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Moments in NRC History: Three Mile Island
On March 28, 1979, the United States suffered its most serious commercial nuclear power
plant accident at the Three Mile Island facility
near Harrisburg, Pennsylvania. The accident released
very small amounts of radioactivity and had negligible effects on public health and the
environment, but it brought about sweeping changes at the Nuclear Regulatory Commission.
The NRC upgraded its regulations related to everything
from operator training, plant design and operations, and emergency response planning.
The lessons of Three Mile Island still resonate today.
Hi, I’m Tom Wellock, the historian for the U.S. Nuclear Regulatory Commission. I study
nuclear power’s history for its relevance to the NRC today and to capture important
insights for future generations. Three Mile Island—which
we usually just call TMI—taught the NRC a lot.
The accident started around 4 o’clock in the morning when pump failures and improperly
closed valves prevented the steam generators from
removing heat produced by the chain reaction in the
nuclear reactor. Removing this heat is essential to prevent reactor damage, and cooling water
is pumped through the reactor and the steam generators
to do so.
When the cooling water stopped flowing to the steam generators, the reactor automatically
shut down – just as it was designed to do in
such a situation. Unfortunately, relief valves that help
lower pressure in the reactor didn’t close properly. Pressure continued to drop and cooling
water escaped from the reactor. Instruments that
should have helped the reactor operators understand the situation provided confusing information,
including false indications that the relief valves
were closed. As a result, the operators did not understand why pressure was falling and
some of their subsequent actions made matters worse
when they erroneously starved the reactor fuel of
cooling water. The fuel began to overheat.
Eventually, the nuclear fuel in the reactor overheated so much the zirconium cladding—long
metal tubes that hold the nuclear fuel pellets—disintegrated and reacted with the steam to create
hydrogen gas. The fuel pellets inside tumbled out and began to melt at over 5000o F. It
was later determined that about half the uranium pellets
melted during the early stages of the accident.
The NRC dispatched a team of experts as soon as news of the accident reached authorities.
Other agencies, such as the Department of Energy
and the Environmental Protection Agency, also mobilized response teams. By midday, TMI’s
owner and the Department of Energy sampled radioactivity in the atmosphere above the
plant.
By evening, the operators had restored cooling to the core, and the reactor appeared stable.
But within a short time, new worries emerged about
the hydrogen created when the zirconium cladding reacted with steam. A large bubble
of hydrogen was lodged at the top of the pressure vessel, the steel container that holds the
reactor core. Experts worried that the hydrogen bubble
might explode, rupturing the pressure vessel and the containment building allowing radiation
to escape. The state’s governor advised pregnant
women and young children within a five-mile radius of the plant to leave the area. The
crisis ended when experts determined on Sunday, April
1, that the bubble could not burn or explode because of the absence of oxygen in the pressure
vessel.
Today, the TMI Unit 2 reactor is permanently shut down. The reactor coolant system is drained,
the radioactive water has been decontaminated and evaporated. The radioactive waste, reactor
fuel, and debris from the core has been shipped off?site.
In the months after the accident, thousands of environmental samples of air, water, vegetation,
soil, and food were collected. Thorough assessments by respected organizations concluded the
actual release of radiation was small and had negligible effects on the physical health
of individuals and the environment.
Nevertheless, the NRC made extensive regulatory changes. New requirements aimed at reducing
human errors affected plant design, human performance standards, and fitness-for-duty
programs. The NRC also expanded incident response staffing, emergency planning, and its
resident inspector program.
The vigilance of the NRC today can be directly tied to the events of 33 years ago. That’s
what makes history so important – it’s not
just about events of the past, but also how it effects the
present and future.
Thanks for watching.