Nuclear Accident at Three Mile Island

Marking Time
This article originally appeared in Pennsylvania Heritage Magazine
Volume XXXV, Number 2 - Spring 2009

PHMC's marker recalls an event of worldwide significance. Photo: PHMC Bureau for Historic Preservation

On March 28, 1979, the worst nuclear accident in the history of the United States occurred at the Three Mile Island (TMI) Nuclear Power Station near Middletown, Dauphin County. The scope of the physical accident was relatively small, but the impact on the country's nuclear power industry was enormous.

On March 28, 1979, the worst nuclear accident in the history of the United States occurred at the Three Mile Island (TMI) Nuclear Power Station near Middletown, Dauphin County. The scope of the physical accident was relatively small, but the impact on the country's nuclear power industry was enormous.

For several days, people around the world anxiously followed the emergency through the eyes of hundreds of reporters who thronged the scene. As the drama unfolded, the media reported that the reactor was out of control and that a total meltdown of the nuclear core was possible. Such a scenario, some reporters speculated, could result in deaths from radiation exposure and render a wide geographical area uninhabitable for at least one hundred thousand years.

On Wednesday, March 28, at 4:00 a.m., a water pump failed in a non-nuclear part of the plant. Without water from the pump to cool the core, a meltdown became likely. A pressure relief valve then opened, but when pressure returned to normal, the valve remained opened, causing water to pour out of the reactor core. Unaware of the problem, operators misinterpreted instrument readings as too much water in the core. They shut off emergency backup cooling water, resulting in an increased temperature that boiled off any remaining cooling water.

The Unit 2 accident impacted the use of nuclear energy in the United States. Photo: Nuclear Regulatory Commission

Operators were able to return cooling water back to the core area by Friday, March 30, but a hydrogen bubble had formed inside the dome of the pressure vessel in an auxiliary building. Experts feared the bubble might explode, releasing massive amounts of radiation, and officials made plans to evacuate nearby residents. President Jimmy Carter dispatched Harold R. Denton, director of the Office of Nuclear Reactor Regulation, to TMI to gather facts and quell rumors. President Carter and First Lady Rosalyn Carter visited Unit 2's control room with Governor Dick Thornburgh to reassure the public that there was no need to panic.

When investigators could safely enter the reactor building in July 1980, they discovered that nearly one-third to one-half of the core had melted. A cleanup program lasted from 1980 to 1983 before Unit 2 was considered decontaminated. Extensive testing of soil, plants, and residents in the vicinity of TMI has not found any problems linked to radiation.

President Jimmy Carter and Governor Dick Thornburgh, among others, visited TMI's control room, in part, to quell public panic. Photo: Nuclear Regulatory Commission

On March 25, 1999, PHMC dedicated a state historical marker commemorating the accident near the entrance to TMI. The plant's four cooling towers still dominate the bucolic landscape, and steam rising from the cooling towers of Unit 1 reactor is discernable, evidence that the plant continues to generate electricity. The damaged Unit 2 reactor building and towers, however, are forever shut down. PHMC's historical marker bears public testimony to the impact of the nuclear industry on twentieth-century American history.

Since 1914, the Pennsylvania Historical and Museum Commission has installed more than 2,500 state historical markers throughout the Commonwealth.

The State Museum of Pennsylvania recently added a Remote Reconnaissance Vehicle (RRV) to the Hall of Industry and Technology. Nicknamed 'Rover,' the RRV was developed by William 'Red' Whittaker at Pittsburgh's Carnegie Mellon University to avoid exposing cleanup workers at TMI to radiation. Over the course of four years, nine vehicles surveyed and treated the heavily contaminated basement of TMI's Unit 2 reactor building. The Rover and several tools now on exhibit at The State Museum were not used in the cleanup, but were employed to train operators and test attachments such as lights, cameras, vacuums, and scoops. GPU Nuclear, which operated the facility at the time of the 1979 accident, donated the RRV and the tools to the museum in 2000. Through 2009, The State Museum is offering a self-guided Energy Tour of its Hall of Industry and Technology, sponsored by Columbia Gas of Pennsylvania, as part of PHMC's annual theme, Energy: Innovation and Impact.