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What Caused the Chernobyl Disaster?

Since its advent, nuclear power has been a contentious topic due to its duplicitous nature. It is a zero-emissions clean energy source meaning that nuclear power plants produce no greenhouse gasses as a byproduct of their use (World Nuclear Association, 2019, p. 1). However, if managed improperly, nuclear energy can result in catastrophic events. In a nuclear reactor, such as a pressurized water reactor, the process of fission is utilized to produce steam, turn turbines and create energy. Fission is the process by which atoms are split apart, typically when hit by a neutron, which releases a large amount of radiation, energy, and heat as well as other neutrons which start a chain reaction; nuclear power plants typically use a specific form of uranium as fuel which is easily split apart in fission reactions (Unknown, U.S. Energy Information Administration, p. 1). When more heat is produced, the atoms in the reaction move more quickly, increasing the reactivity of the fuel, and in turn, increasing heat again in a feedback loop. Therefore, these reactions have to be moderated and controlled in a nuclear power plant through control rods.

Control rods, neutron absorbing movable rods, are used in both reactors to control the chain reaction caused by neutrons because of fissioning atoms being released and creating more fissions. By absorbing neutrons, the control rods sustain the chain reaction and keep it at an efficient rate, (U.S. Energy Information Administration, p. 1).

Figure 1: Structure of a Pressurized Water Reactor. Sarah Harman. 2019.

Throughout the history of nuclear reactors, there have been several disasters involving nuclear power and radioactive material. While nuclear power is a green alternative to burning fossil fuels, communities are often hesitant to live near nuclear power plants after these catastrophes occur. Incidents such as Chernobyl highlight the potential for danger in operating a nuclear power plant (Bromet, 2014, p. 1).

How The Chernobyl Reactors Functioned

The accident at the Chernobyl Nuclear Power Plant located on the Pripyat River in Ukraine in 1986 ─ when the location was a part of the Soviet Union─ is known as the worst nuclear disaster to date. While the estimated death toll of this event reaches 4,000 people due to premature cancer deaths, the USSR party line dictated that the death toll sits at 31 people (World Health Organization, 2005). According to these declarations, in the immediate aftermath, the reported deaths were 28 power plant workers and firefighters dying of acute radiation poisoning (Greenspan, 2022, p. 1). To understand how this disaster occurred, it is first important to understand how the Soviet-era nuclear reactors function in scientific terms.

Figure 2: Structure of an RBMK Reactor. Wikimedia Foundation, Inc. 2008.

The type of reactors used at Chernobyl were known as RBMK reactors which were rarely used outside of the Soviet Union. RBMK reactors are “water-cooled reactor(s) with individual fuel channels and [use] graphite as its moderator,” (World Nuclear Association, 2022). The purpose of a graphite moderator is to slow down the speed at which neutrons travel; if the neutrons travel too fast, a fission reaction is unlikely to occur. Therefore, graphite increases reactivity by decreasing neutron flux (Rhodes, 1993, p. 1). Additionally, the water that cools the core also acts as a neutron moderator, however, when heated and evaporated, steam does not absorb neutrons in the same manner. Steam in this scenario is known as a void, and these specific reactors are associated with a positive void coefficient. When a void is created, the concentration of neutrons increases, enhancing the chain reaction and the reactivity of the system (Hyland, 1987, p. 1-3). Therefore, the more steam in the reactor, the higher the reactivity, creating a positive feedback loop that is typically mediated by the fact that more heat actually reduces the reactivity of the uranium.

Figure 3: Uranium Decay Chain Reaction. OpenStax. 2022.

One of the main flaws of the RBMK reactors lies with their control rods ─though the rods are made of boron which absorbs neutrons to prevent them from colliding with the uranium fuel and lower reactivity. However, the tips are coated with graphite which displaces the water that would otherwise fill the control rod channel if the rod were extracted (Afanasieva et al., 1993, p. 1-7 ). Both water and graphite moderate neutrons to slow neutron flux and increase reactivity, yet graphite is a more effective moderator; additionally, even though water can act as a moderator, it also is used to cool the fuel rods and reduce reactivity. When a control rod is fully retracted, part of the graphite moderator is still within the channel and water fills the lower half (Afanasieva et al., 1993, p. 1-7). As the control rod is reinserted into the channel and the water is displaced reactivity and power increase. Furthermore, one of the final pieces of information that is central to understanding how the disaster occurred is that a by-product of uranium fission is the chemical xenon which usually is burned away by heat and other fission reactions. However, when it is not burned away, xenon gas decreases reactivity (International Atomic Energy Committee, 1992).

Events Leading to The Explosion

The events of late-night April 25th and early morning April 26th, 1986, at the Chernobyl power plant began with a routine safety test that ended disastrously. Reaction number 4 was set to be shut down on April 25th, 1986, when the day shift crew ─who had already begun to reduce power- was told to wait. According to Greenspan (2022, p. 1), “The test is supposed to determine whether, in the event of a power failure, the plant’s still-spinning turbines can produce enough electricity to keep coolant pumps running during the brief gap before the emergency generators kick in”. The test did not continue until April 25th, 11:10 p.m., after the night shift had arrived and the reactor had been held at half power for nearly half a day. This was a particularly unstable situation due to the xenon that had built up in the reactor—due to the reactor being held at half power for so long, the xenon that was created did not burn away and created a situation known as the poisoning of the core (Greenspan, 2022, p. 1). By 12:28 p.m., the power plant operators were unable to bring the power back up to stable levels due to the xenon poising and they began to pull the control rods part way, halfway, then all the way out in an attempt to bring the power up. Most of the 211 rods were pulled out.

Figure 4: Control Room of Chernobyl Nuclear Power Plant Reactor 4. Patrick Landmann. 2006.

At 1:00 a.m., the power stabilized at a level lower than what is considered stable and the emergency safety features were turned off to proceed with the test even though the power was still too low to yield significant results (Greenspan, 2022, p. 1). The xenon concentration was still high enough to prevent the power from rising, and xenon was still being created without being burned away. There was also a high concentration of water in the core at this time due to the pumps continuing to run. When the test began at 1:23:04, the water stopped being pumped into the core, and without sufficient control rods, the reactivity and temperature rose rapidly (Greenspan, 2022, p. 1). The remaining water is converted to steam creating a void that also increased reactivity and burns away the xenon. As the power beings to surge, at 1:23:40, a power plant employee presses the emergency shut down button which instantly inserts all of the control rods. However, since the control rods were tipped with graphite, the already rising reaction increases beyond what was thought possible (Greenspan, 2022, p. 1). Any water left was converted to steam which expanded the fuel rods and channels; the rods are jammed in place and the reaction was endlessly rising. At 12:23 and 45 seconds, the reactor lid was thrown off and the core of Chernobyl reactor 4 exploded, scattering radioactive material such as graphite across the roof and ground of the power plant and carrying it in the smoke and winds from the fire to communities across the USSR (Greenspan, 2022, p. 1). The amount of radioactive material that was released was estimated to be equivalent to ten of the Hiroshima bomb (Rhodes, 1993, p. 1).

Figure 5: Chernobyl Reactor Four After the Explosion. Igor Kostin & Laski Diffusion. 1986.

The explosion at the Chernobyl nuclear power plant and its aftermath had a considerable human toll, as well a human cause. Firemen who rushed to the scene died of acute radiation poisoning while 350,000 people were evacuated and countless other workers who were conscripted to help with cleaning up the scene would likely die of radiation poisoning (World Health Organization, 2022). Chemically, the way something like this disaster can occur is clear-cut. It is the human aspect that is more difficult. The design of the RBMK reactor in regard to the graphite tipped control rods were classified as state secrets and even the men running the power plant weren’t aware of that flaw (Rhodes, 1993, p. 1). While they may have disregarded the safety features of the reactor to complete a test and created the circumstances for the explosion, they did so with the knowledge that there was an undo button. “Without question, the accident at Chernobyl was the result of a fatal combination of ignorance and complacency,” (Rhodes, 1993, p. 1).

Bibliographic References

Afanasieva, A. A., Burlakov, E. V., Krayushkin, A. V., & Kubarev, A. V. (1993). The Characteristics of the RBMK Core. Nuclear Technology, 103(1), 1–9. Bromet, E. J. (2014). Emotional Consequences of Nuclear Power Plant Disasters. Health Physics, 106(2), 206–210. Greenspan, J. (2022). Chernobyl Timeline: How a Nuclear Accident Escalated to a Historic Disaster. HISTORY. Hyland, M. (1987). Reactivity Coefficients in Nuclear Reactors. Europhysics News, 18(11–12), 133–137. Rhodes, R. (1993). Chernobyl | Nuclear Reaction | FRONTLINE | PBS. PBS. International Atomic Energy Agency, International Nuclear Safety Advisory Group, International Atomic Energy Agency, & International Nuclear Safety Advisory Group. (1992). The Chernobyl Accident. International Atomic Energy Agency. World Nuclear Association. (2019). Nuclear energy and climate change. World Nuclear Association.,electricity%20when%20compared%20with%20solar.

World Nuclear Association. (2022). RBMK Reactors | reactor bolshoy moshchnosty kanalny | Positive void coefficient. World Nuclear Association.

World Health Organization. (2005). Chernobyl: the true scale of the accident. World Health Organization.

U.S. Energy Information Administration. (2022). Nuclear explained. U.S. Energy Information Administration (EIA).

Visual References

Cover Image: Mazin, C. Chernobyl. (2019). HBO. [Photograph]. Image retrieved from

Figure 1: Harman, S. Pressurized Water Reactors. (2019). [Illustration]. Image retrieved from

Figure 2: Unknown. Schematic Drawing of an RBMK Reactor. Wikimedia. (2008). [Illustration]. Image retrieved from

Figure 3: Unknown. U-235 Fission Chain Reaction. OpenStax. (2022). [Illustration]. Image retrieved from

Figure 4: Landmann, P. Control Panel of Reactor Unit 4 Inside The Chernobyl Exclusion Zone. Getty Images. (2006). [Photograph]. Image retrieved from

Figure 5: Kostin, I., Diffusion, L. Chernobyl Nuclear Power Plant Weeks After Disaster. Getty Images. (1986). [Photograph]. Image retrieved from


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Erica Littman

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