Three Mile Island accident

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Three Mile Island Nuclear Generating Station consists of two nuclear power plants, each with its own containment building and cooling towers. TMI-2, which suffered a partial meltdown, is in the background.
Three Mile Island Nuclear Generating Station consists of two nuclear power plants, each with its own containment building and cooling towers. TMI-2, which suffered a partial meltdown, is in the background.

Three Mile Island Nuclear Generating Station sits on an island of area of 3.29 km² (814 acres) in the Susquehanna River in Dauphin County, Pennsylvania, near Harrisburg. On March 28, 1979, the Unit 2 nuclear power plant (a pressurized water reactor manufactured by Babcock & Wilcox) on Three Mile Island suffered a partial core meltdown. This was the worst accident in US commercial nuclear power generating history. Approximately 25,000 people lived within five miles of the island at the time of the accident.

The accident unfolded over the course of five tense days, as a number of agencies at local, state and federal level tried to diagnose the problem and decide whether or not the on-going accident required a full emergency evacuation of the population. The full details of the accident were not discovered until much later. In the end, the reactor was brought under control. No identifiable injuries due to radiation occurred, and a government report concluded that "the projected number of excess fatal cancers due to the accident... is approximately one". But the accident had serious economic and public relations consequences, and the cleanup process was slow and costly. It also furthered a major decline in the public popularity of nuclear power, exemplifying for many the worst fears about nuclear technology and, until the Chernobyl accident seven years later, it was considered the world's worst civilian nuclear accident.

Three Mile Island nuclear accident

The accident occurred in the TMI-2 reactor. The plant had two reactors; TMI-1 was at the time shut down for refueling. The accident began when the plant's main feedwater pumps in the secondary non-nuclear cooling system failed at exactly 4:00 a.m. on March 28, 1979. This was "to-the-minute" the first anniversary of start up. The exact cause of the failure was never ascertained, although it was speculated that water entered a pneumatic airline that controlled the flow of water through a water filter known as the condensate polishers. In fact, the NRC stated it did not need to know how the accident started since it could have been prevented at many points along the way. The stoppage of water flow through the condensate system caused the steam generators to cease removing heat. First the turbine, then the nuclear 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, located at the top of the pressurizer, opened. The valve should have closed again when the excess pressure had been released but it did not do so. The indication to the plant's operators that the signal to close the valve had been sent was, in the absence of any indication to the contrary, taken by them to mean that the valve had closed. (A "positive feedback" lamp in the control room indicating the true position of the valve was eliminated in original construction to save time but has now been backfitted to all other similar plants.) As a result of this design error, the valve allowed to remain open and caused the pressure to continue to decrease in the system.

The operators did not initially recognize the accident as a Loss of Coolant Accident (LOCA) since they had only ambiguous reactor water level indication and failed to properly interpret other indications of a LOCA.

Meanwhile, another problem appeared elsewhere in the plant with the emergency feedwater system, the main feedwater system's backup. Three emergency feedwater pumps started automatically following the loss of the main feed pumps, but two valves on the emergency feedwater lines were shut, preventing the feedwater from reaching the steam generators. The emergency feedwater system had been tested 42 hours prior to the accident. As part of the test, these valves were closed but should then have been reopened at the end of the test. But on this occasion it appeared that through either an administrative or human error, the valves were not reopened. The President's Commission requested that the FBI investigate whether these valves were closed by an act of sabotage. The valves were discovered closed about eight minutes into the accident. Once they were reopened, emergency feedwater was restored to the steam generators. This lack of emergency feed water for eight minutes did not have a significant effect on the outcome of the accident but did add to the confusion faced by the operators. The restoration of feedwater did not return cooling capability, however, as steam voids that had formed in the primary loop prevented heat transfer from the reactor to the secondary loop via the steam generator. This problem hampered efforts to cool the reactor for days.

As the pressure in the primary system continued to decrease, voids, areas where there is no water present, began to form in portions of the system other than the pressurizer, most notably in the reactor vessel. Because of these voids, the water in the system was redistributed and the pressurizer water level rose while overall system water inventory decreased. Thus the pressurizer level indicator, which tells the operator the amount of coolant capable of heat removal, incorrectly indicated that system water level was rising. This caused the operators to stop adding water by turning off the Emergency Core Cooling pumps, which had automatically started following the initial pressure decrease, due to fears the system was being overfilled. They were unaware that, because of the voids forming in the reactor vessel, the indicator could, and in this instance did, provide false readings. This erroneous indication blinded operators to the fact that water level was dropping and the reactor core was becoming uncovered.

With the PORV still open, the quench tank that collected the discharge from the PORV overfilled, causing the containment building sump to fill and sound an alarm at 4:11 a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that the pressurizer relief valve was stuck open, but these indications were initially ignored by operators. At 4:15 a.m. the quench tank relief diaphragm ruptured, and radioactive coolant began to leak out into the general containment building. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the sump pumps were stopped at 4:39 AM.

Simplified Schematic Diagram of the TMI-2 plant, from the NRC Fact Sheet.
Simplified Schematic Diagram of the TMI-2 plant, from the NRC Fact Sheet.

After almost 80 minutes of slow temperature rise, the primary loop pumps began to cavitate 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. About 130 minutes after the first malfunction, the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the zirconium nuclear fuel rod cladding. This reaction damaged the nuclear fuel rod cladding, which released more radioactivity to the reactor coolant, and produced hydrogen gas which is believed to have caused a small explosion in the containment building later that afternoon. 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 called a block valve to shut off the coolant venting, but around 950 m³ (250,000 US gallons) of coolant had already leaked 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 that time the radiation levels in the primary coolant water were around 300 times expected levels, and the plant was seriously contaminated.

At 7:00 a.m. a "Site Area Emergency" was declared and at 7:24 a.m. the incident was upgraded to a "General Emergency". Harrisburg radio station WKBO announced a problem with the plant at 8:25 a.m. and the Associated Press announced the general emergency at 9:00 a.m.

It was still not clear to the control room staff that the primary loop water levels were low and that over half of the core was exposed: a Loss Of Coolant Accident. 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 relief valve was opened to reduce pressure. At around nine hours 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 melted, and the system was still dangerously radioactive. Over the next week the steam and hydrogen were removed from the reactor using a recombiner and, more controversially, by venting straight to the atmosphere. It is estimated that a maximum of 13 million curies (480 petabecquerels) of radioactive noble gases were released by the event, though very little of the hazardous iodine-131 was released.

The molten fuel did not break through in a " China Syndrome" (see below). "Despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel."


The full damage to the reactor core was not known for many years after the accident.
The full damage to the reactor core was not known for many years after the accident.

The scientific community continues to be split into two camps over the aftermath of the Three Mile Island Accident. One camp believes that no member of the public was injured by the accident. "The average radiation dose to people living within ten miles of the plant was eight millirem, and no more than 100 millirem to any single individual. Eight millirem is about equal to a chest X-ray, and 100 millirem is about a third of the average background level of radiation received by US residents in a year."

The other focus cites evidence observed by locals of a massive radiation release from the plant . However, "to this day [late 2004], Wing's article remains the only one to present original health data supporting a link between Three Mile Island radiation exposure and cancer." A baby teeth study is to be done. This camp also notes that hundreds of out-of-court settlements have been reached with victims.

Three Mile Island has been of interest to human factors engineers as an example of how groups of people react and make decisions under stress. There is consensus that the accident was exacerbated by wrong decisions made because the operators were overwhelmed with information, much of it irrelevant, misleading or incorrect. As a result of the TMI-2 incident, nuclear reactor operator training has been improved. Before the incident it focused on diagnosing the underlying problem; afterwards, it focused on reacting to the emergency by going through a standardized checklist to ensure that the core is receiving enough coolant under sufficient pressure.

In addition to the improved operating training, improvements in quality assurance, engineering, operational surveillance and emergency planning have been instituted. Improvements in control room habitability, "sight lines" to instruments, ambiguous indications and even the placement of "trouble" tags were made; some trouble tags were covering important instrument indications during the accident. Improved surveillance of critical systems, structures and components required for cooling the plant and mitigating the escape of radionuclides during an emergency were also implemented. In addition, each nuclear site must now have an approved emergency plan to direct the evacuation of the public within a ten mile Emergency Planning Zone (EPZ) and to facilitate rapid notification and evacuation. This plan is periodically rehearsed with federal and local authorities to ensure that all groups work together quickly and efficiently.

US President Jimmy Carter ordered a full investigation of the TMI incident. The Pennsylvania House of Representatives conducted its own investigation, which focused on the need to improve evacuation procedures. House investigators also visited the Three Mile Island site, including the Control Room. Many of the instruments viewed were marked with "error tags," explaining how the instrument was supposed to work, and how it actually worked. A member of the investigating committee, State Rep. Mark B. Cohen of Philadelphia, said it would be "virtually impossible" for any Control Room operator to keep track of the many variations between the equipment's intended and actual functioning.

There was also the psychological effect on the nation. Before the accident approximately seventy percent of the general public approved of nuclear power. After it, support for nuclear power across the country fell to about fifty percent, where it remained for decades. Recently public support for nuclear power has been on the rise, and the George W. Bush administration has been particularly supportive of nuclear power, encouraging power companies to begin considering plans for the first new nuclear reactors in the United States in decades.

The 1979 TMI accident did not, however, initiate the demise of the US nuclear power industry. As a result of post-oil-shock analysis and conclusions of overcapacity, 40 planned nuclear power plants had already been canceled between 1973 and 1979. No US nuclear power plant had been authorized to begin construction since the year before TMI. Nonetheless, TMI added a clearly strong impact on this demise: of 129 plants approved at the time of TMI just 53 of those not already operating were ever completed. Federal requirements became more stringent, local opposition became more strident and construction times were vastly lengthened. The two shortest nuclear power plant construction projects were in this same volatile period, River Bend and St. Lucie-2.

Viewed from the west, Three Mile Island currently uses only one nuclear generating station, TMI-1, which is on the left. TMI-2, to the right, has not been used since the accident.
Viewed from the west, Three Mile Island currently uses only one nuclear generating station, TMI-1, which is on the left. TMI-2, to the right, has not been used since the accident.

The TMI cleanup started in August 1979 and officially ended in December 1993, having cost around US$975 million. From 1985 to 1990 almost 100 tons of radioactive fuel were removed from the site. However, the contaminated cooling water that leaked into the containment building had seeped into the building's concrete, leaving the radioactive residue impossible to remove. TMI-2 had been online only three months, but now had a ruined reactor vessel and a containment building that was unsafe to walk in — it has since been permanently closed. Many similar Babcock and Wilcox reactors on order were canceled — in total, 51 American nuclear reactors were canceled from 1980 to 1984.

In the end, a few simple water level gauges on the reactor vessel might have prevented the accident. The operators' focus on a single misleading indication, the level in the pressurizer, was a significant contributing factor to the meltdown.

Unit 1, which had had its license temporarily suspended following the incident at Unit 2, was permitted to resume operations in 1985. General Public Utilities Corporation, the plant's owner, formed General Public Utilities Nuclear Corporation (GPUN) as a new subsidiary to own and operate the company's fleet of nuclear facilities, including Three Mile Island. The plant had previously been operated by Metropolitan Edison Company (Met-Ed), one of GPU's regional utility operating companies. In 1996, General Public Utilities shortened its name to GPU Inc. Three Mile Island Unit 1 was sold to AmerGen Energy Corporation, a joint venture between Philadelphia Electric Company (PECO), and British Energy, in 1998. In 2000, PECO merged with Unicom Corporation to form Exelon Corporation, which acquired British Energy's share of AmerGen in 2003, dissolving the company in the process. TMI Unit 1 is now owned and operated by Exelon Nuclear Inc., an Exelon Corp. subsidiary.

Three Mile Island Unit 2 was too badly damaged and contaminated to resume operations. The reactor was gradually deactivated and mothballed in a lengthy process completed in 1993. Initially, efforts focused on the cleanup and decontamination of the site, especially the defueling of the damaged reactor. In 1988, the Nuclear Regulatory Commission announced that, although it was possible to further decontaminate the Unit 2 site, the remaining radioactivity had been sufficiently contained as to pose no threat to public health and safety. Accordingly, further cleanup efforts were deemed unnecessary. The defueling process was completed in 1990, and the damaged reactor was removed and disposed of in 1993. General Public Utilities was legally obliged to continue to maintain and monitor the site, and therefore retained ownership of Unit 2 when Unit 1 was sold to AmerGen in 1998. GPU Inc. was acquired by First Energy Corporation in 2001, and subsequently dissolved. First Energy then contracted out the maintenance and administration of Unit 2 to AmerGen. Unit 2 has been administered by Exelon Nuclear since 2003, when Exelon Nuclear's parent company, Exelon, bought out the remaining shares of AmerGen, inheriting First Energy's maintenance contract.

The China Syndrome

The accident at the plant occurred a few days after the release of the movie The China Syndrome, which featured Jane Fonda as a news anchor at a California TV station. In the film, a nuclear accident almost happens while Fonda's character and her cameraman are at a plant doing a series on nuclear power. She goes on to raise awareness of how unsafe the plant was. Coincidentally, there is a scene in which Fonda's character speaks with a nuclear safety expert who says that a meltdown could force an area "the size of Pennsylvania" to be evacuated. Also, the fictional near-accident in the movie stems from plant operators misunderstanding the amount of water within the core.

Soon after the release of the film, Fonda began lobbying against nuclear power — the only actor in the film to do so. In an attempt to counter her efforts, the nuclear physicist Edward Teller, "father of the hydrogen bomb" and long-time government science advisor, himself lobbied in favour of nuclear power, and eventually the 71-year-old scientist suffered a heart attack, which he later blamed on Fonda: "You might say that I was the only one whose health was affected by that reactor near Harrisburg. No, that would be wrong. It was not the reactor. It was Jane Fonda. Reactors are not dangerous." Rhetoric based on the movie is still used to debate for and against nuclear power.

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