1979: Three Mile Island

25 years ago

Around 4:00 a.m. on March 28, maintenance workers at the Three Mile Island nuclear-power plant, near Harrisburg, Pennsylvania, accidentally blocked the flow of water into the reactor core. This water acted as a coolant, absorbing the tremendous heat created there; without it, temperatures would build up dangerously, and the core could even melt down. Events like this had been planned for in the reactor’s design, and the staff was trained in dealing with them. The reactor shut down automatically, emergency water pumps were turned on, and control-room workers looked forward to resuming normal operations in a few hours.

What their faulty instruments didn’t show was that a pair of valves in the emergency cooling system were shut, preventing water from reaching the core, while a valve that allowed water to drain from the core remained open. With coolant levels dropping, the core started to overheat dangerously as engineers and technicians struggled, amid a cacophony of alarm horns and a forest of blinking lights, to figure out what was going on. After 16 hours, plant operators finally managed to restore the flow of coolant. The next day a spokesman assured the public that all was well, the situation was routine, and the danger was past.

But it wasn’t. Unanticipated reactions had released hydrogen gas into the reactor vessel, where it accumulated at high pressure and temperature. If it reacted with oxygen, the liberated energy could blow the reactor dome open and spew radioactive material across central Pennsylvania. It took several tense days, during which 140,000 residents left the area, for plant workers to dissolve the hydrogen gas and eliminate the threat.

No one died or was injured at Three Mile Island, and there was no dangerous release of radiation. Yet in the accident’s aftermath, investigators uncovered a widespread disregard for safety throughout the nuclear industry. The government responded with strict and voluminous new regulations.

In retrospect, Three Mile Island was a watershed for nuclear power, though an unusual one. The industry was already in trouble; with costs mounting, orders for new plants had virtually ceased by the late 1970s. On the other hand, despite Three Mile Island, nuclear power generation increased through the 1980s and 1990s as plants ordered earlier were completed; today it supplies about 20 percent of America’s electricity. So Three Mile Island did not kill nuclear power in America, but it did drive a stake through the industry’s heart by greatly boosting both public opposition and the costs of building and running a plant.