Coolant flowed down from the core to the pressuriser, making the instruments return confusing information to the reactor operators. Because there was no instrument showing the amount of coolant in the core, the operators determined the level of water by the amount of water shown in the pressuriser. As the level indicated was high enough, they assumed that the core was covered with coolant and stopped adding water. They were unaware that, because of the stuck valve, the indicator was providing misleading readings.
Warning lights started to flash and alarms rang, as the operators failed to realise that the plant was suffering from loss of coolant. Due to unavailability of proper cooling, the nuclear fuel overheated and the long metal tubes holding the nuclear fuel pellets (zirconium cladding) burst, and the fuel pellets started melting. Little did they know that about one half of the core melted in the very beginning of the accident, which was found later that day. Even though the TMI plant suffered a severe core meltdown, it wasn’t as bad a situation as experts would predict for such a dangerous kind of nuclear accident. The expected scenario would be a breach of the walls of the containment building, releasing large quantities of radiation to the environment.
Federal and state authorities were concerned about the small releases of radioactive gases measured late morning on the 28th of March off-site. Even though unaware of the core meltdown, the authorities were rather anxious about the potential threat the reactor was to the population surrounding the plant. The Nuclear Regulatory Commission (NRC) dispatched a team of inspectors from the regional office to the site of the accident, later joined by inspectors from the Department of Energy and the Environmental Protection Agency. Helicopters were hired to determine the levels of radioactivity in the atmosphere above the plant, by noon that day.
In the evening of the 28th, order appeared to be re-established in the plant; the core seemed to be cooled properly, and the reactor looked stable. But new concerns would arise on the morning of March the 30th, when a release of radiation originating from the plant’s auxiliary building took place. This building was designed to relieve pressure on the primary system and prevent flow reduction of coolant to the core. Evacuation orders were given to those members of the population more vulnerable to radiation.
After a short time, a large bubble of hydrogen was identified in the dome of the pressure vessel, the container that holds the reactor core. The possibility that the hydrogen bubble might burn or explode was a big concern, as it would rupture the pressure vessel causing the core to fall into the containment building, which might result in a potential breach of containment. This incident spread anxiety among both authorities and population throughout the day of 31st of March. On Sunday, 1st of April, experts determined that the hydrogen bubble could not burn or explode thanks to the absence of oxygen in the pressure vessel. It was eventually removed by opening the vent valve on the reactor cooling system pressuriser periodically, putting an end to the crisis at the Three Mile Island nuclear generating station.
3. The aftermath: impact of the accident
The accident at Three Mile Island has been used throughout the years by engineers as an example of how groups of people react and make decisions under stress. It became of general knowledge that the accident was caused by bad decisions made because the operators were provided with misleading and sometimes incorrect information.
No injuries were ever identified due to radiation releases. Three Mile Island was, however, a serious economic and political disaster. Coincidently it occurred just a few days after the release of a film called “The China Syndrome”, which depicted a fictional nuclear accident.
As consequences of this accident several aspects of personnel training and plant design were improved. The equipment requirements and plant design were strengthened and upgraded, where major changes included the ability of plants to shut down automatically, fire protection, containment building isolation, among many other improvements. Before the accident operator training was mainly focused on the diagnosis of the problem at hand, whereas afterwards it became an important component how the trainee responds to an emergency.
A full investigation was ordered by the U.S. government regarding the TMI incident. The full report presented so many security lapses that most nuclear plants would have to be closed, in case it was released. The results on this report were smoothed down at request of the U.S. president Jimmy Carter.
Despite all of the amendments made to security protocols and plant design and equipment, the public confidence in nuclear energy declined in a large scale, especially in the country where the accident took place. Before TMI nearly 70% of the general public approved nuclear energy, decreasing to 50% afterwards.
4. Current status
The TMI-2 reactor is currently shut down and de-fueled. The coolant system was drained, the radioactive water decontaminated and evaporated and radioactive waste was taken to a proper disposal site. The fuel and core remains of the reactor were shipped off-site to a Department of Energy facility. The plant will be kept operational until the operating license of the TMI-1 expires, where both plants will be decommissioned.
5. References
U.S. Nuclear Regulatory Commission (1979) Fact sheet on the Accident at Three Mile Island.
World Nuclear Association (2001) Three Mile Island: 1979. Viewed on 14-11-04 at
Mark Stencel (1999) A Nuclear Nightmare in Pennsylvania, The Washington Post, March 1999.
Three Mile Island. Viewed on 14-11-04 at
Uranium Information Centre (2001) Three Mile Island. Viewed on 13-11-04 at
