4:00 a.m. local time. Three Mile Island, Pennsylvania, nuclear accident. A partial core meltdown occurred at the Three Mile Island nuclear power plant in Dauphin County. It was the most serious nuclear accident in the United States, resulting in the release of small amounts of radioactive gases and radioactive iodine into the environment. The plant was owned and operated by General Public Utilities and Metropolitan Edison (Met Ed). The reactor involved in the accident, Unit 2, was a pressurized water reactor (PWR) manufactured by Babcock & Wilcox. At the time of the accident, the reactor was operating at 97% capacity. The accident began in the secondary coolant circuit, blocking feed to the steam generators. This blockage led to a significant increase in coolant pressure in the primary core cooling circuit, first causing the opening of a PORV relief valve on the pressurizer, and then the SCRAM, the emergency shutdown of the reactor by inserting the control rods. At this point, the relief valve failed to close, and the operators were unaware of the problem, also because the instrumentation did not indicate the valve’s actual position. As a result, the primary cooling circuit partially emptied, and the residual heat from the reactor core could not be dissipated. As a result, the radioactive core suffered serious damage. The operators were unable to correctly diagnose what was happening and respond appropriately. Poor control room instrumentation and inadequate training proved to be the primary causes of the accident. During the accident, a partial core meltdown occurred, resulting in some very serious damage; Unit 2 was shut down and is still under monitoring, awaiting future decommissioning measures.
