Three Mile Island accident

Early on 28 March 1979, a chain of equipment faults and human misjudgements caused the core of the Unit 2 reactor at the Three Mile Island nuclear plant, near Harrisburg in Pennsylvania, to partly melt. Small amounts of radioactive gas and iodine escaped. No one was killed and the release is not known to have harmed anyone's health, yet it remains the most serious accident in the history of commercial nuclear power in the United States, and it sharply slowed the growth of the American nuclear industry.

The plant stands on an island in the Susquehanna River. Its second reactor, TMI-2, was a pressurised water reactor that had been running for only about three months, and that night it was working at 97 per cent of full power. The trouble began in the non-nuclear part of the system. Workers trying to clear a blocked filter let water into an air line, and at about 4:00 a.m. the main feedwater pumps stopped. The turbine tripped and, seconds later, the reactor shut itself down.

The reactor still produced heat from radioactive decay. Pressure rose and a relief valve opened as designed, but when pressure fell again the valve stuck open, while a control-room light that went out led the operators to think it had shut. Cooling water drained away through it. At the same time, valves on the emergency feedwater lines had been left closed, apparently after a test two days earlier.

Confusing instruments and alarms led the staff to believe that the reactor held too much water rather than too little, and they cut back the emergency cooling. About 80 minutes into the accident the main coolant pumps began to shake as steam formed and were switched off. Soon after 6 a.m. the top of the core was uncovered. The hot fuel cladding reacted with steam, producing hydrogen and more heat, and the core began to melt. A new shift closed a backup valve around 6:00 a.m., but by then roughly 120,000 litres of coolant had escaped. Radiation alarms sounded at 6:45, and a general emergency was declared soon after 7 a.m. Only after about 16 hours were the coolant pumps restarted and temperatures brought down.

The plant's operator, Metropolitan Edison, issued vague and conflicting statements, and state officials came to distrust it. The federal Nuclear Regulatory Commission was poorly organised for a crisis. On the third day a hydrogen bubble found in the reactor vessel raised fears of an explosion; experts soon concluded that, without oxygen, it could not burn, and it was gradually reduced. On the advice of the commission's chairman, Governor Dick Thornburgh advised pregnant women and small children within five miles of the plant to leave. Within days about 140,000 people left, though most returned within three weeks.

President Jimmy Carter set up a twelve-member commission under John G. Kemeny, president of Dartmouth College, which reported on 31 October 1979. Later examination showed that about half the core had melted. Official figures put the radioactive release at a small fraction of what the reactor contained, mostly noble gases such as xenon, and studies found the extra dose to people living nearby was very low. Some anti-nuclear groups and later researchers disputed these figures, and studies of local cancer rates have reached different conclusions.

The accident brought new regulations for the industry, and the commission criticised the companies and regulators for poor training, maintenance and management. Construction of new reactors in the United States declined steeply: 52 planned reactors were cancelled between 1980 and 1984, although rising costs and the 1973 oil crisis had already led to many cancellations. The clean-up of TMI-2 lasted from August 1979 to December 1993 and cost about one billion dollars. The undamaged Unit 1 ran again from 1985 until 2019 and is expected to restart to supply power for Microsoft data centres. The disaster at Chernobyl in 1986, and later the Fukushima accident of 2011, were far more serious.

Our 1985 map shows the United States a few years after the accident, during the late Cold War.