The Three Mile Island Nuclear Power Plant accident is one of the largest accidents in the history of nuclear energy that occurred on March 28, 1979, at the Three Mile Island Nuclear Power Plant located on the Susquehanna River near Harrisburg (Pennsylvania, USA).
The accident occurred at the second power unit of the NPP due to the late detection of a leakage of coolant in the first reactor circuit and, accordingly, the loss of nuclear fuel cooling. The accident resulted in a meltdown of about 50% of the reactor core, after which the power unit was never restored. The NPP premises were heavily contaminated, but the radiation consequences for the public and the environment were insignificant. The accident was assigned level 5 on the INES scale.
The accident exacerbated the existing crisis in the US nuclear power industry and caused a surge in anti-nuclear sentiment in society. Although all this did not lead to an immediate cessation of the growth of the US nuclear power industry, its historical development was halted. From 1979 to 2012, no new nuclear power plant construction licenses were issued, and the commissioning of the 71st previously planned plant was canceled.
Prior to the Chernobyl accident that occurred seven years later, the Three Mile Island accident was considered the largest in the history of global nuclear power and is still considered the worst nuclear accident in the United States, which severely damaged the reactor core and melted some of the nuclear fuel.
Consequences
Although the nuclear fuel partially melted, it did not burn through the reactor vessel. Therefore, the radioactive substances mostly remained inside. According to various estimates, the radioactivity of noble gases released into the atmosphere ranged from 2.5 to 13 million Ci (480.1015 Bq). However, the release of hazardous nuclides, such as 131I, was insignificant. The territory of the station was also contaminated by radioactive water leaking from the first circuit. It was decided that there was no need to evacuate the population living near the plant, but the governor of Pennsylvania advised that pregnant women and preschool children leave the eight-kilometer zone around the plant. The average equivalent radiation dose for people living in the 16-kilometer zone was 8 millirems (80 μSv) and did not exceed 100 millirems (1 mSv) for any of the residents. For comparison, 8 millirems is approximately the same as the dose received during a fluoroscopy, and 100 millirems is ⅓ of the average dose received by a US resident in a year due to background radiation.
A thorough investigation of the accident was conducted. It was recognized that the operators made a number of mistakes that seriously aggravated the situation. These mistakes were caused by the fact that the operators were overloaded with information, some of which was not relevant to the situation, and some of which was simply incorrect. After the accident, changes were made to the operator training system. While before the accident the main focus was on the operator’s ability to analyze the situation and determine what caused the problem, after the accident the training was focused on the operator’s ability to follow predefined technological procedures. Control panels and other plant equipment were also improved. All U.S. NPPs have developed emergency action plans that provide for rapid notification of residents within a 10-mile zone.
Work to eliminate the consequences of the accident began in August 1979 and was officially completed in December 1993. They cost $975 million. The territory of the NPP was decontaminated and the fuel was unloaded from the reactor. However, part of the radioactive water was absorbed into the concrete of the containment, and this radioactivity is almost impossible to remove.
The operation of another reactor (TMI-1) was resumed in 1985.