Three Mile Island Nuclear Reactor Accident

The Three Mile Island Nuclear reactor accident was an event that occurred on 28 March 1979 at Three Mile Island Nuclear Power Plant in the U.S. state of Pennsylvania and is regarded as a critical dunum point in nuclear energy history. Due to equipment failures and a chain of human errors in the plant’s second unit (TMI-2), large quantities of radiation were released into the environment. As a result, the TMI nuclear reactor accident is remembered in history as a turning point that led to the complete rewriting of all nuclear regulations reason event.

Historical Background

In the late 1970s USA, nuclear energy was a rapidly growing electricity production source. By 1979, more than 70 commercial nuclear reactors were in operation worldwide and they supplied approximately 12% of the country’s total electricity. During this period, government and nuclear industry officials were confident and complacent about plant safety. Within this political environment, the Three Mile Island (TMI) nuclear plant was constructed in Pennsylvania near the city of Harrisburg.

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The TMI facility had two separate pressurized water water reactors (PWRs); the first unit, designated TMI-1 with a capacity of 819 MWe, began operation in 1974 and became one of the best-performing nuclear reactors in the United States until 2019. The second unit, TMI-2, had a capacity of 880 MWe and was a very yet new facility at the time of the accident in 1979.

Following the oil crises, nuclear energy was promoted as a reliable alternative in energy policy. However, the Three Mile Island accident, occurring only a few month after TMI-2’s commissioning, dramatically ended the period of growth in commercial nuclear power use in the United States. Accident accelerated global discussions on nuclear safety.

Technical Details and Reactor Characteristics

Three Mile Island Unit 2 (TMI-2) was a pressurized water reactor (PWR) manufactured by Babcock & Wilcox. The reactor featured a “low-pressure loop” design. In PWR systems, water in the primary circuit is maintained under high pressure; the heat generated is transferred to steam generators to produce steam that drives turbines. In TMI-2, the primary (nuclear) circuit was completely separate from the secondary (steam) circuit. Primary circuit pressure was controlled via a pressurizer, while main feedwater pumps continuously supplied water to the steam generators to cool the reactor core.

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The TMI-2 reactor design assumed the core would always remain covered by water; however, there was no direct system to measure the water level inside the reactor vessel. The plant was equipped with an Emergency Core Cooling System (ECCS), which was designed to automatically supply additional cooling water in the event of a loss of coolant. Additionally, a pilot-operated relief valve (PORV) on the pressurizer was installed to reduce pressure if it rose excessively. TMI-2 also featured a steel and concrete containment structure (containment vessel). This building, possible radioactive leakage, and was designed to fully isolate the reactor in emergency situations.

At the time of the accident, most safety systems activated as designed, but failed to achieve the expected outcome due to operator errors and mechanical failures. During the accident, the nuclear chain reaction in the reactor was automatically stopped (SCRAM), and control rods successfully inserted to terminate the fission reaction. However, after shutdown, the decay heat generated in the core—approximately complete power of the reactor’s full capacity—needed to be removed by cooling systems. A series of fault prevented adequate heat removal. Consequently, the TMI-2 accident is remembered in history as an event that caused core meltdown due to failures in active cooling systems and operator wrong interventions, despite the largely passive safety features of the Babcock & Wilcox PWR design.

Development of the Accident (Hour-by-Hour Timeline)

March 28, 1979, 04:00 (Start of the Accident): While operating at 97% power, the second unit of Three Mile Island experienced a failure in the main feedwater pumps. Due to a probable mechanical or electrical problem, water supply to the steam generators ceased. The interruption of feedwater flow prevented heat generated in the reactor from being transferred to the steam circuit, causing the turbine generator to shut down automatically for protection. As a result, the reactor automatically entered emergency shutdown mode (SCRAM).

After shutdown, fission reactions ceased, but radioactive fuel continued to produce decay heat, causing pressure to rise in the primary circuit. The pressurizer relief valve (PORV), designed to control pressure, opened but failed to close after pressure returned to normal levels around 04:01. Indicators in the control room showed the valve as closed, but operators did not difference its actual real position. Consequently, operators failed to realize the valve remained open, and continued loss of steam and coolant substance from the primary circuit persisted.

04:02 – 04:30: After reactor shutdown, the Emergency Core Cooling System (ECCS) automatically activated and began injecting high-pressure cooling water into the core. However, operators observed the water level in the pressurizer tank rising rapidly and misinterpreted this as a sign of a dangerous condition known as “solidification” (solid core). Fearing that if the pressurizer became completely filled with water, pressure control would be lost due to water’s incompressibility, operators manually disabled the ECCS. In reality, this caused a critical reduction in cooling water in the core, which operators failed to recognize but.

Around 04:15, steam pockets formed in the primary circuit, causing severe vibrations in the circulating pumps. Operators interpreted this as a sign of pump damage and shut down the pumps as well. From this point onward, coolant flow into the reactor was nearly completely cut off.

04:30 – 06:00: Due to the failure of the cooling system, core temperature rose rapidly. The zirconium alloy cladding of the fuel rods reacted with steam at high temperatures, producing hydrogen gas. Part of the fuel rods began to melt. Approximately three hours after the accident began (around 07:00), a significant rise in radiation levels was detected inside the containment building and auxiliary building.

At around 08:00, the containment building’s automatic safety system activated isolation mode, closing valves and doors throughout the building. However, due to the earlier valve failure, a mixture of radioactive water and steam had already leaked into the auxiliary building. This radioactive water became the primary source of radiation released to the environment in the following days.

14:00: Hydrogen gas accumulated in the containment building reacted with oxygen, causing a small scale explosion. The explosion occurred in a controlled manner and did not compromise the integrity of the containment vessel, and operators did not immediately notice it. However, analysis the next day confirmed the explosion. On March 29, engineers detected a “hydrogen bubble” inside the pressurizer and, due to this misinterpretation, raised fears of a new explosion risk. Initial statements by the Nuclear Regulatory Commission (NRC) caused panic, but subsequent technical analyses revealed no such risk existed.

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March 30, 1979 (Day 3): Technical uncertainties and public fear reached a peak. Pennsylvania Governor Richard Thornburgh, acting on NRC advice, recommended that pregnant women and young children evacuate from within an 8 km radius of the plant, and schools in the area were holiday. This recommendation was interpreted by the media as an “evacuation order,” triggering panic, and thousands of people fled the area without planning. On the same day, the NRC, after detailed analysis, announced there was no hydrogen explosion risk, helping to reduce panic.

April 1, 1979 (Day 4) and Beyond: U.S. President Jimmy Carter visited Three Mile Island to reassure the public and ring confidence. Carter’s decision to walk through the plant without protective gear was perceived as a calming message. In the first week of April, the plant’s cooling system stabilized, and the hydrogen bubble shrank and disappeared. On April 9, the Pennsylvania Governor’s office lifted the evacuation advisory, announcing that normal life could resume in the area.

Three Mile Island’s second unit was permanently shut down due to severe damage and never restarted. After the crisis ended, detailed investigations, radiation measurements, and inquiries into the causes of the accident were launched.

Engineering and Scientific Analyses

The Three Mile Island accident is remembered in nuclear energy history as a complex sequence of cascading failures offering important lessons. Post-accident technical investigations revealed detailed analyses from thermodynamic, nuclear, and human factors perspectives.

First, although the reactor’s automatic shutdown (SCRAM) halted the chain reaction of fission, heat production continued due to radioactive isotope decay in the fuel. This decay heat, at approximately 6% of normal work power immediately after shutdown, was critical for maintaining cooling. However, equipment failures during the accident prevented effective heat removal, causing the core temperature to rise rapidly, water to boil, and fuel harm to occur.

A series of thermohydraulic phenomenon influenced the accident’s progression. With no feedwater supplied to the steam generators, heat accumulated in the primary circuit, rapidly increasing pressure. The pressurizer’s security valve (PORV) opened to control pressure but, due to a technical fault, remained stuck open, causing significant coolant loss.

This situation resembled a scenario known in literature as a “small break loss-of-coolant accident” (small break LOCA). The Emergency Core Cooling System (ECCS) was designed to activate automatically in such scenarios. However, operators, misled by incorrect indicator information, manually disabled the ECCS, causing the primary circuit water level to drop critically and exposing the core. At this stage, zirconium alloy fuel rods reacted with steam at temperatures above 1200°C, producing hydrogen gas via the reaction Zr + 2H₂O → ZrO₂ + 2H₂. These exothermic reactions further increased core temperature.

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Partial core meltdown became the central focus of engineering analyses. After approximately 2.5 hours of core uncovery, fuel temperatures reached about 2800°C, and portions of the fuel pellets and cladding began to melt. Subsequent analyses determined that approximately 35–50% of the fuel had melted and settled at the bottom of the pressure vessel. However, the pressure vessel maintained its integrity, and molten fuel did not escape into the environment.

This finding demonstrated, from an engineering standpoint, that the public fear known as the “China Syndrome”—the belief that molten fuel would melt through the reactor structure and into the ground—had an extremely low probability. Indeed, no such event occurred during the TMI accident.

Detailed analyses were also conducted on the human factors involved. At the time of the accident, numerous alarms and indicators activated simultaneously in the control room, overwhelming operators with alarm confusion. One of the most critical indicators—the PORV status—was misleading; operators failed to realize the valve remained open. Similarly, since water level was not directly measurable, operators misinterpreted the pressurizer level gauge and made incorrect interventions.

These shortcomings in the human-machine interface and inadequate operator training were identified as fundamental causes of the accident. In particular, operator training manuals did not sufficiently emphasize scenarios in which the PORV could remain open. In fact, a similar situation had previously occurred at the Davis-Besse plant in Ohio, but this experience was not communicated to TMI operators.

Another important scientific analysis concerned the “hydrogen bubble.” Initially, NRC engineers believed the hydrogen bubble formed in the reactor posed an explosion risk; however, subsequent thermochemical calculations showed insufficient free oxygen was present within the bubble to support combustion, eliminating the explosion risk. This hydrogen bubble was later safely removed. However, this phenomenon spurred the development of hydrogen control systems in nuclear plants.

In conclusion, the Three Mile Island accident is an example of multidimensional engineering failure. The accident escalated due to unexpected failures in physical equipment (pumps, valves, indicators) and operator misjudgments. However, the reactor’s final defense—the containment vessel—prevented the accident from becoming a far greater catastrophe. This incident prompted deep investigations in nuclear engineering literature on safety culture, human factors, and multilayered safety systems, paving the way for significant technological advancements.

Environmental Impacts and Radiation Release

The environmental impacts and radiation releases following the Three Mile Island accident were extensively studied. At the time of the accident, approximately 2 million people lived within an 80 km radius of the plant. Analyses showed the average radiation dose received by these individuals was only slightly above natural background radiation levels.

Research conducted by the NRC and other official agencies determined that the average additional radiation dose in the region was only about 1 millirem (0.01 mSv); this value is lower than the dose received during a standard chest X-ray. The highest recorded radiation dose at the boundary is estimated to have been slightly above 100 millirem (1 mSv) of natural background radiation. It was concluded that this level caused no observable health effects (for comparison, the average American receives about 300 millirem of natural radiation annually).

The vast majority of radioactive materials released into the environment during the accident consisted of radioactive gases from the noble gas family. Radioactive xenon and krypton gases accumulated in the containment building and were deliberately released to the atmosphere. Since these gases are biologically inert, they rapidly diluted in the air without causing lasting biological effects.

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Most of the radioactive iodine (particularly I-131), the substance of greatest public concern after the accident, remained contained within the containment building. No iodine-related radioactive contamination was detected in environmental measurements. Comprehensive analyses by the NRC and EPA like agencies found no detectable levels of radioactive contamination in air, water, soil, plant cover, and milk samples. For example, none of the 377 food samples collected between March 29 and April 30, 1979, showed any radioactivity. These results demonstrated that radiation released into the environment was at very low levels and caused no measurable negative impact on the regional ecosystem.

Although the environmental impacts of the accident were generally limited, serious radioactive contamination occurred within the plant site. In particular, radioactive coolant water leaking from the reactor’s primary circuit caused intense radiation residues in the auxiliary building and other systems. Cleaning the radioactive water and damaged fuel within the reactor containment structure took years; a large-scale decontamination program lasting approximately 12 years followed the accident. The collected molten fuel and radioactive residues were transported in special containment vessels to the Idaho National Engineering Laboratory.

The total cost of cleanup and decontamination activities was estimated at approximately $973 million. This high cost highlighted the economic dimension of the accident, while detailed environment measurements clearly demonstrated no long-term radioactive effects on soil, water, or local wildlife in the region.

In conclusion, the Three Mile Island accident is regarded as an incident with very low environmental impact, unlike the major ecological disasters of Chernobyl (1986) and Fukushima (2011).

Effects on Human Health

The effects of the Three Mile Island (TMI) accident on human health have been extensively studied over many years. Immediately after the accident, widespread concern arose in the region population about possible radiation-related health problems. Media reports and rumors led to claims of animal deaths and various health issues. To scientifically evaluate these claims, the Pennsylvania Department of Health established a health monitoring system covering more than 30,000 individuals living in the area during the accident.

Over an 18-year period from 1979 to 1997, this study found no unusual health problems or trends in the local population. In other words, no significant increase in cancer or other health problems was found among people living near TMI.

Post-accident independent epidemiological studies also supported the findings from health records. Dozens of comprehensive studies conducted by different institution and universities found no abnormal increase in cancer cases or death rates in the region. For example, a research study involving 32,000 individuals over 13 years found no significant health impact attributable to the accident.

Experts noted that the radiation doses released during the accident were so low that long-term health problems were not expected. Even theoretical calculations indicated that the additional cancer risk for the most exposed individuals was at a level of only a few per million—negligible. While the lifetime cancer risk for the average American is around 38–40%, the additional risk from the TMI accident is not measurable.

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The most significant health impact of the TMI accident was psychological stress and trauma. Panic and anxiety surged in the region due to fears of explosion and radiation. Surveys conducted by the Presidential Commission and Pennsylvania State Commission showed that local residents experienced intense stress and concern in the aftermath, although these effects diminished over time. Particularly in the first months after the accident, psychological stress was widespread among residents, but most returned to normal in the following period. However, long-term trauma and psychological issues were observed among plant workers.

This demonstrates that the psychosocial impacts of the TMI accident were far greater than its physical health effects. Nevertheless, legal cases were filed over the years alleging health problems caused by the accident. In late 1980s and 1990s legal assessments, no reliable evidence supporting radiation-related health claims was found.

In the 1996 class-action lawsuit, court, the plaintiffs’ claims that cancer cases were caused by the accident were dismissed, and the court concluded there was no evidence of radiation-related health effects in the region.

In conclusion, the scientific community broadly agrees that the Three Mile Island accident had negligible measurable health effects. Even years later, regional health statistics remained consistent with general population averages. However, the accident left important lessons on radiation risk communication and the management of psychological trauma, revealing that public responses to nuclear accidents are shaped not only by physical risks but also by perceived danger and trust.

Legal Proceedings and Legal Outcomes

Comprehensive legal and administrative processes were initiated following the Three Mile Island accident. First, the Presidential Commission established by then-U.S. President Jimmy Carter, chaired by distinguished scientist Dr. John Kemeny, produced a comprehensive report in 1979. The Kemeny Commission Report emphasized that the Nuclear Regulatory Commission (NRC) had failed to prevent the accident and identified serious deficiencies in its organizational structure and safety culture. The report noted that the NRC had not effectively pursued its safety goals and recommended a thorough review of its structure and practices.

The report also recommended that the nuclear industry adopt a new structure to improve operator training and safety culture. In response, by the end of 1979, the U.S. nuclear industry established the Institute of Nuclear Power Operations (INPO), focused on self-regulation.

The NRC also launched its own investigation and evaluation process. The 1980 Rogovin Report provided a detailed analysis of technical and administrative shortcomings. As a result, the NRC restructured its inspection and operator licensing divisions. Congressional hearings at the federal level questioned the NRC’s account, and recommendations were made to establish independent oversight bodies and management committees.

As a result of these processes, numerous structural and legal changes were made to the NRC’s operations and regulations for nuclear facilities. In particular, beginning in 1980, emergency response plans at state and local levels became mandatory before new nuclear facilities could receive operating licenses. This aimed to prevent the communication and coordination failures experienced during the accident.

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Legal aspects of the accident included criminal investigations and compensation lawsuits. Metropolitan Edison Company (a subsidiary of GPU Nuclear), held responsible for technical negligence and providing false or incomplete information to the NRC during the incident, faced investigations. In the early 1980s, the company was issued various administrative money penalties and paid fines after admitting to some charges. These penalties set a precedent for transparency and accurate reporting in the nuclear sector.

Residents and businesses affected by the accident also initiated legal action, filing lawsuits for radiation-related health damages, evacuation expenses, job losses, property value declines, and non-economic damages. In 1981, a partial peace settlement was reached, establishing a $20 million compensation fund for non-health claims and a $5 million people health monitoring fund. Through these funds, compensation was paid to more than 15,000 individuals and businesses near the plant. For example, those who lost income received two weeks’ compensation, and property owners received payments for real estate value losses.

Health-related claims remained subject to litigation for many years. A group of plaintiffs, asserting the accident caused cancer, filed a class-action lawsuit in federal court between 1993 and 1996. In a 1996 ruling by Judge Sylvia Rambo, the court found the evidence presented over nearly twenty years insufficient for a jury to consider and concluded no material proof existed to link health problems to the accident. The ruling stated the radiation exposure during the accident posed no statistically significant health risk.

In conclusion, the legal dimension of the Three Mile Island accident led to significant regulatory changes and raised industry standards. While compensation addressed economic losses, legal liability for health damages was not established against the companies.

The Three Mile Island incident was a pivotal case that prompted a reevaluation of nuclear insurance and compensation systems (the Price-Anderson Act). This encouraged the adoption of preventive measures for larger disasters and led to a reassessment of liability limits for companies and insurers.

Political and Social Consequences

The most prominent social consequence of the Three Mile Island accident was a sharp and sudden decline in public trust in nuclear energy. Before the accident, support for nuclear energy in the United States was high; however, after the March 1979 event, anti-nuclear movements gained rapid strength at both local and national levels. Just a few months after the accident, on 23 September 1979, approximately 200,000 people participated in a work anti-nuclear performance in New York City.

This event became the largest anti-nuclear protest in U.S. history up to that point. Combined with “No Nukes” concerts at Madison Square Garden, these demonstrations reflected the profound public reaction to the Three Mile Island accident. Similar protests involving thousands of people were held in states such as California, Massachusetts, and Michigan, and many local governments adopted a more cautious stance toward new nuclear plant projects.

The accident had a long-lasting impact, particularly on the local population. In the early 1980s, serious cancellations and delays occurred in nuclear energy investments nationwide. Many planned nuclear projects were suspended or canceled entirely. The nuclear industry struggled to secure new orders, and a pronounced stagnation in nuclear power plant construction lasted from the 1980s through the end of the 1990s. During this period, no new nuclear plant licenses were issued, and some ongoing construction projects were halted due to economic and social pressures.

The Three Mile Island accident also strengthened anti-nuclear movements globally. In particular, in Germany and Sweden, West Europe countries, these movements gained momentum, prompting many nations to reassess their energy policies. For example, Sweden decided in 1980, following a national referendum, to phase out nuclear energy. Italy fully abandoned its nuclear program in 1987 after the Chernobyl disaster, but the seeds of such political decisions were planted by the Three Mile Island accident.

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In terms of social impact, the Three Mile Island accident left lasting impressions on the local population. Civil society organizations such as “Three Mile Island Alert,” established after the accident, played a key role in information sharing and public pressure campaigns regarding nuclear facilities. Public perception of nuclear technology became more skeptical and questioning. This highlighted the need for the nuclear industry to be more transparent and sensitive in public relations. For example, the plant operator, GPU Nuclear, increased its public relations activities and established committees to strengthen communication with the local community. Similarly, the NRC held public meetings and information sessions to rebuild trust.

Politically, the Three Mile Island accident accelerated debates on diversifying energy policy. After the accident, some states increased investment in diversifying energy sources and energy efficiency programs. At the federal level, incentives for nuclear energy were sidelined, and fossil fuels and renewable energy resources were prioritized.

Although U.S. President Jimmy Carter’s active response and knowledge of nuclear energy helped mitigate public reaction, public confidence never fully returned to pre-accident levels. Nuclear energy debates continued during the 1980 presidential elections, and the industry’s stagnation persisted even under the Reagan administration.

Internationally, the Three Mile Island accident prompted institutions such as the International Atomic Energy Agency (IAEA) to improve safety standards. From the 1980s onward, the IAEA reviewed its safety protocols and issued new safety recommendations for member countries. Many nations tightened nuclear plant inspections and began taking “worst-case scenario” analyses seriously for the first time. In this regard, the Three Mile Island accident laid the groundwork for international preparedness for the later Chernobyl disaster.

In conclusion, the Three Mile Island accident was a major social and political turning point against nuclear energy across the United States. The loss of public trust and the wave of protests caused the nuclear industry to stagnate for years. This accident revealed that energy policy is not limited to economic or technical dimensions but critically depends on social acceptance and political will. The nuclear sector was forced to undertake a prolonged effort consumption to regain public confidence after the accident.

Lessons Learned and Safety Improvements

Following the Three Mile Island accident, comprehensive reforms were implemented from both technical and managerial perspectives. The weaknesses and deficiencies revealed by the accident were detailed in “lessons learned” reports and shared across the entire nuclear industry. Significant improvements were made in many areas, including emergency management, operator training, human factors engineering, and radiation safety.

The first improvement was the renewal of operator training and emergency procedures. Before the TMI accident, operators relied on procedure manuals focused on specific failure scenarios and struggled with complex situations. After the accident, the training philosophy changed; operators were now trained primarily to prioritize reactor cooling and safety. In this new approach, operators were directed to quickly assess key safety parameters (cooling, pressure, radiation levels, etc.) and stabilize the situation.

Another innovation in training was the mandatory use of full-scale control room simulators. The NRC required every nuclear plant operator to undergo regular training on simulators that were exact replicas of their own plant. For example, in the 1980s, a sophisticated simulator for TMI-1 was built at a cost of approximately $18 million. This allowed operators to experience numerous scenarios—including LOCA (loss-of-coolant accident), power outages, and instrument failures—in a safe environment. Additionally, team communication and teamwork training became integral parts of the curriculum.

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In technical areas, design flaws related to human factors were addressed. The NRC mandated that all plant control room instrumentation be made more understandable and easier to interpret. Specifically, systems were required to directly indicate the actual position of critical valves such as the PORV (Pilot-Operated Relief Valve). Alarm systems were also redesigned to be more comprehensible and manageable.

Another important lesson from the accident was hydrogen gas control. Although the hydrogen explosion in TMI-2 was small-scale, it became clear that similar accidents could pose greater risks. In response, the NRC required the installation of hydrogen recombiners or ventilation systems in the containment structures of pressurized water reactors (PWRs). These systems prevented hydrogen accumulation and eliminated explosion risks. Recommendations to add filters to pressure relief systems were also introduced on international platforms.

Perhaps the most enduring impact of the TMI accident was the birth of the concept of “safety culture.” Post-accident reports criticized not only technical factors but also management approaches, leading to the adoption of a continuous safety-oriented culture in the nuclear industry. The U.S. nuclear industry embraced the philosophy of self-regulation and continuous improvement under the INPO umbrella. Regular inspections and evaluations by INPO helped maintain high safety standards at facilities. In 1985, the National Nuclear Training Academy was established, and operator training standards were standardized.

The NRC also tightened its inspection processes and began maintaining permanent on-site inspectors. Incident reporting systems between facilities were improved, and information sharing increased. This reduced the risk of similar events recurring.

These measures produced concrete results. The number of serious incidents at U.S. nuclear facilities dropped dramatically from the 1980s to the 1990s. The average number of serious incidents per year fell from 2.38 in 1985 to 0.1 in 1997. Meanwhile, plant capacity factors rose from the mid-60s to the 90s during the same period—a clear indicator of improved safety and efficiency.

The improvements and lessons learned from TMI achieved world scale echo. France, Germany, and other countries adopted TMI experiences in areas such as control room ergonomics and symptom-based emergency procedures. In 1989, the World Association of Nuclear Operators (WANO) was established, assuming a role similar to INPO on a global scale and facilitating international sharing of TMI and later Chernobyl experiences.

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In conclusion, the Three Mile Island accident led to lasting and positive changes across a broad spectrum—from operator training and technical design to safety culture and international cooperation. Thanks to the lessons learned from this accident, the safety of nuclear energy facilities was enhanced, and the likelihood of similar accidents was greatly reduced. Although TMI-2 was never restarted, this costly experience contributed significantly to making nuclear power plants worldwide safer.