United States Nuclear Regulatory Commission
Office of Public Affairs
Washington DC 20555
The Accident At Three Mile Island
The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near Middletown,
Pennsylvania, on March 28, 1979, was the most serious in U.S. commercial nuclear power plant
operating history(1), even though it led to no deaths or injuries to plant workers or members of the nearby
community. But it brought about sweeping changes involving emergency response planning, reactor
operator training, human factors engineering, radiation protection, and many other areas of nuclear power
plant operations. It also caused the U.S. Nuclear Regulatory Commission to tighten and heighten its
regulatory oversight. Resultant changes in the nuclear power industry and at the NRC had the effect of
The sequence of certain events - - equipment malfunctions, design related problems and worker errors - -
led to a partial meltdown of the TMI-2 reactor core but only very small off-site releases of radioactivity.
Summary of Events
The accident began about 4:00 a.m. on March 28, 1979, when the plant experienced a failure in the
secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, caused by either
a mechanical or electrical failure, which prevented the steam generators from removing heat. First the
turbine, then the reactor automatically shut down.
Immediately, the pressure in the primary system (the
nuclear portion of the plant) began to increase. In order to prevent that pressure from becoming excessive,
the pilot-operated relief valve (a valve located at the top of the pressurizer) opened. The valve should
have closed when the pressure decreased by a certain amount, but it did not. Signals available to the
operator failed to show that the valve was still open. As a result, cooling water poured out of the stuck-
open valve and caused the core of the reactor to overheat.
As coolant flowed from the core through the pressurizer, the instruments available to reactor operators
provided confusing information.
There was no instrument that showed the level of coolant in the core.
Instead, the operators judged the level of water in the core by the level in the pressurizer, and since it was
high, they assumed that the core was properly covered with coolant.
In addition, there was no clear signal
that the pilot-operated relief valve was open.
As a result, as alarms rang and warning lights flashed, the
operators did not realize that the plant was experiencing a loss-of-coolant accident.
They took a series of
actions that made conditions worse by simply reducing the flow of coolant through the core.