The Three Mile Island accident was a partial meltdown of the Three Mile Island, Unit 2 (TMI-2) reactor on the Susquehanna River in Londonderry Township, Pennsylvania, near the Pennsylvania capital of Harrisburg. It began at 4:00 a.m. on March 28, 1979, and released radioactive gases and radioactive iodine into the environment. It is the worst accident in U.S. commercial nuclear power plant history. On the seven-point logarithmic International Nuclear Event Scale, it is rated Level 5 ("Accident with Wider Consequences").
The accident began with failures in the non-nuclear secondary system followed by a stuck-open pilot-operated relief valve (PORV) in the primary system that allowed large amounts of water to escape from the pressurized isolated coolant loop.