The March 1979 accident—a near meltdown—at the nuclear power station at Three Mile Island, outside Middletown, Pennsylvania (near Harrisburg), was eventually contained. Had it not been, the damage would have been on a level with that of the Chernobyl (Ukraine) disaster, which happened some seven years later. Instead, Three Mile Island served as a wake-up call, reminding the American public and its utility companies of the potential risks involved in nuclear energy.

The sequence of events at the nuclear power plant, which is located on an island in the Susquehanna River, was as follows: At 4:00 A.M. on Wednesday, March 28, an overheated reactor in Unit II of the power plant shut down automatically (as it should have); Metropolitan Edison Company operators, guided by indicators that led them to believe water pressure was building (and an explosion was therefore imminent), shut down those pumps that were still operating; the shut-down of all the pumps caused the reactor to heat further; then, tons of water poured out through a valve that was stuck open; this water overflowed into an auxiliary building through another valve that was mistakenly left open. This final procedure, which took place at 4:38 A.M., released radioactivity.

Since there was no cooling system in operation, the reactor in Unit II was damaged. But this was not the end of it: The radiation within the buildings was released into the atmosphere, and at 6:50 A.M. a general emergency was declared. Early that afternoon, the hydrogen being created by the uncovered reactor core accumulated in a containment building and exploded. Since hydrogen continued to be emitted, officials feared another—catastrophic—explosion. Worse yet, they feared the reactor would become so hot that it would melt down. The effect of a meltdown would be that the superheated material would eat its way through the bottom of the plant and bore through the ground until it hit water, turning the water into high-pressure steam, which would erupt, spewing radioactivity into the air.

As technicians worked to manage the crisis, radiation leaked into the atmosphere off and on through Wednesday and Thursday. On Friday the governor of Pennsylvania ordered an evacuation: Some 144,000 people were moved from the Middletown area. The situation inside the plant remained tenuous as a hydrogen bubble developed and increased in size, again raising fear of explosion. Meantime, public alarm was mounting as the media attempted to monitor the ongoing crisis. Finally, on Sunday, April 1, the plant was visited by President Jimmy Carter (1924-). At about the same time, the hydrogen bubble began to decrease in size, ending the crisis.