Three Mile Island Nuclear Accident

The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near Middletown, Pennsylvania, on March 28, 1979, was the most serious in a commercial nuclear power plant in the United States, even though it led to no deaths or injuries to plant workers or members of adjacent communities. It had major implications for the nuclear industry because it resulted in major changes in the regulatory requirements involving emergency response planning, reactor operator training, radiation monitoring, human factors engineering, radiation protection, and other areas of nuclear plant operations. It also caused the U.S. Nuclear Regulatory Commission (NRC) to tighten and heighten its regulatory oversight.

The accident at TMI-2 was initiated at 4 a.m. by a minor malfunction, or “transient,” in the nonnuclear part of the reactor. The main feed-water pumps stopped running, which prevented the steam generators from removing heat. This event evolved into a series of automated responses in the reactor's coolant system, and during this time, the relief valve on top of a piece of equipment called “the pressurizer” became stuck in an open position. Misreading of the plant conditions by the operators over a 2¼-hour period before the relief valve was closed and the turning off of an automatic emergency cooling system caused [Page 2826]the reactor core to become partially uncovered and severely damaged. The major consequences of the accident unfolded over the next week, and it took 1 month to bring the reactor to a cold shutdown.

The Three Mile Island nuclear power plant near Middletown, Pennsylvania, which was the site of a March 28, 1979, power plant accident.

Source: iStockphoto.

During the first several days of the accident, communications between the NRC and the site were problematic, which made it extremely difficult for the NRC to obtain up-to-date information from the plant and the utility. The suggestion by the NRC of a possible large-scale evacuation out to 20 mi. (miles) was quite different from the 5-mi. planning requirements imposed by the NRC and Pennsylvania before the accident.

On March 30, midmorning, Pennsylvania Governor Richard Thornburgh's press secretary told reporters that there was no need for evacuation and that people in a 10-mi. vicinity of the plant could remain inside for a while. The only official warning to the public to evacuate came at approximately 12:30 p.m., when the governor advised pregnant women and preschool children to leave the area within a 5-mi. radius of TMI until further notice. He also ordered schools to close. The advisory to pregnant women and preschool children was lifted on April 9. The governor's warning was the only official warning issued by the government. People in the vicinity of the plant were bombarded by media coverage, which suggested that a major evacuation was imminent. As a result, many people decided to evacuate despite the limited recommendation made by the governor. Approximately 144,000 people within a 15-mi. radius evacuated.