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THREE MILE ISLAND AND BHOPAL: LESSONS LEARNED AND NOT LEARNED 197 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Three Mile Island and Bhopal: Lessons Learned and Not Learned John F. Ahearne The Three Mile Island Nuclear Station (TMI) consists of an 840-megawatt (MW) reactor (Unit 1) and a 960-MW reactor (Unit 2) located about 10 miles southeast of Harrisburg, Pennsylvania, on an island in the Susquehanna River. At about 4:00 a.m., March 28, 1979, the main feed-water pumps connected with one of the two Unit 2 steam generators shut down, causing an automatic and almost simultaneous shutdown of the Unit 2 turbine. What caused the shutdown is not definitely known, but it may have been a pressure perturbation in the system caused by maintenance work being done at that time. With the feedwater flow stopped, the steam generators stopped removing heat from the primary system, the closed system of pressurized water that carries heat from the reactor to the steam generators and then returns to the reactor. As the heat in the primary system rose, the water pressure rose, causing a pressurizer relief valve to open and the reactor to shut down automatically. This reactor ''scram" took place eight seconds after the original pump stopped. With the reactor scrammed, the nuclear fissioning in the reactor core stopped, but a very large amount of decay heat remained (equivalent to a 55-MW reactor). This decay heat cannot be turned off, and it is necessary that sufficient water and pressure be maintained after shutdown to cool a reactor. During these first few seconds, the equipment acted according to design, and the automatic responses to the interruption of heat transfer were normal. After the reactor scrammed and the relief valve lifted, however, the primary water pressure fell to a level at which the relief valve was supposed to closeâ
THREE MILE ISLAND AND BHOPAL: LESSONS LEARNED AND NOT LEARNED 198 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. but it stuck open. Because the relief valve was open, the pressure in the primary system continued to fall. When the pressure had dropped to about 75 percent of normal, an emergency core cooling system (ECCS) came on automatically and injected cool water under high pressure into the reactor. Believing that the relief valve was closed, and seeing the water level in the pressurizer rise as the ECCS water was injected, the operators in the reactor room feared that the pressurizer would fill up with water and the system would lose its normal behavior. They did not really understand what went on in a reactor, and they did not understand what the pressurizer valve did. So, consistent with their understanding and training, they shut off one ECCS pump and throttled back the other. The water in the core then boiled. The pressurizer relief valve stayed open for about 2 hours and 20 minutes. In addition, early in the accident, the operators were also letting water out because they believed there was too much water going in. More than 30,000 gallons of water escaped from the reactor vessel. Eventually the upper portion of the core became uncovered. There were sharp increases of temperature, heavy damage to fuel, and release of radioactive fission products. The extent of the damage would be uncertain until the reactor was examined, but preliminary measurements indicated extreme damage since much of the upper fuel assembly was rubble on the bottom of the reactor vessel. The operators had several indications of unusual events. About eight minutes into the accident, a sump pump that removes overflow in the bottom of the containment building came on. Three minutes later the sump overflowed. About 20 minutes later, the operators turned off the sump pump, but had not yet reached the conclusion that water was escaping from the reactor. (Over 8,000 gallons of water had been pumped out of the sump by this time.) About 1 hour and 15 minutes into the accident, the four big pumps that push the water through the reactor began to vibrate, because they were now pushing a water- steam mixture rather than water. The operators shut down two pumps. About half an hour later they shut down the other two. At this stage, there was no water going into the reactor core. By 6: 00 a.m., radiation alarms showed that radioactive gas was in the containment building (U.S. Nuclear Regulatory Commission, 1980a, p. 11â15). The operators during this period, including the Superintendent of Technical Support, had been on duty for from one to two hours and had not diagnosed what was happening. A shift supervisor arrived two hours into the accident and within 20 minutes concluded that the pressurizer relief valve was stuck open. He closed a block valve, thus stopping the loss of water nearly 20 hours and 30 minutes into the accident. Twenty-five minutes later, a reactor coolant pump was started again (U.S. Nuclear Regulatory Commission, 1980b, Vol. 1, p. 19).
