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Three Mile Island

Three Mile Island is an island in the Susquehanna River in Dauphin County, Pennsylvania, near Harrisburg, of area 3.29 kmē (814 acres). The name is most commonly associated with an accident at the Three Mile Island Nuclear Generating Station on March 28, 1979, when reactor TMI-2 suffered a partial core meltdown.

The Three Mile Island generation station consists of two pressurized light-water reactors built by Babcock & Wilcox[?] with electrical design capacities of 786 MW (TMI-1) and 900 MW (TMI-2). The plant was at the time run by Metropolitan Edison[?] Company. As of early 2003, TMI-1 was fully operational (operator: AmerGen Energy Co., LLC) and scheduled for decommissioning in 2014.

No injuries due to radiation occurred, but it was a serious economic and public relations disaster. It also furthered a serious decline in the public popularity of nuclear power. By coincidence, the event occurred a mere three days after the release of the movie The China Syndrome, which portrayed a similar fictional incident.

Three Mile Island nuclear accident

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 pressurizer 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, the stuck-open valve caused the pressure to continue to decrease in the system.

Meanwhile, another problem appeared elsewhere in the plant. The emergency feedwater system (backup to main feedwater) was tested 42 hours prior to the accident. As part of the test, a valve is closed and then reopened at the end of the test. But this time, through either an administrative or human error, the valve was not reopened - - preventing the emergency feedwater system from functioning. The valve was discovered closed about eight minutes into the accident. Once it was reopened, the emergency feedwater system began to work correctly, allowing cooling water to flow into the steam generators.

As the system pressure in the primary system continued to decrease, voids (areas where no water is present) began to form in portions of the system other than the pressurizer. Because of these voids, the water in the system was redistributed and the pressurizer became full of water. The level indicator, which tells the operator the amount of coolant capable of heat removal, incorrectly indicated the system was full of water. Thus, the operator stopped adding water. He was unaware that, because of the stuck valve, the indicator can, and in this instance did, provide false readings.

After almost eighty minutes of slow temperature rise the primary loop pumps begin to shudder as steam rather than water began to pass through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system locked the primary loop, and as the water stopped circulating it was converted to steam in increasing amounts. After around 130 minutes since the first malfunction, the top of the reactor core was exposed and the heat and steam drove a reaction involving hydrogen and other radioactive gases with the zirconium rod cladding. The quench tank ruptured, and radioactive coolant began to leak out into the general containment building. At 6 a.m. there was a shift change in the control room. A new arrival noticed that the temperature in the holding tanks was excessive and used a backup valve to shut off the coolant venting. Around 250,000 gallons of coolant had already been lost from the primary loop. It was not until 165 minutes after the start of the problem that radiation alarms activated as contaminated water reached detectors, by which time the radiation levels in the primary coolant water were around 300 times expected levels.

It was still not clear to the control room that the primary loop water levels were low and that over half of the core was exposed. A group of workers took manual readings from the thermocouples and obtained a sample of primary loop water. Around seven hours into the emergency, new water was pumped into the primary loop. The backup release valve was opened to reduce pressure. At around nine hours the hydrogen within the reactor exploded, but this was largely unnoticed. After almost sixteen hours the primary loop pumps were turned back on and the core temperature began to fall. A large part of the core had either melted or vapourized, and the system was still dangerously radioactive. Over the next week the steam and hydrogen were removed from the reator using a recombiner and, more controversially, by venting straight to the atmosphere. It is estimated that 2.5 million curies of radioactive gas were released by the accident.


Three Mile Island has been of interest to human factor engineers[?] as an example of how groups of people react and make decisions under stress. There is general consensus that the accident was execerbated by incorrect decisions made because the operators were overwhelmed with information, much of it irrelevant, misleading, or incorrect. As a result of the TMI, nuclear reactor operator training has been changed. Before TMI, operator training focused on diagnosing the underlying problem. After TMI, operating training has focused on reacting to the emergency by going through a standardized checklist to ensure that the core is receiving enough coolant.

The reactor cleanup started in August 1979 and officially ended in December 1993 at a cost of around $975 million. From 1985 to 1990 almost 100 tonnes of radioactive fuel were removed from the site. TMI-1 was restarted in 1985.

Note: the accident at the plant occurred a few days after the release of the movie "The China Syndrome", which featured Jane Fonda as a newsanchor at a California TV station. She is doing a series on nuclear energy and while she and her cameraman are at the plant, an accident almost happens, and she attempts to raise awareness of how unsafe the plant is. During one scene, she is talking with a nuclear safety expert, who ironically says that a meltdown could force an area "the size of Pennsylvania" to be evacuated. In another irony, the fictional near accident in the movie also occurred when plant operators misunderstood the amount of water within the core.

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