- OAK HARBOR, Ohio (AP) -- 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 fibre 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-sized emergency diesel
generators provided power to avert a total plant blackout, but temperatures
in the room housing one generator rose about one degree Celsius 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 seven-metre-deep pool that cools spent reactor fuel.
The temperature reached 60 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 weather. The U.S. 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 130-145 km-h 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 centre,
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 260 km-h.
-
- 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 worst nuclear accident in the United States.
-
- 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.
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- Davis-Besse managers said the emergency
proved the value of their frequent accident drills.
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