In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system Of the reactor because the operator mistakenly believed that there was too much coolant water present in the reactor and causing the steam pressure release. The scope and complexity of the accident became clear over the course of five days, as employees of Met Deed, Pennsylvania state officials, and members of the LIST.
Nuclear Regulatory Commission (NRC) tried to understand the problem, communicate the situation to the press and local community, decide whether the accident required an emergency evacuation, and ultimately ND the crisis, The NRC authorization of the release of 40,000 gallons (about 150,000 liters) to radioactive waste water directly in the Susquehanna River led to a loss of credibility with the press and community, Critical human factors and user interface engineering problems were revealed in the investigation of the reactor control system’s user interface.
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Despite the valve being stuck open, a light on the control panel indicated that the valve was closed. In fact the light did not indicate the position of the valve, only the status of the solenoid, thus fining false evidence of a closed valve. As a result, the operators did not correctly diagnose the problem for several hours. The design of the POOR indicator light was fundamentally flawed. The bulb was simply connected in parallel with the valve solenoid, thus implying that the POOR was shut when it went dark, Without actually verifying the real position of the valve.
When everything was operating correctly, the indication was true and the operators became habituated to rely on it. However, when things went wrong and the main relief valve stuck open, he unlighted lamp was actually misleading the operators by implying that the valve was shut. This caused the operators considerable confusion, because the pressure, temperature and coolant levels in the primary circuit, so far as they could observe them via their instruments, were not behaving as they would have done if the POOR was shut as they were convinced it was.
This confusion contributed to the severity of the accident because the operators were unable to break out of a cycle of assumptions that conflicted with what their instruments were telling them. It was not until a fresh shift came in Who did not have the mind-set of the first shift of operators that the problem was correctly diagnosed. By this time, major damage had occurred. The operators had not been trained to understand the ambiguous nature of the POOR indicator and to kick for alternative confirmation that the main relief valve was closed.
There was a temperature indicator downstream of the POOR in the tail pipe between the POOR and the pressurize that could have told them the valve was stuck open, by showing that the temperature in the tail pipe remained higher than t should have been had the POOR been shut. This temperature indicator, however, was not part of the “safety grade” suite of indicators designed to be used after an incident, and the operators had not been trained to use it. Its location on the back of the desk also meant that it was effectively out of sight of the operators.
The Kenny Commission noted that Babcock and Willow’s POOR valve had previously failed on II occasions, nine of them in the open position, allowing coolant to escape. More disturbing however, was the fact that the initial causal sequence of events at TMI had been duplicated 18 months earlier at another Babcock and Wilcox reactor, the Davis-Bess Nuclear Power Station owned at that time by Toledo Edison, The only difference was that the operators at Davis-Bess identified the valve failure after 20 minutes, where at TMI it took 80 minutes; and the Davis-Bess facility was operating at 9% power, against Tm’s 97%.
Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue. In the aftermath of the accident, investigations focused on the amount of radiation released by the accident. In total approximately 2. Million curies Of radioactive gases, and approximately 15 curies of iodine-131 was released into the environment. According to the American Nuclear Society, using the Official radiation emission figures, “The average radiation dose to people living within ten miles of the plant was eight milliner, and no more than 100 milliner to any single individual.
Eight milliner is about equal to a chest X-ray, and 100 milliner is about a third of the average background level of radiation received by LIST residents in a year. ” Based on these emission figures, early scientific publications, according to Mango, on the health effects of the fallout estimated one or two additional cancer deaths in the 10 mi (16 km) area around TMI. Disease rates in areas further than 10 miles from the plant were never examined Local activism in the 19805, based on anecdotal reports of negative health effects, led to scientific studies being commissioned.
A variety to epidemiology studies have concluded that the accident has had no observable long term health effects. Proper concluded that the failure at Three Mile Island was a consequence of the system’s immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer vatu he termed a ‘normal accident’.