SUMMARY OF EVENTS

The secondary (non-nuclear) section of the plant started to experience failure unknowingly, which blocked the main feedwater pumps from delivering water to the steam generators that cool the reactor core. This sent the reactor to automatically shut down, and the pressure in the primary section began to rise. in order for the pressure to be stabilized the relief valve had opened, but once pressure was stable the valve was stuck open and could not close. Workers were not aware of this valve being stuck open with coolant pouring out of it.

Video Vault: 1979 Three Mile Island

The reactant coolant pumps were shut off because the valve being stuck greatly decreased the primary system pressure. With the reactor coolant pumps inactive and the primary system deprived of sufficient emergency cooling water, the water level in the pressure vessel fell, leading to core overheating.

"The challenge in the future will be to cool down the reactor core itself to a safe level.And at the present time, all those who are involved here, who are highly qualified, tell me that the reactor core is, indeed, stable."

 -Governor Thornburgh and President Carter statements, followed by Harold Denton NRC press conference, April 1, 1979

The cleanup process at Three Mile Island after the 1979 accident was a long and complex operation that lasted over a decade and cost around $1 billion. Workers stabilized the reactor, removed over 100 tons of melted nuclear fuel, and decontaminated radioactive materials. Thousands of gallons of radioactive water were treated and safely stored, while radiation levels at the site were closely monitored throughout the process. By 1993, the cleanup was complete, leaving the damaged reactor permanently shut down, while the event started improvements in nuclear safety and regulation.