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The Causes of Chernobyl

by Joy Liang

(via BBC)

On April 26, 1986, the world experienced the explosion of Chernobyl’s Reactor 4 in Soviet Ukraine, which caused 400 times the radiation of the Hiroshima bombing to spread across most of Europe, including into Belarus, Sweden, and Germany. There are thirty one recorded deaths as a result of this, but this does not include the numerous thyroid cancers and illnesses as a result of radiation poisoning. A combination of small and large mistakes that ultimately led to the biggest nuclear disaster to date.

(This bridge is nicknamed the “Bridge of Death” in Pripyat, Ukraine. The people that stood there to watch Chernobyl’s fire were affected by the radiation carried by the wind and died shortly after. via Chernobyl Story Tours)

April 25, 1986, 13:05

This story starts with Chernobyl operators beginning preparations for the turbine test.

The turbine was supposed to keep the plant stable, in case of a defect, for twenty seconds until the emergency generators are activated. This test had failed a few years earlier. In order to run the test, the reactor’s capacity had to be reduced.


A dispatcher from the Ukraine electricity network delays the test by ten hours because they need electricity from Chernobyl’s Reactor 4.

In doing this, there were consequences. At midnight, there is a shift change. The night shift was less experienced than the day shift and did not know the proper procedure for the turbine test. It was stated by the day shift operators, Achier Razachkov, Yuri Tregub, and A. Uskov, that the procedure was only told to the day and evening shift, and not the night shift. This was the first mistake that ultimately led to the explosion at Chernobyl.

April 26, 1986, 1:00

Preparations for the turbine test begin again, this time with the night shift in charge. The operators have trouble keeping the plant stable. This is partly due to the design of the RBMK-1000 reactor which is infamous for being difficult to control, and partly due to the inexperience of the night shift workers. They make critical mistakes.

(A RBMK nuclear plant’s core in Lithuania. via Energy Education)

The control rods were raised higher than regulations allowed. Chernobyl’s RBMK-1000 reactor came with 211 control rods made out of boron, a non-reactive material, in order to stop a reaction. Raising the control rods meant that in the case of an accident and a reaction needed to be stopped, the rods would take too long to descend.

The reactor’s capacity drops below the lower limit of 720 MW thermal to 30 MW thermal. The personnel do not think anything is wrong and they continue the experiment instead of giving all attention to stabilizing the reactor.

To raise the capacity, a circulation pump is activated. The capacity of the reactor is further reduced instead of raised because of the strong cooling.

The automatic shut-down system is deactivated for the test. This leaves the plant with only one manual emergency shut-down system, the AZ-5 button. The automatic stabilization systems are also turned off, and so is the emergency cooling system.


The test begins. The plant’s capacity increases dramatically.


One of the operators responsible for the control rods presses the AZ-5 button to stop the reaction.

The AZ-5 button was supposed to lower all 211 control rods into the reactor at once.

Due to the high pressure in the reactor, the control rods get stuck halfway down. The tips of the rods were made out of graphite which is highly reactive, so instead of stopping the reaction, pressing the AZ-5 button only caused it to react more violently.


Pressure in the reactor increases and the water used for cooling turns into steam, further increasing pressure. The 1000 ton lid lifts. This is the first explosion. Oxygen gets into the reactor, which produces hydrogen, and the hydrogen explodes. This is the second explosion.

(Chernobyl after the explosions. via The Guardian)

The personnel do not believe the core has exploded, so they send out operators to examine it. They die as a result. Upon hearing this information, the head of the night shift still does not think anything is wrong, and orders the remaining staff to add cooling water. The operators responsible for the control rods die as well.

The rest as we know it is history.




Bannink, Dirk, and Henk van der Keur. Publication. Edited by Peer de Rijk . CHERNOBYL: CHRONOLOGY OF A DISASTER 724. Vol. 724. Takoma Park, MD: World Information Service on Energy, 2011. https://www.nirs.org/wp-content/uploads/mononline/nm724.pdf.

“Chernobyl Accident 1986.” World Nuclear Association. Last modified May 2021. Accessed December 11, 2021. https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident.aspx.

“Chernobyl Accident and Its Consequences.” Nuclear Energy Institute, May 1, 2019. https://www.nei.org/resources/fact-sheets/chernobyl-accident-and-its-consequences.

Malko, Mikhail V. Rep. The Chernobyl Reactor: Design Features and Reasons for Accident. Minsk, Belarus: Joint Institute of Power and Nuclear Research, National Academy of Sciences of Belarus, 2009.

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