The Future of Nuclear Power, Part II: Fission Power Plant Accidents

August 1, 2022

IV.     Fission Power Plant Accidents

In this section, we review the circumstances that led to three of the most serious historical accidents recorded at operating fission power plants. The details of these accidents illuminate some of the safety concerns that remain about fission-fueled power generation.

Three Mile Island:

The Reactor Complex and the Accident:

Three Mile Island (TMI) was a nuclear power plant located by the Susquehanna River south of Harrisburg, Pennsylvania.  The plant had two separate units, which were operated by different companies.  Figure IV.1 shows a photo of the Three Mile Island complex as of 1979. 

Figure IV.1: The Three Mile Island Nuclear Generating Station as of 1979.  The two large cooling towers are seen in the background, and the Susquehanna River is on the right. 

Figure IV.2 shows the schematic diagram of the nuclear reactors at Three Mile Island.  The containment building houses the reactor, with its fuel bundles and control rods.  The coolant for the reactor is water.  The heated pressurized water is pumped into steam generators, where it transfers heat to a secondary water reservoir that produces steam.  The steam then leaves the containment building, where it spins a turbine that in turn powers a generator.  The used steam is then sent through a condenser where some of the resulting water is returned to the reactor. 

Figure IV.2: Schematic diagram of the nuclear reactor at Three Mile Island.  The fuel is contained in the reactor vessel in the center of the diagram.  Control rods are shown at the top of the reactor vessel.  Water is heated by the fission reactions, and the superheated water is sent to a steam generator.  The resulting steam leaves the containment building and spins a turbine that powers an electric generator.  Steam from the turbine is sent through a condenser; some of the resulting water is sent back to the reactor vessel, while the remainder is sent to a cooling tower.  

On the evening of Mar. 28, 1979, plant operators were attempting to remove a blockage of resin in a set of filters that cleaned the water in the secondary loop (as shown schematically in Fig. IV.2).  In this process, some water got into an instrument air line.  This caused a set of pumps to turn off around 4 am on March 29.  Turning off the pumps meant that feedwater was no longer flowing into the steam generators, which in turn meant that the reactor coolant system (RCS) became much hotter.  The reactor coolant at Three Mile Island was pressurized water.  Eventually the increased heat and pressure in the reactor caused an automated reactor safety procedure to be activated. The control rods dropped all the way down into the reactor and halted the fission process.  When the RCS pressure reached a certain point, a pilot-operated relief valve (PORV) automatically opened up.  This allowed steam to be routed to a drain tank in the basement of the containment building.  After the control rods dropped, there was no longer heat generated by fission of the uranium.  However, the fuel rods were filled with radioactive elements whose decay continued to generate heat even after the fission process stopped. 

Since the feedwater was no longer flowing, three emergency feedwater pumps were automatically activated.  However, the operators did not notice that blocking valves were closed in each of the emergency feedwater lines.  Thus, none of the emergency feedwater was entering the reactor vessel. The fact that the valves were closed represented a major failure of the TMI reactor system protocol; the system was supposed to be shut down whenever the auxiliary pumps were closed. 

Secondary steam valves opened up, which reduced the temperature and pressure from the steam.  Both the temperature and pressure of the RCS decreased.  At this point, the PORV should have closed; however, it was stuck open, so coolant water continued to be released from the reactor.  Because of flaws in the system design, the reactor operators did not realize that the PORV was stuck open; they assumed that it had closed.  Apparently, lights that indicated the status of the PORV valve were not in a location where they could be easily seen by the operators. Coolant was escaping from the reactor vessel through the open PORV.  The TMI-2 reactor was experiencing a so-called loss of coolant accident (LOCA), one of the most serious emergencies that can occur at a nuclear reactor.  Operators had been trained that the signs of LOCA were increased pressure in the RCS and increased pressurizer level.  However, in this case the RCS pressure was temporarily dropping; so, the operators did not realize they were experiencing a LOCA. 

The operators turned off the emergency core cooling pumps, because they did not realize that coolant was escaping through the open PORV valve.  Since they did not understand that coolant was being lost, they were concerned that the cooling pumps would inject too much coolant into the reactor.  The operators believed that water would continue to flow through the system and prevent overheating.  However, the rising temperatures converted much of the water to steam, and the pressure of the steam further prevented water from flowing. As the temperature and pressure increased, much of the cooling water in the reactor vessel was converted to steam.  At 6 am on March 29, this caused the top of the reactor core to be exposed.  When this happened, the zircalloy nuclear fuel rod cladding underwent a reaction that produced zirconium dioxide, hydrogen, and additional heat.  This melted the fuel rod cladding and damaged the fuel pellets, thus releasing radioactive isotopes into the reactor coolant.  

A significant amount of radioactive coolant (roughly 120,000 Liters) was pumped out of the containment building.  When new operators arrived at 6 am, they noticed that temperatures in various areas of the reactor greatly exceeded normal levels.  They closed a valve that prevented more radioactive coolant escaping the containment vessel; however, by this time the containment building was seriously contaminated with radiation.  At 6:56 am, a site supervisor declared a site-area emergency.  At this point, both Metropolitan Edison and state officials were notified and began issuing statements about the incident.  Unfortunately, those statements were both confusing and contradictory.  The Nuclear Regulatory Commission (NRC) was also notified, and they sent a team to the facility.  The resulting early actions were hampered by the fact that the plant operators did not realize that by 8 am, when the NRC was first alerted by the Three Mile Island staff, nearly half of the uranium in the TMI-2 reactor had melted. 

As they were investigating conditions in the reactor vessel, officials found that a large bubble of hydrogen had formed, as a result of the reaction of the zircalloy fuel rod coating with steam in the top of the reactor.  If oxygen was present in the core, the hydrogen could have exploded, with the possibility of releasing radioactive material from the containment building.  To prevent a hydrogen explosion, the operators removed steam and hydrogen from the reactor; however, some of these gases, containing some radioactive elements, were vented straight into the atmosphere. 

The Three Mile Island incident was rated as Level 5 on the International Nuclear Event Scale; that scale is shown in Fig. IV.3.  The Nuclear Event Scale has seven levels, defined as: 0 = Deviation; 1 = Anomaly; 2 = Incident; 3 = Serious Incident; 4 = Accident with Local Consequences; 5 = Accident with Wider Consequences; 6 = Serious Accident; 7 = Major Accident.  For comparison with the other nuclear accidents that we review below, both the Chernobyl and Fukushima accidents were rated as level 7; they are the only level 7 accidents that have yet been recorded.  A more detailed description with examples of what constitute incidents or accidents at each level is provided on the Website of the International Atomic Energy Association. 

Figure IV.3: The International Nuclear Event Scale.  The levels are defined in numbers ranging from 0 to 7, where 7 (the top level) is the most serious.  The lowest three levels, called incidents, are defined as 0 = Deviation; 1 = Anomaly; 2 = Incident; 3 = Serious Incident.  The top four levels are called accidents, with 4 = Accident with Local Consequences; 5 = Accident with Wider Consequences; 6 = Serious Accident; 7 = Major Accident. 

Within a day of the incident, a team from the Environmental Protection Agency (EPA) began monitoring the area surrounding the Three Mile Island plant for increased levels of radioactivity.  They eventually determined that the vast majority of the release of radioactive material involved the noble gases Xenon and Krypton.  The EPA estimated that a person living in the vicinity of Three Mile Island,within a nearby population of the reactor) received an average dose of 1.4 milli-rem (mrem) of radiation.  This can be compared to the 3.2 mrem that a patient receives from a chest X-ray, or the average yearly dose of radiation from natural sources of 310 mrem.  However, anti-nuclear activists have disputed some of these conclusions.  In addition, cow’s milk and goat’s milk were tested and researchers did not find elevated levels of the radioactive isotopes Iodine-131 or Cesium-137.  The EPA later claimed that the release of radioactive materials in the TMI incident did not increase the dose of radiation enough to cause even one additional cancer death in the population.  The EPA also measured no contamination in samples of soil, water, sediment or plants in the area. 

On advice from the NRC, Pennsylvania governor Dick Thornburgh urged that pregnant women and pre-school children living within five miles of the TMI facility be temporarily evacuated.  That distance was subsequently extended to a radius of 20 miles from the nuclear plant.  Within a few days, over 140,000 people had temporarily left that area; within three weeks, it was estimated that 98% of the evacuees had returned to their homes.  

Aftermath of the Three Mile Island Incident:

In April 1979, Jimmy Carter created the Presidential Commission on the Accident at Three Mile Island.  The commission was chaired by John Kemeny, the president of Dartmouth College. The Kemeny Commission reviewed the data following the TMI incident.  They concluded that the reactor vessel had contained the vast majority of the uranium that melted.  The commission report criticized Babcock & Wilcox, the company that built the Three Mile Island reactors.  They also criticized “Met Ed, GPU [the General Public Utilities Nuclear Corporation], and the NRC for lapses in quality assurance and maintenance, inadequate training, lack of communication of important safety information, poor management and complacency.

A particularly damaging section of the Kemeny Report dealt with the company Babcock and Wilcox.  They noted that the pilot-operated relief valve (PORV) had previously been stuck in the open position on 11 different occasions, allowing coolant to escape.  Furthermore, they noted that another Babcock & Wilcox facility, the Davis-Besse Nuclear Power Station in Toledo, OH, had experienced a very similar set of failures just 18 months before the TMI incident.  However, in that case operators had detected the PORV failure within 20 minutes, and the Davis-Besse plant was operating at only 9% capacity.  Babcock and Wilcox had not alerted staff at their other facilities of this problem after it surfaced in Toledo. 

Another serious failure was the fact that the emergency feedwater pumps were closed by blocking valves.  Operating procedures stated that the reactor should never have been operated when those valves were closed.  The combination of the open PORV valve, and the blocking valves closing off the emergency feedwater, led to this very scary incident.  Yet another lapse was that the operators failed to realize that temperatures and pressures in the reactor had reached dangerous levels.  When the morning operators arrived at 6 am, they immediately recognized that the situation was an emergency, and took steps to close down the reactor. 

In summary, the Three Mile Island incident was the result of poor reactor design (operators did not realize that various valves were open or shut), lack of attention to reactor maintenance (running the reactor when the emergency feedwater lines were blocked), poor training of the reactor operators (conflicting information about the system meant that operators did not realize that a loss of coolant accident was occurring), and complacency.  A significant amount of radioactive material, mainly in the form of gases, escaped from the containment building.  Also, coolant containing radioactive material also left the containment building; however, it appears that almost all this material remained on site.  Many tons of the enriched uranium fuel melted and fell to the bottom of the reactor vessel.     

In 1985, a television camera was set up to take photos of the damaged reactor vessel.  In 1986, samples were taken of the materials inside the reactor.  It was found that some 20 tons of uranium had melted and fallen to the bottom of the reactor vessel.  At the extremely high temperatures that had occurred in the reactor vessel, some of the fuel rods had melted.  The fuel, zircalloy cladding of the rods, and various parts of the reactor vessel had reacted at high temperatures and formed a liquid substance called corium (the scientific term for this mixture of reactor fuel, cladding and other elements).  At TMI-2, the corium flowed down to the bottom of the reactor vessel and formed a lava-like solid with a thickness of 5 to 45 centimeters. This confirmed the conclusions of researchers that the top of the fuel rods had been uncovered, and had reacted to high-pressure steam in the vessel.  But it was a pleasant surprise to realize that despite the extreme conditions that existed in the reactor after this incident, nearly all of the molten reactor products had been contained within the reactor vessel. 

The Three Mile Island incident probably had a significant effect on the American nuclear power industry.  Until 1979, the number of U.S. nuclear power plants was generally increasing.  At the time of the TMI incident, 129 new nuclear power facilities were in the planning stage, but only 53 of those were ever completed.  Federal requirements on safety measures became more stringent; in addition, protests from anti-nuclear power groups increased the costs of construction and the length of time before a nuclear plant would open.  The net effect was that after Three Mile Island, it was not until 2012 that a new nuclear power plant was approved for construction.  Furthermore, the cleanup process for the TMI-2 reactor lasted from 1979 until 1993 and cost $1 billion. 

Various groups of researchers disputed the official conclusions that the release of radioactive materials from the Three Mile Island incident was sufficiently low that it likely caused no additional cancer deaths in the area surrounding the plant.  Researcher Joseph Mangano claimed that there was a spike in infant deaths downwind of the TMI plant following the incident.  John Gofman used his model of radiation damage to estimate that there were 333 additional cancer deaths from the TMI incident.   

In estimating the damage produced by exposure to radiation, one has to make assumptions regarding the health effects of very small doses of radiation (for very small doses of radiation, one does not have enough statistics to calculate directly the damage to health).  Initially, Gofman advocated for the “Linear No-Threshold model” (LNT) of radiation damage.  This assumes that the health damage vs. radiation dose behaves in a linear fashion, right down to zero radiation dose.  We know that radiation damage results from the ability of radiation to break strands of DNA and damage them.  It is entirely possible that there is some “threshold” of radiation dose, and that below this threshold radiation does not possess enough energy to break a strand of DNA.  In estimating health damage, the linear no-threshold method is probably a prudent way to proceed, even though it most likely predicts more cases of cancer than actually occur.  However, Gofman’s calculation assumed that very low levels of radiation can produce large adverse health effects.  There is little evidence to suggest that Gofman’s model is correct.   

Nuclear power advocates such as Edward Teller claimed that the Three Mile Island incident was a great success for the nuclear power industry.  He pointed to the fact that the reactor vessel had contained the molten uranium, and that the EPA estimated that the radioactive gases released from the plant did not cause even one additional case of cancer in the surrounding community.  Teller stated that this incident should cause the public to have even more confidence in the safety of nuclear power. 

Few people agreed with Teller.  The public had been assured that the many levels of safety built into a nuclear reactor meant that an accident of this magnitude should never have occurred.  They worried that a more dangerous accident was likely to take place in the future, and they were skeptical of assurances that the release of radioactive gas was relatively harmless.  Immediately after the TMI incident, the public had been subjected to a number of conflicting and confusing statements from plant officials, civil servants, and representatives of the Nuclear Regulatory Commission.  Adding further to the confusion was the fact that the statements changed from day to day, as more information about the incident came to light. 

In an eerie coincidence, just twelve days before the Three Mile Island incident, an anti-nuclear power movie The China Syndrome was released.   In that movie, a journalist played by Jane Fonda and her cameraman (Michael Douglas) film a major accident at a nuclear power plant.  The government then attempts to cover up the incident.  In The China Syndrome, a meltdown and explosion occur at the nuclear plant, and highly radioactive solids are released from the reactor.  Those radioactive solids then pollute the surrounding countryside, and Fonda’s character is told that this incident “could render an area the size of the state of Pennsylvania permanently uninhabitable.” 

Figure IV.4: A scene from the 1979 movie The China Syndrome.  From L: nuclear plant staff supervisor Jack Godell (Jack Lemmon); news cameraman Richard Adams (Michael Douglas); reporter Kimberly Wells (Jane Fonda).  

Coming immediately before the TMI incident, this apocalyptic film increased the public’s fear of nuclear power, and it increased fears that the safety risks from nuclear power might be too great.  Although The China Syndrome greatly exaggerated the safety risk from American nuclear plants, it will turn out that this movie predicted many of the outcomes from the Chernobyl nuclear disaster. 

The cleanup at Three Mile Island took 12 years to complete, at a cost of about $973 million.  In August 1979, low-level radioactive waste was transported to Richland, Washington.  Beginning in October 1985, about 100 tons of damaged fuel in the reactor vessel was lifted into fuel canisters, and then transported to long-term storage at the Idaho National Laboratory.  The process was completed in April 1990.  There were also 2.8 million gallons of contaminated water remaining from the reactor and the cleanup.  The water was processed and stored, and eventually evaporated.  The evaporation was carried out from January 1991 to August 1993.  As costly and challenging as the cleanup at Three Mile Island was, it was dramatically less severe than the level-7 accidents at Chernobyl and Fukushima.   

Nuclear Accident at Chernobyl:

The Reactor Complex and the Accident at Chernobyl:

The Russians built a nuclear reactor complex in Chernobyl, Ukraine.  Construction began in August 1972 and the reactor was commissioned in September 1977.  Chernobyl was about 20 km south of the Ukraine-Belarus border.  The Chernobyl reactors were of the RBMK-1000 design; a schematic of the RBMK reactors is shown in Fig. IV.5.   The fuel in the Chernobyl reactors was slightly enriched (2% U-235) uranium; it was packed into bundles surrounded by zirconium alloy cladding. The fuel bundles in the RBMK reactors were surrounded by graphite moderators, which served to slow down the neutrons arising from fission of U-236.  Water was pumped into the bottom of the reactor, and as it moved up past the fuel bundles, it heated and turned into steam.  The steam powered turbines that generate electricity.  The steam was then sent through a condenser, and pumped back into the bottom of the reactor. 

Figure IV.5: A schematic diagram of the RBMK reactor.  Fuel bundles contained enriched uranium, with graphite moderators and control rods that could be raised and lowered to control the rate of the fission reaction.  Water was injected into the bottom of the reactor vessel; as it rose, it was converted to steam.  The steam was sent through a turbine that powered a generator.  The steam was then sent through a condenser and the water was returned to the reactor. 

On April 25, 1986, reactor 4 at Chernobyl was to be shut down for routine maintenance.  The operators of the reactor decided to prepare reactor 4 for a test; they wanted to see how long turbines would spin and power would continue to be generated if the main electrical power supply was turned off.  The issue was whether the slowing turbines would be able to provide sufficient electrical power to operate the core cooling water circulation pumps, until the emergency power supply became activated.  Unfortunately, there was insufficient communication between the team preparing the test and the personnel operating the reactor.  This was aggravated by the fact that the test was carried out by night shift operators, who were less skilled in operating the reactor and understanding the requirements of the reactor. 

At the start of the test the reactor should have been stabilized to about 700 Megawatts thermal (MWt, a measure of input thermal power in an engine).  However, the power fell to about 30 MWt.  It is not understood precisely why the power fell so dramatically.  The team running the test had shut off the reactor’s emergency core cooling system.  In order to increase the reactor power to 700 MWt, the operators withdrew most of the control rods from the reactor.   The operators had been trained that there should never be less than 15 control rods inserted in the reactor; however, because the reactor power was so much less than desired, the operators left only 8 control rods in the reactor. 

It is believed that “Xenon-135 poisoning” was a major reason for the inability of the operators to raise the reactor power to the desired level of 700 MWt.  Xe-135 is produced by the decay of Iodine-135.  Under normal operating conditions, the amount of Xe-135 in the reactor is stable (it is produced at the same rate that it decays).  However, since I-135 has a half-life of 6.7 hours (half the atoms of a sample of I-135 will decay every 6.7 hours), after the reactor power is turned off, Xe-135 will continue to increase for a few hours.  Xe-135 is a very strong absorber of neutrons.  Thus, the difficulty of restoring the reactor power was likely due to the strong absorption of neutrons by the Xe-135 present in the reactor. 

At 1:03 am on April 26, the reactor temperature stabilized at about 200 MWt and the shutdown test was begun at this power level.  It was not known by the reactor operators, and presumably not known by those who designed the reactors, that the RBMK reactors had a fatal flaw.  The water coolant in the reactor had the function of cooling the fuel bundles; the water also slowed down and absorbed some neutrons from fission reactions.  As the water rose past the fuel bundles, it turned to steam, and after the steam left the reactor area it powered the turbines.  But the reactor possessed a positive void coefficient.”  As water turned into steam, the increase in steam bubbles (called “voids”) increased the speed of the fission reaction.  This was because the steam was much less dense than water, so the steam was less effective in absorbing neutrons. 

The increased number of neutrons meant that the rate of fission increased.  This increased the temperature in the reactor, which increased the amount of steam.  This in turn increased the rate of fission reactions, and hence the temperature.  Under the right conditions, this could produce a runaway fission reaction, where the rate of uranium fissions, the temperature inside the reactor, and the fraction of cooling water that turned to steam, all increased together.  The rapid increase in steam also produced an “overpressure” inside the reactor. 

A well-designed reactor would have insured that the “void coefficient” was always negative; that is, safety mechanisms would automatically kick in if the temperature and fission rate began to accelerate, so that a dangerous and uncontrolled rise in the rate of fission could never occur.

As the temperature inside the reactor and the rate of fissions increased drastically, the control rods were programmed to be fully inserted among the fuel rods.  This condition occurred at 1:23 am on April 26.  In principle, inserting the control rods should have stopped the fission reaction and safely shut down the reactor.  However, the RMBK reactors had even more design flaws, which would prove deadly.  First, the control rods were designed in such a way that they caused a dramatic surge in power and pressure as they were first inserted into the reactor.   The increased pressure caused the cover plate of the reactor to become detached.  This ruptured some of the fuel assemblies, which subsequently jammed the control rods when they were only halfway down the reactor.  So, the control rods failed and the fission, temperature and pressure continued to rise out of control.  Yet another design flaw meant that significant damage to even three or four of the fuel bundles could result in the reactor being destroyed. 

The tremendous increase in temperature and pressure ruptured the emergency cooling circuits, which caused the reactor to be inundated with even more water, which turned to steam and increased the pressure.  The reactor output jumped to around 30,000 MWt, some 10 times the normal power output of the reactor.  This was the last recorded output from the reactor; some later reactor simulations suggest that the power spike may have been at least 10 times the maximum recorded level. The massive amount of steam under high pressure appears to have caused an explosion of the steam.  The explosion blew a hole in the top of the reactor, blasting off the upper biological shield.  The first explosion ejected materials from the fuel, 25% of the graphite moderator and materials from the reactor structure.  It also released massive amounts of gases containing radioactive isotopes. 

Two or three seconds later, a second even more powerful explosion occurred.  The exact nature of the second explosion is not certain (conclusions about the reactor explosions are based on later computer simulations of the reactor behavior).  The most widely accepted idea is that chemical reactions of the zirconium cladding of the fuel bundles with the high-pressure steam produced a large amount of hydrogen.  When the reactor vessel ruptured, the hydrogen combined with oxygen in the atmosphere and exploded.  A second hypothesis was that the second blast was also a steam explosion.  A third, more speculative idea was that the out-of-control fission reactions produced a nuclear explosion, similar in magnitude to an atomic bomb test that “fizzled,” i.e., which led to an explosion but did not produce the gigantic blast from a full-fledged nuclear bomb. 

Regardless of the nature of the explosions, one operator died immediately and a second worker died a few hours later from his injuries.  The plume of material ejected from Chernobyl reactor 4 reached 1 km into the air.  The heavier material landed near the site of the explosion; however, gases and particles that contained fission products and radioactive isotopes were blown from the plant on the prevailing northwesterly winds. 

Figure IV.6: A photo of Chernobyl reactor 4 following the explosion on April 26, 1986.  The giant hole marks the former position of the reactor vessel. 

Firemen were immediately summoned.  The first contingent of 14 firemen from the reactor complex arrived at 1:28 am.  More than 100 firefighters from the site and from the nearby town of Pripyat were brought in to control the fires on site.  Water and foam were used to put out the fires produced by the burning ejecta from the reactor.  Many of the first cadre of fire-fighters received massive doses of radiation.  In addition to the two staff who died on the first day, by the end of July 1986 six firemen and 22 reactor staff died from acute radiation poisoning. 

The Aftermath of the Chernobyl Accident:

On April 28 a massive accident management and cleanup was begun.  As many as 1,800 helicopter flights were made over the reactor complex.  The helicopters dumped neutron-absorbing materials and fire-control material into the crater formed by the reactor explosion.  Initially, the helicopters hovered over the reactor while dumping their loads; however, it was soon realized that the pilots were receiving massive doses of radiation, so on subsequent flights the materials were dropped while the copters flew over the reactor.  Unfortunately, the materials landed over a wide area, and so caused damage to other parts of the reactor complex, which likely resulted in the release of more radioactive material.  Also, some of the fire-control material may have acted as thermal insulators for the damaged reactor.  This may have increased the temperature of the remaining core elements and increased the amount of radioactive materials subsequently released from the Chernobyl site. 

The radioactive isotopes that were most damaging to people living in the vicinity of the Chernobyl plant were Iodine-131, cesium-137 and strontium-90.  Iodine-131 has a half-life of 8 days (in 8 days, half of the atoms of I-131 will have decayed; in 24 days only 1/8 of an original sample of I-131 will not have decayed).  When humans are exposed to Iodine-131 it tends to collect in the thyroid.  Children especially who are exposed to I-131 tend to have a greater risk of developing thyroid cancer.  If correctly diagnosed, thyroid cancer is slow-growing and rather easily treated. 

Cesium-137 has a half-life of 30 years.  People who are exposed to Cs-137 may suffer from exposure to high-energy gamma radiation from decay products of Cs-137.  The most dangerous of these decay products is Barium-137.  It emits gamma radiation, which spreads through the body and can cause damage to DNA in tissues throughout the body.  If people are exposed to significant amounts of Cs-137, the resulting gamma rays can cause skin burns.  The National Radiological Protection Board of Great Britain estimates that over the next 70 years, people exposed to Cs-137 from the Chernobyl accident will likely develop 1,000 additional cases of cancer. 

Strontium-90 has a half-life of 28.8 years.  It was likely expelled from the Chernobyl reactor in the cloud of radioactive gases and particles.  It can be dangerous if inhaled, but the greatest concern for health is if it gets ingested in food or water.  In the body, strontium-90 acts like calcium and so is absorbed in teeth and bones.   There, it can produce cancers of the bone and bone marrow, and soft tissues surrounding the bones. 

The Chernobyl disaster occurred before the breakup of the Soviet Union. The Russian government then embarked on a shameful attempt to keep the Chernobyl accident a secret. About 120,000 people lived within 30 km of the nuclear plant. Although the town of Pripyat was immediately evacuated on April 27, Ukrainian government officials were told that a fire had broken out at the Chernobyl plant, but that it had been extinguished and that “everything was fine.”  The citizens of Pripyat were told that their evacuation would last just three days.  However, the following day the evacuation area was expanded to a 10-km zone surrounding the blast.  And ten days after the accident the evacuation zone was further expanded to 30 km. 

The area surrounding the Chernobyl blast has been converted into the Chernobyl Nuclear Power Plant Zone of Alienation.  Although the current Exclusion Zone has been modified somewhat from the original 30-km area around the blast, the borders have been changed slightly to include more of Ukraine.  The Exclusion Zone includes those areas that still have the highest contamination from radioactive materials.  The Exclusion Zone today has one of the most highly contaminated areas from radioactive debris.  The area initially held a population of roughly 120,000 people.  Figure IV.7 contains a map that shows the levels of radiation at various points within the Exclusion Zone.  The units are Curies/km2, where one Curie of radioactive material contains decays of 37 billion nuclei per second.

Figure IV.7: A map of the area surrounding the Chernobyl nuclear plant, showing the levels of radiation as of 1996.  The units of radiation are Curies/km2.  The different colors denote in order of decreasing radiation the Confiscated Zone, the Permanent Control Zone, the Periodic Control Zone, and the Unnamed Zone.  At present the areas inside the Exclusion Zone remain forbidden for people to live in or enter; however, it is estimated that roughly 200 samosely (returnees) have re-entered the Exclusion Zone and currently live there.  Surrounding the Exclusion Zone are areas that were designated for voluntary resettlement once the radiation had died down sufficiently.  However, those areas have never been reopened.

When can one expect that settlers will once again be allowed to inhabit the Exclusion Zone?  There are vastly different estimates for this.  In 2011, Ukraine government authorities estimated that re-settlement could take place in 320 years.  On the other hand, science writers for the Christian Science Monitor estimated 3,000 years before re-settlement; and Greenpeace claimed that the area could not be inhabited for tens of thousands of years.  Nonetheless, during the ongoing Russian invasion of Ukraine, a significant number of Russian soldiers were encamped within the Exclusion Zone for more than a month in March and April 2022.

The Soviets did not alert their European neighbors that a dangerous and deadly accident had occurred at Chernobyl.  The first indication from outside the Soviet Union occurred on April 28 when radiation monitors at the Forsmark, Sweden nuclear power plant recorded the presence of a cloud containing radioactive particles.  The Swedes contacted the Russians, who denied that any radioactivity was coming from the Soviet Union.  Only when the Swedes announced their intention to notify the International Atomic Energy Agency did the Soviets admit that the radioactive materials were originating from an accident at Chernobyl. 

Eventually, Soviet officials installed a thick layer of concrete under the reactor; this was an attempt to prevent radioactive materials from burning down and contaminating the groundwater.  A “sarcophagus” was also constructed and installed over the top of the reactor facility, in order to prevent further radioactive material from being released into the atmosphere.  It was initially decided that robots would be deployed to work around the intensely high levels of radiation in the former reactor; however, the radiation damaged the robots and destroyed their batteries.  As a result, many workers were deployed to do this work.  They were equipped with bio-hazard suits but apparently the levels of radiation were much higher than the protection provided by their suits.  It was decided that the workers should spend only a single shift working on the reactor, but many of them reported working five or six shifts.  The workers were also poorly informed about the health dangers they faced. 

The International Atomic Energy Agency authorized two investigations of the Chernobyl disaster by the International Nuclear Safety Advisory Group (INSAG).  The first report, issued in 1986, blamed the accident almost entirely on operator error.   This had been the Soviets’ claim regarding the accident.  However, a second report, INSAG-7, issued in 1992, was based on much more information and more sophisticated computer models of the Chernobyl reactor.  That report concluded: “The Accident is now seen to have been the result of concurrence of the following major factors: specific physical characteristics of the reactor; specific design features of the reactor control elements, and the fact that the reactor was brought to a state not specified by procedures or investigated by an independent safety body.  Most importantly, the physical characteristics of the reactor made possible its unstable behavior.”   The investigation concluded that the operators had turned off various safety systems and their actions showed that they ignored regulations to expedite the reactor test, and communication between the safety officers and officials conducting the test was poor.  However, the accident also occurred because of poor design of the reactor that led to an instability that caused the reactor failure.   

The four design elements that contributed to the disaster were: 1) Use of a graphite moderator in a water-cooled reactor, which allowed the reactor to go critical when a loss of coolant accident occurred (when water also serves as the moderator, its loss would reduce the fission rate);  2) A positive steam void coefficient that allowed the reactor to undergo a destructive power surge;  3) The control rods required 18-20 seconds to be fully inserted, and the rods possessed graphite tips that increased reactivity when first inserted; 4) No reinforced containment building surrounded the reactor. 

 These faulty design elements, when combined with a culture that disregarded safety guidelines, and with an atmosphere where communications between the plant’s reactor operators and its safety officers were poor, led to the catastrophic set of circumstances that produced this deadly accident. 

What about the long-term threats to the health of people who lived near the reactor, and to people who were “downwind” of the Chernobyl plant, and who might have been exposed to radioactive gases and dust particles?  Here, the answer is problematic; various groups have produced radically different estimates of the number of cases of cancer due to the radioactivity.  Two scientists, Anders Moller and Timothy Mousseau, have published articles claiming that the mutation rate of animals in the Exclusion Zone is many times higher than normal.  However, Moller had previously been reprimanded for publishing fraudulent articles; this may cast some doubt on his estimates.

One definite consequence of the Chernobyl disaster was the increased number of elective abortions by pregnant women in the Ukraine, Belarus and Russia, but also in countries in western Europe over which the radioactive cloud had passed.  Although the exact numbers are not known, there were at least 150,000 additional abortions because of fears of fetal deformities from the radiation; some have claimed that there were as many as one million elective abortions following the Chernobyl disaster.  However, scientists have claimed that no fetus should have received a threshold dose of radiation.  This is partly because all of the front-line Chernobyl staff and fire fighters, who received the highest doses of radiation, were male.  However, no statistically significant increase in developmental abnormalities has been observed in the children of those front-line workers who received and survived massive doses of radiation. 

The U.N Scientific Committee on the Effects of Atomic Radiation has estimated that the Chernobyl disaster exposed the average citizen on Earth to an additional 21 days of radiation compared to natural background radiation.  However, the “Chernobyl Liquidators,” reactor lab staff and firefighters who received the largest doses of radiation, averaged an increased radiation dose equivalent to 50 years of normal background exposure.  The U.N. Committee estimated that the Chernobyl fallout will lead to 4,000 additional deaths above the norm for the roughly 600,000 people who lived in the Exclusion Zone area. 

The Union of Concerned Scientists (UCS) estimated that worldwide, one should expect an additional 50,000 cancer cases and 25,000 deaths.  However, the UCS scientists calculated this using the collective dose, while the International Commission on Radiological Protection states that using the collective dose “is inappropriate to use in risk projections.”  The UCS estimate can be compared to the World Health Organization’s estimate of 4,000 future cancer deaths in countries bordering Ukraine.  Greenpeace, which carried out its own study, claimed that the Chernobyl accident could lead to 10,000 – 200,000 additional cancer deaths in Ukraine, Belarus and Russia between 1990 and 2004.  The largely discredited book Chernobyl: Consequences of the Catastrophe for People and the Environment, concluded that there were 985,000 premature deaths around the world resulting from the radiation released from Chernobyl.  That number is almost certainly orders of magnitude too large.  However, it is difficult to obtain accurate information following the collapse of the Soviet Union and the low level of current public health resources in former Soviet-bloc countries. 

It is also difficult to obtain accurate figures for the cost of the cleanup at Chernobyl.  Former president of the Soviet Union Mikhail Gorbachev has claimed that the initial costs to the Soviet Union of containment and decontamination were about 18 billion rubles, or about $11.1 billion in 2019 dollars.  The claim was that this nearly bankrupted the Soviet Union.  In April 2006 Gorbachev stated “The nuclear meltdown at Chernobyl 20 years ago this month, even more than my launch of perestroika, was perhaps the real cause of the collapse of the Soviet Union.”  It is estimated that over a 30-year period the cost to Belarus was $235 billion.  Even today, it is claimed that Ukraine spends between 5% and 7% of its budget on Chernobyl-related activities.  At present, the bulk of the costs from Chernobyl come from the continued social benefits paid to 7 million inhabitants living in Russia, Ukraine and Belarus. 

The Fukushima Reactor Disaster:

The Fukushima Reactor disaster occurred on March 11, 2011 and affected the Fukushima Daiichi Nuclear Power Plant in Okama, Fukushima prefecture, Japan.  The proximate cause was the Tohoku earthquake that took place off the coast of Honshu Island in Japan (see Fig. IV.8).  This was a quake of magnitude 9.1 on the Richter Scale, making it one of the largest earthquakes in recent memory.  The epicenter of the quake was 72 km east of the Oshika Peninsula of Tohoku, Japan and it took place just 32 km below the surface.  In this region, the Pacific Plate is subducting beneath the plate of northern Honshu.  The quake was sufficiently powerful that it moved the island of Honshu 2.4 meters to the East.  The earthquake generated some enormous tsunami waves.  The highest crests of the tsunamis were estimated at 40.5 meters, at the town of Miyako (see Fig. IV.9).  The tsunami traveled at 700 km/hour and traveled as far as 10 km inland. 

Figure IV.8: A map showing the location of the 2011 Tohoku earthquake, off the cost of Japan’s Honshu Island. 


Figure IV.9: A photograph of the 40-m high tsunami as it first made landfall in Miyako, Japan, the location of the highest crest of the tsunami. 

The latest official figures for damage from the earthquake and tsunami were 19,747 deaths, 6,242 injured and 2,556 people missing.  The Fukushima Daiichi reactor complex also experienced two tsunamis, with the larger having a height of 14 m.  Since the area is the site of regular volcanic action, a seawall had been built off the coast of the Fukushima reactor complex.  It was sufficient to contain a tsunami of height 5.7 m.  As the maximum height of the tsunami was roughly 14 meters, it swept over the seawall and inundated the reactors.   This is shown in Fig. IV.10.  The size of the earthquake and tsunami were a surprise to geologists, as it was predicted that earthquakes in this region were unlikely to exceed 8.1 on the Richter scale. 

Figure IV.10:  A schematic cross-section of the Fukushima reactors and the tsunami from the Tohoku earthquake.  D denotes the normal sea level; the seawall E was 5.7 m above normal sea level.  C marks the ground level.  B is the maximum height of the tsunami that struck Fukushima, and A are the buildings in the Fukushima Daiichi reactor complex. 

Unfortunately, the administration at TEPCO (the Tokyo Electric Power Company) ignored several earlier recommendations that they make changes in the protections of Fukushima Daiichi from large tsunamis.  In 1991 the U.S. Nuclear Regulatory Commission warned that the Fukushima complex might be in danger of losing emergency power; TEPCO took no action on this recommendation.  In 2000, an in-house TEPCO study recommended that the reactor complex be modified to account for the possibility of a 15-m tsunami, rather than the 5.7 m seawall protecting the plant.  TEPCO did not make any of the suggested changes and did not publicize this request, saying later that “Announcing information about uncertain risks would create anxiety” among the population. And the Active Fault and Earthquake Research Center (AFERC) urged TEPCO to revise upwards their estimates of possible tsunami heights. AFERC used the example of a massive tsunami in 869 at Sanriku; once again, TEPCO took no action on this request. 

As seen in Fig. IV.11, “The Fukushima Daiichi Nuclear Power Plant consisted of six General Electric light water boiling water reactors (BWRs) with a combined power of 4.7 gigawatts.”  This made it one of the 25 largest nuclear power stations in the world.  In a boiling-water reactor, the water that removes heat from the core (the coolant) turns into steam already in the reactor core, and that steam goes directly to the turbines that drive the generators.  In the Fukushima reactors the control rods are introduced from the bottom of the reactor vessel; those rods can vary the rate of fission reactions that occur in the reactor vessel.  When the control rods are fully inserted, the fission reactions stop altogether.  The safety systems include safety measures for the steam and water systems, the reactor vessel which is constructed of steel, and a thick concrete containment building with a watertight steel lining that surrounds the reactor vessel and the spent fuel pool. 

Figure IV.11: A schematic of the Fukushima Daiichi nuclear reactors.  The reactor vessel in the center holds the nuclear fuel rods, which use a water moderator.  Control rods can be raised or lowered to change the rate of fission in the reactor.  The reactor has a steel containment structure surrounding it, and there are primary and secondary concrete containment structures surrounding the reactor.  To the right above the reactor vessel is the spent fuel pool, where fuel rods are cooled in water until their level of radioactivity drops to a point where they can be disposed of off-site. 

The reactor buildings were constructed to withstand a relatively large earthquake.  Because an earthquake shakes the ground, it causes structures to accelerate.  During an earthquake, scientists measure the maximum or peak acceleration of structures.  These are expressed in terms of the acceleration due to gravity at the Earth’s surface, g = 9.8 m/s2..  After the 1978 Miyagi earthquake (which produced a peak acceleration of 0.125 g), the Fukushima buildings sustained no damage.  The design basis for the reactor buildings was to withstand accelerations of between 0.42 g and 0.46 g.   

The Fukushima reactors were initially constructed with the emergency diesel generators and DC batteries, vital safety backup systems for the reactor, located in the basement of the reactor building.  However, staff at GE lobbied TEPCO to change this, as the generators and batteries would be subject to flooding.  So TEPCO moved the diesel generators and batteries to buildings located on the hillside behind the reactor complex.  The diesel generators supplied all six Fukushima reactors with emergency power.  However, the switching stations for reactors 1 through 5 were located in the turbine buildings, and those were poorly protected from flooding.  This turned out to be a fatal error by TEPCO.  Another nearby nuclear reactor complex, the Fukushima Daini Nuclear Power Plant, had moved their generators and backup batteries into the watertight reactor building.  The Fukushima Daini complex emerged from the earthquake and tsunami with far less damage than was experienced by Fukushima Daiichi. 

Another issue was the temporary storage of used fuel rods.  Over time, the amount of fissionable uranium in the fuel rods decreases.  During the operation of the reactor, the rods accumulate radioactive isotopes of many elements, which result from decay processes of the heavy elements produced in nuclear fission.  The used fuel rods are highly radioactive.  Over time, many of the radioactive elements will have decayed; but for a period of 18 months, the fuel rods were too hot to be placed in long-term dry cask storage.  Thus, as a temporary measure, used fuel rods were placed in pools of water on the top of the reactors.  After 18 months, when the rods had cooled sufficiently, they were scheduled to be transferred to dry cask storage. 

On the day that the Tohoku earthquake hit, reactors 5 and 6 at the Fukushima Daiichi plant were shut down for scheduled maintenance, and the core in reactor 4 was unloaded.  No power was required to run those reactors; however, they still needed power to provide cooling for the used fuel rods in the storage pools atop the reactors.  As soon as the Tohoku earthquake occurred, seismic sensors at the reactor site inserted the control rods fully, shutting down the fission reactions in the operating reactors 1, 2 and 3.  Although the fission stopped, the reactors continued to generate heat from the spontaneous decay of radioactive elements in the fuel rods.  As power was lost in the reactors, the emergency generators came on and provided electricity to run the backup cooling systems.  As far as can be determined, the Fukushima reactors did not sustain any significant damage from the earthquake itself. 

However, when two tsunamis arrived one hour later (eight minutes apart), they knocked out the emergency generators.  The DC backup batteries in reactors 1 and 2 were flooded and failed, leaving these units with no instrumentation and producing a station blackout.  The batteries in unit 3 continued to function for another 30 hours.  With no backup power, the temperature in reactors one, two and three rose, until the cooling water in the reactors boiled and the steam pressure increased.  Eventually, the fuel rods became uncovered while the temperature continued to rise.  In unit 1, the water level dropped to the top of the fuel approximately 3 hours after the control rods had shut down the fission reactions.  Another 90 minutes later, the water no longer covered any of the fuel.  When the temperature in the rods exceeded 2,800°C, the fuel rods melted.  The uranium fuel, the zircalloy coating of the rods and various reactor vessel materials melted and mixed together to form a substance called corium (this was discussed in the Three Mile Island section).  While the corium at Three Mile Island had been contained within the reactor vessel, at Fukushima the material pierced the reactor vessel and fell to the bottom of the containment building. 

At sufficiently high temperatures, the zirconium alloy that serves as the cladding for the fuel rods interacts with the steam.  This reaction releases hydrogen gas.  If the hydrogen mixes with oxygen, it will cause an explosion.  The containment buildings contained a mixture of nitrogen and other gases; as long as the hydrogen remained inside the containment building and no oxygen entered the buildings, an explosion would not occur.  However, eventually the pressure inside the reactor caused various lines in the reactor to rupture. 

At this point, hydrogen explosions occurred.  Those occurred on March 12 in reactor 1, March 14 in reactor 3 and March 15 in reactor 4.  The hydrogen explosion in reactor 4 occurred because it was connected to reactor 3 by an adjoining pipe, so hydrogen from reactor 3 was piped to reactor 4 where it led to a hydrogen explosion.  On March 14, the containment unit failed in unit 2.  Although it appears there was no hydrogen explosion in unit 2, nevertheless the leaking system released large amounts of radioactive material both into the air and into the seawater. 

The Fukushima reactors had various backup safety systems.  The steel reactor vessel itself, called the dry well, was designed to withstand a great deal of pressure.  Should the reactor vessel be breached, it was connected to wet well suppression pools (shown at the bottom of the containment building in Fig. IV.11).  The wet well was connected to the reactor vessel through a series of vents.  If a LOCA occurred, steam at high pressure would be released from the reactor vessel through a series of pipes and relief valves.  The steam would then condense in the suppression pools. 

The ‘spent’ fuel rods being stored in pools above the reactors released radioactive elements after the power failed and the system was no longer able to cool the pools.  When those fuel rods became exposed, they released radioactive gases.  Because the spent fuel rods were not inside the reactor vessel, radiation released from the spent fuel pool was more likely to be released into the atmosphere or the groundwater.  Since Iodine-131 has a half-life of 8 days, most of the I-131 in the spent fuel rods will have decayed.  The most dangerous element released by the spent fuel pools was likely Cesium-137, which has a half-life of about 30 years.

Aftermath of the Fukushima Disaster: 

There were three types of releases of radioactive material from the Fukushima Daiichi complex.  One source was deliberate discharge of radioactive coolant water into the sea.  This was done in April 2011 and May 2011, in order to free up storage space in the complex for water that was even more contaminated than was released.  As of March 2021, the Fukushima complex had accumulated 1.25 million tonnes (1 metric tonne is the weight of 1000 kilograms) of waste water that was held in 1,061 tanks in the reactor complex.  A second source of radioactive liquids came from accidental discharge of liquids from leaks or ruptures in the containment vessels.  Between March and April 2011, 270,000 to 540,000 Curies of Iodine-131 had been released from the complex, along with 80,000 to 160,000 Curies of Cesium-137.  The vast majority of that material was released into the ocean.  Furthermore, there have been continuing releases of radioactive liquids from leaks in the reactor water tanks.  TEPCO had denied the existence of such leaks until July 2013. 

There was also significant radioactivity released through the venting of gases into the atmosphere.  Much of this was done deliberately, in an attempt to reduce gas pressure in the reactors.  However, there were also accidental releases of gases through leaks and explosions.  The total release of Iodine-131 gases is estimated at 2.7 million to 13.5 million Curies, along with 160,000 to 540,000 Curies of Cesium-137.  One potential source of contamination is from coastal sediment that becomes contaminated with these products, and subsequently is ingested by fish and shellfish.  In 2011, 41% of marine life caught off the Fukushima coast had Cs-137 levels above the legal limit, while in 2015 0.05% of marine life had Cs-137 levels above the limit.  This reflects the 30-year half-life of Cs-137, and the dispersal of radioactive elements in the ocean.  In Sept. 2011, TEPCO measured the radioactivity released by the Fukushima reactors, and announced that it was 4 million times lower than was measured in March 2011, immediately following the reactor disaster. 

Given the magnitude of releases of radioactive substances into the air and water, together with the large evacuation zone around the reactor complex, the Fukushima Daiichi accident was rated at level 7 on the International Nuclear Event Scale (INES).  The Fukushima and Chernobyl disasters are the only ones ever to receive a level 7 rating on the INES. The Japanese government has a plan by which radioactive water will be pumped into the ocean over a 30-year period.  The water is treated to remove Cs-137 before being discharged.  One of the radioactive isotopes in this water is tritium, which has a half-life of 12.3 years.  The Japanese government claims that, even if all the water was discharged in a single year, the dose received by local citizens would be 0.81 microsieverts (or 0.08 milli-rem); it was claimed that this was negligible compared to the dose from natural causes of 2,100 microsieverts/year. 

Immediately following the inundation of the Fukushima reactor complex, the Japanese government evacuated citizens from the immediate area.  Eventually, 160,000 people were temporarily relocated from the area.  Figure IV.12 shows the relocation areas around the Fukushima plant.  The red area is the “Special Decontamination Area (SDA);” it is estimated that individuals in this area could receive 10 mSv (milli-Sieverts) of radiation per year.  For calibration, that level corresponds to 20% of the maximum annual whole-body radiation dose the U.S. Nuclear Regulatory Commission allows for radiation workers. A second area (shown in yellow) is the “Intensive Contamination Survey Area (ICSA);” in the ICSA area, individuals could receive 1 mSv of radiation.  In the ICSA area, municipalities are responsible for determining their own disposition of contaminated areas, with support and advice from the federal government.   


Location of the Special Decontamination Area (SDA) shown in red, and the Intensive Contamination Survey Area (ICSA) set up by the Japanese authorities at the end of 2011

Figure IV.12: The areas around the Fukushima Daiichi Power Plant where population was relocated following the 2011 reactor explosions.  The red area is the SDA or “Special Decontamination Area.”  It is estimated that individuals in this area could receive an additional 10 mSv (milli-Sieverts) of radiation annually.  The yellow area is the ICSA or “Intensive Contamination Survey Area.” Individuals in the ICSA could receive 1 mSv annually of radiation.  

By the end of 2015, decontamination was considered complete in six municipalities.  Former residents were told that they could return to their villages.  However, one year later only 9% of the evacuees had returned to their homes.  And as of 2021, 37,000 of the 160,000 evacuees had not returned to their homes.  Some have passed away while the remainder have opted not to return. 

The Fukushima reactor disaster was reviewed by a Japanese committee, the Investigation Committee of the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company.  The report claimed that the authorities “had grossly underestimated tsunami risks” that followed the Tohoku earthquake; in fact, the 14-m tsunami that struck the Fukushima plant was twice the height of the highest waves predicted by the officials.  Furthermore, the report said that plant officials had assumed that backup power would continue to function following the tsunami.  “Plant workers had no clear instructions on how to respond to such a disaster, causing miscommunication.”    

A 2012 report in The Economist concluded, “The operating company was poorly regulated and did not know what was going on.  The operators made mistakes.  The representatives of the safety inspectorate fled.  Some of the equipment failed.  The establishment repeatedly played down the risks and suppressed information about the movement of the radioactive plume, so some people were evacuated from more lightly to more heavily contaminated places.” 

What about the short-term and long-term effects on human health for those living close to the Fukushima reactor complex?  During the evacuation of the population, there were about 1,600 deaths, although virtually all of these were not due to radiation exposure.  In 2014, the radioactive water plume reached the western U.S.  Scientists from the National Oceanic and Atmospheric Administration measured the levels of Cs-134 radioactivity and announced that the detected levels posed no threat to the health of Americans.  A research group from Woods Hole Oceanographic Institution found that levels of radioactivity in this plume were far lower than EPA guidelines.  Dr. Jay Cullen, who established a Web site where radioactivity levels were published and who stated that levels measured on the West Coast of the U.S. and Canada posed no threat to human health, received hate mail and death threats from people who believed that radioactive isotopes from the Fukushima disaster would cause a wave of cancer deaths across America. 

The World Health Organization issued a report estimating that girls in the area around Fukushima would likely experience a 70% increase in thyroid cancer, due mainly to Iodine-131 exposure as infants.  The 167 Fukushima plant workers who participated in cleanup operations and received high doses of radiation are expected to have an increased risk of developing cancer.  However, the risk may be sufficiently small that it cannot be detected by studying those workers.  Researchers who used a Linear No-Threshold (LNT) model estimated that the Fukushima disaster would lead to 15 – 1,100 additional cancer deaths.  However, it has been noted that the LNT model grossly overestimated cancer deaths from Hiroshima, Nagasaki and Chernobyl. 

In 2013 the Fukushima Medical University compared cases of thyroid cancer from the Fukushima prefecture before and after the nuclear accident and concluded that “it is unlikely that these cancers were caused by the exposure from the nuclear power plant accident in March 2011.”   However, there are very real mental-health issues from the population close to the Fukushima plant.  As was the case of Chernobyl survivors, there were dramatic increases in the rate of illnesses such as “depression, anxiety, post-traumatic stress disorder (PTSD), medically unexplained somatic symptoms, and suicide.”  This is unsurprising since these individuals were evacuated from their homes, may have lost friends and family from the tsunami, and underwent a life-threatening experience from the quake and tsunami.  These individuals were also less likely to trust their government or the administrators of the Fukushima reactor complex, and they were also worried about potential long-term health hazards from radiation exposure.  

Mothers who were pregnant during the Fukushima disaster were particularly prone to anxiety and depression, worried about potential health hazards to their children.  There was also a sharp rate of increase of suicides by people who had lived in Fukushima prefecture, relative to people in areas more distant from the reactor disaster.  People who have studied this phenomenon believe that one factor in the rate of suicides is the Japanese cultural stigma against people with mental health disorders; this may prevent people who experience suicidal ideations from seeking the professional help they need. 

In Dec. 2011, the Japanese government imposed more stringent limits on allowable amounts of Cs-137 in food products.  For rice, meat, vegetables and fish the limit was decreased from 500 becquerel (Bq) per kilogram to 100 Bq/kg; for milk, milk powder and infant food, the limit was decreased from 200 Bq/kg to 50 Bq/kg; and for drinking water the limit was decreased from 200 Bq/kg to 10 Bq/kg.  (For calibration, 100 Bq is equivalent to 0.27% of a microCurie.) The new limits arose from several instances where “hot spots” were discovered – areas, plants or animals that were found to have anomalously high levels of Cesium.  Cesium-137 reacts in the same way as potassium, so when Cs-137 enters the body it can be taken up in bones, where it can cause cancers. 

In 2016, it was estimated that the bill for cleaning up the Fukushima disaster had increased to 22.6 trillion yen, equivalent to $201 billion.  The costs included compensation to those evacuated from the disaster area and those who lost their jobs as a result of the disaster.  The cost also included decontamination of radiation and for decommissioning the Fukushima Daiichi Nuclear Reactor complex.  The estimated cost of decommissioning the Fukushima plant had increased by a factor of four from earlier estimates.  The overall cost had doubled since an earlier estimate in 2013.  There is an exhaustive Wikipedia article on all the efforts taken to clean up the Fukushima plant and to prevent further releases of contaminated water into the ocean adjacent to the Fukushima reactor complex.

The Fukushima disaster has had an enormous impact on Japan’s nuclear power industry, as well as considerable impacts in other countries. Nuclear energy had been a strategic priority in Japan since the 1960s and was supplying nearly a third of the country’s electricity before 2011.

In the wake of the accident Japanese authorities suspended operations at 46 of the country’s 50 operating nuclear power plants, and only nine of those have resumed operations in the years since. In 2019, fission reactors were supplying only 7.5% of Japan’s electricity. In the wake of Fukushima, Germany and Belgium decided to phase out nuclear power completely by the mid-2020s, while other European countries shelved plans to build new nuclear reactors. According to the International Atomic Energy Agency, “Between 2011 and 2020, some 48 GWe of nuclear capacity was lost globally as a total of 65 reactors were either shut down or did not have their operational lifetimes extended.”

Summary, Nuclear Accidents:

What lessons can we draw from the three nuclear-reactor accidents that we have covered here?  First, we can separate Chernobyl from the other two accidents.  It appears that Chernobyl was an accident waiting to happen.  The design of the reactors at Chernobyl was seriously flawed; the reactor design allowed a “runaway” scenario, where increasing reactor temperature increased the amount of steam generated in the reactor; since steam absorbed far fewer neutrons from fission than water, an increase in the amount of steam increased the number of fission reactions.  This in turn raised the temperature of the reactor and produced more steam, creating a vicious cycle.  Furthermore, the control rods also had a serious design flaw; at the instant when they were inserted among the fuel rods, they caused the fission reactions to increase rather than decrease.  Finally, the containment structures outside the reactor vessel were seriously inadequate in containing the resulting steam explosions, which resulted in release of radioactive materials into the countryside surrounding Chernobyl, and also threw a “plume” of gas containing radioactive material, which then spread across Ukraine, Belarus, Russia and western Europe. 

The Chernobyl reactor operators were badly trained and failed to communicate well with the safety officers at the lab.  And finally, the Russians tried to cover up the accident by not announcing it until Swedish scientists measured a radioactive cloud passing over their territory, and they threatened to alert the International Atomic Energy Agency.  It is hard to imagine a situation more conducive to a major accident than the culture and reactor design at Chernobyl. 

The situation at Three Mile Island was marked by defects in the reactor design, and by the culture of safety at that reactor.  The operators did not realize that a crucial valve was open, and thus the reactor suffered a loss of coolant accident (LOCA).  Since the operators failed to recognize the emergency situation inside the reactor, temperature rose to a point where coolant water was converted to steam.  Eventually, the top of the fuel rods became exposed to air; when this occurred, the fuel rods and their cladding melted, and molten material fell to the bottom of the reactor vessel.  The midnight-shift reactor staff failed to take action before the meltdown occurred.  Once the morning shift arrived, they immediately realized the dire situation in Three Mile Island reactor 2, and took steps to alleviate the situation.  However, in the process some gases containing radioactive materials were released into the atmosphere.  In addition, some radioactive water was also released from the reactor vessel. 

The Three Mile Island accident was not as serious as either Chernobyl or Fukushima, because the reactor vessel and containment structure prevented explosions from releasing major amounts of radioactive materials into the surrounding countryside.   However, this accident revealed some design flaws in the reactor; it also illustrated bad communications between the reactor staff and the safety officers, and it showed that the reactor staff was not sufficiently prepared for this emergency.  And the U.S. nuclear power community should not be complacent just because we have never had a level-7 reactor incident.  Over the years, half of the nuclear-reactor incidents in the world have occurred at U.S. facilities.  This suggests that the safety culture at U.S. nuclear power reactors needs to be dramatically improved.  

The situation at Fukushima was triggered by a major disaster.  The Tohoku earthquake on March 11, 2011 was one of the largest ever recorded off Japan, measuring 9.1 on the Richter scale.  And the quake was followed by a massive tsunami that killed nearly 20,000 Japanese.  The maximum height of the tsunami reached 39 meters at Miyako city, and 14 meters at the Fukushima reactor complex.  The Fukushima complex was run by the Tokyo Electric Power Company or TEPCO.  They had taken some steps to insure that a tsunami would not knock out the emergency power systems at the plant; however, those backup power systems were connected to the lab’s turbine buildings, which were not watertight. 

The Fukushima reactor was located right by the Pacific Ocean. The TEPCO designers had built a seawall capable of stopping a tsunami with a height of 5.7 m; however, this was completely breached by the massive waves following the Tohoku earthquake.  The tsunami knocked out the emergency power systems that were designed to supply coolant to the reactor in the event that the normal systems failed.  The control rods were automatically activated as soon as the earthquake hit, and they shut down the fission reactions in the reactor.  However, without coolant circulating in the reactor, the temperature rose rapidly inside the reactor. 

Eventually, the tops of the fuel rods were exposed to the air; the temperature increased to a point where the fuel rods melted, and the zircalloy coating of the rods underwent a reaction with the steam that produced hydrogen.  Three or four of the reactors at Fukushima experienced a hydrogen explosion, which sent gases containing radioactive material across the surrounding area.  In addition, the plant operators injected seawater into the reactors, in an attempt to cool down the fuel rods.  Also, a great deal of contaminated water was pumped into the sea; part of this was deliberate releases of contaminated water, and part of this was through leaks in the reactor pipes. 

It has proved difficult to estimate the number of increased cancers that can be expected from the release of radioactive material from the reactors.  One method of estimating the expected number of cancers is the Linear No-Threshold (LNT) model.  The number of cancers from exposure to large or moderate amounts of radioactive materials is well known.  However, for exposure to very small amounts of radioactive material, we do not have accurate statistics to know their effect.  The LNT model assumes that the response vs. dose curve follows a straight line, right down to zero radioactive material.  The LNT model most likely gives an overestimate of the number of cancers. 

Estimates of expected cancers and deaths from these nuclear accidents depend upon the political views of those who perform the estimates.  Mainstream scientists and physicians tend to produce the lowest estimates of deaths.  Scientists with anti-nuclear views, such as the Union of Concerned Scientists, often arrive at estimates that may be an order of magnitude more than their peers.  And outspoken environmentalists such as Greenpeace have come up with estimates that can be orders of magnitude larger than those of mainstream scientists.   

These reactor accidents have caused great strains on the mental health of those involved.  The resulting evacuations of populations around the nuclear power stations, dislocation from friends and family, prohibitions on growing crops or raising animals in contaminated areas surrounding the reactors, loss of jobs, and concerns about radiation damage to children and pregnant women have caused severe and lasting damage to mental health. 

The accidents have also been tremendously expensive.  There are the costs of halting and remediating the release of radioactivity from damaged reactors.  There are also costs of evacuating people and providing relief to families that have been displaced.  There are also continuing costs of monitoring the status of soil and water surrounding the nuclear plants.  These costs have been in the hundreds of billions of U.S. dollars for the Chernobyl and Fukushima plants.  And there have been claims that the cost of remediation of the Chernobyl disaster was so high that it became a major factor in the collapse of the Soviet Union that began with the destruction of the Berlin Wall in November 1989. 

Nevertheless, there are countries that have never experienced a reactor incident that was rated as more serious than 3 on the International Nuclear Event Scale (on the INES scale, level 3 denotes a “serious incident.” It is determined by radiation exposure greater than 10 times the annual limit for workers, and non-lethal health effects from radiation (i.e., radiation burns).  It can also refer to exposure rates of greater than 1 Sievert/hr in an operating area, or severe contamination in an area where it was not expected, but with a low probability of exposure to the general public.  Countries such as Germany, South Korea, Taiwan and Pakistan have never reported a nuclear accident that was rated higher than 3 on the INES event scale.   

Nonetheless, these three incidents reveal vividly that fission reactors are complex instruments that can be subject to extremely dangerous runaway conditions. Reactor designers have seldom foreseen all the ways things might go wrong. The nuclear power industry and government regulatory agencies have typically underestimated dangers and failed to report the effects of meltdowns accurately or even consistently. Operators are not always sufficiently well trained to recognize or handle anomalies that may lead to meltdown. “Fail-safe” automated safety systems sometimes fail. Temporary storage of spent fuel rods on site at nuclear power plants pose an additional threat for radiation release, and permanent storage facilities for the radioactive waste have seldom materialized. Estimates of long-term health hazards from radiation exposure vary over such a wide range that the general public is justifiably fearful of releases from fission power plants. Hence, if fission power is to play a significant role in replacing fossil fuels, new, more robust reactor designs are essential. That is the topic we address in Part III, while we survey the options for nuclear power from thermonuclear fusion in Part IV.