Three Mile Island nuclear power plant is situated three miles downriver from the small town of Middletown, Pennsylvania. Consisting of two separate reactors, the second began commercial operation on 30 December 1978.

Just three months later, on 28 March 1979, the second reactor’s cooling system malfunctioned. The reactor shut down almost immediately but, as pressure built from residual heat, a relief valve opened. However a mechanical fault caused this to fail to close again, resulting in so much coolant draining away that emergency pumps had to add additional cooling water. Human operators, unaware that more water was necessary due to the open relief valve, turned the pumps off. As the coolant continued to drain and boil away, the reactor core was exposed and the intense heat damaged the fuel rods. Radioactive material entered the water, which turned to steam and released radioactive contaminant into the surrounding area.

By the evening plant operators had managed to reduce the temperature of the core, avoiding a total meltdown. Nevertheless, the announcement of a potentially explosive hydrogen bubble that could cause more radioactive contamination led more than 100,000 people to leave the area over the next few days. Fortunately the hydrogen was able to be removed as the reactor cooled.

Subsequent investigations of possible adverse effects from the radiation leak on humans, animals and plants couldn’t find any link to the accident. Nevertheless, the incident at Three Mile Island heralded a dramatic shift in public attitudes towards nuclear power and the second reactor never returned to service.

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