Three Mile Island, Pennsylvania

The Three Mile Island Nuclear Generating Station is the site of the most serious accident to befall the U.S. nuclear power industry to date.  It has two nuclear generating plants on site: Unit 1, with a 837MW reactor; and Unit 2 with a 906MW reactor. Unit 1's operating license was granted in 1974, while Unit 2's operating license was granted in 1978. The accident occurred on March 28, 1979, at a time when Unit 2's reactor was operating at 97% power. An unspecified mechanical or electrical fault in the reactor's secondary cooling system, caused the primary cooling system to fail, when pumps were unable to supply it with water. This resulted in the turbine-generator as well as the reactor itself, shutting down automatically. In response to the increasing pressure in the primary system, a critical component known as the pilot-operated relief valve opened, as it was designed to do. Unfortunately, the valve became stuck open, and its failure to close resulted in a serious loss of critical cooling water. Operators were unaware of the ongoing loss of coolant, however, due to malfunctioning monitoring equipment in the control room. Based in part on additional erroneous information, operators then undertook further measures which deprived the reactor core of even more coolant, causing it to overheat, resulting in a partial meltdown. One month later, operators managed to cool the reactor core sufficiently, to place the plant in cold shutdown. Remediation took almost twelve years, at a cost of around $973 million, including the removal of the reactor core. For much of this time, Unit 1 continued to produce electricity, and wasn’t closed until 2019.

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