NRC: Backgrounder on the Three Mile Island Accident Home > NRC Library > Document Collections > Fact Sheets > Backgrounder on the Three Mile Island Accident Backgrounder on the Three Mile Island Accident Printable Version (does not include Animated Diagram of the Sequence of Events) Plant Diagram The Three Mile Island Unit 2 (TMI-2) reactor, near Middletown, Pa., partially melted down on March 28, 1979. This was the most serious accident in U.S. commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public. 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. It also caused the NRC to tighten and heighten its regulatory oversight. All of these changes significantly enhanced U.S. reactor safety. A 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. Summary of Events The accident began about 4 a.m.

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). Either a mechanical or electrical failure prevented the main feedwater pumps from sending water to the steam generators that remove heat from the reactor core. This caused the plant's turbine-generator and then the reactor itself to automatically shut down. Immediately, the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to control that pressure, the pilot-operated relief valve (a valve located at the top of the pressurizer) opened. The valve should have closed when the pressure fell to proper levels, but it became stuck open. Instruments in the control room, however, indicated to the plant staff that the valve was closed. As a result, the plant staff was unaware that cooling water was pouring out of the stuck-open valve. As coolant flowed from the primary system through the valve, other instruments available to reactor operators provided inadequate information. There was no instrument that showed how much water covered the core. As a result, plant staff assumed that as long as the pressurizer water level was high, the core was properly covered with water.

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. The water escaping through the stuck valve reduced primary system pressure so much that the reactor coolant pumps had to be turned off to prevent dangerous vibrations. To prevent the pressurizer from filling up completely, the staff reduced how much emergency cooling water was being pumped in to the primary system. These actions starved the reactor core of coolant, causing it to overheat. Without the proper water flow, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes that hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt. It was later found that about half of the core melted during the early stages of the accident. Although TMI-2 suffered a severe core meltdown, the most dangerous kind of nuclear power accident, consequences outside the plant were minimal. Unlike the Chernobyl and Fukushima accidents, TMI-2's containment building remained intact and held almost all of the accident's radioactive material.

Federal and state authorities were initially concerned about the small releases of radioactive gases that were measured off-site by the late morning of March 28 and even more concerned about the potential threat that the reactor posed to the surrounding population. They did not know that the core had melted, but they immediately took steps to try to gain control of the reactor and ensure adequate cooling to the core. The NRC's regional office in King of Prussia, Pa., was notified at 7:45 a.m. on March 28. By 8 a.m., NRC Headquarters in Washington, D.C., was alerted and the NRC Operations Center in Bethesda, Md., was activated. The regional office promptly dispatched the first team of inspectors to the site and other agencies, such as the Department of Energy and the Environmental Protection Agency, also mobilized their response teams. Helicopters hired by TMI's owner, General Public Utilities Nuclear, and the Department of Energy were sampling radioactivity in the atmosphere above the plant by midday. A team from the Brookhaven National Laboratory was also sent to assist in radiation monitoring.

At 9:15 a.m., the White House was notified and at 11 a.m., all non-essential personnel were ordered off the plant's premises. By the evening of March 28, the core appeared to be adequately cooled and the reactor appeared to be stable. But new concerns arose by the morning of Friday, March 30. A significant release of radiation from the plant's auxiliary building, performed to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core, caused a great deal of confusion and consternation. In an atmosphere of growing uncertainty about the condition of the plant, the governor of Pennsylvania, Richard L. Thornburgh, consulted with the NRC about evacuating the population near the plant. Eventually, he and NRC Chairman Joseph Hendrie agreed that it would be prudent for those members of society most vulnerable to radiation to evacuate the area. Thornburgh announced that he was advising pregnant women and pre-school-age children within a five-mile radius of the plant to leave the area. Within a short time, chemical reactions in the melting fuel created a large hydrogen bubble in the dome of the pressure vessel, the container that holds the reactor core.

NRC officials worried the hydrogen bubble might burn or even explode and rupture the pressure vessel. In that event, the core would fall into the containment building and perhaps cause a breach of containment. The hydrogen bubble was a source of intense scrutiny and great anxiety, both among government authorities and the population, throughout the day on Saturday, March 31. The crisis ended when experts determined on Sunday, April 1, that the bubble could not burn or explode because of the absence of oxygen in the pressure vessel. Further, by that time, the utility had succeeded in greatly reducing the size of the bubble. Animated Diagram of the Sequence of Events The following animated diagram graphically depicts the sequence of events associated with the accident at TMI-2. Health Effects 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. Several independent groups also conducted studies. 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.

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 accident's maximum dose to a person at the site boundary would have been less than 100 millirem above background. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. However, comprehensive investigations and assessments by several well respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment. Impact of the Accident A combination of personnel error, design deficiencies, and component failures caused the Three Mile Island accident, which permanently changed both the nuclear industry and the NRC.

Public fear and distrust increased, NRC's regulations and oversight became broader and more robust, and management of the plants was scrutinized more carefully. Careful analysis of the accident's events identified problems and led to permanent and sweeping changes in how NRC regulates its licensees – which, in turn, has reduced the risk to public health and safety. Here are some of the major changes that have occurred since the accident: Upgrading and strengthening of plant design and equipment requirements. This includes fire protection, piping systems, auxiliary feedwater systems, containment building isolation, reliability of individual components (pressure relief valves and electrical circuit breakers), and the ability of plants to shut down automatically; Identifying the critical role of human performance in plant safety led to revamping operator training and staffing requirements, followed by improved instrumentation and controls for operating the plant, and establishment of fitness-for-duty programs for plant workers to guard against alcohol or drug abuse; Enhancing emergency preparedness, including requirements for plants to immediately notify NRC of significant events and an NRC Operations Center staffed 24 hours a day.

Drills and response plans are now tested by licensees several times a year, and state and local agencies participate in drills with the Federal Emergency Management Agency and NRC; Integrating NRC observations, findings, and conclusions about licensee performance and management effectiveness into a periodic, public report; Having senior NRC managers regularly analyze plant performance for those plants needing significant additional regulatory attention; Expanding NRC's resident inspector program – first authorized in 1977 – to have at least two inspectors live nearby and work exclusively at each plant in the U.S.

to provide daily surveillance of licensee adherence to NRC regulations; Expanding performance-oriented as well as safety-oriented inspections, and the use of risk assessment to identify vulnerabilities of any plant to severe accidents; Strengthening and reorganizing enforcement staff in a separate office within the NRC; Establishing the Institute of Nuclear Power Operations, the industry's own "policing" group, and formation of what is now the Nuclear Energy Institute to provide a unified industry approach to generic nuclear regulatory issues, and interaction with NRC and other government agencies; Installing additional equipment by licensees to mitigate accident conditions, and monitor radiation levels and plant status; Enacting programs by licensees for early identification of important safety-related problems, and for collecting and assessing relevant data so operating experience can be shared and quickly acted upon; and Expanding NRC's international activities to share enhanced knowledge of nuclear safety with other countries in a number of important technical areas. Current Status Today, the TMI-2 reactor is permanently shut down and all its fuel had been removed. The reactor coolant system is fully drained and the radioactive water decontaminated and evaporated.

The accident's radioactive waste was shipped off-site to an appropriate disposal area, and the reactor fuel and core debris was shipped to the Department of Energy's Idaho National Laboratory. In 2001, FirstEnergy acquired TMI-2 from GPU. FirstEnergy has contracted the monitoring of TMI-2 to Exelon, the current owner and operator of TMI-1. The companies plan to keep the TMI-2 facility in long-term, monitored storage until the operating license for the TMI-1 plant expires, at which time both plants will be decommissioned. Below is a chronology of highlights of the TMI-2 cleanup from 1980 through 1993. Date Monitored storage began. Additional Information Further information on the TMI-2 accident can be obtained from sources listed below. The NUREG documents, many of which are on microfiche, can be ordered for a fee from the NRC's Public Document Room at 301-415-4737 or 1-800-397-4209; e-mail pdr@nrc.gov .

The PDR is located at 11555 Rockville Pike, Rockville, Md.; however the mailing address is: U.S. Nuclear Regulatory Commission, Public Document Room, Washington, D.C. 20555. A glossary is also provided below. Additional Sources for Information on Three Mile Island NRC Annual Report - 1979, NUREG-0690 "Population Dose and Health Impact of the Accident at the Three Mile Island Nuclear Station," NUREG-0558 "Environmental Assessment of Radiological Effluents from Data Gathering and Maintenance Operation on Three Mile Island Unit 2," NUREG-0681 "Report of The President's Commission on The Accident at Three Mile Island," October, 1979 "Investigation into the March 28, 1979 Three Mile Island Accident by the Office of Inspection and Enforcement," NUREG-0600 "Three Mile Island; A Report to the Commissioners and to the Public," by Mitchell Rogovin and George T. Frampton, NUREG/CR-1250, Vols.

I-II, 1980 "Lessons learned From the Three Mile Island - Unit 2 Advisory Panel," NUREG/CR-6252 "The Status of Recommendations of the President's Commission on the Accident at Three Mile Island," (A ten-year review), NUREG-1355 "NRC Views and Analysis of the Recommendations of the President's Commission on the Accident at Three Mile Island," NUREG-0632 "Environmental Impact Statement related to decontamination and disposal of radioactive wastes resulting from March 28, 1979 accident Three Mile Island Nuclear Station, Unit 2," NUREG-0683 "Answers to Questions About Updated Estimates of Occupational Radiation Doses at Three Mile Island, Unit 2," NUREG-1060 "Answers to Frequently Asked Questions About Cleanup Activities at Three Mile Island, Unit 2," NUREG-0732 "Status of Safety Issues at Licensed Power Plants" (TMI Action Plan Reqmts.), NUREG-1435 Walker, J. Samuel, Three Mile Island: A Nuclear Crisis in Historical Perspective, Berkeley: University of California Press, 2004.

Glossary Auxiliary feedwater - (see emergency feedwater) Background radiation - The radiation in the natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals. The usually quoted average individual exposure from background radiation is 300 millirem per year. Cladding - The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents the corrosion of the fuel by the coolant and the release of fission products in the coolants. Aluminum, stainless steel and zirconium alloys are common cladding materials. Emergency feedwater system - Backup feedwater supply used during nuclear plant startup and shutdown; also known as auxiliary feedwater. Fuel rod - A long, slender tube that holds fuel (fissionable material) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core. Containment - The gas-tight shell or other enclosure around a reactor to confine fission products that otherwise might be released to the atmosphere in the event of an accident. Coolant - A substance circulated through a nuclear reactor to remove or transfer heat. The most commonly used coolant in the U.S. is water.

Other coolants include air, carbon dioxide, and helium. Core - The central portion of a nuclear reactor containing the fuel elements, and control rods. Decay heat - The heat produced by the decay of radioactive fission products after the reactor has been shut down. Decontamination - The reduction or removal of contaminating radioactive material from a structure, area, object, or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination; (2) letting the material stand so that the radioactivity is decreased by natural decay; and (3) covering the contamination to shield the radiation emitted. Feedwater - Water supplied to the steam generator that removes heat from the fuel rods by boiling and becoming steam. The steam then becomes the driving force for the turbine generator. Nuclear Reactor - A device in which nuclear fission may be sustained and controlled in a self-supporting nuclear reaction. There are several varieties, but all incorporate certain features, such as fissionable material or fuel, a moderating material (to control the reaction), a reflector to conserve escaping neutrons, provisions for removal of heat, measuring and controlling instruments, and protective devices.

Pressure Vessel - A strong-walled container housing the core of most types of power reactors. Pressurizer - A tank or vessel that controls the pressure in a certain type of nuclear reactor. Primary System - The cooling system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine. Radiation - Particles (alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an unstable atom as a result of radioactive decay. Reactor Coolant System - (see primary system) Secondary System - The steam generator tubes, steam turbine, condenser and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation. Steam Generator - The heat exchanger used in some reactor designs to transfer heat from the primary (reactor coolant) system to the secondary (steam) system. This design permits heat exchange with little or no contamination of the secondary system equipment. Turbine - A rotary engine made with a series of curved vanes on a rotating shaft. Usually turned by water or steam. Turbines are considered to be the most economical means to turn large electrical generators.

Plant Diagram Page Last Reviewed/Updated Friday, December 12, 2014 Home Three Mile Island - Facts & Summary - HISTORY.com Three Mile Island A+E Networks Introduction In March 1979, a series of mechanical and human errors at the Three Mile Island nuclear generating plant near Harrisburg, Pennsylvania, resulted in an accident that profoundly affected the utility industry. A combination of stuck valves, misread gauges and poor decisions led to a partial meltdown of the reactor core and the release of radioactive gases into the atmosphere. Although the health effects were not serious, the accident heightened public fears and led to the immediate shutdown of several plants. In addition, a moratorium on the licensing of all new reactors was temporarily imposed, significantly slowing the industry for several years. Google An unlikely series of mechanical and human errors in Unit 2 of the nuclear generating plant at Three Mile Island, near Harrisburg, Pennsylvania , in 1979 resulted in an accident that profoundly affected the utility industry. A complicated combination of stuck valves, misread gauges, and poor decisions led to a partial meltdown of the reactor core and a release of significant amounts of radioactive gases.

The near-total devastation of the nuclear power industry resulted, because the disaster at Three Mile Island tipped the scales in the ongoing controversy over nuclear power in favor of those opposed to it. Massive demonstrations followed the accident, culminating in a rally in New York City that attracted upward of 200,000 people. By the mid-1980s, the construction of nuclear power plants in the United States had virtually ceased. The radioactive gases released by the accident prompted the governor of Pennsylvania to evacuate pregnant women from the area. An investigation by the Nuclear Regulatory Commission claimed that the amount of radioactivity released was not a health threat, but antinuclear activists and many local citizens disputed this. The reactor itself remained unusable-in fact, virtually unapproachable-more than a decade later. The Reader’s Companion to American History. Eric Foner and John A. Garraty, Editors. Copyright © 1991 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Tags Three Mile Island accident | nuclear accident, United States [1979] | Britannica.com Three Mile Island accident nuclear accident, United States [1979] Written By: Eastland disaster Three Mile Island accident, accident in 1979 at the Three Mile Island nuclear power station that was the most serious in the history of the American nuclear power industry. The Three Mile Island power station was named after the island on which it was situated in the Susquehanna River near Harrisburg , Pa. At 4:00 am on March 28, an automatically operated valve in the Unit 2 reactor mistakenly closed, shutting off the water supply to the main feedwater system (the system that transfers heat from the water actually circulating in the reactor core). This caused the reactor core to shut down automatically, but a series of equipment and instrument malfunctions, human errors in operating procedures, and mistaken decisions in the ensuing hours led to a serious loss of water coolant from the reactor core.

As a result, the core was partially exposed, and the zirconium cladding of its fuel reacted with the surrounding superheated steam to form a large accumulation of hydrogen gas, some of which escaped from the core into the containment vessel of the reactor building. Very little of this and other radioactive gases actually escaped into the atmosphere, and they did not constitute a threat to the health of the surrounding population. In the following days adequate coolant water circulation in the core was restored. Aerial view of Three Mile Island near Harrisburg, Pennsylvania. John S. Zeedick/Getty Images The accident at Three Mile Island, though minuscule in its health consequences, had widespread and profound effects on the American nuclear power industry. It resulted in the immediate (though temporary) closing of seven operating reactors like those at Three Mile Island. A moratorium on the licensing of all new reactors was also temporarily imposed, and the whole process of approval for new plants by the Nuclear Regulatory Commission was significantly slowed for years after the accident. No new reactors were ordered by utility companies in the United States from 1979 through the mid-1980s. The accident increased public fears about the safety of nuclear reactors and strengthened public opposition to the construction of new plants.

The unharmed Unit 1 reactor at Three Mile Island did not resume operation until 1985. The cleanup of Unit 2 continued until 1990; damage to the unit was so severe, however (52 percent of the core melted down), that it remained unusable. Learn More in these related articles: in nuclear reactor Nuclear accident at Three Mile Island - Mar 28, 1979 - HISTORY.com This Day In History: 03/28/1979 - Three Mile Island At 4 a.m. on March 28, 1979, the worst accident in the history of the U.S. nuclear power industry begins when a pressure valve in the Unit-2 reactor at Three Mile Island fails to close. Cooling water, contaminated with radiation, drained from the open valve into adjoining buildings, and the core began to dangerously overheat. After the cooling water began to drain out of the broken pressure valve on the morning of March 28, 1979, emergency cooling pumps automatically went into operation. Left alone, these safety devices would have prevented the development of a larger crisis.

However, human operators in the control room misread confusing and contradictory readings and shut off the emergency water system. By early morning, the core had heated to over 4,000 degrees, just 1,000 degrees short of meltdown. The radiation levels, though not immediately life-threatening, were dangerous, and the core cooked further as the contaminated water was contained and precautions were taken to protect the operators. Finally, at about 8 p.m., plant operators realized they needed to get water moving through the core again and restarted the pumps. The temperature began to drop, and pressure in the reactor was reduced. Two days later, however, on March 30, a bubble of highly flammable hydrogen gas was discovered within the reactor building. The bubble of gas was created two days before when exposed core materials reacted with super-heated steam. On March 28, some of this gas had exploded, releasing a small amount of radiation into the atmosphere. After the radiation leak was discovered on March 30, residents were advised to stay indoors. Slowly, the hydrogen was bled from the system as the reactor cooled.

Lead Story Nuclear accident at Three Mile Island Share this: Nuclear accident at Three Mile Island Author Nuclear accident at Three Mile Island URL Publisher A+E Networks At 4 a.m. on March 28, 1979, the worst accident in the history of the U.S. nuclear power industry begins when a pressure valve in the Unit-2 reactor at Three Mile Island fails to close. Cooling water, contaminated with radiation, drained from the open valve into adjoining buildings, and the core began to dangerously overheat. The Three Mile Island nuclear power plant was built in 1974 on a sandbar on Pennsylvania’s Susquehanna River, just 10 miles downstream from the state capitol in Harrisburg. In 1978, a second state-of-the-art reactor began operating on Three Mile Island, which was lauded for generating affordable and reliable energy in a time of energy crises. After the cooling water began to drain out of the broken pressure valve on the morning of March 28, 1979, emergency cooling pumps automatically went into operation. Left alone, these safety devices would have prevented the development of a larger crisis.

However, human operators in the control room misread confusing and contradictory readings and shut off the emergency water system. The reactor was also shut down, but residual heat from the fission process was still being released. By early morning, the core had heated to over 4,000 degrees, just 1,000 degrees short of meltdown. In the meltdown scenario, the core melts, and deadly radiation drifts across the countryside, fatally sickening a potentially great number of people. As the plant operators struggled to understand what had happened, the contaminated water was releasing radioactive gases throughout the plant. The radiation levels, though not immediately life-threatening, were dangerous, and the core cooked further as the contaminated water was contained and precautions were taken to protect the operators. Shortly after 8 a.m., word of the accident leaked to the outside world. The plant’s parent company, Metropolitan Edison, downplayed the crisis and claimed that no radiation had been detected off plant grounds, but the same day inspectors detected slightly increased levels of radiation nearby as a result of the contaminated water leak. Pennsylvania Governor Dick Thornburgh considered calling an evacuation. Finally, at about 8 p.m., plant operators realized they needed to get water moving through the core again and restarted the pumps.

The temperature began to drop, and pressure in the reactor was reduced. The reactor had come within less than an hour of a complete meltdown. More than half the core was destroyed or molten, but it had not broken its protective shell, and no radiation was escaping. The crisis was apparently over. Two days later, however, on March 30, a bubble of highly flammable hydrogen gas was discovered within the reactor building. The bubble of gas was created two days before when exposed core materials reacted with super-heated steam. On March 28, some of this gas had exploded, releasing a small amount of radiation into the atmosphere. At that time, plant operators had not registered the explosion, which sounded like a ventilation door closing. After the radiation leak was discovered on March 30, residents were advised to stay indoors. Experts were uncertain if the hydrogen bubble would create further meltdown or possibly a giant explosion, and as a precaution Governor Thornburgh advised “pregnant women and pre-school age children to leave the area within a five-mile radius of the Three Mile Island facility until further notice.” This led to the panic the governor had hoped to avoid; within days, more than 100,000 people had fled surrounding towns.

On April 1, President Jimmy Carter arrived at Three Mile Island to inspect the plant. Carter, a trained nuclear engineer, had helped dismantle a damaged Canadian nuclear reactor while serving in the U.S. Navy. His visit achieved its aim of calming local residents and the nation. That afternoon, experts agreed that the hydrogen bubble was not in danger of exploding. Slowly, the hydrogen was bled from the system as the reactor cooled. At the height of the crisis, plant workers were exposed to unhealthy levels of radiation, but no one outside Three Mile Island had their health adversely affected by the accident. Nonetheless, the incident greatly eroded the public’s faith in nuclear power. The unharmed Unit-1 reactor at Three Mile Island, which was shut down during the crisis, did not resume operation until 1985. Cleanup continued on Unit-2 until 1990, but it was too damaged to be rendered usable again. In the more than two decades since the accident at Three Mile Island, not a single new nuclear power plant has been ordered in the United States.

Related Videos Lessons From the 1979 Accident at Three Mile Island - Nuclear Energy Institute Lessons From the 1979 Accident at Three Mile Island October 2014 Key Facts The accident at Three Mile Island 2 (TMI 2) in 1979 was caused by a combination of equipment failure and the inability of plant operators to understand the reactor’s condition at certain times during the event. A gradual loss of cooling water to the reactor’s heat-producing core led to partial melting of the fuel rod cladding and the uranium fuel, and the release of a small amount of radioactive material. The TMI 2 accident caused no injuries or deaths. In addition, experts concluded that the amount of radiation released into the atmosphere was too small to result in discernible direct health effects to the population in the vicinity of the plant. At least a dozen epidemiological studies conducted since 1981 have borne this out. Both the industry and the government responded swiftly and decisively to the TMI 2 accident. Among other actions, the industry established the Institute of Nuclear Power Operations (INPO) to promote excellence in training, plant management and operations.

In the final chapter in a series of legal challenges, a federal appeals court in December 2003 dismissed the consolidated cases of 2,000 plaintiffs seeking damages against the plant’s former owners. The court said the plaintiffs failed to present evidence they had received a radiation dose large enough to possibly cause health effects. The Accident: What Happened On March 28, 1979, TMI 2 near Harrisburg, Pa., was operating at about 100 percent power when it automatically shut down after a pump that provided cooling water stopped operating. Pressure and temperature increased in the reactor, causing a pressure relief valve to open. The valve opened as designed, and water and steam began flowing out of the reactor to a tank in the basement of the reactor building. As pressure returned to normal, the valve should have closed. But, unknown to the operators, the valve stuck open. It remained open for more than two hours, allowing water that covered and cooled the fuel core to escape from the reactor system. This caused the fuel to begin to overheat. However, instrumentation in the TMI control room indicated to the operators that the relief valve was closed and that too much water was being injected into the reactor vessel.

Consequently, operators did not replace the water that was lost as a result of the open relief valve. As pressure continued to drop, more and more coolant turned to steam, causing excessive vibration in the main coolant pumps. The vibration prompted operators, who did not realize the reactor was experiencing a loss of coolant, to shut down the pumps. The loss of pressure and water caused a large steam bubble to form in the top of the reactor vessel, further preventing the flow of cooling water through the core. Without coolant, core temperatures rose above the melting point of the fuel cladding and the uranium fuel. About half of the fuel melted before coolant flow was restored. The colder cooling water also shattered some of the hot fuel rods. All of the fuel was damaged. As a result of the TMI 2 accident, 700,000 gallons of radioactive cooling water ended up in the basement of the reactor building and in tanks in the auxiliary building, contaminating them. In addition, a small amount of radioactive material was released into the atmosphere from the ventilation stack of an auxiliary building to relieve pressure inside the reactor building.

No Measurable Health Effects The TMI 2 accident caused no injuries, and at least a dozen epidemiological studies conducted since 1981 have found no discernible direct health effects to the population in the vicinity of the plant. (For details, see the appendix that follows this fact sheet.) A federal appeals court in December 2003 dismissed the consolidated cases of 2,000 plaintiffs seeking damages against the plant’s former owners for health problems they alleged the accident caused. The court said the plaintiffs had failed to present evidence they had received a radiation dose large enough to cause possible health effects. Decades of research and scientific studies have shown no negative health effects on the population surrounding the plant. People who suffered financial losses as a result of the precautionary evacuation following the incident were promptly paid, demonstrating the effectiveness of the industry’s liability insurance protection under the Price-Anderson Act. In addition, businesses were compensated for loss of revenue, and the state and local community were compensated for the expenses incurred during the response to the threat. TMI 2 Placed in Monitored Storage After cleaning up the damaged TMI 2 reactor, GPU Nuclear placed the plant in monitored storage in December 1993.

In December 1999, GPU sold TMI 1 to AmerGen Energy Co., a joint venture of Exelon and British Energy Co. British Energy subsequently sold its interest in TMI 1 to Exelon. In 2008, AmerGen Energy Co. was integrated into Exelon Generation, and the AmerGen legal entity was dissolved. Under the terms of the sale, GPU retained ownership of TMI 2. GPU subsequently merged with FirstEnergy, making First Energy financially responsible for the decommissioning of TMI 2. In-plant and off-site monitoring of TMI 2 will continue until it is fully decommissioned, with regular reports made to the U.S. Nuclear Regulatory Commission, the commonwealth of Pennsylvania and the public. The two reactors will be decommissioned jointly when TMI 1 is taken out of service. Industry, Government Act to Improve Safety Two weeks after the 1979 accident, President Jimmy Carter appointed a 12-member commission, headed by the late John Kemeny, then president of Dartmouth College, to investigate what had happened and the possible impact on the health and safety of the public and plant personnel.

The commission’s report on its investigation, issued in October 1979, recommended that the industry develop its own standards of excellence. The commission also cited a need for agency-accredited training institutions for nuclear plant operators and immediate supervisors of operations. The NRC also moved quickly, setting up a group to study the accident. The group, headed by attorney Mitchell Rogovin, reached many of the same conclusions as the Kemeny Commission. Industry Changes Increase Safety Within nine months of the accident, the industry had formed INPO, whose mission is to promote the highest levels of safety and reliability in the operation of nuclear power plants. To improve training, INPO in 1985 formed the National Academy for Nuclear Training. The academy reviews and accredits nuclear utilities’ training programs for all key positions at each plant. INPO has had a profound impact on the way nuclear plants are managed and operated. The proof is the steady improvement in plant performance in the nearly 30 years since the accident at TMI. Today, the nation’s 104 operating reactors maintain high levels of safety and reliability, as evidenced by the NRC’s reactor oversight program and performance indicators tracked by the World Association of Nuclear Operators.

Appendix: Scientists Find No Health Effects State Public Health Department Studies TMI Several studies were conducted by the Pennsylvania Department of Health. The conclusions are summarized below: A 1981 study reported that if the accident had any effect on infant death rates, there would have been a significant increase in the six months after the accident. Instead, the infant death rate was lower than normal. A 1982 study found that the incidence of congenital hypothyroidism within a 10-mile radius of the plant was well within a normal range in the year after the accident. A 1982 study found no measurable impact on infant mortality within a 10-mile radius of the plant, compared with infant mortality rates for Pennsylvania for 1977-79. A 1985 study found no significant difference in cancer mortality within a 20-mile radius of the plant during the five years preceding the accident and the five years following it. In a more detailed analysis of four communities downwind of the plant, the study found no significant abnormalities in either cancer mortality or cancer incidence among residents considered to be at potentially higher risk.

A 1988 study found no connection between radiation or psychological stress and failed and complicated pregnancies, such as fetal and neonatal mortalities and other problems. A 1989 study found no significant abnormalities in cancer mortality or incidence among residents of selected communities near the plant. Two 1991 studies showed no increased cancer incidence among people who lived near the plant in 1979. One study involved the general population living within a 5-mile radius of the plant; the other involved women of child-bearing age who lived within a 10-mile radius. Although the studies have found no increased incidence of cancer as a result of the accident, they did find evidence of psychological stress, lasting in some cases for five to six years. According to the Pennsylvania Department of Health’s Three Mile Island Health Research Program, people suffering from stress believed their health was poorer than it actually was when the health department checked their medical records. Other Studies In addition to the Pennsylvania Health Department studies, several other studies have examined the health impact of the TMI accident on the population: A study presented by K.

Ramaswamy at the 1988 annual meeting of the American Public Health Association compared post-accident cancer deaths over a six-year period for residents within a 5-mile radius of the plant with cancer deaths of a large control population. The study concluded that the normal death rate and life expectancy for people around TMI were not affected by the accident. A study presented by E. Digon at the 1988 annual meeting of the American Public Health Association, concluded that—based on a comprehensive analysis of statistical data by health researchers—fetal and infant mortality in the vicinity of the plant were neither significantly higher than expected nor significantly different from those in the years before the accident. Several prominent scientists from Columbia University and the National Audubon Society studied cancers among the nearly 160,000 residents within a 10-mile radius of the TMI plant. The principal cancers considered were leukemia and childhood malignancies.

The study, issued in September 1990, concluded: “Overall, the pattern of results does not provide convincing evidence that radiation releases from the Three Mile Island nuclear facility influenced cancer risk during the limited period of follow-up.” In 1990, the National Cancer Institute of the National Institutes of Health released the results of a two-year study of cancer data in 107 U.S. counties that contained, or were adjacent to, major nuclear facilities that had begun operations before 1982. Among the counties were York, Lancaster and Dauphin near the TMI plant in Pennsylvania. The study, which compared cancer mortality rates in the 107 counties with rates in counties with no nuclear facilities, found no increased cancer mortality for people living near the nuclear installations. The study also found no evidence that leukemia for any age was linked to routine operations at the TMI reactors or to the accident at TMI 2. In 2002, researchers at the University of Pittsburgh’s Graduate School of Public Health (GSPH) conducted a 20-year follow-up study of mortality data on residents living within a 5-mile radius of the plant. The study found no significant increase in overall deaths from cancer.

“This survey, which covers the normal latency period for most cancers, confirms our earlier analysis that radioactivity released during the nuclear accident at TMI does not appear to have caused an overall increase in cancer deaths among residents of that area over the follow-up period, 1979 to 1998,” said Evelyn Talbott, professor of epidemiology at GSPH and principal investigator on the study. The NRC has called upon the National Academy of Sciences (NAS) to provide a review of cancer incidence and mortality risk among people living near operating, decommissioned or proposed nuclear facilities. The requested study will update the 20-year-old National Cancer Institute (NCI) report that currently serves as the NRC's primary source for communicating to the public about radiation issues. The NCI report found no indication of increased cancer risk among people living in counties around nuclear energy facilities. The NAS study will take into account newer nuclear units and demographic changes among plant neighbors. The first phase of the study began in summer 2010 and identified a number of technical challenges and limitations associated with the attempt to study potential cancer risk among populations that have received little or no exposure due to nuclear facility operations.

To follow up, the NAS is carrying out a two-year pilot study to determine how best to address the challenges and limitations. The pilot study is expected to conclude by 2016. Three Mile Island | TMI 2 |Three Mile Island Accident. - World Nuclear Association Three Mile Island Accident (March 2001, minor update Jan 2012) In 1979 at Three Mile Island nuclear power plant in USA a cooling malfunction caused part of the core to melt in the #2 reactor. The TMI-2 reactor was destroyed. Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. There were no injuries or adverse health effects from the Three Mile Island accident. The Three Mile Island power station is near Harrisburg, Pennsylvania in USA. It had two pressurized water reactors. One PWR was of 800 MWe (775 MWe net) and entered service in 1974. It remains one of the best-performing units in USA. Unit 2 was of 906 MWe (880 MWe net) and almost brand new.

The accident to unit 2 happened at 4 am on 28 March 1979 when the reactor was operating at 97% power. It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down automatically. Shut down took about one second. At this point a relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away that the residual decay heat in the reactor core was not removed. The core suffered severe damage as a result. The operators were unable to diagnose or respond properly to the unplanned automatic shutdown of the reactor. Deficient control room instrumentation and inadequate emergency response training proved to be root causes of the accident The chain of events during the Three Mile Island Accident Within seconds of the shutdown, the pilot-operated relief valve (PORV) on the reactor cooling system opened, as it was supposed to. About 10 seconds later it should have closed. But it remained open, leaking vital reactor coolant water to the reactor coolant drain tank. The operators believed the relief valve had shut because instruments showed them that a "close" signal was sent to the valve.

However, they did not have an instrument indicating the valve's actual position. Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, cooling water surged into the pressuriser, raising the water level in it. (The pressuriser is a tank which is part of the primary reactor cooling system, maintaining proper pressure in the system. The relief valve is located on the pressuriser. In a PWR like TMI-2, water in the primary cooling system around the core is kept under very high pressure to keep it from boiling.) Operators responded by reducing the flow of replacement water. Their training told them that the pressuriser water level was the only dependable indication of the amount of cooling water in the system. Because the pressuriser level was increasing, they thought the reactor system was too full of water. Their training told them to do all they could to keep the pressuriser from filling with water. If it filled, they could not control pressure in the cooling system and it might rupture. Steam then formed in the reactor primary cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate.

Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps. This ended forced cooling of the reactor core. (The operators still believed the system was nearly full of water because the pressuriser level remained high.) However, as reactor coolant water boiled away, the reactor?s fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the cooling water. At 6:22 am operators closed a block valve between the relief valve and the pressuriser. This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system. Throughout the morning, operators attempted to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water. During the afternoon, operators attempted to decrease the pressure in the reactor system to allow a low pressure cooling system to be used and emergency water supplies to be put into the system. Cooling Restored, radioactive releases to air By late afternoon, operators began high-pressure injection of water into the reactor cooling system to increase pressure and to collapse steam bubbles.

By 7:50 pm on 28 March, they restored forced cooling of the reactor core when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations. Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked, and some radioactive gas was released to the environment. These went through high-efficiency particulate air (HEPA) filters and charcoal filters which removed most of the radionuclides, except for the noble gases, the estimated total of which was about 370 PBq (the Kemeny Commission said ‘a maximum of 480 PBq noble gases’ and NRC also quotes 1.6 PBq of krypton rlelease in July). With short half-life and being biologically inert, these did not pose a health hazard.

The Hydrogen Bubble When the reactor's core was uncovered, on the morning of 28 March, a high-temperature chemical reaction between water and the zircaloy metal tubes holding the nuclear fuel pellets had created hydrogen gas. In the afternoon of 28 March, a sudden rise in reactor building pressure shown by the control room instruments indicated a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel. From 30 March through 1 April operators removed this hydrogen gas "bubble" by periodically opening the vent valve on the reactor cooling system pressuriser. For a time, regulatory (NRC) officials believed the hydrogen bubble could explode, though such an explosion was never possible since there was not enough oxygen in the system. Cold Shutdown and investigation After an anxious month, on 27 April operators established natural convection circulation of coolant. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping. The plant was in "cold shutdown", ie with the water at less than 100°C at atmospheric pressure. The head of the reactor pressure vessel was removed in July 1984 allowing access to the remains of the core.

Subsequent investigation revealed that at lest 45% of the core - 62 tonnes - had melted and 19 tonnes of this had ended up in the lower plenum, mostly in the lower head of the reactor pressure vessel, but without seriously damaging the vessel. Most of the melted core material (corium) had remained in the core region. In 1988 a multinational OECD Vessel Investigation Project (VIP) took samples to evaluate the situation in detail and confirmed that there was much less damage than anticipated. Public concern and confusion When the TMI-2 accident is recalled, it is often in the context of what happened on Friday and Saturday, March 30-31. The drama of the TMI-2 accident-induced fear, stress and confusion on those two days. The atmosphere then, and the reasons for it, are described well in the book "Crisis Contained, The Department of Energy at Three Mile Island," by Philip L Cantelon and Robert C. Williams, 1982. This is an official history of the Department of Energy's role during the accident.

"Friday appears to have become a turning point in the history of the accident because of two events: the sudden rise in reactor pressure shown by control room instruments on Wednesday afternoon (the "hydrogen burn") which suggested a hydrogen explosion? became known to the Nuclear Regulatory Commission [that day]; and the deliberate venting of radioactive gases from the plant Friday morning which produced a reading of 1,200 millirems (12 mSv) directly above the stack of the auxiliary building. "What made these significant was a series of misunderstandings caused, in part, by problems of communication within various state and federal agencies. Because of confused telephone conversations between people uninformed about the plant's status, officials concluded that the 1,200 millirems (12 mSv) reading was an off-site reading. They also believed that another hydrogen explosion was possible, that the Nuclear Regulatory Commission had ordered evacuation and that a meltdown was conceivable. "Garbled communications reported by the media generated a debate over evacuation. Whether or not there were evacuation plans soon became academic.

What happened on Friday was not a planned evacuation but a weekend exodus based not on what was actually happening at Three Mile Island but on what government officials and the media imagined might happen. On Friday confused communications created the politics of fear." (Page 50) Throughout the book, Cantelon and Williams note that hundreds of environmental samples were taken around TMI during the accident period by the Department of Energy (which had the lead sampling role) or the then-Pennsylvania Department of Environmental Resources. But there were no unusually high readings, except for noble gases, and virtually no iodine. Readings were far below health limits. Yet a political storm was raging based on confusion and misinformation. No Radiological Health Effects The Three Mile Island accident caused concerns about the possibility of radiation-induced health effects, principally cancer, in the area surrounding the plant. Because of those concerns, the Pennsylvania Department of Health for 18 years maintained a registry of more than 30,000 people who lived within five miles of Three Mile Island at the time of the accident. The state's registry was discontinued in mid 1997, without any evidence of unusual health trends in the area.

Indeed, more than a dozen major, independent health studies of the accident showed no evidence of any abnormal number of cancers around TMI years after the accident. The only detectable effect was psychological stress during and shortly after the accident. The studies found that the radiation releases during the accident were minimal, well below any levels that have been associated with health effects from radiation exposure. The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts, with no more than 1 millisievert to any single individual. The level of 0.08 mSv is about equal to a chest X-ray, and 1 mSv is about a third of the average background level of radiation received by U.S. residents in a year. In June 1996, 17 years after the TMI-2 accident, Harrisburg U.S. District Court Judge Sylvia Rambo dismissed a class action lawsuit alleging that the accident caused health effects. The plaintiffs have appealed Judge Rambo's ruling. The appeal is before the U.S. Third Circuit Court of Appeals.

However, in making her decision, Judge Rambo cited: · Findings that exposure patterns projected by computer models of the releases compared so well with data from the TMI dosimeters (TLDs) available during the accident that the dosimeters probably were adequate to measure the releases. · That the maximum offsite dose was, possibly, 100 millirem (1 mSv), and that projected fatal cancers were less than one. · The plaintiffs' failure to prove their assertion that one or more unreported hydrogen "blowouts" in the reactor system caused one or more unreported radiation "spikes", producing a narrow yet highly concentrated plume of radioactive gases. Judge Rambo concluded: "The parties to the instant action have had nearly two decades to muster evidence in support of their respective cases.... The paucity of proof alleged in support of Plaintiffs' case is manifest. The court has searched the record for any and all evidence which construed in a light most favourable to Plaintiffs creates a genuine issue of material fact warranting submission of their claims to a jury.

This effort has been in vain." More than a dozen major, independent studies have assessed the radiation releases and possible effects on the people and the environment around TMI since the 1979 accident at TMI-2. The most recent was a 13-year study on 32,000 people. None has found any adverse health effects such as cancers which might be linked to the accident. The TMI-2 Cleanup The cleanup of the damaged nuclear reactor system at TMI-2 took nearly 12 years and cost approximately US$973 million. The cleanup was uniquely challenging technically and radiologically. Plant surfaces had to be decontaminated. Water used and stored during the cleanup had to be processed. And about 100 tonnes of damaged uranium fuel had to be removed from the reactor vessel -- all without hazard to cleanup workers or the public. A cleanup plan was developed and carried out safely and successfully by a team of more than 1000 skilled workers. It began in August 1979, with the first shipments of accident-generated low-level radiological waste to Richland, Washington.

In the cleanup's closing phases, in 1991, final measurements were taken of the fuel remaining in inaccessible parts of the reactor vessel. Approximately one percent of the fuel and debris remains in the vessel. Also in 1991, the last remaining water was pumped from the TMI-2 reactor. The cleanup ended in December 1993, when Unit 2 received a license from the NRC to enter Post Defueling Monitored Storage (PDMS). Early in the cleanup, Unit 2 was completely severed from any connection to TMI Unit 1. TMI-2 today is in long-term monitored storage. No further use of the nuclear part of the plant is anticipated. Ventilation and rainwater systems are monitored. Equipment necessary to keep the plant in safe long-term storage is maintained. Defueling the TMI-2 reactor vessel was the heart of the cleanup. The damaged fuel remained underwater throughout the defueling. In October 1985, after nearly six years of preparations, workers standing on a platform atop the reactor and manipulating long-handled tools began lifting the fuel into canisters that hung beneath the platform.

In all, 342 fuel canisters were shipped safely for long-term storage at the Idaho National Laboratory, a program that was completed in April 1990. It was put into dry storage in concrete containers. TMI-2 cleanup operations produced over 10.6 megalitres of accident-generated water that was processed, stored and ultimately evaporated safely. In February 1991, the TMI-2 Cleanup Program was named by the National Society of Professional Engineers as one of the top engineering achievements in the U.S. completed during 1990. In 2010 the generator was sold by FirstEnergy to Progress Energy to upgrade its Shearon Harris nuclear power plant in North Carolina. It was shipped in two parts, the rotor, which weighs 170 tonnes, and the stator, which weighs about 500 tonnes. The NRC web site has a factsheet on Three Mile Island . TMI-1: Safe and World-Class From its restart in 1985, Three Mile Island Unit 1 has operated at very high levels of safety and reliability. Application of the lessons of the TMI-2 accident has been a key factor in the plant's outstanding performance.

In 1997, TMI-1 completed the longest operating run of any light water reactor in the history of nuclear power worldwide - 616 days and 23 hours of uninterrupted operation. (That run was also the longest at any steam-driven plant in the U.S., including plants powered by fossil fuels.) And in October 1998, TMI employees completed three million hours of work without a lost-work day accident. At the time of the TMI-2 accident, TMI-1 was shut down for refueling. It was kept shut down during lengthy proceedings by the Nuclear Regulatory Commission. During the shutdown, the plant was modified and training and operating procedures were revamped in light of the lessons of TMI-2. When TMI-1 restarted in October 1985, General Public Utilities pledged that the plant would be operated safely and efficiently and would become a leader in the nuclear power industry. Those pledges have been kept. The plant's capability factor for 1987, including almost three months of a five-month refueling and maintenance outage, was 74.1 percent, compared to an industry average of 62 percent.

(Capability factor refers to the amount of electricity generated compared to the plant's maximum capacity.) In 1988 a 1.3% (11 MWe) uprate was licensed. For 1989, TMI-1's capability factor was 100.03 percent and the best of 357 nuclear power plants worldwide, according to Nucleonics Week. In 1990-91, TMI-1 operated 479 consecutive days, the longest operating run at that point in the history of US commercial nuclear power. It was named by the NRC as one of the four safest plants in the country during this period. By the end of 1994, TMI-1 was one of the first two plants in the history of US commercial nuclear power to achieve a three-year average capability factor of over 90% (TMI-1 had 94.3%). In October 1998, TMI workers completed two full years without a lost workday injury. Since its restart, TMI-1 has earned consistently high ratings in the NRC's program, Systematic Assessment of Licensee Performance (SALP).

In 2009, the TMI-1 operating licence was renewed, extending it life by 20 years to 2034. Immediately following this, both steam generators were replaced as TMI's "largest capital project to date" In 1999, TMI-1 was purchased by AmerGen, a new joint venture between British Energy and PECO Energy. In 2003 the BE share was sold so that the plant became wholly-owned by Exelon, PECO's successor. It is now listed as producing 786 MWe net. Training improvements Training reforms are among the most significant outcomes of the TMI-2 accident. Training became centred on protecting a plant's cooling capacity, whatever the triggering problem might be. At TMI-2, the operators turned to a book of procedures to pick those that seemed to fit the event. Now operators are taken through a set of "yes-no" questions to ensure, first, that the reactor's fuel core remains covered. Then they determine the specific malfunction. This is known as a "symptom-based" approach for responding to plant events.

Underlying it is a style of training that gives operators a foundation for understanding both theoretical and practical aspects of plant operations. The TMI-2 accident also led to the establishment of the Atlanta-based Institute of Nuclear Power Operations (INPO) and its National Academy for Nuclear Training. These two industry organisations have been effective in promoting excellence in the operation of nuclear plants and accrediting their training programs. INPO was formed in 1979. The National Academy for Nuclear Training was established under INPO's auspices in 1985. TMI's operator training program has passed three INPO accreditation reviews since then. Training has gone well beyond button-pushing. Communications and teamwork, emphasizing effective interaction among crew members, are now part of TMI's training curriculum. Close to half of the operators' training is in a full-scale electronic simulator of the TMI control room. The $18 million simulator permits operators to learn and be tested on all kinds of accident scenarios. Increased safety & reliability Disciplines in training, operations and event reporting that grew from the lessons of the TMI-2 accident have made the nuclear power industry demonstrably safer and more reliable.

Those trends have been both promoted and tracked by the Institute for Nuclear Power Operations (INPO). To remain in good standing, a nuclear plant must meet the high standards set by INPO as well as the strict regulation of the US Nuclear Regulatory Commission. A key indicator is the graph of significant plant events, based on data compiled by the Nuclear Regulatory Commission. The number of significant events decreased from 2.38 per reactor unit in 1985 to 0.10 at the end of 1997. On the reliability front, the median capability factor for nuclear plants - the percentage of maximum energy that a plant is capable of generating - increased from 62.7 percent in 1980 to almost 90 percent in 2000. (The goal for the year 2000 was 87 percent.) Other indicators for US plants tracked by INPO and its world counterpart, the World Association of Nuclear Operators (WANO) are the unplanned capability loss factor, unplanned automatic scrams, safety system performance, thermal performance, fuel reliability, chemistry performance, collective radiation exposure, volume of solid radioactive waste and industrial safety accident rate.

All are reduced, that is, improved substantially, from 1980. Summary What Happened: After shutting down the fission reaction, the TMI-2 reactor's fuel core became uncovered and more than one third of the fuel melted. Inadequate instrumentation and training programs at the time hampered operators' ability to respond to the accident. The accident was accompanied by communications problems that led to conflicting information available to the public, contributing to the public's fears A small amount of radiation was released from the plant. The releases were not serious and were not health hazards. This was confirmed by thousands of environmental and other samples and measurements taken during the accident. The containment building worked as designed. Despite melting of about one-third of the fuel core, the reactor vessel itself maintained its integrity and contained the damaged fuel. What did not happen: There was no "China Syndrome". There were no injuries or detectable health impacts from the accident, beyond the initial stress. Longer-Term Impacts: Applying the accident's lessons produced important, continuing improvement in the performance of all nuclear power plants. The accident fostered better understanding of fuel melting, including improbability of a "China Syndrome" meltdown breaching the reactor vessel and the containment structure.

Public confidence in nuclear energy, particularly in USA, declined sharply following the Three Mile Island accident. It was a major cause of the decline in nuclear construction through the 1980s and 1990s. Sources GPU Nuclear Corp., 10 briefing papers, 1999. NEI, Nuclear Energy Overview 8/5/00. Share Three Mile Island: 35 years later | StateImpact Pennsylvania Three Mile Island: 35 years later March 28, 2014 | 12:01 AM Joe Ulrich/ WITF Three Mile Island was the most serious accident in U.S. commercial nuclear power plant operating history. Today marks the 35th anniversary of the worst nuclear accident in U.S. history. On March 28th, 1979 one of the reactors at the The Three Mile Island nuclear power plant in Dauphin County partially melted down. Back then, no one knew exactly what was going to happen. The incident was unprecedented. Former Governor Dick Thornburgh was at the center of the crisis. He marked the anniversary at an event Thursday at Penn State’s Harrisburg campus in Middletown–not far from the plant.

Thornburgh remains troubled the country still has no long-term plan to handle its nuclear waste. “The solution to the problem of nuclear waste has to be at the top of anybody’s to-do list, when it comes to ensuring we’re going to have a vital nuclear energy industry that can contribute to our further economic growth,” he says. Former Middletown Mayor Robert Reid was also in the thick of the emergency response. He’s spent decades living in the shadow of the plant’s massive cooling towers. Reid had just been elected when the accident happened. Like many people at the time, he had trouble getting credible information about the threat it posed. “Back, 35 years ago, I was well… anti-nuclear,” he says. “But with the operation of the plant now, by this company– I think it’s a good neighbor. I’ve changed my mind about nuclear energy.” The plant’s still operational Unit 1 reactor is now owned by Exelon, while the mothballed Unit 2 reactor is owned by FirstEnergy. In the years since the accident, no new plants have been commissioned in the United States, and regulatory oversight has increased, according to Neil Sheehan a spokesman for the U.S. Nuclear Regulatory Commission.

“This did have significant impacts across the US nuclear power industry and for nuclear power plants around the world.” The decommissioning of the plant and restoration of the site is still decades away. The license for the Unit 1 reactor expires in 20 years. At that point, both reactors will be dismantled together, over a 10 year period. Plans call for a full restoration of the site by 2054 — 75 years after the accident. “Thirty-five years later, we’re a bookmark,” says Eric Epstein, of the nuclear watchdog group, Three Mile Island Alert. ”Chernobyl happened, then Fukushima happened. Unfortunately we’re part of an exclusive club that has experienced a nuclear catastrophe.” He says since there is currently nowhere to take the waste, it has be indefinitely entombed on the island in the middle of the Susquehanna River and he’s not hopeful it will be removed this century. “This is an inter-generational problem for people who have yet to be born, and that’s not fair,” he says.

“I think as adults we have to come to grips with the fact that we’ve created a hazardous waste site and we’ll have to guard it—isolate it—for hundreds, if not thousands of years.” The decommissioning plans were outlined in a report that was submitted last summer to the NRC. Although the agency isn’t required to give an official approval, it’s still reviewing the proposal.

Question: Three Mile Island is in which US state?
Answer: Pennsylvania