Three Mile Island Accident

"'Nuclear Nightmare,' screamed the April 9, 1979, cover of TIME magazine. On March 28, the Three Mile Island nuclear reactor near Harrisburg, Pa., partially melted down. Coming two weeks after the release of the Jane Fonda film The China Syndrome, the Three Mile Island incident became the natural outlet for fears about the nuclear-power industry. The ironic thing is that while it has become known as one of America's worst nuclear accidents, nothing much really happened. No one died, and the facility itself is still going strong. While the near meltdown is often cited as the reason no new nuclear plant has been built in America in the past 30 years, the industry had begun to slow down construction before Three Mile Island ever happened." see:

From the Beginning

The Three Mile Island nuclear power plant was made up of two connecting installations. Construction of TMI-1 began in 1968 while TMI-2 began in 1969. In 1974, TMI-1 began operation and is still running. In 1975, TIM-2 began operation but has gone through many difficulties since.

The accident on March 28, 1979 was due to another minor problem that turned out to be a catastrophe. This was mainly caused because the workers were misled which increased the danger of the problem. From this problem, the TMI-2’s reactor was ruined. This catastrophe captured the attention of the whole world.

The Accident: March 28, 1979

Around 4 a.m. the plant was experiencing difficulties in the secondary, non nuclear part of the plant. The steam generators were not able to remove heat because of the main feedwater pumps which stopped working due to mechanical or electrical failure. The turbine, and then the reactor, shut down. The pressure in the nuclear part of the plant increased. The pilot-operated release valve opened to inhibit excessive heat. When the release valve was supposed to close, it did not. Because of this, cooling water poured out of the broken release valve and overheated the core of the reactor. Many signals and alarms provided unclear information and the level of coolant in the core was not able to be read. There was no signal that told the operators the pilot-operated release valve was still open. The operators couldn't recognize the loss of coolant they were experiencing.
Because of the insufficient amount of cooling, the nuclear fuel overheated so much that the zirconium cladding that held the nuclear fuel pellets broke and the fuel pellets began to melt. One-half of the core turned out to be melted from the beginning of the accident.




The section of the TMI nuclear plant where the accident occured. The building to the right is the TMI-2 reactor building which contained the melted core.

Health Effects

Several studies have been formed to look at the radiological consequences of the accident. Many estimates are that the average dose of this accident is less than exposure to a normal chest x-ray, the collective dose to the community from the accident was very small. In the months following the accident there were no problems that affected the humans and surrounding species directly involving the accident. Many of environmental samples of air, water, milk, and food were collected by groups monitoring the area. Low levels of radionuclides could be credited to releases from the accident. Most of the radiation was contained and that the actual release had small effects on the physical health of individuals or the environment.

Impact of the Accident

The accident was caused by a combination of personnel error, design deficiencies, and element failures. A small amount of radioactive gas was let into the air but not enough to be harmful. “There is no doubt that the accident at Three Mile Island permanently changed both the nuclear industry and the NRC. Public fear increased, NRC’s regulations and oversight became broader and more robust, and management of the plants was scrutinized more carefully.” (Walker, J. Samuel) The problems identified from careful analysis of the events during those days have led to changes in how NRC controls its workers. This also has reduced the risk to public health and safety.

Current Status of the Plant

Today, the TMI‑2 reactor is permanently shut down. They then drained coolant system, the radioactive water and evaporated radioactive waste to an appropriate disposal site, the reactor fuel and core was shipped to a Department of Energy facility and the remainder of the plant is being monitored. Exelon is the present owner of the TMI-1 plant, but the company is monitoring the plant plan to keep the TMI-2 facility in long‑term. They have decided to use it as monitored storage until the license for the TMI‑1 plant expires. This will then make both plants shut down and not used.

For More Information:


"NRC: Backgrounder on the Three Mile Island Accident." NRC: Home Page. Web. 28 Apr. 2011. < sheets/3mile-isle.html>.

"Three Mile Island | TMI 2 |Three Mile Island Accident." World Nuclear Association | Nuclear Power - a Sustainable Energy Resource. Web. 28 Apr. 2011. <>.

" Three Mile Island." Pennsylvania Highways. Web. 28 Apr. 2011. <>.

"Top 10 Environmental Disasters." M.RISHAN SHAREEF COLLECTION OF ARTICLES. Web. 28 Apr. 2011. <>.