Accident at Three Mile Island - March 29, 1979


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Photograph of Harold Denton with Governor Dick Thornburgh

school children's cards expressed both fears and gratitude to Harold Denton

school children's cards expressed both fears and gratitude to Harold Denton


Manuscript Group 471: Harold and Lucinda Denton Papers. Photograph of Harold Denton with Governor Dick Thornburgh responding to questions concerning, the nation's worst commercial nuclear accident at Three Mile Island, March 29, 1979, and school children's cards expressed both fears and gratitude to Harold Denton in the aftermath of the accident at Three Mile Island.

In the early hours of March 28, 1979 an equipment malfunction occurred at the Unit 2 nuclear generating station on Three Mile Island (TMI) near Harrisburg, Pennsylvania. A combination of equipment failure and operator error allowed the malfunction to lead to a dangerous situation. This situation was compounded by a failure to understand what was happening at the time and to communicate that information afterward. The problem was caused by the failure of a regulating valve to close after relieving high pressure in the primary cooling system, the one that keeps the nuclear core from overheating. An emergency cooling system correctly came on line as it was designed to do. The operators, however, not accurately interpreting the system's instrument readings, shut down the emergency system. This led to a partial melting of the core and contamination of the containment vessel.

Though the problem was contained by midday when the emergency system was restarted, company officials were not fully aware of what had happened and the extent of damage suffered by the core. Incorrect information was initially disseminated to reporters who themselves had little understanding of technical matters. The public was alarmed by the specter of a nuclear reactor out of control and it was some days before calm was restored. In the meantime, citizens reacted in many different ways. A large number left the area after Governor Richard Thornburgh ordered a limited evacuation of pregnant women and preschool children living within a five mile radius of the plant. Others offered assistance of various sorts - from housing for the evacuees to technical advice. Many, especially children, later wrote to express their fears and gratitude that the emergency had been contained.

A principal factor in the restoration of calm was the assignment of Harold R. Denton by the Nuclear Regulatory Commission to oversee the actions of the company and to advise officials responsible for public safety. Denton's technical expertise and unflappable manner finally provided an anchor in a sea of uncertainty. Though the immediate problem of cooling the reactor core had been solved by the afternoon of the incident, concerns over the existence of a hydrogen bubble within the containment vessel lingered for several days until technicians found a way to reduce its potential for further trouble. During this time, Denton ably assessed the situation and provided sage advice and understandable explanations that allayed the public's immediate fears. In the long term, however, the incident at Three Mile Island began the erosion of public support for this type of electrical power generation. Applications for construction of new nuclear plants fell sharply, and those in the planning phase received vigorous questioning from nearby residents.