Key Issues
The TMI 2 Accident: Its Impact, Its Lessons
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The Accident: What Happened
On March 28, 1979, TMI 2 near Harrisburg, Pa., was operating at about 100 percent power when it automatically shut down after a pump that provided cooling water stopped operating. Pressure and temperature increased in the reactor, causing a pressure relief valve to open. The valve opened as designed, and water and steam began flowing out of the reactor to a tank in the basement of the reactor building.
As pressure returned to normal, the valve should have closed. But, unknown to the operators, the valve stuck open. It remained open for more than two hours, allowing water that covered and cooled the fuel core to escape from the reactor system. This caused the fuel to begin to overheat.
However, instrumentation in the TMI control room indicated to the operators that the relief valve was closed and that too much water was being injected into the reactor vessel. Consequently, operators did not replace the water that was lost as a result of the open relief valve.
As pressure continued to drop, more and more coolant turned to steam, causing excessive vibration in the pumps. The vibration prompted operators, who did not realize the reactor was experiencing a loss of coolant, to shut down the pumps.
The loss of pressure and water caused a large steam bubble to form inside the reactor vessel, further preventing the flow of cooling water through the core. Without coolant, core temperatures rose above the melting point of the fuel cladding and the uranium fuel.
About half of the fuel melted before the bubble was dispersed and the coolant flow was re-stored. The colder cooling water also shattered some of the hot fuel rods. All the fuel was damaged.
As a result of the TMI 2 accident, 700,000 gallons of radioactive cooling water ended up in the basement of the reactor building and in tanks in the auxiliary building, contaminating them.
In addition, a small amount of radioactive material was released into the atmosphere from the ventilation stack of an auxiliary building to relieve pressure inside the reactor.
The Accident: What Happened
On March 28, 1979, TMI 2 near Harrisburg, Pa., was operating at about 100 percent power when it automatically shut down after a pump that provided cooling water stopped operating. Pressure and temperature increased in the reactor, causing a pressure relief valve to open. The valve opened as designed, and water and steam began flowing out of the reactor to a tank in the basement of the reactor building.
As pressure returned to normal, the valve should have closed. But, unknown to the operators, the valve stuck open. It remained open for more than two hours, allowing water that covered and cooled the fuel core to escape from the reactor system. This caused the fuel to begin to overheat.
However, instrumentation in the TMI control room indicated to the operators that the relief valve was closed and that too much water was being injected into the reactor vessel. Consequently, operators did not replace the water that was lost as a result of the open relief valve.
As pressure continued to drop, more and more coolant turned to steam, causing excessive vibration in the pumps. The vibration prompted operators, who did not realize the reactor was experiencing a loss of coolant, to shut down the pumps.
The loss of pressure and water caused a large steam bubble to form inside the reactor vessel, further preventing the flow of cooling water through the core. Without coolant, core temperatures rose above the melting point of the fuel cladding and the uranium fuel.
About half of the fuel melted before the bubble was dispersed and the coolant flow was re-stored. The colder cooling water also shattered some of the hot fuel rods. All the fuel was damaged.
As a result of the TMI 2 accident, 700,000 gallons of radioactive cooling water ended up in the basement of the reactor building and in tanks in the auxiliary building, contaminating them.
In addition, a small amount of radioactive material was released into the atmosphere from the ventilation stack of an auxiliary building to relieve pressure inside the reactor.
