On April 26th, 1986, the world experienced the worst nuclear disaster in history: the meltdown of the reactors in Chernobyl, Ukraine. The effects of the disaster were both immediate and long-term, affecting not only the then-Soviet Union but a host of other nations in the fallout zone.

Photo: Twitter/ @MrEwanMorrison

Now the subject of everything from horror movies and scientific documentaries, let’s take a look back at what exactly happened on that fateful day in April 1986.

What, and Where, Was Chernobyl?

Chernobyl was a reactor and nuclear power generator located near the city of Pripyat in northern Ukraine, just across the border from Belarus and only a hundred miles from the border with Russia.

Photo: Wikimedia Commons

At the time, Ukraine formed a part of the U.S.S.R, a socialist state that existed from 1922 to 1991, and covered former independent nations including Kazakhstan, Latvia, Lithuania, Estonia and Azerbaijan (among others).

The Chernobyl Nuclear Power Plant had four functioning reactors, each capable of producing about 1,000 megawatts of power. The plant generated about 10% of Ukraine’s electricity when it was in operation. The plant was begun in 1977 and was still under construction at the time of the accident (two more reactors were planned but not yet complete).

What Happened in April 1986?

On the 25th, plant workers were preparing to run a test of the steam turbines (which generated the plant’s power). The purpose of the test (ironically) was to try out a new potential emergency procedure, which would be used if the nuclear core needed emergency cooling.

Photo: Wikipedia

The power plant had generators to kick in if an emergency required them to cool off a core quickly. Risk managers, however, had determined that, in the event of a power grid failure, the cooling necessary to prevent core damage and meltdown in the reactors would be delayed (it would take the generators up to a minute to get up to speed). Therefore, they determined that a rapid cooling in the one-minute gap could be provided by the steam turbine as it wound down under residual steam pressure.

The test to be performed on the 26th would simulate an electrical outage and see if this plan worked. It would begin with a standard emergency shutdown (to simulate power loss) in reactor #4.

What Went Wrong?

In a nutshell, a lack of safety protocols and a serious lack of attention to detail and safety procedure.

The theory and the process were unproven and therefore should have been approved by both the chief designer of the reactor and the scientific manager of the plant to check for potential problems. This was not done, with only the director of the plant signing off on the test procedure.

Photo: Twitter/ @vicenews

Because of this lack of input from the chief scientists, several things were not done that needed to be done for the test to proceed smoothly. First, on the day of the test, a request came in for emergency power near Kiev. This delayed the test, and the day shift that had trained in the procedures were not the ones who carried the test out. It was left to night shift workers who had no training and no experience with the procedure.

Second, the reactor needed to be giving out energy at a rate of no lower than 700 megawatts. This is important because, in order for the “emergency cooling” to be tested, the reactor had to be hot to begin with. The reactor temperature was not checked, so when the experiment started, the reactor was “too cool” because of the delay in the time of the experiment.

The workers, not understanding the dangers, decided to disable automatic safety systems that would have protected from a meltdown so they could try to raise the heat in the reactor for the test to work. Their decisions to withdraw power rods (creating an unstable reactor environment) and to run up the steam generator despite this instability created a very dangerous situation. The control room received multiple and repeated emergency signals about the levels of water and steam from the neutron power controller. They ignored these.

Photo: Wikipedia

The last thing that sealed the fate of the reactor was that workers had disabled all of the fail-safe, automated and even passive safety features of the reactor so that in the event of critical failure, the reactor would not be able to recover automatically.

The details of the final explosion are still a bit murky, as all of the workers involved in the experiment have since died of radiation poisoning.  We do know that there was a power spike, the core overheated, and some of the fuel rods cracked. This series of events (we think) caused an increase in fuel temperature and steam buildup. This eventually caused the temperature of the reactor to jump to approximately 30,000 megawatts thermal (10 times the normal output). In all of this, a series of steam explosions occurred, ejecting radioactive material into the atmosphere.

Because of (again) lack of attention to safety regulations, the hot material landed on the roof of reactor 3, which had been built of flammable material. Reactor 3 caught on fire. The engineer in reactor 3 wanted to initiate an emergency shutdown, but his boss would not let him.


The levels of radiation in the vicinity of the two reactors were deadly. Exposure to 500 rotagens in the course of an hour is enough to kill a person. The levels in and around the plant ranged from 1,000 to 30,000 rotagens.

Managers and chiefs only had the use of dosimeters that would read low levels of radiation. The extremely high levels just read as “off scale.” They, therefore, made the decision that workers could work on cleanup. (They were perhaps not willing to admit how bad the problem was.)

Photo: Wikimedia Commons

Firefighters also arrived to try to put out the fires. They were also unaware of how bad the radiation was. The immediate concern was to stop the fires on reactor 3 to keep it from melting down.

Following the overall theme of lack of concern for safety protocols, the nearby city of Pripyat was not evacuated, and within hours of the explosion, hundreds were sick of radiation poisoning. Three days later, the city was evacuated.

In the end, heavily protected soldiers were sent in to try and clean up the mess, which was so radioactive that they could only work in 40-second shifts. They were only supposed to do this once, but many did 5 or 6 times. Pilots also risked the radiation, dropping bags of sand, lead and boric acid on the reactor from helicopters.

Photo: Instagram/ chernobyltours

Eventually, in October of 1986, the reactor was covered in a large, concrete box that was dubbed a “sarcophagus.”

In the end, many European nations were affected and contaminated with radioactive cesium. Belarus, Ukraine, Russia, and Sweden were the most affected, but other nations from Finland to Italy were also contaminated. Today its estimated that 4,000 people have died of cancer related to the accident and another 237 of acute radiation sickness.

The disaster at Chernobyl is a cautionary tale about the importance of safety features and procedures.