Three Mile Island
Max and Matt
Mr. Rowzee
Standards: 901, 903, 904, 905

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A Look Back: Three Mile Island
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In Dauphin County, Pennsylvania an almost disasterous core meltdown took place at the Three Mile Island Nuclear Generating System

By: MAX and MATT
Published: March 28, 1999

DAUPHIN COUNTY PENNSYLVANIA –Twenty years ago on March 28, 1979, at 4:00 A.M. on Three Mile Island, a potentially dangerous incident occurred in the second nuclear reactor. While the reactor was operating at 97% power, there was a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This malfunction caused the reactor to shut down automatically, as it was designed to do. Within seconds of the shutdown, the pilot-operated relief valve (PORV) on the reactor cooling system opened, as it was also supposed to. About ten seconds later it should have closed. However, it remained open, leaking vital reactor coolant water to the reactor coolant drain tank. This meant that the core could not be cooled. While this was happening the operators believed that the relief valve had shut because instruments showed them that a "close" signal was sent to the valve, but they did not have an instrument indicating the valve's actual position.

In response to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, cooling water surged into the pressurizer, raising the water level in it. (The pressurizer is a tank which is part of the primary reactor cooling system, maintaining proper pressure in the system. The relief valve is located on the pressurizer. In this reactor, water in the primary cooling system around the core is kept under very high pressure to keep it from boiling). Operators responded by reducing the flow of replacement water. Their training told them that the pressurizer water level was the only dependable indication of the amount of cooling water in the system. Because the pressurizer level was increasing, they thought the reactor system was too full of water. Their training told them to do all they could to keep the pressurizer from filling with water. If it filled, they could not control pressure in the cooling system and it might rupture.

Steam then formed in the reactor primary cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate. Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps. This ended forced cooling of the reactor core. (The operators still believed the system was nearly full of water because the pressurizer level remained high.) However, as reactor coolant water boiled away, the reactors fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the cooling water.

At 6:22 am operators closed a block valve between the relief valve and the pressurizer. This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system. Throughout the morning, operators attempted to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water. During the afternoon, operators attempted to decrease the pressure in the reactor system to allow a low pressure cooling system to be used and emergency water supplies to be put into the system. All of these methods were used to try to cool the reactor core so that more radiation would not be released.

By late afternoon, operators began high-pressure injection of water into the reactor cooling system to increase pressure and to collapse steam bubbles. By 7:50 pm on March 28th, they restored forced cooling of the reactor core when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations. Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29th and 30th, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked a little, and some radioactive gas was released to the environment. But after an anxious month, on April 27th operators established a natural convection circulation of coolant in the system. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping.
Applying the accident's lessons produced important, continuing improvement in the performance of all nuclear power plants. The accident also fostered better understanding of fuel melting, including the improbability of a meltdown breaching the reactor vessel or the containment building. However, public confidence in nuclear energy, particularly in the United States, declined sharply following the Three Mile Island accident.

Training reforms are among the most significant outcomes of the TMI-2 accident. Training became centered on protecting a plant's cooling capacity, whatever the triggering problem might be. At TMI-2, the operators turned to a book of procedures to pick those that seemed to fit the event. Now 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 plant operations.

The TMI-2 accident also led to the establishment of the Atlanta-based Institute of Nuclear Power Operations (INPO) and its National Academy for Nuclear Training. These two industry organizations have been effective in promoting excellence in the operation of nuclear plants and accrediting their training programs. INPO was formed in 1979. The National Academy for Nuclear Training was established under INPO's auspices in 1985. TMI's operator training program has passed three INPO accreditation reviews since then. Training has gone well beyond button-pushing. Communications and teamwork, emphasizing effective interaction among crew members, are now part of TMI's training curriculum. Close to half of the operators' training is in a full-scale electronic simulator of the TMI control room. The $18 million simulator permits operators to learn and be tested on all kinds of accident scenarios.

The Three Mile Island accident caused concerns about the possibility of radiation-induced health effects, principally cancer, in the area surrounding the plant. Because of those concerns, the Pennsylvania Department of Health for 18 years maintained a registry of more than 30,000 people who lived within five miles of Three Mile Island at the time of the accident. The state's registry was discontinued in mid 1997, without any evidence of unusual health trends in the area.

Indeed, more than a dozen major, independent health studies of the accident showed no evidence of any abnormal number of cancers around TMI years after the accident. The only detectable effect was psychological stress during and shortly after the accident. The studies found that the radiation releases during the accident were minimal, well below any levels that have been associated with health effects from radiation exposure.

Works Cited:

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