The accident at Three Mile Island took place on March 28, 1979. The night shift team supervisor, operators, and remainder of trained but unlicensed auxiliary operators went to work unaware of what was going to happen to them that night. .
In the Unit # 2 reactor, resin had built up in the feed water decontamination tank. Workers tried to free up the resin but could not get it off the tank. Even the plants back up for this kind of problem failed to remove the resin. Once the cleaning's failed every valve throughout the feed water system closed shut. When the feed water system closed shut the reactor started to tremble violently and the force of it ripped out the valve controls and caused extensive damages to the cooling system. Eventually the reactor over heated and started to rupture and the radioactive water then spilled into the auxiliary building and into the drain system which forced workers to evacuate the building. .
The accident at Three Mile Island could have been very easily avoided. It was human and design errors which created the problems. The first error was the undocumented design change to the air system. If the plant supervisors knew of the design update they most likely would have been able to solve the problems that they encountered. Another error has to do with the emergency pumps; they were serviced by maintenance but were never turned back on. Yet another error was the pressure relief valve, this was designed incorrectly as well as the indicator. Last but not least one of the workers incorrectly transmitted data to the operators. If simple design problems, human error and communication problems had been avoided, the accident at Three Mile Island would have never occurred. Although the Three Mile Island accident was not as great of a disaster as Chernobyl, it was still the United States most severe nuclear disaster that got people thinking about how safe nuclear power really is.