Three Mile Island Nuclear

The Three Mile Island Nuclear Generating Station is currently a single-unit nuclear power plant located on the banks of the Susquehanna River, near Middletown, Pennsylvania. It operated as a dual-unit plant until Unit 2, otherwise known as TMI-2, suffered a partial meltdown on March 28, 1979. This incident, which is regarded as the most significant accident involving nuclear power in U.S. history, permanently crippled TMI-2 and delayed the reactivation of Unit 1, otherwise known as TMI-1, until 1985. It also hindered the development of new nuclear power sites in the U.S. for several decades.

TMI-1, which is a Babcock and Wilcox Pressurized Water Reactor with an installed capacity of 802 MW, currently produces 6,645 GWh of electrical power a year for the residents of Pennsylvania and Maryland. Having entered service in 1974 and licensed until 2034, it is currently owned and operated by the Exelon Corporation. TMI-2 was a slightly larger Babcock and Wilcox PWR rated at 906 MW that was commissioned in 1978, with commercial power production beginning on December 30. Its remnants are owned by the First Energy Company of Akron.

The incident that brought about the partial meltdown of TMI-2 began on the morning of March 28, 1979, when the pumps for one of the condensate polishers, which were receiving maintenance, failed for unknown reasons. This initiated a sequence of events wherein a polisher valve failed to trip, which in turn caused water to stop flowing into TMI-2’s secondary loop’s main feedwater pump, which then ceased operations. This resulted in a loss of water flow to the steam generators, thereby causing the turbine to shut down. Though the reactor performed an immediate emergency shutdown, the disruption of the steam cycle prevented the removal of decay heat from the reactor’s water loop, thereby raising TMI-2’s core temperature.

Auxiliary pumps, which are intended to introduce coolant in the event of such an emergency, happened to be offline for maintenance at the moment of the accident. This oversight, which was noted by the Nuclear Regulatory Commission as the greatest factor in the escalation of the crisis, prevented the flow of water into the reactor vessel and initiated the partial meltdown.  The accident was also compounded by human errors that were later discovered to be the result of poorly designed computer interfaces as, during the crisis, operators were unable to determine the exact status of the reactor’s temperature and pressure.

Unfortunately, this might not be the only crisis the Three Mile Island plant has initiated.  Many nuclear power plants built in the 1960s and 1970s contained asbestos in their insulation or other construction materials.  This mineral helped protect valuable machinery, but exposed plant workers to deadly fibers that, when they accumulate in the lungs, can cause mesothelioma.