The nuclear reactor accident at Three Mile Island, Pennsylvania, occurred on March 28, 1979. It began when the flow of water to the reactor core was inadvertently cut off. Pressure in the core increased rapidly, causing a safety relief valve to open and the reactor to shut down automatically. To understand what had happened, operators tried to read the hundreds of display panels in the operating room. They thought that the safety valve had closed when it had not. As coolant flooded out through the open valve, the Emergency Core Cooling System (ECCS) automatically began to flood the core. However, to prevent too much water from reaching the core, operators manually shut off the ECCS.

This mistake was not discovered for two hours, during which time the core continued to lose coolant. Water in the core began to boil, forming a steam bubble. Part of the core became uncovered, and fuel elements were damaged seriously enough to release fission products. Some of this radioactive material escaped from the core into the containment building through the open safety valve, some was carried by coolant water into an auxiliary building that housed water storage tanks, and a small amount went up a stack, through a charcoal filter, into the open air.

Six different committees investigated the accident. They identified four basic flaws in reactor operations: poor communication within the industry (due to a similar chain of events that had occurred at a different nuclear plant 18 months before), poor operator training, faulty instrumentation, and poor control room design.¹

At 1:00 a.m., on April 26, 1986, operators of the fourth and newest reactor at Chernobyl, in the Soviet Union, were one full day into a special test. They wanted to see whether the residual energy of a spinning turbine could provide sufficient power in case of an emergency shutdown with loss of offsite power. During the course of the test, operators disconnected safety systems and violated operating procedures in order to press the test forward.

Each adjustment caused further departures from the plant's intended performance, making it more unstable: the reactor's power output fell to 6 percent of its normal level, the emergency core cooling system had been shut down along with other safety mechanisms, and all of the control rods had been at least partially pulled out to keep the reactor running. Ultimately, in the space of 4.5 seconds, the reactor's power output jumped more than 2,000-fold to 120 times the reactor's rated capacity. Two explosions ensued, ripping open the 2,000 fuel rod and control channels in the reactor's core. The rupture of the fuel rods caused the cooling water to flash into steam, resulting in a huge explosion that blasted aside a 1,000 ton concrete slab.²

For the first time ever, the inventory of the lethal radioactive contents inside a nuclear reactor were exposed to the atmosphere.³ Two operators died immediately from the force of the explosion, 29 others died from a combination of thermal and radiation burns,⁴ 200 people were hospitalized, and 135,000 people were exposed to an increased long-term risk of cancer. Most of the 200 square mile area within approximately 18 miles of the plant will not be usable for general agriculture for several decades.⁵ Pripyat, a city of 45,000, was evacuated and remains uninhabitable.⁶ The immediate financial cost to the Soviet Union is estimated at between $3 billion and $5 billion, with initial damages from radioactive fallout in neighboring countries estimated at $500 million.⁷ The long-term worldwide health consequences of the accident are still being studied and debated.

The Chernobyl reactor that sustained the destructive accident was a graphite, tube-type reactor which did not have a complete containment structure. At Three Mile Island, the large, high-pressure containment building retained most of the significant radioactive elements released during the accident.⁸

¹ Edward Edelson, The Journalist's Guide to Nuclear Energy, (Bethseda: Atomic Industrial Forum, Inc., 1985), pp. 15-16.
² Flavin, Reassessing Nuclear Power: The Fallout From Chernobyl, op. cit., pp. 8-9
³ Ibid., p. 9.
⁴ Henry N. Wagner, "After Chernobyl," Administrative Radiology, May 1987, Vol. 5, No. 5, p. 20
⁵ E.L. Zebroski, "The Nuclear Accident at Chernobyl, U.S.S.R.," for publication in the Yearbook of Science
and Technology, (McGraw-Hill, 1987), pp. 1-2.
⁶ Robert Scheer, "A Legacy of Ruin: Inside Chernobyl," Los Angeles Times, April 9, 1987, 1:1.
⁷ Flavin, Reassessing Nuclear Power: The Fallout From Chernobyl, op. cit., pp. 19-20.
⁸ Zebroski, op. cit., p. 7