Three Mile Island

* The Three Mile Island accident was a partial meltdown of the Three Mile Island, Unit 2 (TMI-2) reactor on the Susquehanna River in Londonderry Township, Pennsylvania, near the Pennsylvania capital of Harrisburg. It began at 4 a.m. on March 28, 1979, and released radioactive iodine into the environment. It was the worst accident in U.S. commercial nuclear power plant history. On the seven-point International Nuclear Event Scale, it was rated Level 5-Accident with wider consequences.

* The accident began with failures in the non-nuclear secondary system followed by a stuck-open pilot-operated relief valve (PORV)  in the primary system that allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident (LOCA). TMI training and procedures left operators and management ill-prepared for the deteriorating situation. During the event, these inadequacies were compounded by design flaws, including poor control design, the use of multiple similar alarms, and a failure of the equipment to clearly indicate coolant inventory level or the position of the stuck-open PORV.

* The accident crystallized anti-nuclear safety concerns among activists and the general public, and led to new regulations for the nuclear industry. It accelerated the decline of effort to build new reacators.

* The TMI accident inspired Charles Perrow’s Normal Accident Theory, which attempts to describe “unanticipated interactions of multiple failures in a complex system”. TMI was an example of this type of accident because it was “unexpected, incomprehensible, uncontrollable and unavoidable.” Perrow concluded that the failure at TMI was a consequence of the system’s immense complexity. Such modern high-risk systems, he realized, were prone to failure however well they were managed. It was inevitable that they would eventually suffer what he termed a “normal accident.” Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely.



* Which supports and barriers were in play?

* What were the dynamics?

* Who, or what, won the Tug-Of-War?

* Discuss the outcome with your friends and family.

* Use Post #4 as a reference for the dynamics, and the relationships, between supports and barriers.