At about 4:00 a.m. on March 28, 1979, a pump failure in a secondary (non-nuclear) system of the reactorstopped steam generators from removing heat from the unit, increasing the pressure in the primary (nuclear) portion.
A relief valve was opened to release the pressure, but the valve stuck and didn't close. There were no warning alarms indicating that something was wrong with the valve. As a result, vital cooling water was released while the reactor began to overheat. Other warnings lights and alarms signaled that the reactor was not running correctly, but operators were getting confusing messages and they had experienced false alarms before.
It was hours before they acknowledged that there was a problem and even longer before they alerted the public. In the hours that followed, radiation was released from the plant, sparking concern from public officials that residents near the plant might have to be evacuated.
The full damage to the reactor remained unknown until years later when television cameras and a special ultrasonic imaging system provided the first pictures from inside the structure. Cleanup of the accident required $1 billion and longer than 10 years to complete,according to The Washington Post. Officials believed the plant would be repaired and put back into service, but some features of the reactor couldn't be repaired because they had melted.
It concluded, “It is further concluded that the management utility joined the narrow and confined view on the safety items and virtually ignored other vital parts of plant operation. … This illustrated that the utility management had not exhibited the desire or capacity to go beyond the NRC requirements to provide a well-designed, maintained, and staffed plant capable of reliable performance that would not jeopardize the health and safety of the public and its own workers.”
The NRC also conducted an investigation and released a report in 1980. It found that the plant equipment was generally sound, and placed blame for the accident primarily on human error.
“If this system had been allowed to function automatically, as intended, it would have mitigated the effects of the loss-of- coolant and cooled the core,” it stated. “The operators’ actions, which led to the severe core damage that characterized the TMI accident, resulted from their failure to understand basic plant conditions that were indicated to them, or to follow appropriate procedures or prudent operating practices, any one of which could have prevented the severe core damage. This demonstrated a deep and significant weakness of the operating crew on shift.”
The NRC also released a fact sheet in 2004 that summarized its findings. Three Mile Island Alert, a central Pennsylvania-based citizens watchdog group founded in 1977, offers a point-by-point criticism of the NRC report.
Despite the results of studies that called the radiation release negligible, some disagreed with the findings. Rates of certain cancers did increase after the accident, but researchers did not consider the increase statistically significant, The New York Times wrote in 1990. Three Mile Island Alert, a nonprofit citizens' organization, said research didn't consider the physical and psychological effects felt by residents living close to the reactor.