Nuclear Incidents: Three Mile Island Nuclear Plant
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For three days beginning on March 28, 1979, a series of mechanical, electrical, and human failures led to a severe meltdown of the reactor core at the Three Mile Island Nuclear Power Plant.
On this page:
What happened during the meltdown?
A meltdown is the most dangerous type of nuclear power accident. At Three Mile Island (TMI), there was a "loss of coolant" accident, meaning that cooling water that surrounds the core and keeps it cool was lost. The temperature of the core rose so high that the materials actually melted. In the worst-case, a meltdown breaches the containment building resulting in a massive release of radiation. Fortunately, this did not occur during the TMI accident. (Had the reactor at Chernobyl had a containment building, the severity of the accident would have been greatly reduced.)
Some radioactive gases did escape to the atmosphere. The estimated average dose to area residents was about 1 millirem, about 1/6 the exposure from a full set of chest x-rays, and about 1/100th the natural radioactive background dose for the area. The maximum dose to a person at the site boundary would have been less than 100 millirem.
How did EPA Respond to the Incident
On March 28, EPA arrived and immediately stationed experts with radiation monitoring equipment around the power plant to assess the potential for radiation exposure to people living around the plant. After the accident, EPA remained in the area for eight years, maintaining a field office monitoring the air. EPA operated a continuous radiation monitoring network in the area surrounding the plant to ensure that public health and the environment were protected. EPA transferred this activity to the Commonwealth of Pennsylvania in 1989.
What changes were made after the incident?
After the core meltdown at Three Mile Island, the Nuclear Regulatory Commission and other federal agencies moved to correct problems in the areas of staff training, reactor design, and component reliability to prevent recurrence of such an event:
- plant design and equipment requirements
- operator training and staffing, instrumentation and controls for operating the plant, and fitness-for-duty programs for plant workers to guard against alcohol or drug abuse
- emergency preparedness
- public information about plant performance
- regulatory controls, inspections, and enforcement
- self policing by the industry
- early detection of problems and sharing lessons learned in U.S. and abroad.*
Three Mile Island experience led to developing the
current Federal Radiological Emergency
Response Plan, which has since been replaced by
the National Response Framework.