The most serious nuclear reactor accident to date in the United States occurred at 4 A.M. on March 28, 1979, at the Three Mile Island nuclear power plant outside Middletown, Pennsylvania. Operator errors in dealing with a pump that had shut down caused the Unit 2 pressurized-water reactor to lose coolant and overheat. The temperature of the reactor core then rose to the point at which some of the zirconium-alloy fuel cladding failed, fuel itself partially melted, and cladding reacted with steam to produce bubbles of vapor and hydrogen, which then escaped into the reactor building, along with fission products from the reactor core. As a result of the failure to close a backup valve that could be operated manually, coolant was not restored to the reactor core until more than six hours after the accident, by which time enough hydrogen had accumulated in the building to pose the treat of a low-level explosion. The building had been designed to seal automatically in the event of a pressure rise, but no rise occurred, and four hours were allowed to elapse before the building was sealed, during which time radioactive gases escaped into the atmosphere.

Within three hours after the first sign of trouble, elevated radiation levels were detected by monitors in the reactor auxiliary building. A site emergency was declared, and officials enlisted the aid of local, state, and federal emergency personnel. The presence of a large hydrogen bubble in the reactor vessel prompted widespread fear that the reactor might explode, a concern that experts failed to allay although they knew it to be a misapprehension. Adding to the fear, dosimeter readings made in a helicopter three hundred feet above the auxiliary building's ventilation stack were misinterpreted by officials to signify elevated ground levels of radiation, prompting the governor of Pennsylvania to recommend the evacuation of all pregnant women and preschool children residing within five miles of the plant, who then complied.

Although large amounts of radiation were released, the resulting exposure of the public was relatively slight, resulting mainly from xenon-133 that was present in the gaseous plume. The largest dose of radiation any member of the public may have received is estimated to have been smaller than his or her annual dose from natural background irradiation, and the average dose to those living within fifty miles of the reactor is estimated to have been 40 to 50 times smaller than that. Because of the small magnitude of the doses that were received, no demonstrable injuries from the radiation were expected, nor have any actually been observed. Nevertheless, the legacy of fear and resentment left by the accident has adversely affected the well-being of those living nearby, and it has heightened negative attitudes toward nuclear energy.

ARTHUR C. UPTON

(SEE ALSO: Energy; Environmental Determinants of Health; Nuclear Power)