Three Mile Island Accident Case Study

The Three Mile Island accident occurred on March 28, 1979, in reactor number 2 of Three Mile Island Nuclear Generating Station (TMI-2) in Dauphin County, Pennsylvania, near Harrisburg. It was the most significant accident in U.S. commercial nuclear power plant history.[2] The incident was rated a five on the seven-point International Nuclear Event Scale: Accident with wider consequences.[3][4]

The accident began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve in the primary system, which allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors, such as human-computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system of the reactor because the operator mistakenly believed that there was too much coolant water present in the reactor and causing the steam pressure release.[5]

The accident crystallized anti-nuclear safety concerns among activists and the general public, resulted in new regulations for the nuclear industry, and has been cited as a contributor to the decline of a new reactor construction program that was already underway in the 1970s.[6] The partial meltdown resulted in the release of radioactive gases and radioactive iodine into the environment. Worries were expressed by anti-nuclear movement activists;[7] however, epidemiological studies analyzing the rate of cancer in and around the area since the accident, determined there was a small statistically non-significant increase in the rate and thus no causal connection linking the accident with these cancers has been substantiated.[8][9][10][11][12][13] Cleanup started in August 1979, and officially ended in December 1993, with a total cleanup cost of about $1 billion.[14]


Stuck valve[edit]

In the nighttime hours preceding the incident, the TMI-2 reactor was running at 97% of full power, while the companion TMI-1 reactor was shut down for refueling.[16] The main chain of events leading to the partial core meltdown began at 4:37 am EST on March 28, 1979, in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor (PWR).

The initial cause of the accident happened eleven hours earlier, during an attempt by operators to fix a blockage in one of the eight condensate polishers, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and impurities in the water from accumulating in the steam generators and increasing corrosion rates in the secondary side.

Blockages are common with these resin filters and are usually fixed easily, but in this case the usual method of forcing the stuck resin out with compressed air did not succeed. The operators decided to blow the compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument air line. This would eventually cause the feedwater pumps, condensate booster pumps, and condensate pumps to turn off around 4:00 am, which would in turn cause a turbine trip.[17]

With the steam generators no longer receiving feedwater, heat and pressure increased in the reactor coolant system, causing the reactor to perform an emergency shutdown (SCRAM). Within eight seconds, control rods were inserted into the core to halt the nuclear chain reaction. The reactor continued to generate decay heat and, because steam was no longer being used by the turbine, heat was no longer being removed from the reactor's primary water loop.[18]

Once the secondary feedwater pumps stopped, three auxiliary pumps activated automatically. However, because the valves had been closed for routine maintenance, the system was unable to pump any water. The closure of these valves was a violation of a key Nuclear Regulatory Commission (NRC) rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure.[19]

The loss of heat removal from the primary loop and the failure of the auxiliary system to activate caused the primary loop pressure to increase, triggering the pilot-operated relief valve at the top of the pressurizer – a pressure active-regulator tank – to open automatically. The relief valve should have closed when the excess pressure had been released, and electric power to the solenoid of the pilot was automatically cut, but the relief valve stuck open because of a mechanical fault. The open valve permitted coolant water to escape from the primary system, and was the principal mechanical cause of the primary coolant system depressurization and partial core disintegration that followed.[20]

Human factors: confusion over valve status[edit]

Critical human factors and user interface engineering problems were revealed in the investigation of the reactor control system's user interface. Despite the valve being stuck open, a light on the control panel ostensibly indicated that the valve was closed. In fact the light did not indicate the position of the valve, only the status of the solenoid being powered or not, thus giving false evidence of a closed valve.[21] As a result, the operators did not correctly diagnose the problem for several hours.[22]

The design of the pilot-operated relief valve indicator light was fundamentally flawed. The bulb was simply connected in parallel with the valve solenoid, thus implying that the pilot-operated relief valve was shut when it went dark, without actually verifying the real position of the valve. When everything was operating correctly, the indication was true and the operators became habituated to rely on it. However, when things went wrong and the main relief valve stuck open, the unlighted lamp was actually misleading the operators by implying that the valve was shut. This caused the operators considerable confusion, because the pressure, temperature and coolant levels in the primary circuit, so far as they could observe them via their instruments, were not behaving as they would have if the pilot-operated relief valve were shut. This confusion contributed to the severity of the accident because the operators were unable to break out of a cycle of assumptions that conflicted with what their instruments were telling them. The problem was not correctly diagnosed until a fresh shift came in who did not have the mindset of the first shift of operators. By this time major damage had occurred.

The operators had not been trained to understand the ambiguous nature of the pilot-operated relief valve indicator and to look for alternative confirmation that the main relief valve was closed. There was a temperature indicator downstream of the pilot-operated relief valve in the tail pipe between the pilot-operated relief valve and the pressurizer that could have told them the valve was stuck open, by showing that the temperature in the tail pipe remained higher than it should have been had the pilot-operated relief valve been shut. This temperature indicator, however, was not part of the "safety grade" suite of indicators designed to be used after an incident, and the operators had not been trained to use it. Its location on the back of the seven-foot-high instrument panel also meant that it was effectively out of sight of the operators.[23]

Consequences of stuck valve[edit]

As the pressure in the primary system continued to decrease, reactor coolant continued to flow, but it was boiling inside the core. First, small bubbles of steam formed and immediately collapsed, known as nucleate boiling. As the system pressure decreased further, steam pockets began to form in the reactor coolant. This departure from nucleate boiling (DNB) into the regime of "film boiling" caused steam voids in coolant channels, blocking the flow of liquid coolant and greatly increasing the fuel cladding temperature. The overall water level inside the pressurizer was rising despite the loss of coolant through the open pilot-operated relief valve, as the volume of these steam voids increased much more quickly than coolant was lost. Because of the lack of a dedicated instrument to measure the level of water in the core, operators judged the level of water in the core solely by the level in the pressurizer. Since it was high, they assumed that the core was properly covered with coolant, unaware that because of steam forming in the reactor vessel, the indicator provided misleading readings.[24] Indications of high water levels contributed to the confusion, as operators were concerned about the primary loop "going solid," (i.e. no steam pocket buffer existing in the pressurizer) which in training they had been instructed to never allow. This confusion was a key contributor to the initial failure to recognize the accident as a loss-of-coolant accident, and led operators to turn off the emergency core cooling pumps, which had automatically started after the pilot-operated relief valve stuck and core coolant loss began, due to fears the system was being overfilled.[25]

With the pilot-operated relief valve still open, the pressurizer relief tank that collected the discharge from the pilot-operated relief valve overfilled, causing the containment building sump to fill and sound an alarm at 4:11 am. This alarm, along with higher than normal temperatures on the pilot-operated relief valve discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing loss-of-coolant accident, but these indications were initially ignored by operators.[26] At 4:15 am, the relief diaphragm of the pressurizer relief tank 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.[27]

After almost 80 minutes of slow temperature rise, the primary loop's four main reactor coolant pumps began to cavitate as a steam bubble/water mixture, rather than water, passed through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, 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 Zircaloynuclear fuel rodcladding, yielding zirconium dioxide, hydrogen, and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant, and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.[28]

At 6 am, there was a shift change in the control room. A new arrival noticed that the temperature in the pilot-operated relief valve tail pipe and the holding tanks was excessive and used a backup valve – called a "block valve" – to shut off the coolant venting via the pilot-operated relief valve, but around 32,000 US gal (120,000 l) of coolant had already leaked from the primary loop.[29] 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 general containment building was seriously contaminated.

Emergency declared[edit]

At 6:57 am, a plant supervisor declared a site area emergency, and less than 30 minutes later station manager Gary Miller announced a general emergency, defined as having the "potential for serious radiological consequences" to the general public.[30]Metropolitan Edison (Met Ed) notified the Pennsylvania Emergency Management Agency (PEMA), which in turn contacted state and local agencies, GovernorRichard L. Thornburgh and lieutenant governorWilliam Scranton III, to whom Thornburgh assigned responsibility for collecting and reporting on information about the accident.[31] The uncertainty of operators at the plant was reflected in fragmentary, ambiguous, or contradictory statements made by Met Ed to government agencies and to the press, particularly about the possibility and severity of off-site radioactivity releases. Scranton held a press conference in which he was reassuring, yet confusing, about this possibility, stating that though there had been a "small release of increase in normal radiation levels" had been detected. These were contradicted by another official, and by statements from Met Ed, who both claimed that no radioactivity had been released.[32] In fact, readings from instruments at the plant and off-site detectors had detected radioactivity releases, albeit at levels that were unlikely to threaten public health as long as they were temporary, and providing that containment of the then highly contaminated reactor was maintained.[33]

Angry that Met Ed had not informed them before conducting a steam venting from the plant, and convinced that the company was downplaying the severity of the accident, state officials turned to the NRC.[34] After receiving word of the accident from Met Ed, the NRC had activated its emergency response headquarters in Bethesda, Maryland and sent staff members to Three Mile Island. NRC chairman Joseph Hendrie and commissioner Victor Gilinsky[35] initially viewed the accident, in the words of NRC historian Samuel Walker, as a "cause for concern but not alarm".[36] Gilinsky briefed reporters and members of Congress on the situation and informed White House staff, and at 10:00 a.m. met with two other commissioners. However, the NRC faced the same problems in obtaining accurate information as the state, and was further hampered by being organizationally ill-prepared to deal with emergencies, as it lacked a clear command structure and the authority to tell the utility what to do, or to order an evacuation of the local area.[37]

In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point".[38] He further wrote: "We didn't learn for years – until the reactor vessel was physically opened – that by the time the plant operator called the NRC at about 8:00 a.m., roughly half of the uranium fuel had already melted."[38]

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 group of workers took manual readings from the thermocouples and obtained a sample of primary loop water. Seven hours into the emergency, new water was pumped into the primary loop and the backup relief valve was opened to reduce pressure so that the loop could be filled with water. After 16 hours the primary loop pumps were turned on once again, and the core temperature began to fall. A large part of the core had melted, and the system was still dangerously radioactive.

On the third day following the accident, a hydrogen bubble was discovered in the dome of the pressure vessel, and became the focus of concern. A hydrogen explosion might not only breach the pressure vessel, but, depending on its magnitude, might compromise the integrity of the containment vessel leading to large-scale release of radioactive material. However, it was determined that there was no oxygen present in the pressure vessel, a prerequisite for hydrogen to burn or explode. Immediate steps were taken to reduce the hydrogen bubble, and by the following day it was significantly smaller. Over the next week, steam and hydrogen were removed from the reactor using a catalytic recombiner and, controversially, by venting straight to the atmosphere.

Release of radioactive material[edit]

The release occurred when the cladding was damaged while the pilot-operated relief valve was still stuck open. Fission products were released into the reactor coolant. Since the pilot-operated relief valve was stuck open and the loss of coolant accident was still in progress, primary coolant with fission products and/or fuel was released, and ultimately ended up in the auxiliary building. The auxiliary building was outside the containment boundary.

This was evidenced by the radiation alarms that eventually sounded. However, since very little of the fission products released were solids at room temperature, very little radiological contamination was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. According to the Rogovin report, the vast majority of the radioisotopes released were the noble gases xenon and krypton. The report stated, "During the course of the accident, approximately 2.5 MCi (93 PBq) of radioactive noble gases and 15 Ci (560 GBq) of radioiodines were released." This resulted in an average dose of 1.4 mrem (14 μSv) to the two million people near the plant. The report compared this with the additional 80 mrem (800 μSv) per year received from living in a high altitude city such as Denver.[39] As further comparison, a patient receives 3.2 mrem (32 μSv) from a chest X-ray – more than twice the average dose of those received near the plant.[40] Measures of beta radiation were excluded from the report.

Within hours of the accident, the United States Environmental Protection Agency (EPA) began daily sampling of the environment at the three stations closest to the plant. Continuous monitoring at 11 stations was not established until April 1, and was expanded to 31 stations on April 3. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area, but measures of beta radiation were not included. The EPA found no contamination in water, soil, sediment or plant samples.[41]

Researchers at nearby Dickinson College – which had radiation monitoring equipment sensitive enough to detect Chinese atmospheric atomic weapons-testing – collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely due to natural radon plate-out, not the accident).[42] Also, white-tailed deer tongues harvested over 50 mi (80 km) from the reactor subsequent to the accident were found to have significantly higher levels of cesium-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric weapons testing.[43] Had there been elevated releases of radioactivity, increased levels of iodine-131 and cesium-137 would have been expected to be detected in cattle and goat's milk samples. Yet elevated levels were not found.[44] A later scientific study noted that the official emission figures were consistent with available dosimeter data,[45] though others have noted the incompleteness of this data, particularly for releases early on.[46]

According to the official figures, as compiled by the 1979 Kemeny Commission from Metropolitan Edison and NRC data, a maximum of 480 PBq (13 MCi) of radioactive noble gases (primarily xenon) were released by the event.[47] However, these noble gases were considered relatively harmless,[48] and only 481–629 GBq (13.0–17.0 Ci) of thyroid cancer-causing iodine-131 were released.[47] Total releases according to these figures were a relatively small proportion of the estimated 370 EBq (10 GCi) in the reactor.[48] It was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed,[15][49] with 5 ft (1.5 m) of the core gone, and around 20 short tons (18 t) of uranium flowing to the bottom head of the pressure vessel, forming a mass of corium.[50] The reactor vessel – the second level of containment after the cladding – maintained integrity and contained the damaged fuel with nearly all of the radioactive isotopes in the core.[51]

Anti-nuclear political groups disputed the Kemeny Commission's findings, claiming that independent measurements provided evidence of radiation levels up to seven times higher than normal in locations hundreds of miles downwind from TMI.[52]Arnie Gundersen, a nuclear engineer and former nuclear industry manager, said "I think the numbers on the NRC's website are off by a factor of 100 to 1,000".[48][53]

Some other insiders[who?] make the same claim; Gundersen, a former nuclear industry executive who is now an expert witness in nuclear safety issues,[54][55] offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before 2:00 p.m. on March 28, 1979, which would have provided the means for a high dose of radiation to occur.[48] Gundersen cites affidavits from four reactor operators according to which the plant manager was aware of a dramatic pressure spike, after which the internal pressure dropped to outside pressure. Gundersen also notes that the control room shook and doors were blown off hinges. However official NRC reports refer merely to a "hydrogen burn."[48] The Kemeny Commission referred to "a burn or an explosion that caused pressure to increase by 28 pounds per square inch in the containment building".[56]The Washington Post reported that "At about 2:00 pm, with pressure almost down to the point where the huge cooling pumps could be brought into play, a small hydrogen explosion jolted the reactor."[57]


Voluntary evacuation[edit]

Twenty-eight hours after the accident began, William Scranton III, the lieutenant governor, appeared at a news briefing to say that Metropolitan Edison, the plant's owner, had assured the state that "everything is under control".[58] Later that day, Scranton changed his statement, saying that the situation was "more complex than the company first led us to believe."[58] There were conflicting statements about radioactivity releases.[59] Schools were closed and residents were urged to stay indoors. Farmers were told to keep their animals under cover and on stored feed.[58][59]

Governor Dick Thornburgh, on the advice of NRC chairman Joseph Hendrie, advised the evacuation "of pregnant women and pre-school age children...within a five-mile radius of the Three Mile Island facility." The evacuation zone was extended to a 20-mile radius on Friday, March 30.[60] Within days, 140,000 people had left the area.[15][58][61] More than half of the 663,500 population within the 20-mile radius remained in that area.[62][60] According to a survey conducted in April 1979, 98% of the evacuees had returned to their homes within three weeks.[60]

Post-TMI surveys have shown that less than 50% of the American public were satisfied with the way the accident was handled by Pennsylvania State officials and the NRC, and people surveyed were even less pleased with the utility (General Public Utilities) and the plant designer.[63]


Several state and federal government agencies mounted investigations into the crisis, the most prominent of which was the President's Commission on the Accident at Three Mile Island, created by Jimmy Carter in April 1979.[64] The commission consisted of a panel of twelve people, specifically chosen for their lack of strong pro- or anti-nuclear views, and headed by chairman John G. Kemeny, president of Dartmouth College. It was instructed to produce a final report within six months, and after public hearings, depositions, and document collection, released a completed study on October 31, 1979.[65] The investigation strongly criticized Babcock & Wilcox, Met Ed, GPU, and the NRC for lapses in quality assurance and maintenance, inadequate operator training, lack of communication of important safety information, poor management, and complacency, but avoided drawing conclusions about the future of the nuclear industry.[66] The heaviest criticism from the Kemeny Commission concluded that "fundamental changes were necessary in the organization, procedures, practices 'and above all – in the attitudes' of the NRC [and the nuclear industry.]"[67] Kemeny said that the actions taken by the operators were "inappropriate" but that the workers "were operating under procedures that they were required to follow, and our review and study of those indicates that the procedures were inadequate" and that the control room "was greatly inadequate for managing an accident."[68]

The Kemeny Commission noted that Babcock & Wilcox's pilot-operated relief valve had previously failed on 11 occasions, nine of them in the open position, allowing coolant to escape. More disturbing, however, was the fact that the initial causal sequence of events at TMI had been duplicated 18 months earlier at another Babcock & Wilcox reactor, the Davis-Besse Nuclear Power Station owned at that time by Toledo Edison. The only difference was that the operators at Davis-Besse identified the valve failure after 20 minutes, where at TMI it took 80 minutes, and the Davis-Besse facility was operating at 9% power, against TMI's 97%. Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue.[69]

The Pennsylvania House of Representatives conducted its own investigation, which focused on the need to improve evacuation procedures[citation needed].

In 1985, a television camera was used to see the interior of the damaged reactor. In 1986, core samples and samples of debris were obtained from the corium layers on the bottom of the reactor vessel and analyzed.[70]

Effect on nuclear power industry[edit]

According to the IAEA, the Three Mile Island accident was a significant turning point in the global development of nuclear power.[71] From 1963–1979, the number of reactors under construction globally increased every year except 1971 and 1978. However, following the event, the number of reactors under construction in the U.S. declined every year from 1980–1998.[citation needed] Many similar Babcock & Wilcox reactors on order were canceled; in total, 51 U.S. nuclear reactors were canceled from 1980–1984.[72]

The 1979 TMI accident did not initiate the demise of the U.S. nuclear power industry, but it did halt its historic growth. Additionally, as a result of the earlier 1973 oil crisis and post-crisis analysis with conclusions of potential overcapacity in base load, forty planned nuclear power plants already had been canceled before the TMI accident. At the time of the TMI incident, 129 nuclear power plants had been approved, but of those, only 53 (which were not already operating) were completed. During the lengthy review process, complicated by the Chernobyl Disaster seven years later, Federal requirements to correct safety issues and design deficiencies became more stringent, local opposition became more strident, construction times were significantly lengthened and costs skyrocketed.[73] Until 2012,[74] no U.S. nuclear power plant had been authorized to begin construction since the year before TMI.

Globally, the end of the increase in nuclear power plant construction came with the more catastrophic Chernobyl disaster in 1986 (see graph).


Three Mile Island Unit 2 was too badly damaged and contaminated to resume operations; the reactor was gradually deactivated and permanently closed. TMI-2 had been online only 13 months but now had a ruined reactor vessel and a containment building that was unsafe to walk in. Cleanup started in August 1979 and officially ended in December 1993, with a total cleanup cost of about $1 billion.[14]Benjamin K. Sovacool, in his 2007 preliminary assessment of major energy accidents, estimated that the TMI accident caused a total of $2.4 billion in property damages.[75]

Initially, efforts focused on the cleanup and decontamination of the site, especially the defueling of the damaged reactor. Starting in 1985, almost 100 short tons (91 t) of radioactive fuel were removed from the site. The first major phase of the cleanup was completed in 1990, when workers finished shipping 150 short tons (140 t) of radioactive wreckage to Idaho for storage at the Department of Energy's National Engineering Laboratory. However, the contaminated cooling water that leaked into the containment building had seeped into the building's concrete, leaving the radioactive residue too impractical to remove. 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 deferred to allow for decay of the radiation levels and to take advantage of the potential economic benefits of retiring both Unit 1 and Unit 2 together.[14]

Health effects and epidemiology[edit]

Main article: Three Mile Island accident health effects

In the aftermath of the accident, investigations focused on the amount of radioactivity released by the accident. In total approximately 2.5 megacuries (93 PBq) of radioactive gases, and approximately 15 curies (560 GBq) of iodine-131 was released into the environment.[76] According to the American Nuclear Society, using the official radioactivity emission figures, "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."[51][77]

Based on these emission figures, early scientific publications, according to Mangano, on the health effects of the fallout estimated no additional cancer deaths in the 10 mi (16 km) area around TMI.[52] Disease rates in areas further than 10 miles from the plant were never examined.[52] Local activism in the 1980s, based on anecdotal reports of negative health effects, led to scientific studies being commissioned. A variety of epidemiology studies have concluded that the accident had no observable long term health effects.[8][12][78][79]

The Radiation and Public Health Project, an organization with little credibility amongst epidemiologists,[80] cited calculations by its member Joseph Mangano – who has authored 19 medical journal articles and a book on Low Level Radiation and Immune Disease – that reported a spike in infant mortality in the downwind communities two years after the accident.[52][81] Anecdotal evidence also records effects on the region's wildlife.[52] For example, according to one anti-nuclear activist, Harvey Wasserman, the fallout caused "a plague of death and disease among the area's wild animals and farm livestock", including a sharp fall in the reproductive rate of the region's horses and cows, reflected in statistics from Pennsylvania's Department of Agriculture, though the Department denies a link with TMI.[82]

John Gofman used his own, non-peer reviewed low-level radiation health model to predict 333 excess cancer or leukemia deaths from the 1979 Three Mile Island accident.[7] A peer-reviewed research article by Dr. Steven Wing found a significant increase in cancers from 1979–1985 among people who lived within ten miles of TMI;[83] in 2009 Dr. Wing stated that radiation releases during the accident were probably "thousands of times greater" than the NRC's estimates. A retrospective study of Pennsylvania Cancer Registry found an increased incidence of thyroid cancer in counties south of TMI and in high-risk age groups but did not draw a causal link with these incidences and to the accident.[9][10] The Talbott lab at the University of Pittsburgh reported finding only a few, small, mostly statistically non-significant, increased cancer risks within the TMI population, such as a non-significant excess leukemia among males being observed.[11] The ongoing TMI epidemiological research has been accompanied by a discussion of problems in dose estimates due to a lack of accurate data, as well as illness classifications.[84]

Activism and legal action[edit]

See also: List of anti-nuclear groups in the United States § Three Mile Island Alert

The TMI accident enhanced the credibility of anti-nuclear groups, who had predicted an accident,[85] and triggered protests around the world.[86] (President Carter—who had specialized in nuclear power while in the United States Navy—told his cabinet after visiting the plant that the accident was minor, but reportedly declined to do so in public in order to avoid offending left-wing Democrats who opposed nuclear power.[87])

Members of the American public, concerned about the release of radioactive gas from the accident, staged numerous anti-nuclear demonstrations across the country in the following months. The largest demonstration was held in New York City in September 1979 and involved 200,000 people, with speeches given by Jane Fonda and Ralph Nader.[88][89][90] The New York rally was held in conjunction with a series of nightly "No Nukes" concerts given at Madison Square Garden from September 19–23 by Musicians United for Safe Energy. In the previous May, an estimated 65,000 people – including California Governor Jerry Brown – attended a march and rally against nuclear power in Washington, D.C.[89]

In 1981, citizens' groups succeeded in a class action suit against TMI, winning $25 million in an out-of-court settlement. Part of this money was used to found the TMI Public Health Fund.[91] In 1983, a federal grand jury indicted Metropolitan Edison on criminal charges for the falsification of safety test results prior to the accident.[92] Under a plea-bargaining agreement, Met Ed pleaded guilty to one count of falsifying records and no contest to six other charges, four of which were dropped, and agreed to pay a $45,000 fine and set up a $1 million account to help with emergency planning in the area surrounding the plant.[93]

According to Eric Epstein, chair of Three Mile Island Alert, the TMI plant operator and its insurers paid at least $82 million in publicly documented compensation to residents for "loss of business revenue, evacuation expenses and health claims".[94] Also according to Harvey Wasserman, hundreds of out-of-court settlements have been reached with alleged victims of the fallout, with a total of $15 million paid out to parents of children born with birth defects.[95] However, a class action lawsuit alleging that the accident caused detrimental health effects was rejected by HarrisburgU.S. District Court Judge Sylvia Rambo. The appeal of the decision to U.S. Third Circuit Court of Appeals also failed.[96]

Lessons learned[edit]

The Three Mile Island accident inspired Charles Perrow's Normal Accident Theory, in which an accident occurs, resulting from an unanticipated interaction of multiple failures in a complex system. TMI was an example of this type of accident because it was "unexpected, incomprehensible, uncontrollable and unavoidable."[97]

Perrow concluded that the failure at Three Mile Island was a consequence of the system's immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer what he termed a 'normal accident'. Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely.[98]

"Normal" accidents, or system accidents, are so-called by Perrow because such accidents are inevitable in extremely complex systems. Given the characteristic of the system involved, multiple failures which interact with each other will occur, despite efforts to avoid them.[99] Such events appear trivial to begin with before unpredictably cascading through the system to create a large event with severe consequences.[100]

Normal Accidents contributed key concepts to a set of intellectual developments in the 1980s that revolutionized the conception of safety and risk. It made the case for examining technological failures as the product of highly interacting systems, and highlighted organizational and management factors as the main causes of failures. Technological disasters could no longer be ascribed to isolated equipment malfunction, operator error or acts of God.[98]

Comparison to U.S. Navy operations[edit]

Following the Three Mile Island (TMI) power plant's partial core melt on March 28, 1979, President Jimmy Carter commissioned a study, Report of the President's Commission on the Accident at Three Mile Island (1979).[101] Subsequently, AdmiralHyman G. Rickover was asked to testify before Congress in the general context of answering the question as to why naval nuclear propulsion (as used in submarines) had succeeded in achieving a record of zero reactor-accidents (as defined by the uncontrolled release of fission products to the environment resulting from damage to a reactor core) as opposed to the dramatic one that had just taken place at Three Mile Island. In his testimony, he said:

Over the years, many people have asked me how I run the Naval Reactors Program, so that they might find some benefit for their own work. I am always chagrined at the tendency of people to expect that I have a simple, easy gimmick that makes my program function. Any successful program functions as an integrated whole of many factors. Trying to select one aspect as the key one will not work. Each element depends on all the others.[102]

The China Syndrome[edit]

On March 16, 1979, twelve days before the accident, the movie The China Syndrome premiered, and was initially met with backlash from the nuclear power industry, claiming it to be "sheer fiction" and a "character assassination of an entire industry."[103] In the film, television reporter Kimberly Wells (Jane Fonda) and her cameraman Richard Adams (Michael Douglas) secretly filmed a major accident at a nuclear power plant while taping a series on nuclear power. Plant supervisor Jack Godell (Jack Lemmon) discovers potentially catastrophic safety violations at the plant and with Wells' assistance attempts to raise public awareness of these violations.

After the release of the film, Fonda began lobbying against nuclear power. In an attempt to counter her efforts, the then elderly Edward Teller, a nuclear physicist and long-time government science adviser best known for contributing to the Teller-Ulam design breakthrough that made hydrogen bombs possible, personally lobbied in favor of nuclear power.[104] Teller suffered a heart attack shortly after the incident and joked that he was the only person whose health was affected.[105]

Current status[edit]

Unit 1 had its license temporarily suspended following the incident at Unit 2. Although the citizens of the three counties surrounding the site voted by an overwhelming margin to retire Unit 1 permanently in a non-binding resolution in 1982, it was permitted to resume operations in 1985 following a 4-1 vote by the Nuclear Regulatory Commission.[106][107] 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 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. Today, AmerGen LLC is a fully owned subsidiary of Exelon Generation and owns TMI Unit 1, Oyster Creek Nuclear Generating Station, and Clinton Power Station. These three units, in addition to Exelon's other nuclear units, are operated by Exelon Nuclear Inc., an Exelon subsidiary.[citation needed]

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 FirstEnergy Corporation in 2001, and subsequently dissolved. FirstEnergy 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 FirstEnergy's maintenance contract. Unit 2 continues to be licensed and regulated by the Nuclear Regulatory Commission in a condition known as Post Defueling Monitored Storage (PDMS).[108]

The TMI-2 reactor has been permanently shut down with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored. The owner planned to keep the facility in long-term, monitoring storage until the operating license for the TMI-1 plant expired, at which time both plants would be decommissioned.[15] In 2009, the NRC granted a license extension which allowed the TMI-1 reactor to operate until April 19, 2034.[109][110] In 2017 it was announced that operations would cease by 2019 due to financial pressure from cheap natural gas.[111]


April 1974Reactor-1 online
Feb 1978Reactor-2 online
Mar. 1979TMI-2 accident occurred. Containment coolant released into environment.
April 1979Containment steam vented to the atmosphere in order to stabilize the core.
July 1980Approximately 1,591 TBq (43,000 curies) of krypton were vented from the reactor building.
July 1980The first manned entry into the reactor building took place.
Nov. 1980An Advisory Panel for the Decontamination of TMI-2, composed of citizens, scientists, and State and local officials, held its first meeting in Harrisburg, PA.
Dec. 1980U.S. 96th Congressional session passes U.S. legislation establishing a five-year nuclear safety, research, demonstration, and development program.
July 1984The reactor vessel head (top) was removed.
Oct. 1985Defueling began.
July 1986The off-site shipment of reactor core debris began.
Aug. 1988GPU submitted a request for a proposal to amend the TMI-2 license to a "possession-only" license and to allow the facility to enter long-term monitoring storage.
Jan. 1990Defueling was completed.
July 1990GPU submitted its funding plan for placing $229 million in escrow for radiological decommissioning of the plant.
Jan. 1991The evaporation of accident-generated water began.
April 1991NRC published a notice of opportunity for a hearing on GPU's request for a license amendment.
Feb. 1992NRC issued a safety evaluation report and granted the license amendment.
Aug. 1993The processing of accident-generated water was completed involving 2.23 million gallons.
Sept. 1993NRC issued a possession-only license.
Sept. 1993The Advisory Panel for Decontamination of TMI-2 held its last meeting.
Dec. 1993Post-Defueling Monitoring Storage began.
Oct. 2009TMI-1 license extended from April 2014 until 2034.

See also[edit]


  1. ^"PHMC Historical Markers Search"(Searchable database). Pennsylvania Historical and Museum Commission. Commonwealth of Pennsylvania. Retrieved January 25, 2014. 
  2. ^"NRC: Backgrounder on the Three Mile Island Accident". Retrieved 6 March 2018. 
  3. ^Spiegelberg-Planer, Rejane. "A Matter of Degree: A revised International Nuclear and Radiological Event Scale (INES) extends its reach". Retrieved March 19, 2011. 
  4. ^King, Laura; Kenji Hall and Mark Magnier (March 18, 2011). "In Japan, workers struggling to hook up power to Fukushima reactor". Los Angeles Times. Retrieved March 19, 2011. 
  5. ^Minutes to Meltdown: Three Mile Island – National Geographic Archived April 29, 2011, at the Wayback Machine.
  6. ^Michael Levi on Nuclear Policy, in video "Tea with the Economist", 1:55–2:10, on, retrieved April 6, 2011, 3.24pm.
  7. ^ abGofman John W., Tamplin, Arthur R. (December 1, 1979). Poisoned power: the case against nuclear power plants before and after Three Mile Island (updated edition of Poisoned Power (1971) ed.). Emmaus, PA: Rodale Press. p. xvii. Retrieved October 1, 2013.  
  8. ^ abMaureen C. Hatch; et al. (1990). "Cancer near the Three Mile Island Nuclear Plant: Radiation Emissions". American Journal of Epidemiology. Oxford Journals. 132 (3): 397–412. PMID
Simplified schematic diagram of the TMI-2 plant[15]
NRC graphic of TMI-2 core end-state configuration.
A sign dedicated in 1999 in Middletown, Pennsylvania near the plant describing the accident and the evacuation of the residents in the area.
Global history of the use of nuclear power. The Three Mile Island accident is one of the factors cited for the decline of new reactor construction.
Anti-nuclear protest following the Three Mile Island accident, Harrisburg, 1979.
Viewed from the east, Three Mile Island currently uses only one nuclear generating station, TMI-1, which is on the right. TMI-2, to the left, has not been used since the accident.
TMI-2 as of February 2014. The cooling towers are on the left. The spent fuel pool with containment building of the reactor are on the right.

Case NameThree Mile Island Nuclear Accident
DateMarch 28, 1979
PlacePennsylvania, USA
LocationThree Mile Island
OverviewAn accident occurred at Three Mile Island site, Unit 2 nuclear power plant 10 miles (16 km) southeast of Harrisburg, Pennsylvania (See Figure 1). Failure of the cooling system of the Unit 2 nuclear reactor led to overheating and partial melting of the pressurized-water reactor's uranium core and release of radioactive gas and contaminated water. The accident had a number of primary causes, related both to technical malfunction in the condensation system and human error. Three days after the accident, the official issued an advisory to evacuate pregnant women and preschool children living within a 5-mile radius of Three Mile Island, raising fears an explosion and dispersal of radioactivity among residences. The accident increased public concern over the dangers of nuclear power and slowed construction of other reactors in the US.
IncidentAn accident occurred at Three Mile Island site of a nuclear power plant 10 miles (16 km) southeast of Harrisburg, Pennsylvania. Failure of the cooling system of the Unit 2 nuclear reactor led to overheating and partial melting of the pressurized-water reactor's uranium core and release of radioactive gas and contaminated water. Three days after the accident, the official issued an advisory to evacuate pregnant women and preschool children in a 5-mile radius of Three Mile Island, raising fears of an explosion and dispersal of radioactivity among residences.
Sequence(1) The Three Mile Island Generating Station had two pressurized water reactors. Unit 2 was rated at 959 MWe. The accident in the Unit 2 reactor began when the plant's main feedwater pumps in the secondary non-nuclear cooling system failed (4 in Figure 2). In response to the failure, the auxiliary feedwater pumps kicked in, however, the water did not reach the steam generator because the outlet valves were closed (5 in Figure 2). The valve was discovered closed about eight minutes into the accident. Responding to the increase in temperature and pressure in the primary system, the pressurizer relief valve automatically opened in the line between the pressurizer and the quench tank (7 in Figure 2). After the nuclear reactor automatically shut down (8 in Figure 2), pressure in the reactor dropped. The valve, which was supposed to close after the pressure dropped below the set-point for closure, failed to re-close (11 in Figure 2). The valve was left open for 2 hours and 20 minutes, leaking 80 ton of primary coolant water from the quench tank. At this point, over 100 warnings tripped in the control room causing a big panic.
(2) Pressure drop in the reactor caused the Emergency Core Cooling System (ECCS) to pour water into the reactor system at 4ton/min (13 in Figure 3). As water and steam escaped through the relief valves, coolant water surged into the pressurizer, raising the water level in it. Unaware of the stuck valve and the false indicator readings, operators manually turned off the ECCS (15 in Figure 3). Temperature rose, steam then formed in the reactor primary cooling system (22 in Figure 3). The pumping of the mixture of steam and water caused the reactor cooling pumps to vibrate, and the operators shut down the pumps to stop severe vibrations (24 in Figure 3). As the reactor cooling water boiled away, the top of the reactor core was exposed (26 in Figure 3).
(3) Coolant water stopped flowing into the pressurizer, the temperature in the primary system reached 2,000 degrees Celsius, and approximately 45% of the reactor core melted down (32 in Figure 4). The coated tubing and water reacted to generate hydrogen gas, and ten hours later, a hydrogen explosion occurred. Water from the pressure relief valves overfilled the quench tank (18 in Figure 4) and flooded the containment vessel floor. The floor pump sent the radioactive coolant to the auxiliary building ventilation system (20 in Figure 4) where the radioactivity escaped to the outside. Eventually, cooling water was added and the reactor (33 in Figure 4) which then started to cool with natural circulation, however, hydrogen gas and radioactive gas kept generating and approximately 10 million curies of radioactive gas were released into the atmosphere by the event.
Cause(1) Inadequate failure information system
Problems with the failure information system in the control room caused inadequate emergency response by the operators. A warning sign blocked the view of a LED indicating that the auxiliary feedwater valve was closed, and moreover, a green LED was lit when the valve was closed. The instruments in the control room did not have consistent LED color - some LEDs were green and some were red under abnormal conditions.
The instruments only showed that a CLOSE signal was sent to the relief valve without indicating the valve's actual position. The LED instruments were not designed to warn malfunctions, and the operators did not know that the relief valve was stuck open.
Operators generally try to prevent filling up the pressurizer because them they will then not be able to control pressure in the pressurizer filled with water. Note that the pressurizer water level rises with high-pressure injection pumps pushing replacement water into the reactor system and cooling water surging into the pressurizer while water and steam escape through the open relief valves. Gas generation from the heated reactor core also lifts the pressurizer water level as well. The operators mistakenly judged that the pressurizer water level was rising under these situations. The water level indicator did not show the actual amount of water in the pressurizer.
The control panel had more than 1,200 LEDs and over 100 alarms went off during the emergency.
(2) Lack of reliability assurance
The malfunctioned pressurizer relief valve had repeated troubles earlier and was unreliable. Despite of the obvious issue, the plant instructed the operators to "fool" the system without replacing the valves with reliable ones.
(3) Inadequate training of operators
The operators of the Three Mile Island Nuclear Power Plant were not employees of the utilities firm. The utilities firm had contracted out the plant operations, however, the contract operators lacked proper knowledge about nuclear reactors and thermal phenomena. They had hardly been trained for accident situation.
(4) Unexpected event not in the safety design standards
Safety devices of nuclear plants are designed to handle certain nuclear accidents. The chain of events in the Three Mile Island Nuclear accident was far out of the range of assumed failures and no one had thought about how to handle such cases.
ResponseThe above sections describe actions taken by the operators. Because of confused communication among people uninformed about the plant's status, officials concluded that the accident released hazardous amount of radiation and issued an advisory to evacuate pregnant women and preschool children within a 5-mile radius of Three Mile Island. Approximately 140,000 people evacuated the area in panic.
CountermeasuresMany agencies and officials investigated the Three Mile Island Nuclear accident. The U.S. President ordered a full investigation of the incident, and the Accident Investigation Board reported inadequate training of operators.
Nuclear Safety Commission of Japan submitted the first, the second and the third reports on the incident. The reports provided 52 safety measures including safety standards, safety study, safety design, operating control, disaster prevention and safety research. The local government reviewed its nuclear disaster prevention plan.
Knowledge Comment(1) Most of the time, an accident starts from a malfunction and gets aggravated with human misunderstandings and other problems further complicating the event..
It is important to install highly reliable equipment and maintain a backup system so that it functions properly when needed.
(2) It is essential to design safety devices that has easy-to-read indicators accommodating human cognitive constraints. It is also essential to develop safety systems for preventing inadvertent control inputs and erroneous operations.
(3) Outsourcing can involve risks to the operations. It is necessary to prevent outsourcing from degrading operation efficiency and reliability.
BackgroundThe energy sources in the world at the time had shared of 70% oil, 20% coal, and a little less than 10% hydroelectric; oil by far supplied most of the electricity to the world. At the time, OPEC turned its oil pricing policy and the infrastructure of oil supply was vulnerable. Nuclear power then was the star alternate energy source.
Commercial nuclear power generation involves containing and controlling the fission reactions so that the heat can be used to make steam, which in turn drives a turbine and generates electricity. Basically the heat source, which is oil or coal for thermal power stations, was replaced with fission.
Three Mile Island Nuclear Generating Station consists of two pressurized light water reactors, each with its own containment building and cooling towers. The Unit 2 reactor, which suffered a partial meltdown, had 959MWe total capacity. It was constructed by Babcock and Wilson, and its operations were run by Metropolitan Edison Company. There were number of malfunctions reported since its test operation in March 1977 and during the commercial operation until January 1979: 9 malfunctions in the feedwater system, stuck-open main steam relief valve, 9 activation of ECCS (1 manual activation). In particular in March 1978, a station blackout occurred during low-power operation, and the pressurizer relief valves opened by accident. ECCS (high-pressure injection system) was activated automatically, but it was unable to control the reactor pressure and the pressurizer water level because of the station blackout. When the power came back on, the operators responded to the emergency situation appropriately, but not immediately because the control room was not equipped with open/close indicator of the pressurizer relief valve and it took time for the operators to understand the circumstances.
Prior to this accident, similar accidents were reported at a Swiss power station in August 1974, at Oconee Nuclear Station in the US in June 1975 and at Davis-Besse Nuclear Power Station in the US in September 1977, and their reports including analysis and evaluation on the events were available. The utilities firm should have studied these accident reports closely and have taken preventive measures.
Public confidence in nuclear energy declined sharply following the Three Mile Island accident. It was a major cause of the decline in nuclear construction through the 1980s and 1990s.
Primary Scenario Regular Movement, Wrong Movement, Non-Regular Operation, Emergency Operation, Carelessness, Insufficient Precaution, Planning and Design, Poor Planning, Bad Event, Thermo-Fluid Event, Bad Event, Thermo-Fluid Event
Sources Yotaro Hatamura (Editor), Jissai-no Sekkei (Practical Design) Research Foundation (1996) Zoku-Zoku Jissai-no Sekkei (Practical Design III), The Nikkan Kogyo Shimbun, LTD.
Genshiryoku Hyakka Jiten (Nuclear Energy Encyclopedia),
Kunio Yanagida (1986) Kyoofu-no Ni-jikan Juuhappun (2 Hours 18 Minutes in Terror), Bungeishunju, LTD.
"US Nuclear Power Plant Accident Analysis Report III", Nuclear Safety Bureau Monthly Report, June 1981, 34, Nuclear Safety Bureau of Science and Technology Agency of Japan.
Multimedia FilesFigure1.Location of Three Mile Island Generating Station [4]
Figure2.Three Mile Island Nuclear Accident: Sequence of Events 1 [1]
Figure3.Three Mile Island Nuclear Accident: Sequence of Events 2 [1]
Figure4.Three Mile Island Nuclear Accident: Sequence of Events 3 [1]
Field Mechanical Engineering
Author NAKAO, Masayuki (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)

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