The atomic power plant's control room grew dim. Instrument lights flashed red as the storm raging outside knocked out two of three power lines. Then came the urgent warning from security: Cameras showed three distinct funnel clouds taking dead aim.
Moments later the last power link was cut, and the control room briefly went dark. The plant computer failed. A "red phone" hot line to Washington went dead.On June 24, the Davis-Besse Nuclear Station on Lake Erie took the worst direct hit by a tornado ever weathered by a U.S. atomic plant. The 900-megawatt reactor shut down automatically and no radiation leaked, largely because of the staff's quick, competent responses and backup equipment.
But for 41 tense hours, an array of equipment problems complicated efforts to keep the reactor's radioactive fuel core cool and to communicate inside and outside the plant. Among the problems:
- The tornado severed fiber optic lines and knocked out the plant's main phone system. The federal hot line, linked directly to the plant, also failed. A microwave-based phone system provided limited service but slowed plant officials' efforts to notify local and state emergency managers that they had declared an alert - the second of four increasingly serious emergency classifications.
- Because of a faulty switch, a key safety display went black for two hours immediately after the tornado. Heat knocked it out again the next afternoon.
- Two locomotive-size emergency diesel generators provided power to avert a total plant blackout, but temperatures in the room housing one generator rose 2 degrees over the operating limit when a vent got stuck the afternoon after the storm.
"For a few minutes your heart goes up into your throat," said Bob Donnellon, the plant's emergency director when the generator alarm sounded. "But you have a gut feeling that it's OK. Your guys confirm it. Your comfort level comes back a bit."
Hours later, as the plant was switching back to offsite power, the second generator shut down a few seconds early because of a faulty relay.
Plant managers also worried about rising temperatures in a 23-foot-deep pool that cools spent reactor fuel. The temperature reached 140 degrees - roughly the point where evaporation would increase - but enough offsite electricity returned to power the cooling pumps.
In the end, public safety was never threatened, according to the Nuclear Regulatory Commission. The NRC, which activated its emergency command post outside Washington and alerted the White House and other U.S. and international agencies, is preparing a report on the equipment problems. The agency's senior inspector at Davis-Besse, Steve Campbell, said he was aware of no operator error throughout the episode.
An industry watchdog group complimented the plant staff.
"They were faced with a lot of challenges they shouldn't have had to face, and they met those challenges very well," said David Lochbaum of the Union of Concerned Scientists. "They're trying to gain all the lessons learned from this event to make things even better in the future."
One lesson will be to alert plant workers about threatening weath-er. The National Weather Service issued a severe thunderstorm warning at 7:48 p.m. A tornado warning cleared at 8:44 p.m. With winds of 80-90 mph on the leading edge of the storm knocking out power all over Ottawa County, a regional dispatcher for Toledo Edison neglected to pass along both warnings to the Davis-Besse control room.
Inside the fortified command center, workers could not hear the roaring winds or see the sickly yellowish sky. Their first warning was a phone call from an off-duty colleague who lives nearby, at 8:36 p.m., NRC spokesman Jan Strasma said.
Less than 10 minutes later, a guard monitoring outside cameras from the plant's security room saw three funnel clouds converging into one over the plant and called the control room, said Jim McGee, the plant supervisor that night.
Davis-Besse security analyst Jim Theisen went outside to check conditions and saw water and debris being sucked out of the base of the plant's cooling tower. "There was large chunks of stuff coming out," he said.
The tornado destroyed the plant's wind gauge; weather service forecaster Larry Gabric estimated the twister's winds at almost 160 mph.
Most seriously, the tornado ripped power cables from their connectors in the electrical switchyard - in essence the fuse box for the entire plant.
In the control room, McGee watched the indicator for the last power link go from green to red. The room briefly went dark except for instruments and emergency lighting.
"For those few seconds everyone's just standing still," McGee said. "And then the diesels come up, and everyone goes on with their tasks."
Even so, as the backup electricity relighted the control room, the safety display stayed black. This was a system the NRC ordered installed in U.S. nuclear plants after the partial meltdown in 1979 at Pennsylvania's Three Mile Island reactor, where control-room confusion contributed to the nation's worst nuclear accident.
The 21-year-old Davis-Besse plant has a similarly designed reactor to Three Mile Island's. It had serious operational problems in the 1980s but is now considered one of the better-run of 107 operating nuclear plants in the country.
Plant officials declared the emergency over at 1:58 p.m. on June 26 after restoring a second power line.
Davis-Besse managers said the emergency proved the value of their frequent accident drills.
"We may complain about the emergency response guys and how much they make us drill, but we don't now," Donnellon said.