Three Mile Island

Three Mile Island

On March 28, 1979 a series of mechanical, electrical, and human failures led to what has been described as the worst nuclear power plant accident in the history of the United States. News of the accident rocked the nation, and its effects were keenly felt by those who lived in the shadow of the great concrete towers of Three Mile Island.

The accident began about 4:00 a.m. on March 28, 1979, when there was failure in the secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, which was caused by either a mechanical or electrical failure that prevented the steam generators from removing heat. First the turbine, then the reactor automatically shut down. Then the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to stop that pressure from becoming excessive, the pressurizer relief valve opened. The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open. As a result, the stuck-open valve caused the pressure to continue to decrease in the system.

Meanwhile, another problem appeared in another part of the plant. The emergency feedwater system, backup to main feedwater, was tested 42 hours prior to the accident. As part of the test, a valve is closed and then reopened at the end of the test. But this time, because of either an administrative or human error, the valve was not reopened. This mistake prevented the emergency feedwater system from functioning. The valve was discovered closed about eight minutes into the accident. After it was reopened, the emergency feedwater system began to work correctly, allowing cooling water...

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...ining water from the core.

In conclusion, today white steam waves from the twin hourglass-shaped cooling towers of TMI's only functioning unit, which cost $7 million dollars to build. Operators say it is a model of efficiency and safety. But in a deal to be sealed this summer, the plant will be sold for $100 million, a seventh its book value. Three Mile Island was the worst nuclear disaster in the U.S. and because of that accident many nuclear power plants have either been forced to shut down or upgrade their facilities.

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