Three Mile Island Unit 2 nuclear reactor accident

ABSTRACT The analysis of events during the accident at Three Mile Island concluded that the accident was due to confused control room operators with inadequate instrumentation and inaccurate procedures. Therefore, the United States Nuclear Regulatory Commission (USNRC) called for improved nuclear reactor operator training and Emergency Operating Procedures (EOPs). 

The development of nuclear power was in full swing in the 1970s when the accident occurred at the Three Mile Island Unit 2 nuclear power plant near Harrisburg, Pennsylvania, in 1979. The reactor was a PWR supplied by Babcock Wilcox Corporation. As a result of this accident, reactor construction came to a standstill as the cause of the accident was analyzed, and the design of reactors under construction was modified to meet new licensing requirements. Costs increased dramatically and many orders for reactors were canceled. The impact of this accident was felt primarily in the United States. 

The human health consequences of the accident at tire Three Mile Island Unit 2 (TMI-2) nuclear reactor in Pennsylvania in 1979 were minimal. The small radioactive releases at Three Mile Island have had no detectable health effects on plant workers or the public, and a recent study determined that the actual release had negligible effects on the physical health of individuals or the environment (World Nuclear Association report, January 2012). 

Nuclear power has achieved an excellent safety record. Exceptions are the accidents at Three Mile Island in 1979 and at Chernobyl in 1986. In the United States, safety can be attributed in part to the strict regulation provided by the Nuclear Regulatory Commission, which reviews proposed reactor designs, processes appHcations forUcenses to constmct and operate plants, and provides surveillance of all safety-related activities of a utiUty. The utiUties seek continued improvement in capabiUty, use procedures extensively, and analy2e any plant incidents for their root causes. Similar programs intended to ensure reactor safety are in place in other countries. 

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

There have been two major accidents (Three Mile Island in the United States and Chernobyl in the former Soviet Union) in which control was lost in nuclear power plants, with subsequent rapid increases in fission rates that resulted in steam explosions and releases of radioactivity. The protective shield of reinforced concrete, which surrounded the Three Mile Island Reactor, prevented release of any radioactivity into the environment. In the Russian accident there had been no containment shield, and, when the steam explosion occurred, fission products plus uranium were released to the environment—in the immediate vicinity and then carried over the Northern Hemisphere, in particular over large areas of Eastern Europe. Much was learned from these accidents and the new generations of reactors are being built to be passive safe. In such passive reactors, when the power level increases toward an unsafe level, the reactor turns off automatically to prevent the high-energy release that would cause the explosive release of radioactivity. Such a design is assumed to remove a major factor of safety concern in reactor operation, see also Bohr, Niels Fermi, Enrico AIan-HATTAN Project Plutonium Radioactivity Uranium. 

As a result of the analysis of events during the accident at the Three Mile Island nuclear plant (TMI, March 1979), the importance of human error in nuclear plants was better understood. The accident resulted from the confusion of the control room operators with inadequate instrumentation and inaccurate procedures. The most important factor was that they had to act in spite of the weaknesses in the training to respond to unexpected events. Therefore, the United States Nuclear Regulatory Commission (USNRC) called for the improvement of Emergency Operating Procedures (EOPs) and in nuclear reactor operator training. The philosophy of incident response implemented in the improved procedures was to take a symptom-based approach (operators foUow a series of yes - no questions to ensure that the reactor core remains covered and only then determine what was the cause of the problem) (USNRC, 2009). 

Although nuclear power reactors really do have a good safety record, the distrust and fear associated with radiation make most people sensitive to safety issues and accidents. The most serious accident to occur in the United States happened in 1979 at the Three Mile Island Plant in Pennsylvania. A combination of operator error and equipment failure caused a loss of reactor core coolant. The loss of coolant led to a partial meltdown and the release of a small amount of radioactive gas. There was no loss of life or injury to plant personnel or the general population. 

The deterministic approach to the design of nuclear reactors was rapidly supplemented by the development of probabilistic studies, referred to as PSAs and also as PRA. The first study of this kind carried out in the United States was published in 1975 (Rasmussen report—USNRC 1975) and provided the first assessment of the potential risk of core damage for two power reactors. The accident in 1979 at the Three Mile Island plant generated renewed interest in this type of study. One of the recommendations made after the accident was that probabilistic analysis techniques should be used to supplement conventional safety assessment procedures for NPPs, and that probabilistic objectives should be developed in order to facilitate the determination of acceptable safety levels for nuclear facilities. 

Many see the commercial nuclear power station as a hazard to human life and the environment. Part of this is related to the atomic-weapon heritage of the nuclear reactor, and part is related to the reactor accidents that occurred at the Three Mile Island nuclear power station near Harrisburg, Pennsylvania, in 1979, and Chernobyl nuclear power station near Kiev in the Ukraine in 1986. The accident at Chernobyl involved Unit-4, a reactor that was a light water cooled, graphite moderated reactor built without a containment vessel. The accident resulted in 56 deaths that have been directly attributed to it, and the potential for increased cancer deaths from those exposed to the radioactive plume that emanated from the reactor site at the time of the accident. Since the accident, the remaining three reactors at the station have been shut down, the last one in 2000. The accident at Three Mile Island... 

A clear example is the commercial nuclear power industry in the United States. In the 1960s and early 1970s, nuclear power looked like a very promising career and an incredible growth industry. Nuclear power was to be our cheap form of energy. But the industry was stymied by the American public s perception that nuclear power is an unwarranted risk. And the 1979 Three Mile Island nuclear accident sealed its fate. Now commercial nuclear power in the United States is a relatively small industry in comparison to other forms of generating power. And yet, in France, 40% of the electric power is generated by nuclear reactors. 

Nuclear accidents at Three Mile Island, Pennsylvania, in 1979 and Chernobyl, Ukraine, in 1986 had a devastating effect on public opinion in the United States and, to a smaller degree, elsewhere in the world. At Three Mile Island only about 50 curies of radiation were released to the environment and there were no casualties. The explosion at Chernobyl was a very different story. About 100 million curies were released, leading to at least 31 fatalities. Moreover, 135,000 people were permanently evacuated from the region surrounding the reactor. Since then, all the other reactors at Chernobyl—three, in addition to the one that exploded—have been permanently shut down. 

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