![]() Three Mile Island nuclear facility, c. 1979 | |
Date | March 28, 1979 |
---|---|
Time | 04:00 (Eastern Time Zone UTC−5) |
Location | Londonderry Township, Dauphin County, Pennsylvania, U.S. |
Outcome | INES Level 5 (accident with wider consequences) |
Designated | March 25, 1999[1] |
The Three Mile Island accident was a partial nuclear meltdown of the Unit 2 reactor (TMI-2) of the Three Mile Island Nuclear Generating Station on the Susquehanna River in Londonderry Township, near Harrisburg, Pennsylvania. The reactor accident began at 4:00 a.m. on March 28, 1979, and released radioactive gases and radioactive iodine into the environment.[2][3] It is the worst accident in U.S. commercial nuclear power plant history.[4] On the seven-point logarithmic International Nuclear Event Scale, the TMI-2 reactor accident is rated Level 5, an "Accident with Wider Consequences".[5][6]
The accident began with failures in the non-nuclear secondary system,[7] followed by a stuck-open pilot-operated relief valve (PORV) in the primary system,[8] which allowed large amounts of water to escape from the pressurized isolated coolant loop. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident (LOCA). TMI training and operating procedures left operators and management ill-prepared for the deteriorating situation caused by the LOCA. During the accident, those inadequacies were compounded by design flaws, such as poor control design, the use of multiple similar alarms, and a failure of the equipment to indicate either the coolant-inventory level or the position of the stuck-open PORV.[9]
The accident heightened anti-nuclear safety concerns among the general public and led to new regulations for the nuclear industry. It accelerated the decline of efforts to build new reactors.[10] Anti-nuclear movement activists expressed worries about regional health effects from the accident.[11] Some epidemiological studies analyzing the rate of cancer in and around the area since the accident did determine that there was a statistically significant increase in the rate of cancer, while other studies did not. Due to the nature of such studies, a causal connection linking the accident with cancer is difficult to prove.[12][13][14][15][16][17][18] Cleanup at TMI-2 started in August 1979 and officially ended in December 1993, with a total cost of about $1 billion (equivalent to $2 billion in 2023).[19] TMI-1 was restarted in 1985, then retired in 2019 due to operating losses. Its decommissioning is expected to be complete in 2029 at an estimated cost of $1.2 billion.[20]
A revised International Nuclear and Radiological Event Scale (INES) extends its reach.
Level 5: Accident with Wider Consequences; Three Mile Island, USA, 1979 – Severe damage to the reactor core.
The steam generator tubes, steam turbine, condenser, and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation. Most commonly used in reference to pressurized water reactors.
The primary system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping that connects these components to the reactor vessel. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators. A second function is to contain any fission products that escape the fuel.
The commission concluded that Met Ed, GPU, Babcock & Wilcox, and the NRC shared responsibility for the shortcomings in operator training. Those inadequacies were compounded by design flaws that undermined the efforts of the plant staff to deal with the accident. They included the cacophony of undifferentiated alarms, the inconvenient arrangement of instruments and controls, and the absence of clear indicators either of levels of water in the pressure vessel or of the position of the stuck-open PORV.
...we arrive at 333 fatal cancers or leukemias.
Thyroid cancer incidence has not increased in Dauphin County, the county in which TMI is located. York County demonstrated a trend toward increasing thyroid cancer incidence beginning in 1995, approximately 15 years after the TMI accident. Lancaster County showed a significant increase in thyroid cancer incidence beginning in 1990. These findings, however, do not provide a causal link to the TMI accident.
The NRC conducted detailed studies of the accident's radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. Several independent groups also conducted studies. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X-ray is about 6 millirem and the area's natural radioactive background dose is about 100–125 millirem per year for the area. The accident's maximum dose to a person at the site boundary would have been less than 100 milligrams above the background. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. Comprehensive investigations and assessments by several well-respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.