Three Mile Island Accident (1979): America's Worst Nuclear Crisis

The Three Mile Island accident began on 28 March 1979, when the Unit 2 reactor at a nuclear plant near Middletown, Pennsylvania, lost cooling and partly melted down. Although very little radiation escaped and no one was killed, days of confusion and fear made it the most serious crisis in the history of US commercial nuclear power. It transformed nuclear safety rules and deepened public doubts about atomic energy.

Background

The Three Mile Island Nuclear Generating Station stands on an island in the Susquehanna River, about 16 kilometres south-east of Harrisburg, the state capital. It was owned by Metropolitan Edison and other subsidiaries of General Public Utilities. Unit 2, a pressurised water reactor built by Babcock & Wilcox, had entered commercial service only on 30 December 1978.

In a pressurised water reactor, water in a sealed primary circuit is heated by the nuclear fuel and kept under high pressure so that it does not boil. It passes through steam generators, where it heats water in a separate secondary circuit; that water turns to steam and drives the turbine. The fuel must be covered by cooling water at all times, even after the chain reaction has stopped, because radioactive decay continues to produce a great deal of heat for days afterwards. Losing that cooling water was exactly what happened at Unit 2.

By the late 1970s nuclear power supplied around a tenth of US electricity, but it was already controversial. Twelve days before the accident, the film The China Syndrome, about a cover-up of a reactor accident, opened in American cinemas, and it later drew large audiences as events seemed to echo its plot.

What went wrong

At about 4 a.m. on 28 March, pumps feeding water to the steam generators in the plant’s secondary cooling system stopped. The turbine and then the reactor shut down automatically, as designed. Pressure in the primary system rose, and a relief valve on top of the pressuriser opened to release it.

The valve should have closed again after about ten seconds, but it stuck open. A light in the control room showed only that the signal to close had been sent, so operators believed the valve was shut. Cooling water escaped for more than two hours before a later shift identified the problem.

Misled by a rising water level in the pressuriser, operators also cut back the emergency cooling water that had started automatically, fearing the system would overfill. In fact the core was losing water. Part of it became uncovered and overheated, and roughly half of the fuel melted. Investigators only learned the full extent of the damage years later, when cameras were lowered into the reactor in the 1980s.

A confused emergency

Communication from the utility and regulators was contradictory. Officials at first played down the problem, then reported higher radiation readings. On 28 March a hydrogen burn occurred inside the containment building, though its significance was not understood at the time.

On Friday 30 March an unplanned release of radioactive gas and alarming reports that a hydrogen bubble in the reactor vessel might explode caused panic. Governor Dick Thornburgh advised pregnant women and preschool children living within 5 miles of the plant to leave, and closed schools in the area. An estimated 140,000 people left voluntarily for a time. Later analysis showed that the bubble could not have exploded, because there was not enough oxygen inside the vessel.

President Jimmy Carter, who had trained as a nuclear engineer in the navy, sent Harold Denton of the Nuclear Regulatory Commission (NRC) to take charge of technical communication. Carter visited the plant’s control room on 1 April 1979. By then the bubble was shrinking, and the reactor reached a stable cold shutdown on 27 April.

Radiation and health

Radioactive gases, mainly krypton and xenon, and a small amount of iodine were released. Government studies estimated that the roughly two million people living nearby received an average additional dose of about 0.01 millisieverts, and that no one received more than about 1 millisievert, which is less than a year of natural background radiation.

Epidemiological studies over the following decades, including work by Columbia University and the University of Pittsburgh, found no consistent increase in cancer deaths that could be linked to the accident. Some residents and independent researchers remain sceptical, and the question has been the subject of lawsuits, but the scientific consensus is that health effects were very small or undetectable.

The long cleanup

The full condition of the damaged reactor was not known for years. When engineers lowered a camera into the vessel in 1982, they found a cavity where the top of the core had been. Later investigations showed that molten material had flowed down to the bottom of the reactor vessel, but the steel vessel itself had not failed, which was the most important reason why so little radioactivity escaped.

Cleaning up Unit 2 was an engineering challenge without precedent. Hundreds of thousands of litres of contaminated water had to be processed, and the release of radioactive krypton gas from the containment building in 1980, intended to allow workers to enter, caused new local protests. Removal of damaged fuel began in 1985, and about 100 tonnes of fuel and debris were shipped to the Idaho National Laboratory for storage. The work was completed in 1993, and the remains of Unit 2 have been kept under monitoring since, awaiting final dismantling.

Investigation and reform

President Carter appointed a commission led by John G. Kemeny, president of Dartmouth College. Its report of October 1979 blamed not one fault but a combination of equipment failure, poor control-room design, inadequate operator training and complacency in both the industry and the NRC. It concluded that fundamental changes were needed.

The NRC tightened rules on operator training, emergency planning, control-room instruments and inspections. The industry founded the Institute of Nuclear Power Operations in 1979 to set its own standards. The cleanup of Unit 2 lasted until 1993 and cost about one billion dollars.

Impact on the nuclear industry

Orders for new reactors in the United States were already falling because of high costs and slowing electricity demand. After Three Mile Island, dozens of planned plants were cancelled, and no new reactor received permission for construction until 2012. Abroad, the accident encouraged anti-nuclear movements, especially in West Germany, and its lessons were revisited after the far more serious Chernobyl disaster of 1986 and the Fukushima Daiichi disaster of 2011.

On the International Nuclear Event Scale, introduced later, the accident is rated level 5, an “accident with wider consequences”, compared with level 7 for Chernobyl and Fukushima.

Timeline

• 1978, 30 December: Unit 2 begins commercial operation.
• 1979, 16 March: The film The China Syndrome opens in US cinemas.
• 1979, 28 March, c. 4 a.m.: Feedwater pumps stop; a relief valve sticks open and the core begins to lose water.
• 1979, 30 March: Governor Thornburgh advises pregnant women and young children near the plant to leave.
• 1979, 1 April: President Carter visits the plant.
• 1979, 27 April: The reactor reaches cold shutdown.
• 1979, October: The Kemeny Commission publishes its report.
• 1993: Cleanup of Unit 2 is completed.
• 2019, September: Unit 1 is shut down.

Why it matters

Three Mile Island showed that a series of small failures and human misunderstandings could push a reactor to meltdown. It changed nuclear regulation and operator training worldwide and, together with the troubled economics of nuclear power, helped halt the growth of the US nuclear industry for a generation.