1979 - Three Mile Island, Nuclear Meltdown

The Three Mile Island nuclear meltdown was a partial core meltdown that occurred on March 28, 1979, at the Three Mile Island Nuclear Generating Station in Pennsylvania, USA. It remains the most serious accident in U.S. commercial nuclear power plant history.

The Accident

The meltdown occurred in Reactor Unit 2 (TMI-2) at around 4:00 AM due to a failure in the secondary cooling system, which prevented heat removal from the reactor core.

A pressure relief valve (PORV) in the primary cooling system stuck open, allowing coolant water to escape. However, operators were unaware of this due to misleading instrument readings.

The reactor core overheated, leading to a partial meltdown, releasing radioactive gases into the environment.

Causes

Equipment Failures: The relief valve failed, and the emergency cooling systems did not function properly.

Operator Error: Due to misinterpretation of instrument readings, operators reduced cooling water flow, worsening the situation.

Design Flaws: The control panel indicators did not clearly show that the relief valve was open.

Immediate Consequences

A hydrogen bubble formed inside the reactor, raising fears of an explosion, but it eventually dissipated.

Some radioactive gases (xenon and krypton) were released, but no significant health impacts were reported.

The plant was permanently shut down, while TMI-1 (the other reactor) continued operation until 2019.

Long-Term Impact

The accident led to major regulatory changes in the U.S. nuclear industry, including stricter safety measures, improved operator training, and better emergency preparedness.

It fueled public fear and opposition to nuclear energy, slowing down nuclear power plant construction in the U.S.

Cleanup of the damaged reactor took 14 years (completed in 1993) and cost about $1 billion.