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7.4: Nuclear Reactor Accidents

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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 Three Mile Island. The four larger structures in this picture are the cooling towers that allow excess heat to be transfered to the atmosphere.

     Figure \(\PageIndex{2}\): 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 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. 

    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 radioistopes were released to the areas surrounding TMI nuclear power station. Several independent research facilities have quantified the radiation that was released at TMI. These sfudies 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 \(\PageIndex{1}\): Diagnostic x-rays levels
    Type of X-ray Exam Diagnostic Reference level (DRL) in mSv
    Chest 0.96
    Abdomen 7.10
    C-spine 3.56
    T-sping 8.00
    L-Spline 12.0
    Skull 6.00
    Lumb 7.21
    Hip 7.21
    Source: Sonawane AU1, Shirva VK, Pradhan AS.,  2010 Feb;138(2):129-36. doi: 10.1093/rpd/ncp235.

    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 TMI2 started in August of 1979. This 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 1 unit. The approximate clean-up price of 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.

    Chernobyl (Pripyat, Ukraine, April 26, 1986)

    On April 26, 1986, a test was scheduled at the Chernobyl Nuclear Power Plant to test a method of keeping the reactors properly cooled in the event of a power grid failure. At the time of the accident the Chernobyl facility used four RBMK (reaktor bolshoy moshchnosty kanalny or high power channel) reactors to produce a total of 4000 MW of energy. Two more reactors were being constructed to produce additional wattage. RBMKs not only produce large amount of energy, but also produce weapons grade Pu-239. The excessive power, design, and materials of these reactors have discouraged many countries (the United States) from building them.


    Schematic of a RBMK reactor. Image taken from:

    When comparing the differences of RBMK reactors to those of typically used in most other countries, view the table below:

    Table \(\PageIndex{2}\): Types of Nuclear Reactor
    Type of Reactor Containment Fuel Coolant Moderator
    PWR repetitive layers of lead/ concrete and top is closed inside reactor building slightly enriched U-235 (2-4%) LW = light water LW = light water
    BWR  repetitive layers of lead/ concrete and top is closed inside reactor core building

    slightly enriched U-235 (2-4%l

    LW = light water LW = light water
    RBMK layers of lead/concrete and top is open inside reactor core building to allow extraction of rods while reactor is producing energy

    low enriched U-235


    LW = light water Graphite (carbon)

    Unlike PWR and BWRs, RBMK reactors use a graphite to slow the neutrons of the fission process. Also known as the element carbon, graphite can catch on fire quite easily. The lower purity level of U-235 of the RBMK reactor allows for the more fissionable Pu-239 to be produced. Unfortunately, the Chernobyl reactor technicians felt that removing the new Pu-239 fuel would be too difficult with a full containment covering of the unit 4. The new Pu-239 material would extracted while the reactor continued to produce energy. The high power wattage, combustible moderator, and open containment convinced many other countries to steer clear of building RBMKs.

    Meltdown occurred at 1:23 AM, starting a fire that dispersed large quantities of radioactive materials into the atmosphere. The amount of radioactive material released was 400 times more than the amount the atomic bombing of Hiroshima released. The fallout would be detected in almost all parts of Europe.

    Chernobyl's Unit 4 Nuclear Reactor after full meltdown. Image taken from:

    Before the accident

    Nuclear reactors require active cooling in order to remove the heat generated by radioactive decay. Even when not generating power, reactors still generate some heat, which must be removed in order to prevent damage to the reactor core. Cooling is usually accomplished through fluid flow, water in Chernobyl's case.

    The problem at the Chernobyl plant was that following an emergency shutdown of all power, diesel generators were needed to run the cooling pumps. These generators took about a minute to attain full speed, which was deemed an unacceptably long time for the reactor to be without cooling. It was suggested that the rotational momentum of the winding down steam turbine be used to power the pumps in the time between shutdown and the generators being ready. A test was devised to test this method in 1982, but the turbine did not prove to be successful in providing the required voltage as it spooled down. Two more tests would be conducted in the following years, but would also be unsuccessful. The fourth test was scheduled to be run on April 25, 1986.

    The experiment was devised in such a way that if it had gone as planned, the disruption and danger to the plant would be very minimal. First, the reactors would be brought down to low power, between 700 and 800 megawatts. Then the steam turbine would be run up to full speed and then turned off. The power generated by the winding down generators would then be measured to determine if it was sufficient to power the cooling pumps in the time before the diesel generators got up to full speed.

    By 1986, the plant had been running for two years without the implementation of a method to keep the cooling pumps running continuously following an emergency shutdown. This was an important safety measure that the plant was lacking, which presumably gave the plant managers a considerable amount of urgency in completing another test.

    The experiment

    Preparing for the experiment

    The experiment was scheduled to run during the day shift of 1985, while the night shift would only have to maintain cooling of the radioactive decay in the shut-down plant. However, another power generator nearby unexpectedly shut down, necessitating the need for the Chernobyl plant to delay the test and continue producing power. The experiment would be resumed at 11:04 PM, by which time the day shift had departed and the evening shift was about to leave. This meant that the experiment would be conducted in the middle of two shifts, leaving very little time for the night shift employees to be briefed about the experiment and told what to do.

    The power reduction of reactor 4 to 700 MW was accomplished at 00:05 AM on the 26th of April. However, the natural production of a neutrino absorber, Xenon-135, led to a further decrease in power. When the power dropped to about 500 MW, the night shift operator committed an error and inserted the reactor control rods too far. This caused the reactor to go into a near-shutdown state, dropping power output to around 30 MW.

    Since this was too low for the test, it was decided to restore power by extracting the control rods. Power would eventually rise and stabilize at around 200 MW.

    The operation of the reactor at such a low power level would lead to unstable temperature and flow. Numerous alarms and warnings were recorded regarding emergency measures taken to keep the reactor stable. In the time between 0:35 and 0:45 AM, alarm signals regarding thermal-hydraulic parameters were ignored in order to preserve the reactor's power level.

    The test continued, and at 1:05 AM extra water pumps were activated in order to increase the water flow. The increased coolant flow rate led to an increase of the coolant temperature in the core, reducing the safety margin. The extra water flow also led to a decrease in the core's temperature and increased the neutron absorption rate, decreasing the reactor's power output. Operators removed the manual control rods in order to maintain power.

    All these actions led to the reactor being in an unstable state that was clearly outside safe operation protocol. Almost all the control rods had been removed, which reduced the effectiveness of inserting safety rods in an emergency shutdown. The water was very close to boiling, which meant that any power increase would cause it to boil. If it started boiling, it would be less effective at absorbing neutrons, further increasing the reactor's power output.

    Conducting the experiment

    The experiment was started at 1:23:04 AM. The steam to the turbines was shut off, causing the turbines to start spooling down. Four of the eight cooling pumps were also shut down. The diesel generator was started and began powering the cooling pumps after at 1:23:43. Between this time, the four pumps were powered by the slowing steam turbines. As the turbines slowed down, their power output decreased, slowing the cooling pumps. This lead to increased formation of steam voids in the core, reducing the ability of the cooling water to absorb neutrons. This increased the power output of the reactor, which caused more water to boil into steam, further increasing the reactor's power. However, during this time the automatic control system was successful in limiting power increase through the insertion of control rods.

    At 1:23:40, a button was pressed that initiated the emergency shutdown of the reactor and the insertion of all control rods. It is believed that this was done as a routine method to shut down the reactor to conclude the experiment and not as an emergency measure.

    The process of inserting the control rods was initiated, but it took about 20 seconds for the rods to be completely inserted. A flawed design in the graphite-tip control rod meant that coolant was displaced before the neutron absorbing material could be fully inserted and slow down the reaction. This meant that the process of inserting the control rods actually increased the reaction rate in the lower half of the core.

    A massive power spike occurred, causing the core to overheat. Some of the fuel rods fractured, causing the control rods to become stuck before they were fully inserted. Within three seconds the core's power output rose to above 500 MW. According to simulation, it is estimated that power output then rose to 30 GW, ten times the normal power output. This was caused by the rising power output causing massive steam buildup, which destroyed fuel elements and ruptured their channels.

    It is not possible to know precisely what sequence of events led to the destruction of the reactor. It is believed that the steam buildup entered the reactor's inner structure and lifted the 2000 ton upper plate. This steam explosion further ruptured fuel channels, resulting in more coolant turning into steam and leaving the reactor core. This loss of coolant further increased the reactor's power. A nuclear excursion (an increasing nuclear chain reaction) caused a second, even more powerful explosion.

    The explosion destroyed the core and scattered its contents in the surrounding area, igniting the red-hot graphite blocks. Against safety regulations, a flammable material, bitumen, had been used in roof of the reactor. When this was ignited and scattered into the surrounding area, it started several fires on reactor 3. Those working there were not aware of the damage that had been done and continued running the reactor until it was shut down at 5:00 AM.

    Crisis Management

    Radiation Levels

    In the worst-hit parts of the reactor building, radiation levels were high enough to cause fatal doses in a matter of minutes. However, all dosimeters available to the workers did not have the ability to read radiation levels so high and thus read "off scale." Thus, the crew did not know exactly how much radiation they were being exposed to. It was assumed that radiation levels were much lower than they actually were, leading to the reactor crew chief to believe that the reactor was still intact. He and his crew would try to pump water into the reactor for several hours, causing most of them to receive fatal doses of radiation.


    Fire crew were called in to protect the remaining buildings from catching fire and to extinguish the still burning reactor 4. While some firefighters were not aware of the harmful doses of radiation they were receiving and had assumed it to be a simple electrical fire, others knew that they would probably receive fatal doses of radiation. However, their heroic efforts were necessary in order to try to contain the large amounts of radiation being released into the atmosphere. The fires in the surrounding buildings were extinguished by 5:00 AM, but it would take firefighters until May 10 before they could fully extinguish reactor 4.

    In order to prevent a steam explosion from occurring, volunteers were needed to swim through radioactive water and drain a pool of water under the reactor core. While they were successful, they would later succumb to the high doses of radiation that they had received. The worst of the radioactive debris was shoveled back into the reactor by crew wearing heavy protective gear. In total, 600,000 people worked in the cleanup, about 250,000 of which received their lifetimes' limit of radiation. It is estimated that over 10,000 eventually died from the radiation.

    By December, a concrete sarcophagus had been completed that sealed off the reactor. This was never meant to be a completely permanent solution, however, and is now in danger of collapsing. A collapse could cause a large amount of radioactive material to once again be released and spread around the world. This is why it is necessary that a new structure be constructed to contain reactor 4.

    Evacuation of Pripyat

    Pripyat, a city nearby the power plant, was not immediately evacuated. At first, the government denied that the reactor had exploded and insisted that it was only a small accident. By April 27, though, investigators were forced to acknowledge that the reactor had exploded and ordered Pripyat to be immediately evacuated.


    400 times more radiation was released by the disaster than had been by the atomic bombing of Hiroshima. The radiation would later be detected in almost all parts of Europe. Over one million people could have been adversely affected by the radiation. The radiation would cause numerous problems, including Down's Syndrome, chromosomal aberrations, mutations, leukemia, thyroid cancer, and birth defects.

    The radiation would affect all parts of the environment surrounding the plant, killing plants and animals and infecting the soil and groundwater. Life has returned to the area and seems to be flourishing, possibly due to the lack of human intrusion. Remarkably, numerous species have been reported to have adapted to their environment and have developed increased tolerance of radiation, making it possible for them to live with the radiation that is still prevalent in the soil and plants around the plant. It has even been reported that radiotrophic fungi have been growing on the walls of reactor 4.

    Today, radiation levels are still higher than normal in the areas surrounding the plant, but have dropped considerably from the levels that they were at twenty years ago. It is now considered safe to visit the areas immediately surrounding the plant for short periods of time. However, it is estimated that it will take 20,000 years for reactor 4's core to be completely safe.

    Another major nuclear accident involving a reactor occurred in April 1986, at the Chernobyl Nuclear Power Plant in Ukraine, which was still a part of the former Soviet Union. While operating at low power during an unauthorized experiment with some of its safety devices shut off, one of the reactors at the plant became unstable. Its chain reaction became uncontrollable and increased to a level far beyond what the reactor was designed for. The steam pressure in the reactor rose to between 100 and 500 times the full power pressure and ruptured the reactor. Because the reactor was not enclosed in a containment building, a large amount of radioactive material spewed out, and additional fission products were released, as the graphite (carbon) moderator of the core ignited and burned. The fire was controlled, but over 200 plant workers and firefighters developed acute radiation sickness and at least 32 soon died from the effects of the radiation. It is predicted that about 4000 more deaths will occur among emergency workers and former Chernobyl residents from radiation-induced cancer and leukemia. The reactor has since been encapsulated in steel and concrete, a now-decaying structure known as the sarcophagus. Almost 30 years later, significant radiation problems still persist in the area, and Chernobyl largely remains a wasteland.


    In 2011, the Fukushima Daiichi Nuclear Power Plant in Japan was badly damaged by a 9.0-magnitude earthquake and resulting tsunami. Three reactors up and running at the time were shut down automatically, and emergency generators came online to power electronics and coolant systems. However, the tsunami quickly flooded the emergency generators and cut power to the pumps that circulated coolant water through the reactors. High-temperature steam in the reactors reacted with zirconium alloy to produce hydrogen gas. The gas escaped into the containment building, and the mixture of hydrogen and air exploded. Radioactive material was released from the containment vessels as the result of deliberate venting to reduce the hydrogen pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events.

    An evacuation zone around the damaged plant extended over 12.4 miles away, and an estimated 200,000 people were evacuated from the area. All 48 of Japan’s nuclear power plants were subsequently shut down, remaining shuttered as of December 2014. Since the disaster, public opinion has shifted from largely favoring to largely opposing increasing the use of nuclear power plants, and a restart of Japan’s atomic energy program is still stalled (Figure \(\PageIndex{4}\)).

    A photo and a map, labeled “a” and “b,” respectively, are shown. Photo a shows a man in a body-covering safety suit working near a series of blue, plastic coated containers. Map b shows a section of land with the ocean on each side. Near the upper right side of the land is a small red dot, labeled “greater than, 12.5, m R backslash, h r,” that is surrounded by a zone of orange that extends in the upper left direction labeled “2.17, dash, 12.5, m R backslash, h r.” The orange is surrounded by an outline of yellow labeled “1.19, dash, 2.17, m R backslash, h r” and a wider outline of green labeled “0.25, dash, 1.19, m R backslash, h r.” A large area of light blue, labeled “0.03, dash, 0.25, m R backslash, h r” surrounds the green area and extends to the lower middle of the map. A large section of the lower middle and left of the land is covered by dark blue, labeled “less than 0.03, m R backslash, h r.”

    Figure \(\PageIndex{4}\): (a) After the accident, contaminated waste had to be removed, and (b) an evacuation zone was set up around the plant in areas that received heavy doses of radioactive fallout. (credit a: modification of work by “Live Action Hero”/Flickr).

    The energy produced by a reactor fueled with enriched uranium results from the fission of uranium as well as from the fission of plutonium produced as the reactor operates. As discussed previously, the plutonium forms from the combination of neutrons and the uranium in the fuel. In any nuclear reactor, only about 0.1% of the mass of the fuel is converted into energy. The other 99.9% remains in the fuel rods as fission products and unused fuel. All of the fission products absorb neutrons, and after a period of several months to a few years, depending on the reactor, the fission products must be removed by changing the fuel rods. Otherwise, the concentration of these fission products would increase and absorb more neutrons until the reactor could no longer operate.

    Spent fuel rods contain a variety of products, consisting of unstable nuclei ranging in atomic number from 25 to 60, some transuranium elements, including plutonium and americium, and unreacted uranium isotopes. The unstable nuclei and the transuranium isotopes give the spent fuel a dangerously high level of radioactivity. The long-lived isotopes require thousands of years to decay to a safe level. The ultimate fate of the nuclear reactor as a significant source of energy in the United States probably rests on whether or not a politically and scientifically satisfactory technique for processing and storing the components of spent fuel rods can be developed.


    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)