The little Pennsylvania island that almost became America’s Chernobyl in 1979

The 1979 Three Mile Island Accident in Pennsylvania

Three Mile Island sits on the Susquehanna River near Harrisburg, Pennsylvania. On March 28, 1979, at 4:00 a.m., Unit 2 reactor was running at nearly full power when equipment began to fail.

What followed became the worst nuclear accident in U.S. history. Parts of the reactor core melted, and radioactive gases leaked into the air. This one-day crisis changed how nuclear plants operate in America and shifted public opinion about nuclear power for decades to come.

When Cooling Water Pumps Failed At 4:00 A.M.

The trouble started when the main water pumps stopped working at 4:00 a.m. These pumps normally move heat from the reactor to make steam for electricity. When the pumps stopped, heat built up quickly in the reactor.

The reactor shut down automatically as a safety measure, inserting control rods to stop the nuclear reaction. Emergency cooling pumps started on their own, but workers turned them off. They wrongly thought the system had too much water, not too little.

The Critical Pilot-Operated Relief Valve Failure

As pressure rose in the system, a relief valve opened to release the pressure. This valve should have closed once pressure dropped to safe levels. Instead, the valve stuck open. Cooling water drained away from the reactor core for hours.

The control panel falsely showed the valve was closed. Workers had no idea they were losing vital cooling water until it was too late.

Operators Misinterpreting Warning Signals

Over 100 alarms blared in the control room. The noise and flashing lights overwhelmed the workers as they tried to understand what was happening. The control panels showed confusing readings that led workers to make wrong choices.

They believed they had too much water in the system when the opposite was true. Their training never prepared them for multiple problems happening at once.

Radiation Alarms Triggered By 5:00 A.M.

By 5:00 a.m., radiation detectors throughout the plant showed high readings. The tank collecting water from the stuck valve had filled up and burst. Water carrying radioactive material spilled into nearby buildings.

This water released radioactive gases into the air through vents. Workers monitoring radiation levels called for help. By this time, about 32,000 gallons of cooling water had leaked from the system.

Core Begins To Uncover At 6:00 A.M.

By 6:00 a.m., alarms showed radioactive gas inside the main building. At 6:22 a.m., workers finally closed a backup valve, stopping the water leak that had flowed for over two hours.

The water level in the reactor had dropped so low that the top of the fuel rods were no longer underwater. Without water to cool them, the fuel rods began to overheat. The plant manager declared an emergency at 6:56 a.m.

Emergency Declaration At 7:24 A.M.

At 7:24 a.m., the station manager declared a general emergency – the most serious kind. The plant owner, Metropolitan Edison, called the Pennsylvania Emergency Management Agency.

State officials then told Governor Richard Thornburgh and Lieutenant Governor William Scranton III about the growing crisis. The Nuclear Regulatory Commission sent emergency teams to Harrisburg.

Despite these actions, nobody fully understood what was happening inside the damaged reactor.

Public Notification After 8:00 A.M.

News of the accident reached the public after 8:00 a.m. Metropolitan Edison tried to downplay how serious it was, claiming no radiation had escaped the plant. This wasn’t true.

State radiation monitors had already detected higher radiation levels nearby from leaking gases. These early false statements created public distrust. As the day went on, conflicting information from company, state, and federal officials only made things worse.

Fuel Rod Damage Creates Hydrogen

Inside the reactor, the exposed fuel rods heated to over 4,000 degrees. At this extreme heat, the metal tubes holding the uranium fuel reacted with steam around them. This reaction created hydrogen gas and further damaged the fuel.

Radioactive materials from inside the fuel rods leaked into the remaining water. By the end of the day, about half the reactor core had melted, with molten fuel collecting at the bottom.

Hydrogen Explosion At 1:50 P.M.

The hydrogen that built up inside the building ignited at about 1:50 p.m. This caused a sudden pressure spike inside the main structure. Workers felt the building shake. Some heard a sound like a door slamming and didn’t realize it was hydrogen burning.

This explosion released more radioactive materials into the air but stayed contained within the concrete building.

Cooling Restored At 8:00 P.M.

After struggling for 16 hours, workers finally realized they needed to get water flowing through the damaged core again. They restarted one cooling pump at 7:50 p.m. This brought water back to the overheated fuel.

The temperature began to drop as water absorbed heat from the damaged fuel, and pressure returned to safer levels. The immediate danger of a complete meltdown passed, though concerns about hydrogen remained.

In September 2024, Constellation Energy announced plans to restart Unit 1 by 2028, renaming it the Christopher M. Crane Clean Energy Center. The power will supply Microsoft data centers. Unit 2 remains permanently shut down since the accident.

Visiting Three Mile Island Nuclear Generating Station

Three Mile Island is located in the Susquehanna River near Middletown, Pennsylvania, about 10 miles southeast of Harrisburg. The plant is not open for public tours, but you can view it from several spots along River Road (PA-441).

A historical marker on River Road explains the 1979 accident. From there, you can see the cooling towers and reactor buildings across the water. The National Museum of American History in Washington DC has exhibits about the accident with control room equipment and interactive displays.

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