March 28th, 1979

44 years ago today, at the Three Mile Island nuclear power facility outside Harrisburg, Pennsylvania, a pressure valve suddenly failed; allowing cooling water to escape from the radioactive core. 

Unaware of the stuck-open relief valve and unsure whether the radioactive core was covered with cooling water, the staff took a series of actions that exposed it.

First, because of the stuck valve, the primary system pressure dropped so low that the reactor coolant pumps began to vibrate and so the staff turned them off. They then reduced the flow of water because the emergency cooling water being pumped into the primary system threatened to completely fill the pressurizer. As a result of the reactor coolant pumps not circulating water and the primary system being deprived of emergency cooling water, the water level in the pressure vessel dropped and the core overheated.

Temperatures in the reactor hit 4,300 degrees Fahrenheit, dangerously close to meltdown, and the exposed part of the core began to fry. But, when contaminated water spilled into a surrounding building and began to emit radioactive gases inside the plant, Three Mile Island’s supervisor declared the first general emergency.

Soon, Governor Dick Thornburgh advised the evacuation of “pregnant women and children under the age of five…within a five-mile radius of the Three Mile Island site.” On Friday, March 30, the evacuation zone was expanded to a 20-mile radius. After a few days, 140,000 people had evacuated the area. 

A clean-up plan was developed and carried out safely and successfully by a team of more than 1000 skilled workers. Three Mile Island Unit 2’s damaged nuclear reactor system required nearly 12 years and $973 million to clean up.

Training reforms are one of the most significant outcomes of the Three Mile Island accident. Whatever the triggering problem, training became focused on protecting a plant’s cooling capacity to prevent similar disasters.