Three Mile Island accident

Three Mile Island accident
Carter TMI-2.jpg
President Jimmy Carter touring the TMI-2 control room on April 1, 1979, with NRR Director Harold Denton, Governor of Pennsylvania Dick Thornburgh and James Floyd, supervisor of TMI-2 operations
DateMarch 28, 1979
(39 years ago)
Time04:00 (Eastern Time Zone UTC-5)
LocationThree Mile Island, Dauphin County, Pennsylvania
DesignatedMarch 25, 1999[1]

The Three Mile Island accident occurred on March 28, 1979, in reactor number 2 of Three Mile Island Nuclear Generating Station (TMI-2) in Dauphin County, Pennsylvania, near Harrisburg. It was the most significant accident in U.S. commercial nuclear power plant history.[2] The incident was rated a five on the seven-point International Nuclear Event Scale: Accident with wider consequences.[3][4]

The accident began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve in the primary system, which allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors, such as human-computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. 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.[5]

The accident crystallized anti-nuclear safety concerns among activists and the general public, resulted in new regulations for the nuclear industry, and has been cited as a contributor to the decline of a new reactor construction program that was already underway in the 1970s.[6] The partial meltdown resulted in the release of radioactive gases and radioactive iodine into the environment. Worries were expressed by anti-nuclear movement activists;[7] however, epidemiological studies analyzing the rate of cancer in and around the area since the accident, determined there was a small statistically non-significant increase in the rate and thus no causal connection linking the accident with these cancers has been substantiated.[8][9][10][11][12][13] Cleanup started in August 1979, and officially ended in December 1993, with a total cleanup cost of about $1 billion.[14]


Stuck valve

Simplified schematic diagram of the TMI-2 plant[15]

In the nighttime hours preceding the incident, the TMI-2 reactor was running at 97% of power, while the companion TMI-1 reactor was shut down for refueling.[16] The main chain of events leading to the partial core meltdown began at 4:37 am EST on March 28, 1979, in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor (PWR).

The initial cause of the accident happened eleven hours earlier, during an attempt by operators to fix a blockage in one of the eight condensate polishers, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and impurities in the water from accumulating in the steam generators and increasing corrosion rates in the secondary side.

Blockages are common with these resin filters and are usually fixed easily, but in this case the usual method of forcing the stuck resin out with compressed air did not succeed. The operators decided to blow the compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument air line. This would eventually cause the feedwater pumps, condensate booster pumps, and condensate pumps to turn off around 4:00 am, which would in turn cause a turbine trip.[17]

With the steam generators no longer receiving feedwater, heat and pressure increased in the reactor coolant system, causing the reactor to perform an emergency shutdown (SCRAM). Within eight seconds, control rods were inserted into the core to halt the nuclear chain reaction. The reactor continued to generate decay heat and, because steam was no longer being used by the turbine, heat was no longer being removed from the reactor's primary water loop.[18]

Once the secondary feedwater pumps stopped, three auxiliary pumps activated automatically. However, because the valves had been closed for routine maintenance, the system was unable to pump any water. The closure of these valves was a violation of a key Nuclear Regulatory Commission (NRC) rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure.[19]

The loss of heat removal from the primary loop and the failure of the auxiliary system to activate caused the primary loop pressure to increase, triggering the pilot-operated relief valve at the top of the pressurizer – a pressure active-regulator tank – to open automatically. The relief valve should have closed when the excess pressure had been released, and electric power to the solenoid of the pilot was automatically cut, but the relief valve stuck open because of a mechanical fault. The open valve permitted coolant water to escape from the primary system, and was the principal mechanical cause of the primary coolant system depressurization and partial core disintegration that followed.[20]

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Bahasa Indonesia: Musibah Pulau Three Mile