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The Three Mile Island Nuclear Accident

Updated: Jul 21, 2023

#51 Science in History

28th March 1979

Today in 1979, America faced a nuclear crisis - the worst nuclear accident in a commercial, nuclear, power plant.

The Three Mile Island nuclear accident of 1979 was one of the most significant nuclear accidents in the history of the United States. The accident occurred on March 28, 1979, at the Three Mile Island nuclear power plant located in Pennsylvania, USA. It was caused by a combination of mechanical and human errors that resulted in a partial meltdown of one of the reactors.

The accident started at 4:00 a.m. in reactor number 2 when a cooling malfunction in the secondary cooling system caused the temperature in the primary cooling system to rise rapidly. This led to the automatic shutdown of the reactor as the safety systems detected a fault. However, a valve that was supposed to close during the shutdown process malfunctioned and remained open, causing coolant water to leak from the reactor. The plant operators did not notice the leak until it was too late, and the reactor core had begun to overheat.

As the temperature in the reactor core increased, the fuel rods began to melt, releasing radioactive gases and iodine into the containment vessel. The operators then made a series of mistakes that exacerbated the situation. They failed to recognize the severity of the problem and did not follow proper procedures. The operators also misinterpreted the readings on the control panel, which led them to believe that the reactor was cooling down when it was really overheating.

The accident, classified as a level 5 accident on the International Nuclear Event Scale (INES), was finally brought under control after several days of intense efforts by the plant staff and outside experts. However, a significant amount of radioactive material had already been released into the atmosphere. Although no immediate deaths occurred as a result of the accident, the long-term effects on the health of the plant workers and local residents remain a subject of debate.

The Three Mile Island nuclear accident had far-reaching consequences. It led to a renewed focus on safety regulations for nuclear power plants and a decrease in public support for nuclear energy. It also contributed to the growth of the anti-nuclear movement in the United States and around the world.

Thankfully, the radioactive release from the plant had negligible effects on the population and the environment - radioactive material was largely contained. The undamaged reactor number 1 remained operational until September 2019, when it was shut down permanently. The damaged reactor (Unit 2) has been inactive since the accident and was officially decommissioned in 1993. The clean-up and decommissioning of the damaged Unit 2 reactor has been ongoing since then and is expected to take several decades and cost billions of dollars. Currently, the plant is being maintained in a dormant state while the radioactivity decays over time. This phase is expected to last until 2078. Exelon Generation, the current owner, plans to dismantle Unit 2 starting in the mid-2040s, after the radioactivity has decayed sufficiently.


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