On the morning of Friday April 25th, 1986, the fourth attempt for a safety test was planned for reactor No. 4 in the Chernobyl Nuclear Power Plant in the northern part of the Ukrainian Soviet Socialist Republic [fn1].  The test wanted to simulate an electrical power outage since there was a concern that water circulation was insufficient to cool the reactor during such a failure.  Back-up generators took about one minute to provide full power, and this minute was considered a risk factor for the nuclear reactor core to overheat.  

In an RBMK-type nuclear reactor, commonly used in the Soviet Union and still in use in Russia today [fn2], most heat is generated from nuclear fission, with less than 10% generated by radioactive decay of the accumulated fission products.  This additional heat, called decay heat, continues after the fission chain reaction is shut down.  Continued circulation of coolant after shutdown is still vital to prevent the core from overheating. RBMK reactors use water as coolant, which is circulated by electrically powered pumps. Each nuclear reactor has three backup diesel generators which take about one minute to reach full speed and provide full coolant power.  

This “minute gap” was deemed a safety risk of the design, and so the coolant capability during this gap required experimental testing.  A test manual existed, but the authors were not aware of the detailed nuances of the RBMK-1000 reactor behavior during the planned real operating conditions.  The test was considered a simple test of the generator, not a complex test of the entire unit although it still involved critical systems. The three previous tests had failed to suggest a solution. The test necessitated disabling of certain safety systems (emergency core cooling system) which received approval from the chief engineer.  

The experimental procedure from the test procedure manual included: [fn3]

  • The test would take place during the scheduled reactor shutdown
  • Reactor power reduced to between 700-800 MW
  • Steam turbine generator to run at normal speed
  • Steam supply to turbines to be shut off
  • Residual turbine performance monitored to evaluate whether coolant power was sufficient for this nearly one minute gap until the diesel generators reached full power

The test was scheduled for the morning of Friday, April 25th, 1986, as part of the planned reactor shut-down.  The daytime workers and a special team of electrical engineers was prepared to run the gap test after planned conditions had been met.  Reactor No 4 was reduced to 50% power, the teams completed remaining preparations for the test, and the emergency core cooling system was shut down.  Additional unrelated normal maintenance tasks were performed in the morning, with the gap test scheduled for the afternoon.   Unexpectedly that afternoon, another regional power station went offline, and the Kiev power grid requested that no additional power reductions occur until after the evening peak demand.  The test was accordingly postponed from the afternoon to the late evening.  

The evening shift replaced the day workers.  The emergency core cooling system was left disabled, as changing it required multiple workers manually turning sailboat-sized valve wheels. At 11:04 PM local time, Kiev allowed the additional power decrease.  The day shift had left hours previously, the evening shift was preparing to leave, and the night shift would arrive at midnight. The initial plan had scheduled the night shift to only be responsible for the maintenance of the decay heat cooling systems on the shut-down reactor No 4, as the test would have normally been completed hours previously. The night shift had very little time to prepare, and the highest local authority overruled any objections from the workers. 

Reactor No 4 reduced power to 720 MW at 12:05 AM on Saturday April 26th.  RBMK reactors produce a fission byproduct, xenon-135, which absorbs neutrons and thereby inhibits fission. (This is normally burned off during normal operations.) During this low power state, the xenon-135 continued to increase, causing the core power to automatically decrease to less than 30 MW. This power level was insufficient for the test, and so workers raised power by disconnecting most of the reactor control rods from the automatic control system and manually moved the rods to their upper limits to counteract the effect of the xenon-135.  The power slowly increased to 200 MW.  

As part of the test, extra water pumps were activated at 1:05 AM.  The increased coolant flow rate on the lower powered reactor hindered the ability of the water to release energy as steam – the coolant water did not have enough time to release heat within the turbines or cooling towers.  Water absorbs neutrons more effectively than steam, which additionally decreased the reactor power. Workers counteracted this by turning off two of the pumps and removing more manual control rods to maintain a sufficient power level, leaving almost no safety margin.    

The test finally began at 1:23:04 AM.  The steam to the turbines was shut off, and the diesel generators started, expected to provide sufficient coolant pump power at 1:23:43 AM. At 1:23:40 AM, an emergency shutdown of the reactor was manually initiated.  This engaged the drive mechanism on all control rods to fully insert them into the water of the core and stop the fission reaction.  

SCRAM button at the Experimental Breeder Reactor I in Idaho, USA.
Sometimes the switch will have a flip cover to prevent inadvertent operation.
By Alan Levine, CC BY 2.0, rid=17466801

The rods require 18-20 seconds to travel the length of the core.  The design of an RBMK reactor includes a graphite end at each rod, which helps boost reactor output when the rod is removed.  In an emergency shutdown, the rods are rapidly inserted back into the water.  Water is a neutron absorber, and so it by default decreases fission chain reactions.  The graphite tips are neutron moderators rather than absorbers, and so they boost reactor output relative to the water.  As the rods moved down and displaced the water with graphite, the fission reaction rate increased in the lower part of the core, spiking the power output and overheating.  This overheating caused some of the fuel rods and channels to fracture, preventing the control rods from inserting the rest of the way.  Within three seconds, the reactor power nearly tripled to 530 MW.  

The reactor quickly jumped to 30,000 MW, 10 times the normal operational output.  (This was the last reading of the power meter on the control panel.)  Explosive steam pressure from the damaged fuel channels likely escaped into the exterior cooling structure and blew out the reactor casing, causing the first heard explosion.  

A second explosion occurred several seconds later, which dispersed the damaged core, terminating the majority of the fission reaction.  This ejected hot chunks of graphite which drastically increased radioactive fallout in the adjacent area.  The explosion and fire ejected nuclear fuel, radioactive isotopes, and other radionuclides into the air.  One dosimeter was buried in the rubble of the building, and another was non-functional.  Remaining dosimeters only operated on much smaller limits = all of them reading “off-the scale.”  Following the inaccurate low readings, the reactor crew chief strongly felt that the reactor core was still intact. Worker testimony of distributed graphite and reactor fuel was rejected. An additional dosimeter that was brought in at 4:30 AM showed significantly higher radiation, but the findings were dismissed by the reactor crew chief as defective.  Crew members were sent without protective gear to try to pump water into the reactor. 

Material landed on the roof of the adjacent reactor No 3 and ignited additional fires.  At 1:45 AM, firefighters arrived.  They were not told about the radioactivity of the smoke or debris.  The exterior fires were extinguished by 5 AM, but the fire inside reactor No 4 continued to burn [fn4]. Several firefighters noted that the area “tasted like metal” and that they felt “pins and needles” all over their face.   

The nearby city was not immediately evacuated. After just a few hours, dozens of people became sick, reporting to medical centers with severe headaches, metallic tastes in their mouths, and some with uncontrollable coughing and vomiting.  Moscow authorities centrally ran nuclear power plants and so the government of Ukraine was not promptly informed..  

In a recent dramatization, during a meeting of communist leaders shortly after the explosion, one of the members recommends that the nearby city of Pripyat be evacuated because firefighters outside are showing signs of radiation exposure.  A local Communist Party elder countered: 

Our faith in Soviet Socialism will always be rewarded.
The state tells us the situation here is not dangerous. Have faith, comrades.
The state tells us it wants to prevent a panic. Listen well.
It is in my experience that when the people ask questions that are not in their own best interests, they should simply be told to keep their minds on their labor and leave matters of the state to the state. That is how we keep the people from undermining the fruits of their own labor.
Yes comrades, we will all be rewarded for what we do here tonight. [fn5]

On April 27th, over 36 hours after the initial explosion, the evacuation of the area was ordered, after multiple people had already died and dozens were hospitalized.  No public notification was made until April 28th. [fn6]  [fn7]

“That scene from HBO’s Chernobyl miniseries is more terrifying than anything I’ve seen in a horror movie in decades. Or maybe not, because Chernobyl really is a horror movie: not just about errant technology, but also a maleficent portrait of an ideology that denies the existence of error.” [fn8]


  • The tension around ideology will always exist.  What are some productive ways to engage with this tension?
  • Postmortems are nearly always easier than evaluation during the moment. How does one appropriately do a postmortem and then do better during future challenges? 
  • In this situation, something less than one minute made a difference.  Detailed nuances like the design quirks of an RBMK nuclear reactor made a difference from the experiment handbook. How does one engage with any potential conflicts between ideology and context/nuance?
  • The policy of openness (glasnost) and ‘uncompromising criticism’ of outmoded arrangements had been proclaimed at the 27th Congress (of the Communist Party of Soviet Union), but it was only in the aftermath of Chernobyl that glasnost began to change from an official slogan into an everyday practice. Are there any ways other than a disaster that will help take inherently good organizational policies from a slogan to a practice? 
  • This explanation of the events of Chernobyl is still an oversimplification.  I deal with radiation professionally, but I’m neither a nuclear physicist nor a nuclear engineer. While simplifications are still necessary in my field, I still have to engage daily in nuance.  In your area or profession, are there nuances that are ultimately vital, that someone outside your field could potentially reject to their detriment? 
Public Twitter, March 3, 2020

[fn1] https://www.iaea.org/newscenter/focus/chernobyl

[fn2] https://en.wikipedia.org/wiki/RBMK These RBMK nuclear reactors are still in use today in cities in and around my mission. 

[fn3]  “INSAG-7: The Chernobyl Accident: Updating of INSAG-1” (PDF). IAEA. 1992. Archived (PDF) from the original on 20 October 2018.

[fn4] Medvedev, Zhores A. (1990). The Legacy of Chernobyl (First American ed.). W.W. Norton & Company. ISBN 978-0-393-30814-3.

[fn5] While this quote is a dramatization from the HBO miniseries “Chernobyl”, Valentyna Shevchenko, then Chairwoman of the Presidium of Verkhovna Rada Supreme Soviet of the Ukrainian SSR, recalls that Ukraine’s acting Minister of Internal Affairs Vasyl Durdynets phoned her at work at 09:00 to report current affairs; only at the end of the conversation did he add that there had been a fire at the Chernobyl nuclear power plant, but it was extinguished and everything was fine. When Shevchenko asked “How are the people?”, he replied that there was nothing to be concerned about: “Some are celebrating a wedding, others are gardening, and others are fishing in the Pripyat River”. Валентина Шевченко: ‘Провести демонстрацію 1 травня 1986–го наказали з Москви’. Istorychna Pravda (in Ukrainian). 25 April 2011. Archived from the original on 26 April 2016. Retrieved 20 August 2011.

[fn6]  https://www.youtube.com/watch?v=sC7n_QgJRks

[fn7] The policy of openness (glasnost) and ‘uncompromising criticism’ of outmoded arrangements had been proclaimed at the 27th Congress (of the Communist Party of Soviet Union), but it was only in the tragic days following the Chernobyl disaster that glasnost began to change from an official slogan into an everyday practice. The truth about Chernobyl that eventually hit the newspapers opened the way to a more truthful examination of other social problems. More and more articles were written about drug abuse, crime, corruption and the mistakes of leaders of various ranks. A wave of ‘bad news’ swept over the readers in 1986–87, shaking the consciousness of society. Many were horrified to find out about the numerous calamities of which they had previously had no idea. It often seemed to people that there were many more outrages in the epoch of perestroika than before although, in fact, they had simply not been informed about them previously.” Kagarlitsky 1989, pp. 333–334

[fn8] https://reason.com/2019/05/03/hbos-chernobyl-presents-catastrophic-history-as-horror/