Three Mile Island Accident

The three mile island (tmi) plant, located on the Susquehanna River, about 10 miles from Harrisburg, Pennsylvania, was the site of what the Nuclear Regulatory Commission (NRC) calls the most serious nuclear accident in American history. Metropolitan Edison then owned the TMI facility, consisting of two reactors. The accident began at about 4:00 a.m. on March 28, 1979, in the nonnuclear section of the power plant, when the pumps that feed water to the system to create steam (which propelled the power turbines) failed. Because these pipes carry away part of the heat from the reactor, the cooling system would be required to carry the extra load. The water and steam pressure increased in the cooling system, which caused a relief valve to open. The valve was supposed to close when pressure reached a safe level, but it did not, and, unknown to controllers, pressure and water in the cooling system was lost, thereby leading to a partial meltdown of the reactor core. The NRC summarizes the situation:

This sequence of events describes what Charles Perrow calls a “normal accident” that results when redundant safety systems interact with human actors to result in unpredictable system accidents. By mid-day, the NRC, Environmental Protection Agency, and Department of Energy inspectors and scientists were at the scene. The utility's and the agencies' efforts appeared to be successful, but, on March 30, there was a small release of radiation due to attempts to release pressure on the coolant system. The radiation had come from an auxiliary building, not the containment, but the radiation was sufficiently worrisome to induce the governor of Pennsylvania to urge an evacuation of school-aged children and pregnant women within five miles of the reactor.

Another concern soon arose when it became evident that a bubble of hydrogen gas had appeared at the top of the containment structure; the highly flammable gas could explode and cause a small breach of the containment, thereby releasing dangerous radiation. By April 1, engineers and scientists had determined that the lack of oxygen in the containment would minimize the chance of an explosion, and, in any case, the size of the bubble had diminished.

What remained for the utility and the regulators was to secure the reactor, assess the damage, and figure out what went wrong. The accident was sufficiently serious that President Jimmy Carter created a commission, popularly known as the Kemeny Commission after its chair, to investigate what happened. The basic conclusion they reached is that the accident was partially caused by mechanical failure, but that failure was greatly compounded by human error. The commission noted that the control room technicians were poorly trained, that they failed to properly interpret the information their instruments provided, and that they did not suspect a loss of coolant accident (LOCA) until quite late in the day. It was not until late in the chain of events that the operators realized that the core was not covered by cooling water, and that a partial meltdown had [Page 1730]taken place. The LOCA was ultimately discovered, and by 3:30, the immediate crisis had passed.

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