|Accident at Three Mile Island
Prior to March 28, 1979, few people
living outside of Central Pennsylvania had ever
heard of Three Mile Island. After that date,
however, just about anyone with access to a
television, radio, or newspaper could take a map
of Pennsylvania and point to a sandbar in the
middle of the Susquehanna River and know that
that was the site of the nation's worst
commercial nuclear reactor accident.
The accident did not get as far
as the ultimate nightmare of meltdown, in which a
liquefied core of uranium drops through its
shielding into the ground and spreads large
amounts of radiation haphazardly across the
countryside. And, although no one died as a
direct result of the accident, it led to a major
shake-up at the Nuclear
and a host of better safety measures at nuclear
power plants across the country. It also led
communities near those plants to draw up detailed
plans to deal with future emergencies.
A Timeline of the Accident
Maintenance workers inadvertently shut off the
water supply carrying heat from the reactor to
the turbine that generates electricity. With no
water being added, steam pressure drops and
safety systems automatically shut down turbine
and the generator it powers.
4:00:02 am Steam flow
reduction raises temperature of reactor coolant,
causing water to expand.
4:00:08 am Pressure
continues to rise inside pressurizer. Emergency
systems shut down reactor.
4:00:09 am Nuclear
reaction stops, but decay heat causes water to
continue to heat up inside the reactor. Because
pumps that normally supply the steam generators
shut down, three emergency pumps automatically
start. Compressed steam in the reactor causes the
pilot-operated relief valve to open. The valve,
which should have closed when pressure decreased,
remains open for 2 hours 22 minutes. Meanwhile,
steam and water flow out of the reactor coolant
system through a drain pipe to a relief tank.
~4:02:00 am Operators
in the control room fail to recognize that the
emergency relief valve was stuck in the open
position. Their instruments seem to indicate that
water is completely flooding the reactor, so they
override the automatic system that had started
the high-pressure water pumps, depriving the core
of cooling water. Although the reactor has been
shut down, atomic reactions inside it continue to
produce heat. The reactor's temperature soars
above 2000° F.
4:05:30 am Steam inside
the reactor displaces water and eventually leaves
Metropolitan Edison issues a "general
emergency" at the plant.
The Nuclear Regulatory Commission dispatches
experts to the scene. Their monitors pick up
signs of radioactivity in the air around the
site, caused by vented steam.
by 8:00 pm
Plant operators have managed to bring the core's
temperature down to about 280° F, but the core
has already been damaged, water has ruptured an
overflow system, and radioactive steam has
escaped into the atmosphere.
Pennsylvania Highways www.pahighways.com/features/threemileisland.html
Three Mile Island Alert www.tmia.com
Three Mile Island: The Inside Story
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