Three Mile Island damaged reactor core. Illustration of the TMI-2 (TMI Unit 2) core end-state configuration, showing the damage and how the reactor core appeared following the accident on 28 March 1979 at Three Mile Island (TMI) nuclear power station near Harrisburg, Pennsylvania, USA. The accident was caused by a loss of reactor cooling water that resulted in the exposure of fuel rods, some of which melted, burned through the reactor chamber lining and sank to the floor of the reactor building. The clean-up operation took 14 years. The diagram shows the hole in the baffle plate, debris, molten materials, and various damaged areas.