Three Mile Island

THE WORST NUCLEAR accident in the history of the United States occurred at Pennsylvania's Three Mile Island (TMI) nuclear power plant in the predawn hours of March 28, 1979. The plant located approximately 10 miles south of Harrisburg, Pennsylvania, on the Susquehanna River, came dangerously close to a nuclear meltdown that could have had devastating results.

For nearly a year, the TMI plant had been successfully producing electricity. Problems began, however, when TMI's Unit 2 Reactor was hastily opened for operation at the end of December 1987. By January 1979, Unit 2 Reactor had to be shut down for two weeks while operators identified leaks in the piping and pump system. Problems began in the pump system of the Unit 2 Reactor, specifically in the secondary loop feedwater pumps, which are responsible for turning heat and pressure from the primary nuclear sector of the plant into safely emitted steam. Due to unidentified mechanical or electronic failure, the pumps automatically shut down. Without water pushing through the secondary loop, the heat being generated in the radioactive core had no way of escaping. Thus, the water and pressure in the primary loop began to rise. A pressure relief valve opened to release the steam into a holding tank and alleviate the building pressure inside the core. The valve should have closed once the pressure decreased inside the reactor but the valve got stuck ajar. The pressure in the reactor continued decreasing as water and steam drained off the core through the opening. Further compounding the problem, the emergency water pumps, which should have immediately turned on, did not. The cutoff valves connecting the backup pumps to the rest of the system were closed days before during routine tests and never reopened. It took eight minutes for operators to notice that the valves were shut.

In the control room, an indicator light erroneously showed that the pressure relief valve had closed. Since water and steam continued to leak through the open valve, a loss-of-coolant accident was developing. Voids, or areas where there is no water present, began to form. Water was automatically and unsuccessfully redistributed to fill the voids but in the process, the pressurizer filled with water. This caused the level indicator in the control room to register that the system was safely full of water.

Operators had no idea that the pressure and water in the core were continuing to decrease, so they turned off the emergency injection water system despite the fact that the open valve was releasing water and steam. Because of inadequate cooling and the loss of water, the exposed rods warmed to[Page 806] roughly 4,300 degrees, dangerously close to the 5,200-degrees meltdown point. It took almost 16 hours for plant operators to fix the valves and pumps and stabilize the temperature and pressure inside the nuclear reactor. However, one more problem had to be dealt with. A hydrogen bubble had formed above the reactor core. There was concern that the bubble would block the flow of water to the core and cause an explosion if the hydrogen were to mix with the oxygen in the water.

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