The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near Middletown, Pennsylvania, United States, took place on 28 March 1979. It was the most serious nuclear accident in US history and one of the most serious in the world. Rated 5 on the INES scale (Chernobyl is rated 7); it resulted in the partial meltdown of the reactor core. The accident was caused by human error rather than by a technical failure.

The accident was confined within the containment building and it led to no deaths or injuries and had no significant impact on the surrounding environment, which proved the efficiency of the defence-in-depth mechanism. However it brought about major changes in the nuclear industry and was a turning point in the enhancement of safety measures world-wide. It also demonstrated the importance of the human factor in the safety of complex facilities such as nuclear power plants. The lessons learnt and the safety improvements that followed considerably decreased the risk of such an accident happening again.

Three-Mile Island and Chernobyl are the only major nuclear-related accidents to have occurred during the 12,000 cumulative reactors-years of commercial operation in 32 countries. This safety record compares very favourably with any other industry.

Here are a few texts to help you better understand the accident and its consequences:

NucNet fact sheet on the TMI accident  Three Mile Island.pdf (For more information about NucNet and to inquire about subscriptions see www.worldnuclear.org or email This e-mail address is being protected from spambots. You need JavaScript enabled to view it This email address is being protected from spam bots, you need Javascript enabled to view it )

United States Regulatory Commission’s fact sheet on the TMI accident

Dickinson college site on Three Mile Island accident

What happened and what didn’t happen (American Nuclear Society)

Tell it like it is (communications lessons from Three Mile Island, IAEA)

You can also watch a documentary on Three Mile Island accident entitled “Meltdown at Three Mile Island”.