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Three Mile Island Accident

History >> US History 1900 to Present

Reactors at Three Mile Island
Three Mile Island
Source: United States Department of Energy.
What is Three Mile Island?

Three Mile Island is the name of an island in the Susquehanna River in Pennsylvania. It is home to the Three Mile Island Nuclear Power Plant. When people talk about "Three Mile Island" they are usually talking about a nuclear accident that occurred at the power plant on March 8, 1979.

What is nuclear power?

Nuclear power is when electricity is produced from nuclear reactions. Most nuclear power plants use nuclear fission to generate heat. They then use the heat to create steam from water. The steam is used to power electrical generators.

Why is it dangerous?

The nuclear reactions that are used in power plants produce lots of radiation. If radiation gets into the air, the water, or the ground, it can be harmful to people and animals. Too much radiation can cause cancer or even death.

Nuclear power plants have shielding to prevent any radiation from getting out. However, if the reactor should overheat and "meltdown", then radiation could escape.

How did the reactor fail?

Three Mile Island had two nuclear power plants. It was reactor number 2 that failed. A number of things went wrong to cause the reactor to fail. The real problem occurred when a valve got stuck open. The valve was letting water out of the reactor. Unfortunately, the instruments were telling the workers that the valve was closed.

Water is used to keep the reactor from overheating. As more water was being let out by the bad valve, the reactor began to overheat. The workers could see that the reactor was getting hot, but didn't know that the valve was open. Eventually, the entire reactor had to be shut down, but not before it had achieved a partial meltdown.


People around the power plant began to panic. No one knew exactly what was going on. What would happen if the reactor had a meltdown? How much radiation got out? Many people in the surrounding areas were evacuated.

Were people hurt?

In the end, scientists believe that very little radiation escaped from the reactor. It had been shut down in time. No one got immediately sick or died from radiation. Over time, many studies have been made by the government, universities, and independent organizations. It is believed today that the accident had little or no effect on the people and surrounding environment.


The accident had two major effects on the nuclear power industry. First, it scared a lot of people and caused a slowdown in the building of new plants. Second, it forced a number of new regulations on the industry in order to make nuclear power more safe.

Today at Three Mile Island

Despite the failure of reactor number 2, reactor number 1 is still in operation today (as of 2015). It is expected to operate until the year 2034.

Interesting Facts About the Three Mile Island Nuclear Accident Activities

Works Cited

History >> US History 1900 to Present

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