With all of the care and precautions involved in the defense-in-depth design of a nuclear plant, how could the TMI accident happen?

At TMI, the defense-in-depth safety systems operated correctly but were shut down by qualified operators who misinterpreted the chain of events.

The operators consciously turned off emergency cooling systems because they thought additional water would rupture the cooling system.

They were convinced a valve was closed because a control panel light showed the valve had been given a signal to close. Although there were other indications that the valve was actually open, the operators continued to act to protect the system from additional water.

After TMI, the nuclear power plants have expanded their operator training programs. Plants have also modified their control room indicators, and have modified some plant equipment to prevent other accidents from occurring.