Three-Mile Island

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On March 28, 1979, a series of events took place at the nuclear reactor at Three Mile Island, Unit 2 (TMI-2), near Harrisburg, Pennsylvania, that resulted in an accident in which a significant fraction of the nuclear reactor core melted and a small amount of radioactivity was released to the environment. After more than twenty years of government-stimulated development of the nuclear power industry and in the context of increasing public objections, that accident became the focus for an intensely polarized debate about the wisdom of further construction of nuclear reactors. The accident at the Three Mile Island nuclear power station has taken on a key historical role in discussions concerning science, technology, and ethics.

Reactor Design

Understanding the accident requires a general understanding of the way the TMI-2 reactor worked. TMI-2 was a pressurized water reactor. A simple diagram of the system is shown in Figure 1.

The fission process—splitting the atom, with the release of energy—occurs in the reactor core. This generates heat, and so the core is cooled with water under high pressure, which is needed to prevent the water from boiling. The reactor is contained in a thick (ten inches) steel-walled reactor vessel. Two loops circulate the water. The primary loop carries the pressurized water through the reactor, where it is heated, to a device called a steam generator. In the steam generator heat is transferred from the primary loop to water in a secondary loop, which is not under pressure, and thus is converted to steam. Water in the primary loop does not mix with water in the secondary loop. Radioactivity in the primary loop never mixes with water in the secondary loop. The cooled water in the primary loop then is pumped back to the reactor for reheating. The steam produced in the secondary loop is piped to a turbine, where it hits turbine blades and causes them to spin. The turbine is connected to a generator that produces electricity. The steam then condenses below the turbine and is pumped back to the steam generator for its own reheating.

The primary loop is contained inside a steel-lined, steel-reinforced concrete building in which the walls are three to five feet thick. This containment building, as shown in Figure 1, is designed to prevent or at least minimize radiation leakage to the environment in case of a serious accident. It is a requirement in the United States that all commercial reactors be built inside a containment building. This is part of the "defense-in-depth" philosophy that has been required from the beginning in the design of commercial nuclear power plants in the United States.

The Accident

The accident began at 4:00 A.M., when maintenance activities caused secondary loop pumps to shut down, leading to a buildup of heat in the primary loop. The reactor shut down automatically, but the pressure in the primary loop increased significantly. As is shown in Figure 1, a pressurizer outside the reactor vessel monitors the primary loop. If the pressure gets too high, a valve opens and radioactive water escapes to the drain tank below the reactor.

This is what happened at TMI-2. When the pressure returned to normal, the operator sent an electrical signal to the motor that closes the valve. An indicator light showed this action was taken, causing the operator to believe the valve had closed. Unfortunately the indicator did not show the actual valve position, which was partially stuck open. One of the changes resulting from the accident is an indicator that actually shows closure of the valves. Another sensor in the control room showed high pressure in the reactor drain tank, which indicated a leak, but this indicator was located behind a seven-foot-high instrument panel.

Alarms and warning lights began to go off in the control room, indicating problems in different systems. This confused the operators and made it difficult to diagnose the problem and choose the appropriate corrective action. Water continued to leak through the open valve from the primary loop to the basement, where it overflowed from the reactor drain tank onto the basement floor. It then was pumped to tanks in the adjacent auxiliary building. When those tanks overflowed, radioactive water spilled onto the floor of the auxiliary building, enabling the radioactive gas xenon, an inert gas that is not incorporated into the body tissue, to escape from the building through the ventilation system. This resulted in a low-level exposure to residents in surrounding communities.

Even when a reactor is shut down, residual radioactive fission products in the reactor core continue to produce heat that must be removed. An emergency cooling system turned on automatically and started pumping water into the primary loop. The operators, however, thinking that the valve on the pressurizer was closed and noting that the water level indicator in the pressurizer showed that the pressurizer was full, throttled back and then shut down the emergency cooling system because they feared that the primary loop would overfill with water and cause a dangerous overpressure in the loop.

Actually, the pressure was dropping in the primary loop because of the open valve, and boiling of the remaining water began to occur. The large pumps for the primary cooling water began to vibrate heavily because they were filling with steam from the boiling water. Those pumps were shut down to prevent them from being damaged. Although the primary loop water was boiling off, with the steam going through the open valve, serious damage to the core still would have been avoided if the emergency core cooling system had continued operating.

After about 100 minutes enough water had leaked from the core through the open pressurizer valve that the top of the core was no longer covered with cooling water. The temperature in the uncovered parts of the core began to rise. The fuel is contained in tubes called cladding made of a zirconium alloy, and the uncovered tubes began to react with the steam, releasing hydrogen. Some of that hydrogen escaped into the containment building and later underwent a rapid burn (mild explosion) that caused some equipment damage. Some of the hydrogen accumulated in the top of the vessel that held the reactor and inhibited reactor cooling for several days. It also led to concern by some Nuclear Regulatory Commission (NRC) staff members that the hydrogen might explode. (It turned out that this was not possible because of an oxygen shortage in the system.) Because of uncertainty about the condition of the reactor two days after the accident began Pennsylvania Governor Richard Thornburgh advised pregnant women and pre-school-age children within a five-mile radius of the plant to evacuate.

After 142 minutes the cause of the leak was determined, and a backup valve for the pressurizer was closed, stopping the loss of water. However, by that time about one-third of the primary loop water had escaped. Because of concern that introducing cold water into the intensely heated core would cause the fuel elements to fracture, the emergency core cooling system was not restarted until four and a half hours after the accident began.

As the core overheated and the cladding underwent chemical reactions as well as melting, the core structure began to lose strength and the top of the fuel elements collapsed into a pile, some of which heated to the melting temperature of the fuel, creating a large molten mass in the center. Some of that molten fuel eventually spilled over the side of the core and accumulated below the core. Altogether approximately 50 percent of the core melted. Fortunately, there was sufficient cooling water to prevent the molten fuel from rupturing the reactor vessel. Except for the radioactivity in the cooling water that leaked into the drain tank and then was pumped into the auxiliary building, from which there were small gaseous releases to the environment, almost all the radioactivity was contained within the containment building. The final state of the core at the end of the accident is shown in Figure 2.

Health Effects

The Nuclear Regulatory Commission, the Environmental Protection Agency (EPA), the Department of Health, Education and Welfare, the Department of Energy, and the state of Pennsylvania conducted studies on the health effects of the accident. All those studies concluded that the dose any member of the public received was far less than the natural background radiation. There was no increase in cancer in the surrounding communities.

Some nongovernmental groups and university researchers rejected those reports. Although the accident led to no generally accepted radiation injuries to the public or to workers, it did cause an emotional trauma to the local citizens and indeed to the nation. Without question it led to a loss of public confidence in nuclear power.

Lessons and Changes

Analysis of the accident revealed several significant operations problems in the industry as well as oversight problems at the NRC. Of particular importance was the finding that operator error had resulted from a lack of understanding of how the system behaved, a lack of information at the control panel to help operators make a correct diagnosis, and a control panel design that promoted confusion rather than understanding. Other issues in the accident included poor communication between the reactor site and NRC headquarters, ineffective communication with the public and the press, and an inadequate communication system for the NRC and industry to inform operators of safety problems identified at other plants. For example, the operators did not know that a similar stuck valve incident had occurred at another reactor eighteen months earlier.

In response the industry created an operations oversight organization called the Institute for Nuclear Power Operations (INPO). Among its activities are plant visits by expert teams on a regular basis (twelve to eighteen months), assistance to plant operators to improve their skills, and the creation of the National Academy for Nuclear Training, which accredits nuclear training programs in maintenance and operations to assure high standards. Simulators that replicate the behavior of the plant now exist at each site and are used to train operators on normal operations and accident scenarios. A key goal of INPO is the promotion of a culture at nuclear power plants that emphasizes "safety first" as the basis of decision making.

Finally, the NRC and industry used information from the accident to develop computer models that describe the progression of serious accidents. There are now emergency centers that conduct regular emergency exercises, including the use of local community response teams of emergency workers and fire fighters. All these efforts have transformed the U.S. nuclear industry and its regulation and have resulted in remarkable improvements in safe operations as well as economic performance, both of which were needed.

In the United States the nuclear power industry had developed rapidly in the 1960s and 1970s, with different companies involved and with diverse designs and changes in design with each new reactor. The power output of the reactors increased quickly from the early small reactors, with the belief that there would be an "economy of scale" with larger units. The regulatory process developed in parallel with industry growth, and changes in regulations were made as experience was gained and plants got larger. As a result each reactor was unique, and it was difficult to maximize learning in construction, operation, and maintenance. This contrasts with both the French and the Japanese nuclear power industries, which were initiated later and chose one or a small number of designs for their reactors, which contributed to facilitated learning in building and operations.

Accident Cleanup

Cleanup of the accident included the processing and storing of radioactive contaminated water in the auxiliary and reactor buildings and removal of contaminated building materials and the reactor core to a safe storage site at the Idaho National Engineering Laboratory (INEL). This was a lengthy, expensive, and contentious process. Numerous technical challenges, many of them first of a kind, had to be overcome. Those challenges included (1) building and operating systems to treat the radioactive water; (2) inspecting damage to the core, which revealed a collapse of the top five feet of the reactor material into a rubble bed, with a five-foot-thick section of solidified melted fuel below; (3) development and use of tools to break up the solidified section of the core so that it could be loaded into casks and shipped to INEL; (4) solving a biological growth problem that caused clouding of the water; and (5) the development and use of robotic equipment to decontaminate the reactor building basement. In addition to finding solutions to the technical problems, NRC approval was needed for each step in the cleanup. This often resulted in delays, partly because the NRC frequently sought general public input and acceptance.

Some of the contentious issues that arose delayed the cleanup. One was the venting of radioactive gas from the containment building to allow worker entry and building cleanup to begin. Two raucous public meetings were held before NRC approval of the plan. The public was angry, fearful, and mistrusting, and assurances that radiation exposure to the public would be negligible fell on deaf ears. The venting took place from June 28 to July 11, 1980, and was monitored by the NRC, the EPA, a state agency, the utility company, and a citizen's group. Radiation exposure was determined to be negligible.

Another issue was more technical and involved the use of a crane above the reactor vessel to remove the vessel head to allow access to the fuel. The conditions inside the containment were junglelike, including high humidity and even rain. Extensive maintenance was performed on the crane to ready it for use, but one engineer, Richard Parks, wanted to do a full load test before attempting to lift the multiton vessel head. When management decided against this, Parks went directly to the NRC with his concern and was fired for whistle-blowing by the general contractor, Bechtel. The NRC sided with his concern, and testing was performed before the head was lifted.

Additional public concerns arose about shipping canisters of highly radioactive waste off-site to INEL and about the disposal of the decontaminated water after the rest of the cleanup had been completed. The simplest and least expensive solution would have been to release the water gradually to the river. This would not have presented any hazard to the public, but there was strong citizen opposition to putting the water into the Susquehanna River. In the end the utility agreed to evaporate the water. That operation was completed in August 1993 after a two-and-a-half-year process.

It took approximately eleven years to complete the cleanup and place the building in a monitored shutdown state. The cost was approximately $1 billion. This does not include the cost of replacement electricity or the cost related to TMI-1 being shut down for six years before it was allowed to restart. The cost to the industry was also substantial because the NRC required numerous modifications to the safety systems of all pressurized water reactors as well as changes to operating procedures. Although those changes did enhance plant safety in most cases, making changes in response to a crisis is generally more expensive and undoubtedly drove up the cost of nuclear power generation in the 1980s.

Ethical and Policy Issues

Several ethical and policy issues have arisen regarding the safety of nuclear power plants and whether another accident might occur. The first issue is whether electric power generation companies might put economics before safety. Although the industry has found that the safest plants are also the most economical, decisions to keep a plant operating even though conservative safety considerations suggest it should be shut down occasionally still occur. One example was the Davis-Besse plant in Ohio in 2002, where evidence of continuous corrosion of the reactor vessel was not investigated thoroughly until the corrosion completely penetrated the head. Fortunately, the steel liner was able to hold the reactor pressure until the problem was discovered. The public will have to judge whether the safety record of the industry and the oversight of the NRC are sufficient to justify the continued operation of nuclear power plants.

Second, and perhaps more significant in the early twenty-first century, is whether, in light of potential terrorist attacks against nuclear power plants, the nation should continue to use nuclear power, which in 2000 supplied approximately 20 percent of the electricity consumed in the United States. Could a group of terrorists breach all safety systems and cause a significant radiation injury to the public? After the terrorist attacks of September 11, 2001, security has been enhanced at each nuclear site, including the hiring of additional guards. Also, studies have been made on the effect of an airplane crash into the containment building and other parts of the plant. These studies suggest that the use of standard evacuation procedures would be sufficient to prevent any serious injury to the public. Nonetheless, some public officials and critics of nuclear power lack confidence in the results and believe nuclear power plants should be eliminated.

There are, however, national security and environmental benefits of nuclear power that must be considered. Nuclear power does not require the use of imported fossil fuels such as oil or future imports of natural gas. Furthermore, there are no emissions of sulfur oxides, nitrous oxides, or carbon dioxide as there are with the burning of fossil fuels. Indeed, nuclear power is already the dominant method of avoiding carbon dioxide emissions in the nation. Any replacement of the 20 percent of electricity generated by nuclear power could increase the cost of electricity generation, reduce the reliability of the electrical grid system, and/or increase pollutants emitted to the environment. Nuclear power may be critically needed to reduce the potential consequences of global warming. Also, as the price of natural gas rises and as it is recognized that natural gas may be able to serve as a substitute for oil in transportation, nuclear power may be the most cost-effective means for producing electricity, especially for electrical generation that has a minimum of environmental consequences.


SEE ALSO Chernobyl; Nuclear Ethics; Nuclear Regulatory Commission.


Knief, Ronald Allen. (1992). Nuclear Engineering: Theory and Technology of Commercial Nuclear Power, 2nd edition. Washington, DC: Hemisphere. A technical account of the accident contained in a text on commercial nuclear power. Has a good summary of the lessons learned.

Osif, Bonnie A.; Anthony J. Baratta; and Thomas W. Conkling. (2004). TMI 25 Years Later. University Park: Pennsylvania State University Press. Written for the general public; provides a good summary of how nuclear power plants work as well as a description of the accident, its causes, and responses to it.

President's Commission on the Accident at Three Mile Island. (1979). The Need for Change, the Legacy of TMI: Report of the President's Commission on the Accident at Three Mile Island. Washington, DC: Author. (Also available in paperback and hardcover from Pergamon, Elmsford, NY, 1979.) The first and most important analysis of the accident, with particular emphasis on the failures of the NRC, along with ample discussion of problems in the nuclear industry. The authors identified needed improvements and made numerous specific recommendations to improve the industry and its regulation.

Rees, Joseph V. (1994). Hostages of Each Other: The Transformation of Nuclear Safety since Three Mile Island. Chicago: University of Chicago Press. The history of the Institute for Nuclear Power Operations. The primary emphasis is on the industry's response to the TMI-2 accident.

Walker, J. Samuel. (2004). Three Mile Island: A Nuclear Crisis in Historical Perspective. Berkeley: University of California Press. A well-documented book that describes the accident in great detail, summarizing what happened each day from Wednesday, when the accident started, to the Monday after the accident, when the most intensive crisis phase was over. Effectively captures the human dynamics involving the NRC, the governor of Pennsylvania, and state offices. Also continues the story of the accident through the reviews that followed.