In a PWR like TMI-2, water in the primary cooling system around the core is kept under very high pressure to keep it from boiling.) The relief valve is located on the pressurizer. (The pressurizer is a tank which is part of the primary reactor cooling system, maintaining proper pressure in the system. As water and steam escaped through the relief valve, cooling water surged into the pressurizer, raising the water level in it. Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. However, they did not have an instrument indicating the valve's actual position. The operators believed the relief valve had shut because instruments showed them that a "close" signal was sent to the valve. But it remained open, leaking vital reactor coolant water to the reactor coolant drain tank. About 10 seconds later it should have closed. Within seconds of the shutdown, the pilot-operated relief valve (PORV) on the reactor cooling system opened, as it was supposed to. The chain of events during the Three Mile Island accident Deficient control room instrumentation and inadequate emergency response training proved to be root causes of the accident. The operators were unable to diagnose or respond properly to the unplanned automatic shutdown of the reactor. The core suffered severe damage as a result. At this point a relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away that the residual decay heat in the reactor core was not removed. This in turn caused the reactor to shut down automatically. It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. The accident to unit 2 happened at 4 am on 28 March 1979 when the reactor was operating at 97% power. TMI-2 was of 959 MWe (880 MWe net) and almost brand new at the time of the accident. TMI-1, a PWR of 880 MWe (819 MWe net) entered service in 1974, and remained one of the best-performing units in the USA until it was shut down in 2019.
The Three Mile Island power station is near Harrisburg, Pennsylvania in the USA. There were no injuries or adverse health effects from the Three Mile Island accident.Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents.In 1979 at Three Mile Island nuclear power plant in USA a cooling malfunction caused part of the core to melt in the #2 reactor.
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