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7.7: Three Mile Island Nuclear Accident

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  • The importance of cooling and containment are amply illustrated by three major accidents that occurred with the nuclear reactors at nuclear power generating stations in the United States (Three Mile Island), the former Soviet Union (Chernobyl), and Japan (Fukushima). Other nuclear incidents have occurred around the world that have involved the storage or transportation of nuclear materials.

    Three Mile Island (Harrisburg, Pennsylvania, March 28, 1979)

    The two reactors housed at this facility are located in Middletown, Pennsylvania. This site is approximately 10 miles southeast of Harrisburg, Pennsylvania. Both units are PWRs (pressurized water reactors) are were commissioned in the mid to late 1970's. Before the accident, each of the Three Mile Island (TMI) reactors generated around 800 MW of electrical power. The smaller cylindrical buildings in the photograph below are the unit 1 and 2 reactors at this facility. The four larger structures in this picture are the cooling towers that allow excess heat to be transferred to the atmosphere.

    Figure \(\PageIndex{1}\): Three Mile Island Nuclear Facility. Image taken from:

    On March 28, 1979, Three Mile Island's Unit Two reactor lost cooling water in the secondary loop (note the arrow pointing down in the schematic below). This event occurred around 4:00 a.m. and alarms sounded in the control room immediately. Within one minute, the reactor automatically shutdown. Meanwhile, water in the primary loop (yellow tube) overheated and the pressure of the reactor increased. To relief this excess pressure inside the reactor, the pressure relief valve (top yellow/red tube) opened up. Backup pumps restarted the water in the secondary loop to assist cooling efforts.

    Figure \(\PageIndex{2}\): The following animated diagram graphically depicts the sequence of events associated with the accident at TMI-2. Image used with permission (United State Nuclear Regulatory Commission; Public Domain).

    In the control room, operators noted the decrease in pressure of the reactor and a reduction of cooling water in the primary loop. Within fifteen minutes of the original malfunction, approximately 3000 gallons of cooling water had escaped the reactor but remained in the containment building. The reactor components (fuel and control rods) became exposed with lack of water. These materials were melted to various degrees. Back in the control room, instrumentation noted that the pressure valve remained closed. Operators attempted to cool down the control and fuel rods by pumping more water into the primary loop of the reactor. Unfortunately, the pressure value was still open and most of the water converted to steam. This made temperatures increase greatly in the reactor core.

    Figure \(\PageIndex{3}\): Image taken from:

    By 6:22 a.m., the operators closed a valve before the pressure release valve. This allowed water to remain in the primary loop. Unfortunately, most of the water in the core was in the form of steam and did not circulate efficiently throughout the core. In addition, the steam reacted with metals in the control and fuel rods. These reactions produced combustible hydrogen gas which rose to the top of the reactor core. By 7:50 a.m., all cooling water was restored to the core successively, but a large hydrogen bubble hovered over the top of the reactor. At this time, operators were fearful the bubble might ignite and then explode the core to the outside environment. To prevent this explosion from happening, operators and engineers vented the hydrogen gas outside the nuclear reactor into the atmosphere. This venting occurred from March 30 until April 1, 1978.

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    Residents in nearby communities were informed of a nuclear issue around 7:24 a.m. on March 28, 1979. Radio and television broadcasts started within the next hour. By 12:45 p.m., representatives of the Department of Energy (DOE) arrived to evaluate the current status of the reactor. DOE agents put together a plan of action to keep the reactor under control. Numerous state governmental officials met with the DOE and devised safety protocols (remain indoors) and emergency evacuation plans if the hydrogen bubble were to burst. To see a comprehensive timeline of these events, please click here.

    In the venting process, minimal amount of radioisotopes were released to the areas surrounding TMI nuclear power station. Several independent research facilities have quantified the radiation that was released during the accident These studies state that residents within 10 miles of TMI were exposed to radiation within the range of 0.08 mSv to 1 mSv. Referring back to Section 5.8, this level of radiation is comparable to having a chest x-ray exam (Table \(\PageIndex{2}\)).

    Table \(\PageIndex{1}\): Fission products vented to the atmosphere from TMI 2
    Isotope Emits Half-life (t1/2)
    Kr-85 beta particles, gamma rays 10.8 years
    Cs-137 beta particles, gamma rays 30.2 years
    I-131 beta particles, gamma rays 8.0 days
    Xe-133 beta particles, gamma rays 5.3 days

    In 1984, nuclear scientists were able to survey the destruction of the reactor core. They concluded that approximately 1/3 of the fuel rods melted inside the reactor. This accident was labeled a partial meltdown (full meltdown would indicate the majority of fuel rods were melted). The contaminant structure remained in tact and protected the citizens of the area.

    The clean-up process for TMI unit 2 started in August of 1979. It was a twelve year project that employed over 1000 people. All radioactive fuel and water were shipped to existing nuclear waste storage facilities in the United States (Washington State and Idaho). The containment building for this reactor still remains and houses waste for the TMI unit 1. The approximate clean-up price for this accident was 973 million dollars.

    The accident at Three Mile Island halted all construction of new nuclear power plants in the United States. Many citizens became fearful of nuclear power in general and became supportive of other types of energy sources. Films like the China Syndrome catalyzed this feeling for decades after this catastrophe.

    At the time of the partial meltdown of TMI Unit 2, reactor 1 was shutdown for refueling. Despite much opposition, Unit was restarted in the fall of 1985 has been providing power to the residents of this area since then. The NRC has extended the license of this reactor through the spring of 2034.


    1. Harwood, William, Herring, Geoffrey, Madura, Jeffry, and Petrucci, Ralph, General Chemistry: Principles and Modern Applications, Ninth Edition, Upper Saddle River,New Jersey, Pearson Prentice Hall, 2007.
    2. Larabee, Ann. Decade of Disaster. Chicago: Board of Trustees of the University of Chicago, 2000.


    • Abheetinder Brar (UCD)