What Caused the Accident at Three Mile Island?

Nuclear power has been a safe source of energy production in the United States. But what caused the Three Mile Island accident?

What Caused the Accident at Three Mile Island?
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Nuclear power generated about 18.9% of electricity consumed in the United States in 2021, but many people are concerned that huge accidents cause by these nuclear power plants are extremely dangerous. So what caused the accident at Three Mile Island, the biggest nuclear power incident in the United States?

The accident at Three Mile Island was caused by a combination of human error, faulty or insufficient measuring tools, and a breakdown of vital equipment. On March 28, 1978, a steam pump valve failed to close, leading to a chain reaction of events that stopped water from cooling down the nuclear core. The core eventually heated to levels close to meltdown before cooling down, barely averting a massive disaster.

In this article, we will talk about how nuclear power plants work, what went wrong at Three Mile Island, and the implications that followed the incident.

How the Three Mile Island power plant worked

In order to understand what went wrong, we need to understand how the power plant itself worked. Nuclear power is produced by nuclear reactors, which produce heat through fission. The fission process begins by firing a neutron into the uranium's atom, which splits the atom, releasing a crazy amount of energy in the form of heat.

This process happens in a huge pool of water, which is designed to control and sustain the process’s chain reaction. The water is heated to such extreme temperatures that it turns into steam, spinning a turbine to produce electricity. The core water is then pumped back into the reactor, and the process is repeated.

The Three Mile Island plant in Harrisburg, Pennsylvania had two water reactors. With these came a series of systems to slow down, accelerate, or even shut down the nuclear reaction to balance the heat it produces. This is usually in the form of neutron-absorbing materials like boron or silver which are placed into the uranium core and a complex water pumping system.

There are additional safety measures in place if a nuclear core overheats or radiation-soak water escapes. There are automatic cooling systems that flood the core with water, automatic shutdowns, and consistent safety checks run by the workers who are at power plants on a 24/7 basis and government officials.

Issues before the meltdown

Three Mile Island nuclear generation station was built by General Public Utilities Corporation. The construction began in 1968 with the building of unit 1, also known as TMI-1. The construction of this unit had no issues. However, with unit 2, TMI-2, there were huge issues throughout the building process, including massive delays. Originally supposed to take six years to build, it was constructed in nine.

TMI-2 was completed in 1978 but was plagued with a series of shutdowns. In fear of losing their jobs and the government shutting down the plant, workers on TMI-2 began falsifying safety tests and data on the plant’s performance. They submitted altered information to the Nuclear Regulatory Commission in order to keep the plant running, despite consistent errors and a lack of data-collecting equipment available to them.

What went wrong at Three Mile Island?

On March 28, 1979, the worst nuclear power incident on U.S. soil took place. This only happened due to a series of minor errors which cascaded into a near catastrophe. Thouh every automatic failsafe worked properly, the lack of operator training and proper equipment made the problem worse.

That day, the steam pump which led water to the reactor’s generator stopped working because a pressure valve failed to close. The reactor began operating at 97% power, producing dangerous heat levels. The reactor immediately shut down automatically and began pumping cooling water into the system. However, the instrumentation did not show the reason behind this shutdown, showing that the valve had shut when in reality it remained open.

Without the proper diagnosis, operators were confused about what happened. They believed the valve shut, so they reduced the amount of water. This created steam in the reactor’s cooling system, which caused the pumps to vibrate. The operators were forced to shut down the pumps, or else the vibration could have damaged them further, making them unusable.

As the water began boiling away without being replaced, the reactor heated to 4,000 degrees, only 1,000 degrees away from a complete meltdown like Fukushima or Chornobyl. Luckily, though, emergency cooling pumps automatically kicked on. The operators were also eventually able to force water into the reactor from outside systems, avoiding an all-out disaster.

The crisis continues after shut-down

Fortunately, there were enough fail-safes and crisis management for the levels of radiation to be safe for humans. However, a chemical reaction created a huge hydrogen gas bubble that built up in an auxiliary building, which was only discovered two days after the initial incident. Operators and government officials discovered that part of the gas exploded on the day of the initial incident, which released radiation into the atmosphere.

Only at this time were residents in the area told to stay indoors. Nuclear power experts were worried that the hydrogen bubble could explode, causing a bigger meltdown. The operators worked the next few days releasing small amounts of the gas bubble by opening a vent valve. Though some radiation was released, there was never enough to harm humans

On April 27, operators finally reported that the reactor core was receiving enough water through natural means rather than pumping water in. The cleaning and investigation process could finally start. Reports say that the cleanup process was finally finished in August 1993, 14 years after the initial meltdown.

Impacts of the accident

With any incident involving radiation, there were fears that there would be health effects on the workers at the plant and people who lived in the surrounding area. Government agencies continually measured the amount of radiation released during the accident. They found that there was no more radiation over 1 millisievert, about the same as a chest X-ray, in a single individual.

For the next 18 years, the Pennsylvanian Department of Health monitored and registered 30,000 people living within a five-mile radius of Three Mile Island during the incident. During this time, public officials found no evidence of abnormal cancer rates and eventually stopped collecting data in the 1990s.

The biggest impact of the accident, though, was the public opinion of nuclear power. Before this event, people believed that nuclear power would be the only way to avert an energy crisis in the United States. Understandably, people began questioning the safety and reliability of nuclear power. When the company wanted to open TMI-1 in the mid-1980, massive protests took place in retaliation.


Despite this near-disaster, some good came from the Three Mile Island incident. Lessons from the lack of training and suitable equipment were corrected, and the government added more tracking and security checks in nuclear power plants. This led to more reliable energy output and safety in the United States.