Three Mile Island, near Harrisburg, Pennsylvania, was the site of the worst accident in the history of commercial nuclear power in the United States. On March 28, 1979, the nuclear reactor—a device that produces and controls nuclear energy—at Unit 2 of the Three Mile Island Nuclear Generating Station overheated and suffered a massive core meltdown. In an extreme case, a meltdown can overwhelm the safety systems of a plant and release enormous quantities of radioactive materials to nearby areas. At Three Mile Island, however, despite severe damage to the plant, only small amounts of the most dangerous forms of radiation escaped to the environment. The accident did not cause any immediate deaths, and it did not appear to cause an increase in cancer or other diseases among the local population. But the accident tested the readiness of authorities to react to nuclear emergencies and led to increased regulation of the commercial nuclear power industry.
The accident.
Unit 1 of the Three Mile Island nuclear facility, about 10 miles (16 kilometers) southeast of Harrisburg on an island on the Susquehanna River, had been in operation since 1974. The plant’s Unit 2 reactor began commercial operation in December 1978. The Metropolitan Edison Company was the principal owner and operator of both units.
Nuclear reactors like those at Three Mile Island use the heat of a controlled nuclear reaction to produce steam, which then drives turbines to generate electric power. At about 4 a.m. on March 28, 1979, a series of problems in the generating and reactor cooling systems for the plant’s Unit 2 reactor led to a serious accident. A valve that had opened automatically to relieve pressure in the reactor core failed to close. The core contains the long, slender fuel rods that produce heat through nuclear fission. With the valve stuck open, water that cooled the fuel rods escaped from the core.
As lights flashed and alarms sounded, the operators did not realize that they were faced with a “loss-of-coolant accident.” The plant had no instruments that showed clearly that the valve was open or that coolant was rushing from the core. When the level of coolant diminished, the plant’s “emergency core cooling systems” immediately dumped water onto the core, as they were designed to do. But the operators, believing that the problem was an excess of water, slowed the flow of coolant from the emergency systems to a trickle. Consequently, the fuel rods heated up from their normal operating temperature of about 600 °F (315 °C) to 4000 °F (2200 °C) or more, and about half of the core melted. This meltdown released extraordinarily high levels of radiation within the containment building, a huge dome-shaped structure that housed the reactor core.
Response to the accident.
The severity of the accident was unclear for some time after it occurred. Executives of the Metropolitan Edison Company initially made statements that sharply underestimated the potential consequences of the accident. Not until the afternoon of March 29 did they conclude that the reactor had suffered serious damage. But experts from the company and from the United States Nuclear Regulatory Commission (NRC), the federal agency that regulated commercial nuclear plants, did not know that the core had melted. Their primary objective was to find ways to cool the core and to prevent a large release of radiation to the environment. As the company and the NRC worked to bring the crippled plant under control, federal and state government officials considered whether people should be ordered to leave the area. Richard L. Thornburgh, the governor of Pennsylvania, was responsible for making the final decision on a mandatory evacuation.
Thornburgh and his staff, as well as the NRC and the White House, faced a difficult dilemma. The governor feared that if he ordered an evacuation, it would cause disruption, economic loss, injuries, and almost certainly some deaths among the population. But if he did not order an evacuation and there was a major release of radiation, a public health disaster would result. A lack of reliable information about the condition of the plant made the decision even more difficult. On March 30, in response to incorrect reports about an “uncontrolled release” of radiation, Thornburgh advised pregnant women and families with young children within a 5-mile (8-kilometer) radius of the plant to evacuate their homes.
The hydrogen bubble.
The evacuation issue became even more urgent when experts discovered that a large hydrogen bubble had formed in the pressure vessel of the reactor. The pressure vessel holds the core, and authorities at first were concerned that the bubble would prevent cooling of the core. By March 31, however, some NRC authorities feared that the bubble might burn or explode. If this occurred, it would increase the chances of an eventual breach (break) of containment. The officials did not believe that the plant was on the verge of an explosion, but the bubble generated much anxiety for authorities and the general population.
Public fears about nuclear safety were heightened at this time in part because of the popular film The China Syndrome (1979). The film, released less than two weeks before the Three Mile Island incident, chronicled a fictional nuclear disaster in California.
President Jimmy Carter toured the plant on April 1, and his presence helped to reassure the public that the plant was not about to explode. A short time after the president’s visit, the NRC found that the fears of the bubble becoming flammable or explosive were without basis. This finding ended the immediate crisis, and the 144,000 people who voluntarily evacuated the area soon returned home.
Aftermath of the accident.
A series of investigations by a presidential commission, the NRC, the state of Pennsylvania, and the United States Congress sharply criticized Metropolitan Edison, the nuclear industry, and the NRC itself. In the aftermath of the accident, the industry and the NRC took steps to improve operator training, instrumentation, safety systems, reactor oversight, communications, and emergency planning.
The accident spurred new concerns about the safety of nuclear power, which had been the focus of a bitter national controversy during the 1970’s. In a series of rallies—including one that drew about 200,000 protesters in New York City—opponents called for an end to nuclear power in the United States.
The cleanup of the plant took more than a decade and cost about $1 billion. Although the nuclear industry had been in a slump even before Three Mile Island, its credibility and financial outlook fell to new depths after the accident. Plans for a number of new plants were canceled. Not until 2012 did the NRC approve permits for the construction of new nuclear plants. The Three Mile Island nuclear facility permanently closed in 2019.
