2. Generating plants

The accident at Unit 2 of the Three Mile Island (TMI-2) nuclear power plant in Pennsylvania, USA, occurred on March 28, 1979. Unit 2 is a 900 MWe pressurized water reactor of Babcock and Wilcox design.

The initial state was that the reactor was operating at rated power, but in an inadequately maintained condition. The initiating event was trivial: failure of the normal water supply to the steam generators. The automatic systems functioned normally, but indications in the control room misled the operators into manually shutting down the safety injection system for an extended period, even though there was a major leak in the pressurizer, which remained undetected for 2 hours and 20 minutes. The largely flooded reactor core was severely damaged and partially melted, despite the late restoration of safety injection. A 20 t mass of molten core...