On a Wednesday morning, maintenace workers cleaning sludge from a small pipe blocked the flow of water in the main feedwater system of a reactor at Three Mile Island near Harrisburg, Pennsylvania. Since the reactor was still producing heat, it heated the blocked cooling water around its core hot enough to create enough pressure to have popped a relief valve. Some 220 gallons of water per minute began flowing out of the reactor vessel. Within five minutes after the main feedwater system failed, the reactor, deprived of all normal and emergency sources of cooling water, and no longer able to use its enormous energy to generate electricity, it gradually started to tear itself apart.
The loss of coolant at the reactor continued for some 16 hours. About a third of the core melted down. Radioactive water flowed through the stuck relief valve into an auxiliary building, where it pooled on the floor. Radioactive gas was released into the atmosphere. It took a month to stabilize the malfunctioning unit and safely shut it down. The reactor was a total loss and the cleanup required years of repair and hundreds of millions of dollars. No one was reported injured and the little radiation that leaked out was quickly dispersed. Although this accident did cost lots of money and time, no one was hurt.