On March 28, 1979, the nuclear power station on Three Mile Island experienced a serious accident. Three Mile Island, an island in the Susquehanna River, is located near Harrisburg, Pennsylvania. The station suffered a partial meltdown of its nuclear reactor through a combination of mechanical malfunction and human error. On the 7-point International Nuclear Event Scale, the accident was rated a 5, corresponding to an “accident with wider consequences.” At the time it was the worst accident in civilian nuclear technology, and it highlighted several deficiencies in operator training, emergency management, and relief coordination from various government agencies.

According to official records, the amount of radioactive material that escaped from the Three Mile Island power station was minimal and could not have had long-term effects on the local population, but reports from several other sources have since disputed this claim. These reports cite evidence of higher rates of cancer and birth defects in the vicinity of the station in the years following the incident. The accident and the way that it was handled by government agencies left a legacy of mistrust among the local population. Cleanup efforts took 14 years to complete, and questions and fears concerning the true nature of the accident lingered for many years.

The accident at Three Mile Island occurred in the aftermath of energy crises in the 1970s, which highlighted the vulnerability of the United States and other countries to reductions in petroleum supply as a consequence of political actions taken by oil-exporting countries. Proponents of nuclear energy development presented nuclear energy as a way to achieve greater national self-sufficiency. The accident, however, served as a stark reminder of the dangers of nuclear technology and of failures in the institutions responsible for overseeing its use.

In the 1930s scientific research by a number of physicists spurred developments in the field of nuclear energy. This research included the discovery of the Page 606  |  Top of Articleneutron in 1932 by British physicist James Chadwick (1891–1974) and studies of radioactivity in 1934 by French chemists Frédéric Joliot-Curie (1900–1958) and Irène Joliot-Curie (1897–1956). Nuclear fission was discovered in 1938 by German chemist Otto Hahn (1879–1968). During World War II (1939–45) several countries carried out research and development projects to put these new discoveries to military use. Germany's Uranprojekt (1939–45) and the U.S. Manhattan Project (1942–46) aimed to exploit nuclear fission to create a new, highly destructive type of bomb. The U.S. program culminated in 1945 with the creation of bombs that used nuclear fission for their destructive power. While still at war, the United States exploded an atomic bomb over Hiroshima, Japan, on August 6 and another over Nagasaki, Japan, on August 9.

The civilian use of nuclear energy in the United States began with the Atomic Energy Act of 1954, which established the civilian application of nuclear power. The first nuclear power plant to generate electricity, the Shippingport Atomic Power Station, was built 25 miles (40 kilometers) from Pittsburgh, Pennsylvania, and began operation in December 1957. During the 1960s and 1970s several nuclear power plants were built in the United States by General Electric and Westinghouse. The Three Mile Island power plant was constructed during this period.

Three Mile Island lies in the Susquehanna River about 10 miles (16 kilometers) south of Harrisburg. Prior to the building of the plant, the 625-acre (253-hectare) island was used primarily for farming, and most of the surrounding area consisted of farming communities. Plans for the first nuclear reactor at Three Mile Island were approved in 1966, and the construction of a second reactor was decided on two years later. The location proved advantageous for several reasons. The existing infrastructure of coal-fired plants lessened the building costs; a steel mill and chocolate factory in the vicinity would create a large and continuous demand for the electricity generated by the plant; and the proximity of the river guaranteed a steady supply of coolant. For the neighboring communities, construction and operation of a nuclear power plant meant jobs and economic revival, which the region needed after the 1964 announcement that the nearby Olmsted Air Force Base would be closing.

The first reactor at Three Mile Island (Unit 1, or TMI-1) began commercial operation on September 2, 1974, and the second reactor (Unit 2, or TMI-2) started operating on December 30, 1978. Not three months later, the accident occurred at TMI-2.

Most nuclear power plants in the United States are light water reactors. They use nuclear fuel (such as uranium) to heat water and produce steam to power turbines that generate electricity. There are two types of light water reactors: boiling water reactors and pressurized water reactors (PWRs). The two differ in how they use water as a coolant and how their steam is produced. A PWR features two independent coolant loops, so that the water that comes into contact with the reactor core (and is thus radioactive) and the water used to power the turbine generators (which produce electricity) remain separate. The two reactors built at Three Mile Island were PWRs.

Nuclear fission, the process of splitting an atom's nucleus, takes place in the reactor core. Fission occurs when a neutron strikes an atom in the nuclear fuel, typically an atom of uranium called uranium-235. As it is split, the atom ejects new neutrinos. These neutrinos, in turn, strike and split additional atoms of uranium. This ongoing series of nuclear reactions is called a chain reaction. A chain reaction releases large amounts of energy in the form of heat, and it needs to be stabilized with control rods made of neutron-absorbing boron or cadmium.

The heat produced by the chain reaction warms the water in the reactor to a very high temperature, but the water is kept under high pressure to prevent it from boiling. The high-temperature water then passes through a steam generator, where the reactor water heats water from a secondary loop, which produces steam. This steam drives the turbine of a generator to produce electrical power.

Nuclear power plants were designed according to the principle of defense in depth. This principle requires that safety measures be independent of one another so that, if a mechanical failure or human error occurs, problems in one area cannot translate into problems in another. An extremely serious mishap, such as reactor meltdown, would therefore be able to occur only if two or more main systems were to fail at the same time or if human error and mechanical failure happened in consequence of one another. Such a scenario was thought to be unlikely owing to the high engineering standards and the design independence of nuclear plants, such as those at Three Mile Island.

Nevertheless, such a scenario did develop at Three Mile Island on March 28, 1979. Problems began at 4 a.m. in the secondary (nonnuclear) system of the plant. Operators were working to clear a blockage in equipment in which steam from the turbine generator is turned back to liquid water. For reasons that were never fully explained, pumps in this equipment failed, which in turn caused the turbine generator to shut down. These actions caused pressure and temperature to rise in the primary system, so the nuclear reactor also shut down automatically.

Pressure in the primary system was designed to be relieved by a valve in a vessel called the pressurizer, which regulates pressure in the primary coolant system. The valve opened to relieve the pressure, but it failed to close Page 607  |  Top of Articleonce the pressure was reduced. Emergency relief pumps therefore went into action and injected replacement water into the pressurizer. Although the reactor had shut down and was no longer producing heat through a nuclear chain reaction, it still needed coolant water because of the decay heat of its nuclear fuel.

The meltdown at the Three Mile Island nuclear plant was very serious. As of 2015, it was the worst nuclear accident that had occurred in the United States. © A. L. Spangler/Shutterstock.com. © A. L. Spangler/Shutterstock.com.

The water level in the pressurizer remained high, but operators did not know that water was escaping through the stuck open valve. Furthermore, steam began building up, giving inaccurate readings of water levels in the system. Concerned that the water level in the pressurizer might be too high, a situation their training taught them to avoid, the plant operators shut down the emergency pumps that were sending water into the primary coolant system. The operators thus not only failed to interpret the incident for what it was (a loss-of-coolant accident) but also inadvertently made the situation worse.

The buildup of steam was causing dangerous vibrations in the reactor coolant pumps, so operators shut them down as well. At that point no water was flowing into the reactor, and the remaining water was turning to steam. A partial meltdown occurred when fuel rods (which hold nuclear fuel elements) began to melt as a result of these conditions.

The situation stabilized at 6:22 a.m. when the relief shift operators were able to correctly interpret the data to determine that the relief valve was open. The valve was then shut, and water was reintroduced into the cooling system. Although this solved the immediate emergency, several problems remained. A chemical reaction between melted fuel rod components and water was causing the buildup of hydrogen gas in the reactor. Additionally, water overflowing from the relief valve had filled the drain tanks and overflowed into an auxiliary building, and some of the overflow had been exposed to radioactive material. Although the building was designed to handle some amount of radioactive overflow, the higher levels caused by the meltdown resulted in the release of radioactive elements into the atmosphere.

Approximately 35,000 people were residing within 5 miles (8 kilometers) of the plant at the time of the accident. The incident was declared a general-area emergency, meaning that it could have serious consequences for the population at large. Several state and federal agencies were involved in the response to the accident, including the Nuclear Regulatory Commission (NRC), Page 608  |  Top of Articlethe Environmental Protection Agency (EPA), the U.S. Department of Energy (DOE), and the Pennsylvania Emergency Management Agency (PEMA), as well as representatives of the plant operator, Metropolitan Edison.

The days following the accident were marked by great confusion. Poor communication between government officials, scientists, plant operators, and the press made it difficult for anyone to assess the severity of the accident and determine an appropriate response. The local population dealt with contradictory statements from various sources. Some people immediately evacuated the area, but authorities were unsure of the size of the affected region and, therefore, unable to determine the scope of the evacuation order. An announcement was made two days after the accident that pregnant women and small children should leave, but by that time they would already have been exposed to radiation.

The hydrogen bubble in the reactor was also a source of public anxiety because many people feared that it might explode and eject radioactive material from the reactor into the atmosphere. Authorities announced that no significant quantities of radiation had been released because of the accident, yet people living close to the plant reported a metallic taste in their mouths, a sign of high levels of radioactive material. A visit from U.S. president Jimmy Carter (1924–) to TMI-2 on April 1, 1979, helped avert panic because many locals believed that the president's presence meant the area was safe.

After the hydrogen-bubble crisis was resolved, and as the threat of a major meltdown slowly faded, authorities decided on a cold shutdown of the plant, hoping to reestablish cooling through natural circulation. This was accomplished on April 27. Cleanup began in August 1979 and continued until December 1993. In July 1984 workers were able to access the remains of the core and assessed that more than 45 percent of the core had melted. TMI-1, meanwhile, was reactivated in October 1985, with decommissioning scheduled to begin in April 2034, a process expected to take 20 years.

The International Atomic Energy Agency created the International Nuclear and Radiological Event Scale to classify such incidents in terms of the severity of the event. The accident at Three Mile Island rated a Level 5 while the Chernobyl and Fukushima incidents both rated a Level 7.

The accident at Three Mile Island revived many of the fears that the proliferation of nuclear power plants had brought up in the 1960s and 1970s. During that period the development of atomic energy and the construction of several nuclear power plants throughout the world prompted a debate between proponents and opponents of nuclear power. Proponents argued that nuclear power, a renewable form of energy, was cleaner than fossil fuels (such as petroleum and coal) and would allow independence from foreign sources of energy. In contrast, opponents warned of the danger associated with the handling of radioactive materials, the disposal of nuclear waste, and the increased possibility of nuclear warfare.

In the aftermath of the accident at Three Mile Island, several investigations sought to determine its causes. The most prominent was that of the Kemeny Commission, which was appointed by President Carter and chaired by John Kemeny (1926–1992), then president of Dartmouth College. Internal reviews and investigations were also carried out at the NRC and within the nuclear energy industry.

The Kemeny Commission concluded that an original mechanical malfunction had been compounded by poor judgment by plant operators, whose training had not included the analytical skill set necessary to correctly assess the situation. A serious but manageable problem had therefore turned into an accident of near-catastrophic proportions. The commission also severely criticized Babcock and Wilcox, the manufacturer of the malfunctioning relief valve. The company had been aware of the valve's design flaw but had failed to communicate it.

The commission found several flaws in the engineering of control-room equipment that led to operators’ misinterpretation of the situation. The relief-valve signal light, for example, was not intended to reflect the valve's status but only to indicate that a signal had been sent for it to close. The fact that water levels in the reactor did not have a dedicated gauge and could only be inferred from water levels in the pressurizer also contributed to the presentation of information that baffled the operators. As a consequence, the commission recommended modifications in nuclear plant operators’ training programs. The Institute of Nuclear Power Operations, which sets operating standards and safety guidelines for nuclear power plants in the United States, was created Page 609  |  Top of Articleas a consequence of the Three Mile Island accident. The institute funds the National Academy for Nuclear Training program, which trains nuclear professionals.

The commission was also harshly critical of illpreparedness on the part of the government, the NRC, and the nuclear industry and of inadequate and poorly implemented emergency procedures. These observations prompted several reviews and reevaluations of safety and emergency protocols, both on the part of state and federal governments and within the nuclear energy industry. Finally, the commission criticized the confusion prompted by journalists’ lack of scientific training and scientists’ inability to explain complex concepts in simple terms, which greatly contributed to the public's anxiety as events were unfolding.

The more disputed and controversial elements of the Kemeny Commission's conclusions concerned the long-term consequences of the accident. The commission found that the maximum dose of radiation that any individual who was near the accident could have been exposed to was 0.7 millisieverts, a negligible amount that would not have long-term health effects. The conclusions were confirmed by several independent studies, including a 1990 independent review led by Columbia University researchers. The review did not find unusual or higher incidences of leukemia in adults or of childhood cancers (the diseases most commonly linked to radiation exposure) within a 10-mile (16-kilometer) radius of the accident site.

Other reports, however, contend that radiation exposure was much worse. Scientists who used data that were not available to the Kemeny Commission when it filed its findings have pointed to higher incidences of infant mortality, premature births, and underweight babies within the range of wind-borne radiation from the Three Mile Island accident (more specifically, iodine-131, which at high levels could affect the thyroid of unborn babies). A 1997 paper by scientists at the University of North Carolina, reported on by David Williamson, also found that occurrences of cancer were 10 times greater downwind than upwind from the accident site.

Local inhabitants have spoken of their own experience of the accident. From a metallic taste in the air to symptoms of radiation poisoning at the time of the accident to health issues such as respiratory problems, hyperthyroidism, and leukemia, their testimonies implied much higher levels of radiation than have been officially acknowledged. Hundreds of personal injury claims were filed in a class-action lawsuit against Metropolitan Edison. The lawsuit went back and forth between district and appeals courts, as well as the U.S. Supreme Court, for 15 years before being dismissed by a district judge in 1996.

The U.S. nuclear power program was essentially suspended following the Three Mile Island accident. Growing public concern over the safety of nuclear energy resulting from the accident led to the cancellation of subsequent nuclear power plant construction for many years. One plant started operation in the United States in 1996. However, when the NRC approved licenses to build two new reactors in 2012, they were the first such approvals in more than 30 years.

The subsequent nuclear disasters in 1986 at the Chernobyl nuclear power station in Ukraine (then part of the Soviet Union) and in 2011 at the Fukushima Daiichi nuclear power station in Japan resulted in a greater public reaction because of their greater severity and the greater extent. However, the accident at Three Mile Island was first in raising many of the public concerns inherent in nuclear technology.

The accident at Three Mile Island provided valuable lessons that prompted a thorough revision in plant operator training and emergency crisis management, but the debate over its long-term consequences on the local population's health revived many fears linked to nuclear energy and its potential dangers. Furthermore, authorities’ handling of the crisis, ranging from incompetence to potential cover-up, raised many ethical questions regarding the practices of nuclear energy's regulating bodies and of the entities that have an economic interest in the development of nuclear technology.