The accident’s maximum dose to a person at the site boundary would have been less than 100 millirem above background. To put this into context, exposure from a chest X ray is about 6 millirem and the area’s natural radioactive background dose is about 100 125 millirem per year for the area. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. Several independent groups also conducted studies. The NRC conducted detailed studies of the accident’s radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. The following animated diagram graphically depicts the sequence of events associated with the accident at TMI-2. Without the reactor coolant pumps circulating water and with the primary system starved of emergency cooling water, the water level in the pressure vessel dropped and the core overheated.Īnimated Diagram of the Sequence of Events The emergency cooling water being pumped into the primary system threatened to fill up the pressurizer completely-an undesirable condition-and they cut back on the flow of water. The stuck valve reduced primary system pressure so much that the reactor coolant pumps (8) started to vibrate and were turned off. Unaware of the stuck-open relief valve and unable to tell if the core was covered with cooling water, the staff took a series of actions that uncovered the core. As a result, plant staff assumed that as long instruments showed that the pressurizer water level was high enough, the core was properly covered with water too. So there was no need for a water-level instrument to show whether water in the vessel covered the core. 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. Animated Diagram of the Sequence of Events.Printable Version (does not include Animated Diagram of the Sequence of Events) Backgrounder on the Three Mile Island Accident
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