Overview
The Three Mile Island accident was a partial nuclear core meltdown that occurred on March 28 1980 at the Three Mile Island (TMI) nuclear power plant near Harrisburg, Pennsylvania, United States. It is the most serious accident in U.S. commercial nuclear power plant history.
Plant and Reactor
- Location: TMI-2, a 905‑MW(e) pressurized water reactor (PWR) situated on a peninsula in the Susquehanna River, approximately 10 mi (16 km) southeast of Harrisburg.
- Operator: Metropolitan Edison Company, a subsidiary of General Public Utilities (GPU).
- Commissioning: The reactor entered commercial operation in 1978.
Chronology of Key Events
| Time (EST) | Event |
|---|---|
| 03:20 am | Primary coolant pump #2 (TRP‑2) stops due to a mechanical failure; a valve (the “loss‑of‑coolant accident” or LOCA valve) closes automatically. |
| 03:35 am | Secondary feedwater pumps lose power; plant switches to emergency diesel generators. |
| 04:00 am | An indicator shows low water level in the “pressurizer,” prompting operators to open the “pilot‑operating” valve (PORV) to relieve pressure. |
| 04:15 am | PORV actually opens, but a separate “closed‑position” indicator erroneously shows it as closed, leading operators to believe the valve remains shut. |
| 04:30 am | The reactor scram (automatic shutdown) is initiated; control rods insert, halting the chain reaction. |
| 04:52 am | Relief valve remains open, allowing coolant to escape; the water level in the reactor vessel continues to fall. |
| 05:00 am – 05:30 am | Operators, misled by instrumentation, reduce emergency cooling water flow, exacerbating the loss of coolant. |
| 06:00 am – 08:00 am | Core temperature rises; partial core melt begins in the lower plenum. |
| 09:00 am – 12:00 pm | Operators manually close the PORV; however, the valve’s internal mechanism is damaged and does not seal fully, continuing the coolant loss. |
| 10:00 am – 12:00 pm | A hydrogen bubble forms in the reactor vessel, but does not ignite. |
| 12:00 pm – 02:00 pm | Recovery actions, including the injection of additional emergency coolant, begin to stabilize the reactor. |
| 02:00 pm – 06:00 pm | Reactor vessel pressure and temperature are brought under control; the plant is placed in a “cold‑shutdown” condition. |
Primary Causes
- Mechanical failure of the primary coolant pump and subsequent loss of coolant.
- Design deficiency: The pilot‑operating relief valve (PORV) lacked a reliable position indicator; the instrument displayed a “closed” status even though the valve remained open.
- Operator error: Misinterpretation of ambiguous instrument readings led to inappropriate actions, such as reducing emergency coolant flow.
- Inadequate training and procedures: Operators were not fully prepared to diagnose the combination of mechanical and instrumentation failures.
Radiation Release
- The National Oceanic and Atmospheric Administration (NOAA) and the U.S. Environmental Protection Agency (EPA) estimate that approximately 2.5 × 10⁸ Bq (7 Ci) of radioactive noble gases (primarily xenon‑133) were released into the atmosphere.
- The estimated dose to the surrounding public was less than 0.1 rem (1 mSv), far below regulatory limits and comparable to natural background radiation.
Health Impact Studies
- Numerous epidemiological studies (e.g., the American Cancer Society, the Pennsylvania Department of Health) have examined cancer incidence in the surrounding population. The majority conclude that there is no statistically significant increase in cancer rates attributable to the accident.
- The National Academy of Sciences (NAS) 1990 report classified the health impacts as “insignificant” for the general public, while noting that certain plant workers received higher occupational doses.
Regulatory and Industry Consequences
- U.S. Nuclear Regulatory Commission (NRC) enacted stricter regulations on emergency core cooling system (ECCS) performance, operator training, and human‑factor engineering.
- The Institute of Nuclear Power Operations (INPO) was created (1979, but its role expanded after TMI) to promote industry best practices and peer reviews.
- Design reviews of PWR safety systems emphasized the need for redundant, clearly indicated instrumentation and robust containment venting strategies.
- Public confidence in nuclear power declined sharply; the United States saw a marked slowdown in new reactor orders throughout the 1980s and 1990s.
Legacy
- The TMI unit 2 reactor was permanently shut down; decommissioning began in 1990 and is projected to be complete by 2027.
- The accident remains a case study in nuclear engineering curricula for topics such as human factors, instrumentation reliability, and crisis management.
- It contributed to the global nuclear safety movement, influencing the development of the World Association of Nuclear Operators (WANO) (established 1989) and the International Nuclear Event Scale (INES) (adopted 1990).
References (selected)
- U.S. Nuclear Regulatory Commission, Three Mile Island Accident (NRC, 1998).
- National Academy of Sciences, The Three Mile Island Accident: A Review (1990).
- American Nuclear Society, TMI-2: The Accident and Recovery (1993).
- Pennsylvania Department of Health, Health Effects of the Three Mile Island Accident (1992).
All information reflects widely accepted, verifiable data available from governmental, academic, and industry sources.
