Three Mile Island Unit 2

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The Three Mile Island Unit 2 (TMI-2) reactor, near Middletown, Pa., partially melted down on March 28, 1979. It was the most serious accident in U.S. commercial nuclear power plant operating history. Its consequence brought about tremendous changes involving emergency response planning, reactor operator training, ergonomics, radiation protection, and many other areas of nuclear power plant operations. All of these changes significantly improved U.S. reactor safety.
Chain of events during the Accident:
The nuclear station had two pressurized water reactors. The accident began in the Unit 2 reactor when the plant's main feed water pumps in the secondary non-nuclear cooling system failed. In response to the failure, the auxiliary feed water pumps kicked in. However, the water did not reach the steam generator because the outlet valves were closed. The valve was discovered closed about eight minutes into the accident. Responding to an increase in temperature and pressure in the primary system, the pressurizer relief valve automatically opened in the line between the pressurizer and the quench tank. This caused the nuclear reactor to shutdown automatically, thereby dropping the pressure in the reactor. The valve, which was supposed to close after the pressure dropped below the set-point for closure, failed …show more content…
As the water and steam escaped through the relief valves, coolant water surged into the pressurizer, raising the water level in it. Unaware of the stuck valve and the false indicator readings, operators manually turned off the ECCS. Temperature raised steam then formed in the reactor primary cooling system. The pumping of the mixture of steam and water caused the reactor cooling pumps to vibrate and the operators shutdown the pumps to stop severe vibrations. As the reactor cooling water boiled away, the top of the reactor core was

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