The name “Three Mile Island” is synonymous with the most significant accident in the history of commercial nuclear power in the United States. But for many, the immediate question is simply: where is Three Mile Island?
Three Mile Island (TMI) is located in the Susquehanna River, in central Pennsylvania, just a few miles southeast of Harrisburg, the state capital. More precisely, it sits near Middletown, Dauphin County. It’s not actually an island in the traditional sense of being isolated in the ocean, but rather a landmass within a river. This location was chosen for its access to the Susquehanna River’s cooling water, essential for the operation of a nuclear power plant.
The site housed two pressurized water reactors, Unit 1 and Unit 2. While Unit 1 continued operating for many years after, it is Unit 2 that became infamous due to the partial nuclear meltdown on March 28, 1979. This incident, although releasing minimal radioactivity with no detectable health effects on workers or the public, profoundly changed the landscape of nuclear power regulation and safety in the US.
The 1979 Accident at Three Mile Island: A Summary of Events
The accident at Three Mile Island Unit 2 began in the early hours of March 28, 1979, triggered by a failure in the plant’s secondary, non-nuclear system. This initial malfunction, either mechanical or electrical, stopped the main feedwater pumps from supplying water to the steam generators, which are crucial for removing heat from the reactor core.
Animated diagram showing the progression of the Three Mile Island accident, highlighting equipment malfunctions and human responses.
This feedwater disruption caused the turbine-generator and subsequently the nuclear reactor to automatically shut down as designed. However, immediately following the shutdown, pressure within the primary system (the part of the plant containing nuclear piping) began to rise. To manage this pressure increase, the pilot-operated relief valve, situated at the top of the pressurizer, opened to release pressure.
Crucially, this valve malfunctioned and became stuck in the open position, even after the pressure had returned to normal levels. The control room instruments, unfortunately, provided misleading information, indicating to the operators that the valve had closed correctly. Unaware that the relief valve was stuck open and continuously releasing coolant in the form of steam, the plant staff failed to recognize they were facing a loss-of-coolant accident.
Adding to the confusion, other instruments gave inadequate or misleading data. Normally, the large pressure vessel encasing the reactor core was completely filled with water. Plant operators relied on the pressurizer water level as an indicator of core water coverage, assuming a high pressurizer level meant the core was also adequately cooled. This assumption proved to be false.
Operating under the mistaken belief that the stuck-open relief valve was closed and lacking accurate information about the core’s water level, the staff took actions that ultimately worsened the situation and led to the uncovering of the reactor core. The continuous coolant loss through the stuck valve reduced pressure in the primary system to a point where the reactor coolant pumps started vibrating excessively, leading to their shutdown by the operators. Furthermore, emergency cooling water, automatically injected into the primary system, threatened to overfill the pressurizer – an undesirable condition. In response, the operators reduced the flow of emergency cooling water.
With the reactor coolant pumps off, halting water circulation, and with a reduced supply of emergency cooling water, the water level inside the reactor pressure vessel dropped critically, causing the reactor core to overheat and partially melt down.
Health and Environmental Effects: Minimal Impact
Following the accident, numerous detailed investigations into the radiological consequences were conducted by the NRC, the Environmental Protection Agency (EPA), the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, the Commonwealth of Pennsylvania, and various independent research groups.
These studies consistently concluded that the radiation release from the TMI-2 accident was minimal. It is estimated that the approximately 2 million people living within a 50-mile radius of the plant during the accident received an average radiation dose of only about 1 millirem above the usual background radiation levels. To provide context, a chest X-ray exposes an individual to about 6 millirem, and the natural background radiation in the area is around 100-125 millirem per year. The maximum possible dose to a person at the plant’s boundary during the accident was calculated to be less than 100 millirem above background levels.
In the aftermath, while concerns were raised about potential adverse effects of radiation on humans, animals, and plant life in the TMI area, no direct correlation to the accident could be established. Extensive environmental sampling of air, water, milk, vegetation, soil, and food items by government agencies revealed very low levels of radionuclides attributable to the accident releases. However, comprehensive assessments by reputable institutions like Columbia University and the University of Pittsburgh concluded that despite the severe damage to the reactor, the actual radioactive release had negligible effects on the physical health of individuals or the surrounding environment.
Impact and Legacy: Transforming Nuclear Safety
The Three Mile Island accident, stemming from a combination of human error, design flaws, and equipment malfunctions, profoundly and permanently changed both the nuclear industry and the Nuclear Regulatory Commission (NRC). Public fear and distrust of nuclear power surged. In response, the NRC significantly broadened and strengthened its regulations and oversight of nuclear power plants, and plant management practices underwent much stricter scrutiny.
Thorough analyses of the accident’s sequence of events identified critical areas for improvement, leading to sweeping and lasting changes in how the NRC regulates its licensees. These changes were specifically designed to minimize risks to public health and safety.
Key improvements implemented since the TMI accident include:
- Enhanced Plant Design and Equipment: Upgrades and strengthening of requirements for fire protection, piping systems, auxiliary feedwater systems, containment building isolation, component reliability (pressure relief valves, electrical circuit breakers), and automatic plant shutdown capabilities.
- Human Performance Focus: Recognizing the crucial role of human factors, operator training and staffing requirements were revamped. This included improved instrumentation and control systems in control rooms and the implementation of fitness-for-duty programs to prevent alcohol or drug abuse among plant personnel.
- Emergency Preparedness Enhancements: Stricter requirements for emergency preparedness were established, mandating immediate NRC notification of significant events and a 24/7 NRC Operations Center. Regular drills and response plans are now conducted, involving state and local agencies in coordination with the Federal Emergency Management Agency (FEMA) and the NRC.
- Performance Monitoring and Transparency: The NRC integrated its observations, findings, and conclusions regarding licensee performance and management effectiveness into regular public reports.
- Proactive Regulatory Attention: Senior NRC managers now regularly analyze plant performance to identify and address plants requiring increased regulatory attention.
- Resident Inspector Program Expansion: The NRC’s resident inspector program, initiated in 1977, was expanded to ensure at least two NRC inspectors reside near and work full-time at each US nuclear plant, providing daily oversight of regulatory compliance.
- Risk-Informed Inspections: Inspections were broadened to include performance-oriented and safety-focused assessments, and risk assessment methodologies were adopted to identify plant vulnerabilities to severe accidents.
- Enforcement Authority Strengthening: The NRC’s enforcement staff was strengthened and reorganized into a separate office to enhance its effectiveness.
- Industry Self-Regulation and Unified Approach: The nuclear industry established the Institute of Nuclear Power Operations (INPO) as a self-regulating body. The Nuclear Energy Institute (NEI) was also formed to provide a unified industry approach to nuclear regulatory issues and to facilitate interactions with the NRC and other government agencies.
- Accident Mitigation Equipment: Licensees installed additional equipment to better mitigate accident conditions and to enhance radiation level and plant status monitoring.
- Operating Experience Programs: Programs were implemented by licensees for the early identification of safety-related problems and for the systematic collection and analysis of operating experience data to enable rapid sharing of lessons learned and prompt corrective actions across the industry.
- International Safety Collaboration: The NRC expanded its international collaborations to share its enhanced nuclear safety knowledge with other countries across various technical domains.
Current Status: Decommissioning and Monitoring
Today, Three Mile Island Unit 2 is permanently shut down, with 99% of its nuclear fuel removed. The reactor coolant system has been fully drained, and the radioactive water has been decontaminated and evaporated. Radioactive waste from the accident was shipped to appropriate disposal sites, and the reactor fuel and core debris were sent to the Department of Energy’s Idaho National Laboratory. TMI-2 Solutions acquired the license for Unit 2 in 2020 and is currently responsible for the remaining decommissioning activities.
Unit 1 at Three Mile Island also permanently ceased operations in September 2019. Constellation Energy Company (formerly Exelon Generation) is now responsible for decommissioning Unit 1.
The table below outlines key milestones in the cleanup of TMI Unit 2 from 1980 to 1993:
| Date | Event |
|---|---|
| July 1980 | Approximately 43,000 curies of krypton vented from the reactor building. |
| July 1980 | First manned entry into the reactor building. |
| Nov. 1980 | Advisory Panel for the Decontamination of TMI‑2 held its first meeting. |
| July 1984 | Reactor vessel head (top) removed. |
| Oct. 1985 | Fuel removal began. |
| July 1986 | Off-site shipment of reactor core debris began. |
| Aug. 1988 | GPU submitted request to amend TMI‑2 license to “possession‑only” license. |
| Jan. 1990 | Fuel removal completed. |
| July 1990 | GPU submitted funding plan for $229 million escrow for decommissioning. |
| Jan. 1991 | Evaporation of accident-generated water began. |
| April 1991 | NRC published notice of hearing opportunity on license amendment request. |
| Feb. 1992 | NRC issued safety evaluation report and granted license amendment. |
| Aug. 1993 | Processing of accident-generated water completed (2.23 million gallons). |
| Sept. 1993 | NRC issued possession-only license. |
| Sept. 1993 | Advisory Panel for Decontamination of TMI-2 held its last meeting. |
| Dec. 1993 | Monitored storage began. |
Further Information and Resources
For more detailed information on the TMI-2 accident, NUREG documents are available, many on microfiche. These can be ordered for a fee from the NRC’s Public Document Room.
Additional Sources for Information on Three Mile Island: (These are implicitly the original NRC documents, given the source material)
Glossary of Nuclear Terms
Auxiliary feedwater ‑ (see emergency feedwater)
Background radiation ‑ The radiation in the natural environment, including cosmic rays and radiation from naturally radioactive elements. Average individual exposure is about 300 millirem per year.
Cladding ‑ Thin‑walled metal tube forming the outer jacket of a nuclear fuel rod, preventing corrosion and release of fission products.
Emergency feedwater system ‑ Backup feedwater supply used during plant startup and shutdown; also known as auxiliary feedwater.
Fuel rod ‑ Long, slender tube holding fuel (fissionable material) for nuclear reactor use, assembled into fuel elements or assemblies.
Containment ‑ Gas‑tight shell around a reactor to confine fission products in case of an accident.
Coolant ‑ Substance circulated through a reactor to remove or transfer heat, typically water in US reactors.
Core ‑ Central portion of a reactor containing fuel elements and control rods.
Decay heat ‑ Heat produced by radioactive fission product decay after reactor shutdown.
Decontamination ‑ Reduction or removal of radioactive material from a structure, area, object, or person.
Feedwater ‑ Water supplied to the steam generator to remove heat from fuel rods by boiling into steam for the turbine generator.
Nuclear Reactor ‑ Device for sustained and controlled nuclear fission reaction, including fuel, moderator, reflector, heat removal, controls, and protective devices.
Pressure Vessel ‑ Strong‑walled container housing the reactor core in most power reactors.
Pressurizer – Tank controlling pressure in certain nuclear reactor types.
Primary System ‑ Cooling system removing energy from the reactor core and transferring it to the steam turbine.
Radiation ‑ Particles or photons emitted from an unstable atom’s nucleus due to radioactive decay.
Reactor Coolant System ‑ (see primary system)
Secondary System ‑ Steam generator tubes, turbine, condenser, and associated components converting reactor coolant heat energy into electrical energy.
Steam Generator ‑ Heat exchanger transferring heat from the primary to the secondary system in some reactor designs.
Turbine ‑ Rotary engine with curved vanes on a shaft, typically turned by steam or water to drive electrical generators.
Plant diagram illustrating the components of Three Mile Island Unit 2, including the Reactor Building, Turbine Building, and Cooling Tower, along with the direction of cooling water flow.
April 2022
Page Last Reviewed/Updated Thursday, March 28, 2024
