The Three Mile Island accident was the most significant accident in the history of the American commercial nuclear power generating industry. It resulted in the release of a significant amount of radioactivity, an estimated 43,000 curies of radioactive krypton (1.59 PBq), but under 20 curies (740 GBq) of the particularly hazardous iodine-131, to the environment.[2] However, there are no deaths or injuries to plant workers or members of the nearby community which can be attributed to the accident.[3][4] Public reaction to the event was probably influenced by at least three factors: first, the release (a few weeks before the accident) of a popular movie called The China Syndrome, concerning an accident at a nuclear reactor; secondly, what was felt to be a lack of official information in the initial phases of the accident; and lastly, many of the statements made by political and social activists long opposed to nuclear power. Whatever the sources of the local fear and outrage, public reaction to the event is judged by some epidemiologists to have induced stresses in the local population that could have caused adverse health effects.
The accident began on Wednesday, March 28, 1979, and ultimately resulted in a partial core meltdown in Unit 2 of the nuclear power plant (a pressurized water reactor manufactured by Babcock & Wilcox) of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg.
Jack Herbein, Metropolitan Edison's then Vice President for Power Generation initially and erroneously called the accident "a normal aberration."The scope and complexity of this reactor accident became clear over the course of five days, as a number of agencies at the local, state and federal levels tried to solve the problem and decide whether the on-going accident required a full emergency evacuation of the local community, if not the entire area to the west/southwest. In the end, the reactor was brought under control, although full details of the accident were not discovered until much later.
Although 25,000 people lived within five miles (8 km) of the site at the time of the accident, no identifiable injuries due to radiation occurred, and a government report concluded that "There will either be no case of cancer or the number of cases will be so small that it will never be possible to detect them. The same conclusion applies to the other possible health effects."
The accident was followed by essentially a 100% cessation of nuclear construction in the US. The impact of news stories about the accident was no doubt a factor, but other factors were the availability of cheap natural gas, a transition away from manufacturing and toward importation of consumer products, and federal policies that tolerated air pollution in the interest of keeping coal-fired electricity cheap.
Accident description
The accident occurred in the TMI-2 reactor. Although the plant had two reactors, TMI-1 was shut down for refueling at the time of the accident. The accident began when the plant's main feedwater pumps in the secondary non-nuclear cooling system failed at exactly 4:00 a.m. EST on March 28, 1979. Remarkably, this was, to-the-minute, the first anniversary of the unit's startup. The exact cause of the failure has never been determined, although it was speculated that water entered a pneumatic air line that controlled the flow through a filter known as a condensate polisher. In fact, the NRC stated that it did not need to know how the accident started since it could have been prevented at many points along the way. Because water was no longer flowing through the secondary loop, the steam generators no longer removed heat from the reactor. First the turbine, then the nuclear reactor, automatically shut down. Due to the loss of heat removal from the primary loop (the dry steam generators), the primary side pressure began to increase, which is normal and expected during shut down.
To prevent primary side pressure from becoming excessive, the pilot-operated pressurizer relief valve (PORV), at the top of the pressurizer, opened automatically. The valve should have closed again when the excess pressure had been released, but it did not do so. The indication to the plant's operators that the signal to close the valve had been sent was, in the absence of any indication to the contrary, taken by them to mean that the valve had closed. (A "positive feedback" lamp in the control room indicating the true position of the valve was eliminated in original construction to save time, but has been retrofitted onto all similar plants after the accident.) As a result of this design error, the PORV remained open, unnoticed by the operators for several hours, and caused the pressure to continue to decrease in the system. Cooling water poured out of the stuck-open valve at the top of the pressurizer and caused the core of the reactor to overheat.
There was no dedicated instrument to measure the level of water in the core. Instead, the operators judged the level of water in the core by the level in the pressurizer (which is mounted higher than the reactor in the closed pressure loop, so intuitively, if there is water in the pressurizer then there is water in the core), and since it was high, they assumed that the core was properly covered with coolant.[3] Due to the stuck-open PORV, the water inside the pressurizer was turbulent causing a false water level indication. Control room operators did not initially recognize the accident as a Loss of coolant accident (LOCA) since they had only ambiguous reactor water level indication and failed to properly interpret other indications of a LOCA.
Meanwhile, another problem appeared elsewhere in the plant with the emergency feedwater system, which is the backup for the main feedwater system. Following the loss of the main feed pumps, three emergency feedwater pumps started automatically, but two valves on the emergency feedwater lines were closed, preventing the feedwater from reaching the steam generators. The emergency feedwater system had been tested 42 hours prior to the accident; and, as part of the test, these valves were closed. They should have been reopened at the end of the test, but on this occasion it appeared that the valves were not reopened, through either an administrative or human error. The President's Commission requested that the FBI investigate whether these valves were closed by an act of sabotage.The valves were discovered closed about eight minutes into the accident. Once they were reopened, emergency feedwater was restored to the steam generators.
This lack of emergency feed water for eight minutes did not have a significant effect on the outcome of the accident, but did add to the confusion faced by the operators.The restoration of feedwater did not return cooling capability, however, as steam voids (areas where there is no water present) had formed in the primary loop and prevented heat transfer from the reactor to the secondary loop via the steam generator. This problem hampered efforts to cool the reactor for days.
As the pressure in the primary system continued to decrease, voids began to form in portions of the system other than the pressurizer, most notably in the reactor vessel. Because of these voids, the water in the system was redistributed and the pressurizer water level rose while overall system water inventory decreased. Thus, the pressurizer level indicator, which tells the operator the amount of coolant capable of heat removal, incorrectly indicated that system water level was rising. This caused the operators to stop adding water by turning off the emergency core cooling pumps, which had automatically started after the initial pressure decrease, due to fears the system was being overfilled. They were unaware that, because of the voids forming in the reactor vessel, the indicator could, and in this instance did, provide false readings. This erroneous indication blinded operators to the fact that water level was dropping and the reactor core was being uncovered.
With the PORV still open, the quench tank that collected the discharge from the PORV overfilled, causing the containment building sump to fill and sound an alarm at 4:11 a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing LOCA, but these indications were initially ignored by operators. At 4:15 a.m., the quench tank relief diaphragm ruptured, and radioactive coolant began to leak out into the general containment building. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the sump pumps were stopped at 4:39 a.m.
