Today in 1979, the TMI-2 reactor located at the Three Mile Island nuclear power plant experienced the most serious accident in US commercial nuclear power plant operating history. No deaths or injuries occurred because of the accident, either to workers or to people living in nearby communities. Nevertheless, the events of that day marked the beginning of the end of the growth of nuclear power in the United States.
The Three Mile Island Nuclear Generating Station occupies 814 acres on an island in the Susquehanna River near Harrisburg, Pennsylvania. In 1979, the number 2 reactor at the site was just a year old and had only been operating for a few months. At 4AM on March 28th, the main feedwater pumps on the secondary side of the plant stopped running. This prevented the steam generators, the giant heat exchangers that remove heat from the primary, or reactor, side of the plant, from removing heat. The reactor shut down, but pressure in the primary system began to increase. A relief valve opened to relieve the pressure; it was designed to close when pressure decreased to an operational level. However, the valve remained open, allowing primary cooling water to pour out of the reactor side of the plant. This exposed the core of the reactor, causing it to overheat.
The reactor operators on duty, like ROs everywhere, could only react to what their instruments told them. In this reactor design, the water level in the core was determined by the pressure reading in the primary side of the plant. Since the pressure was high, it was assumed that the core was still covered with water. In fact, one-half of the fuel pellets in the core were already melting. This is what is known as a core meltdown. Fortunately, the containment building was not breached, a situation that would’ve released toxic amounts of radiation into the outside environment.
There were other concerns which arose over the next three days that were serious enough to cause the Governor of Pennsylvania and the Chairman of the Nuclear Regulatory Commission to call for an evacuation of those most vulnerable to radiation within a 5-mile radius of the plant.
Studies over the years have shown that no significant damage was caused by the small amount of radiation released during the crisis. Despite this, the accident led to a re-evaluation of training and safety standards at all nuclear power plants in the United States. But no amount of redesign and retraining would curtail the psychological damage done to the American public in its opinion of nuclear power. Support for nuclear power dropped 20 percent to a point where only half the population supported the building of more nuclear plants. Of the 129 plants that had been approved at the time of the accident, only 53 were ever completed. It can be argued that nuclear power was already in danger the late 1970’s due to inexpensive oil and a nation that was already beginning to forget about the oil embargo just a few years earlier. If this is so, then Three Mile Island helped to nail the coffin shut.
Today, TMI-1, the first reactor at Three Mile Island, continues to operate. The cleanup of TMI-2 lasted for more than 14 years and cost approximately $975 million. In the end, it was determined that a few simple water level gauges (already standard on some designs) would have prevented the accident.