommendation of its consultants.

But, at the time of the mishap, he said, "all the forward-looking management things came a cropper. There's no question the extra pump would have been a handy device to have had."

Davis-Besse is equipped with a pressurized water reactor. Inside, uranium atoms split and create intense heat transferred to water covering the reactor core. This "primary" reactor water flows through thousands of narrow metal tubes that form the steam generator.

Cool water is simultaneously pumped, or fed over the hot tubes, producing steam that turns the turbines to generate electricity.

The "feedwater" is important not only to create steam but to cool the primary water, which flows back to the core. Unless heat is removed in this fashion, the reactor will overheat and eventually melt down. Even after the plant shuts down, residual heat must be removed to prevent core damage.

On June 9, Davis-Besse's two main feedwater systems malfunctioned, automatically shutting down the reactor. An operator, trying to turn on one of the two emergency feedwater pumps, hit the wrong buttons. His mistake prevented the backup system from working.

"At this point," the NRC report said, "things in the control room were hectic. The plant had lost all feedwater; reactor pressure and temperature were increasing, and a number of unexpected equipment problems had occurred.

"The seriousness of the situation was fully appreciated."

Without feedwater, the reactor core temperature rose. If no corrective actions had been taken, the reactor core would have lost its protective covering of water in 41 minutes, exposing it to danger. Certain remedial actions could extend the time 30 minutes.

The Davis-Besse crew managed to recover 12 minutes after the feedwater stopped. Two workers dashed to the emergency pump room in hope of manually restarting the system. The pumps were three floors below the control room in a locked chamber. The slower operator, lagging 10 feet behind his colleague, tossed him the padlock key to speed his entry, the report said.

Inside the pump room, they were unable to get the equipment started because they had never performed the task, the report said.

The assistant shift supervisor, meanwhile, decided to try another backup system. A small, motor-driven pump, used to restart the plant after shutdowns, could be put into service to cool the steam generators temporarily while operators worked on the emergency feedwater system.

But the startup pump had to be assembled. That required turning four valves in different rooms and inserting fuses, a job that normally took 15 to 20 minutes.

On June 9, the pump was started in four minutes.

By 1:51 a.m., the startup pump began sending water to the generators. One of the emergency pumps was restarted two minutes later. At 2:04 a.m., the plant was "essentially stable," the report said, despite failure of eight valves, two pumps, nuclear instrumentation and other equipment.

"It wasn't even close to a meltdown," Williams said. "If the operators had sat on their duffs and done nothing for 30 minutes or so, you'd have had some serious problems. But except for the wrong buttons, the operators did exactly what they were supposed to do."

Nevertheless, the feedwater breakdown jarred members of Congress. Toledo Edison had failed to install the extra emergency pump that many lawmakers regarded as the best defense against another Three Mile Island.

And, the NRC was accused of standing idly by while, in the words of Rep. Edward J. Markey (D-Mass.), "Davis-Besse was an accident waiting to happen."

Markey, chairman of the House Energy and Commerce energy conservation and power subcommittee, blamed the commission for "benign neglect" in failing to force changes at Davis-Besse despite knowledge of its management problems and the risks posed by "design defects."

The NRC staff first recommended the additional backup pump in July 1979. The commission had shut down the plant after the Three Mile Island accident because of their similar reactors. When Davis-Besse was allowed to reopen, the NRC urged the utility to put in an extra pump.

An internal memorandum signed Aug. 8, 1980, by the NRC's director of safety technology said a new pump should be the "highest priority" for Davis-Besse and installed "as soon as possible by issuing an order."

The memorandum, noting that Toledo Edison was still pondering the 1979 recommendation, added, "This is too long a time to merely study such an important issue."

In December 1981, the utility presented the NRC with a consultant's study rejecting a new pump in favor of improvements to the existing emergency system. The commission staff took 14 months to review the analysis, then another 14 months to respond to Toledo Edison.

The NRC's reply, April 23, 1984, reaffirmed its stance: The existing system was not sufficiently reliable and a new pump would improve safety.

Toledo Edison agreed in November 1984 to put in a new motor-driven pump -- but not until the plant was closed for refueling in the spring of 1986.

Frank Miraglia, the NRC's director of pressurized water reactor licensing, acknowledged in an interview that the staff "should have handled it a bit more expeditiously." At the time, he said, "we didn't assign it the priority we should have."

He said the staff was then confronted with a "tough call." Toledo Edison was arguing that the new pump would be costly while adding only "incremental reliability."

"Events and history have shown we should have done something sooner and we didn't," Miraglia said.