During normal operations, the large pressure vessel (7) that held the reactor core was always filled to the top with water. Other instruments available to plant staff provided inadequate or misleading information. As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident. As a result, the plant staff was unaware that cooling water in the form of steam was pouring out of the stuck-open valve. Instruments in the control room, however, indicated to the plant staff that the valve was closed. The valve should have closed when the pressure fell to proper levels, but it became stuck open. It was located at the top of the pressurizer (6). In order to control that pressure, the pilot-operated relief valve (5) opened. Immediately, the pressure in the primary system (the nuclear piping portion of the plant shown in orange) began to increase. This caused the plant's turbine-generator (4) and then the reactor itself to automatically shut down. Either a mechanical or electrical failure prevented the main feedwater pumps-component (1) in the animated diagram)-from sending water to the steam generators (2) that remove heat from the reactor core (3). on Wednesday, March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site).
reactor safety.Ī combination of equipment malfunctions, design-related problems and worker errors led to TMI-2's partial meltdown and very small off-site releases of radioactivity. All of these changes significantly enhanced U.S. It also caused the NRC to tighten and heighten its regulatory oversight. Its aftermath 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. commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public. This was the most serious accident in U.S. The Three Mile Island Unit 2 reactor, near Middletown, Pa., partially melted down on March 28, 1979.
Backgrounder on the Three Mile Island Accident
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