Nuclear power station and the island on which it is situated, in the Susquehanna
River near Harrisburg, Pa., U.S. The most serious accident in
the history of the American
nuclear power industry occurred there in 1979. At 4:00 AM on
March 28, an automatically operated valve in the Unit 2 reactor
mistakenly closed, shutting off the water supply to the main
feedwater system (the system that transfers heat from the water
actually circulating in the reactor core). This caused the reactor
core to shut down automatically, but a series of equipment and
instrument malfunctions, human errors in operating procedures,
and mistaken decisions in the ensuing hours led to a serious
loss of water coolant from the reactor core. As a result, the
core was partially exposed, and the zirconium cladding of its
fuel reacted with the surrounding superheated steam to form
a large accumulation of hydrogen gas, some of which escaped
from the core into the containment vessel of the reactor building.
Very little of this and other radioactive gases actually escaped
into the atmosphere, and they did not constitute a threat to
the health of the surrounding population. In the following days
adequate coolant water circulation in the core was restored.
The accident at Three Mile Island, though miniscule in its
health consequences, had widespread and profound effects on
the American nuclear power industry. It resulted in the immediate
(though temporary) closing of seven operating reactors like
those at Three Mile Island. A moratorium on the licensing of
all new reactors was also temporarily imposed, and the whole
process of approval for new plants by the Nuclear Regulatory
Commission was significantly slowed for years after the accident.
No new reactors were ordered by utility companies in the United
States from 1979 through the mid-1980s. The accident increased
public fears about the safety of nuclear reactors and strengthened
public opposition to the construction of new plants. The unharmed
Unit 1 reactor at Three Mile Island did not resume operation
until 1985. The cleanup of Unit 2 continued until 1990; damage
to the unit was so severe, however (52 percent of the core melted
down), that it remained unusable.