Three Mile Island Nuclear Accident – Pennsylvania – March 28, 1979

Failure of water pumps caused a chain reaction of higher and higher temperatures and some radioactive gas began to escape into the surrounding area.

The worst accident in the history of U.S. commercial nuclear power generation occurred at 4:00 A.M. on March 28, 1979. It happened at the Three Mile Island installation near Middleton, Pennsylvania, when the plant experienced a failure in the nonnuclear section. The two main water pumps stopped running and a chain reaction followed. Before long radioactive gas escaped into the surrounding area and the governor of Pennsylvania ordered preschool children and pregnant women to stay away from an area within five miles of the nuclear installation.

There are dozens of nuclear power plants operating in the United States, providing electrical power to more than half of the nation’s states. An accident in any one of them sends alarms across the country. Hence, when something went wrong with the installation at Three Mile Island in Pennsylvania in March of 1979 it cast a shadow on every nuclear plant in the rest of the country. The companies that operate these plants knew that these power generators have backup systems to cope with accidents and they also knew that if these systems failed there could be a serious tragedy. The Three Mile Island event was what one expert called a common mode accident. By that he meant an event that bypasses all the backup systems either because of its rarity or because an operator interferes with the safety systems by disconnecting them.

The basic rule in nuclear generators is to have duplicate safety systems for all the important operations so that failure of one will not cause irreparable damage before repairs can be done. These duplicate systems were installed in the Three Mile Island plant but because nuclear power generators were still relatively new at the time not every eventuality was provided for. The heart of the power generator is the fuel cell that is heated by nuclear fission. This heat is used to generate electricity. However, the heat generated by the fuel cell can rise to uncontrollable levels if it is not kept at a safe level and cooling water is the agent that regulates this temperature. A common-mode accident would be an event that cuts off the supply of cooling water. None of all the other safety systems matter if this were to happen; the power plant would be out of control.

The fundamental weakness that gave rise to the common-mode accident at Three Mile Island was the competence of the plant workers. These maintenance people who looked after the installation day and night were well trained for keeping everything in good order, for conducting routine adjustments and repairs on equipment, and for reporting anything that was abnormal. They had spent a year in a training course. Some of them had previous experience on nuclear-powered submarines. They were not qualified nuclear engineers and they had no training for coping with complex emergencies, nor were they supposed to. The automatic systems were designed for these emergencies but, as it turned out, they could not anticipate every eventuality.

The pipes that provide water to the fuel cells have to be cleaned periodically to remove impurities and the workers had been attending to this for some time when one of the pipes became blocked. While trying to clear the pipe, one man accidentally cut off the main flow of water to the fuel cells and, true to form, the power plant shut down. The workers heard some loud noises that confirmed it. This happened around 4:00 A.M. on March 28, 1979. Within a few seconds of the shutdown emergency, water supplies would normally have gone into operation just as the designers had planned and the plant would be ready to start up again. This was the theory and, indeed, all the systems seemed to work as intended, except that a pressure surge due to the sudden cut-off of water popped open a relief valve. This valve should have returned to normal when operations resumed but instead it got stuck in the open position allowing two hundred gallons of water to escape from the reactor core every minute.

The problem was compounded by an error when the operators cut off the emergency water pumps. They had misunderstood what was happening and thought that the reactor core was receiving too much water. They also opened a drain line and this released still more water from the reactor core. These actions stemmed from a lack of information about how much water was in the core at any time. There was no instrumentation to provide this data. In the absence of this information they had been instructed to check the levels in another water tank and conclude that if it were full then the water around the fuel cells would be adequate. Within a few minutes, temperatures within the core rose sharply, the remaining water turned into steam, and the nuclear core fuel rods overheated and began to disintegrate. Three hours later as the plant manager arrived, a state of emergency was declared. Radioactive material and water continued to escape.

No “meltdown” took place as happened at Chernobyl in the Ukraine. That is, nuclear fuel did not “melt” through the floor beneath the containment or through the steel reactor vessel. However, a substantial amount of fuel did melt. Radioactivity in the reactor coolant increased dramatically. Radioactive gas began to spread through small leaks. It reached all parts of the plant and went out into the surrounding environment. Two days after the accident, Governor Thornburgh of Pennsylvania ordered a precautionary evacuation of preschool children and pregnant women from within a five-mile zone around the plant. People living within ten miles were urged to stay inside and keep their windows closed. These measures lasted for about a week until the situation at the plant was completely under control and the danger from radiation was eliminated.

Detailed studies of the consequences of the accident were conducted by a number of government agencies and several independent agencies. The general conclusion was that the average exposure to about two million people in the area was 1 millirem. This corresponds to one-sixth of the amount of radiation one would be exposed to in the course of having a full set of chest x-rays. Besides, the natural levels of radioactivity in and around the Three Mile installation are a little over one hundred millirem per year. Clearly, the average amount of radiation was trivial and the maximum that anyone was likely to experience was one hundred millirem. A consensus gradually emerged over the possible long-term damage. It was a risk of one additional cancer death over a time period of thirty years.

In the months that followed, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the area, none could be directly related to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected and monitored by various groups. Today, the damaged reactor is permanently shut down and the reactor coolant system has been decontaminated. Radioactive liquids have been treated, most components shipped to a licensed low-level waste disposal site, and the whole location carefully monitored. Costs of cleanup have been running at 70,000 dollars annually ever since the accident.

Causes of the Three Mile Island accident continue to be debated to this day. The main factors appear to have been a combination of personnel error, design deficiencies, and component failures. There is no doubt that the accident permanently changed the nuclear industry. Public fear and distrust increased. This was the most serious in U.S. commercial nuclear power plant operating history, even though it led to no deaths or injuries to either plant workers or members of the nearby community. It brought about sweeping changes in emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations. Reactor operator training was high on the list of reforms. All electric utilities expanded their training for personnel who work at and support nuclear plant operations.

The cleanup of the damaged nuclear reactor took nearly twelve years and cost almost a billion dollars. The work was challenging technically and with regard to the handling of radiation. Plant surfaces as well as the water used in the cleanup had to be decontaminated. One hundred tons of damaged uranium fuel was removed from the reactor vessel without any harm being done to the workers involved. Waste nuclear material was sent to Richland, Washington, for storage. After the cleanup, reactor number two in the Three Mile Island Plant was placed on long-term monitored storage. It was kept completely free from number one in that, though unaffected by what had happened, it had also been shut down at the time of the accident. The Three Mile Island number one unit was restarted in 1985 and has been working efficiently and safely ever since.

The National Nuclear Academy was instituted to accredit the training of plant staff for all programs. Utilities purchased simulators for the training of personnel who work in the main control rooms. Training reforms centered on protecting a plant’s cooling capacity, whatever the triggering problem might be. In the 1979 accident, operators turned to a book of procedures to pick those that seemed to fit the event. In the new training operators are taken through a set of “yes-no” questions to ensure, first, that the reactor’s fuel core remains covered. Then they determine the specific malfunction. This is known as a “symptom-based” approach for responding to plant events. Underlying it is a style of training that gives operators a foundation for understanding both theoretical and practical aspects of nuclear installations.