It would be wrong to take for granted that everyone, or even most people know about the series of events that lead to what would become one of the major catastrophes of modern times, and that's why I want to give you a short briefing on what went on in Reactor 4 on that early morning of April 1986.The reactors used in the Chernobyl NPP (Nuclear Power Plant) were of the type called RBMK, Реактор Большой Мощности Канальный (Reaktor Bolshoy Moshnosti Kanalniy), or to be more precise - RBMK-1000, and to put it short, you could say that the accident was due to partly the construction of the reactor, partly due to the lack of knowledge of the operating personnel present, but as such a statement without explanation could lead to wrong conclusions and assumptions being made, I will try to explain further.
It's said that back then "no one really knew how these reactors worked" and even though the amount of truth of statements as such could be doubted, one thing is for certain: The RBMK reactors were completely differently constructed compared to any other nuclear reactor in the world. In a RBMK reactor every fuel element is surrounded by a pressure tube, where water is pumped through the fuel element to make it boil. Every pressure tube is surrounded by graphite which acts as the neutron moderator in the chain reaction. At low effects this construction leads to that the reactor easily becomes unstable and an increased effect will lead to increasement of the amount of steam, which in turn leads to further increased effect. This is called positive void coefficient. Most reactors run a negative void coefficient, which means that increased heat and steam leads to poorer moderation, thus also decreased fission and thereby a lesser effect.
At the night of the accident, an experiment was run in the 4th reactor. The 
purpose of the test was to determine whether the turbines could keep on delivering power if the external power supply was interrupted and in order to perform this experiment, certain security systems had been temporarily disconnected. During the experiment, the 4th reactor's providing of power to Ukraine's capital Kiev was supposed to be interrupted, but as there occured a lack of power in the grid, new orders were received to continue the power supply and return to normal production. When the effect again suddenly increased, the core lacked of certain neutron absorbing isotopes that slow down the effect. The rapid and unplanned increased effect could thus happen at a much faster pace than during normal circumstances.
The cooling water in the reactor vaporized, which further increased the effect and the result was an explosion that destroyed the very reactor vessel and lit the moderating graphite.
The upper part of the reactor building had a thin roof and thin walls and the lower parts of the building's walls were made of concrete. The steam explosions and the intense heat lifted the 2000 tons heavy reactor lid and dislocated it, which made it possible for radioactive particles to spread over a large area.
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The reactor contained many radioactive elements with varying half life - from seconds to weeks and when these decay, they produce heat and due to the extreme temperatures it became very difficult to put out the graphite on fire. Water could not be used. If water would have been used in the extinguishing process, the river Pripyat, close to the nuclear power plant, would have transfered radioactive substance to other towns by the river. Instead they tried smothering the fire from the sky...
Helicopters flew back and forth over the reactor, carrying soldiers who threw down sand bags into the broken chimney (later the sand was replaced by lead), thus exposing themselves to lethal levels of radiation. A pilot would take several turns a day and there are witnesses' reports of pilots throwing up and fainting during flights.
The extingushing process went on for nine days and during this time, a large amount of radioactive dust was spread through the air and the fire was the main cause of this.
purpose of the test was to determine whether the turbines could keep on delivering power if the external power supply was interrupted and in order to perform this experiment, certain security systems had been temporarily disconnected. During the experiment, the 4th reactor's providing of power to Ukraine's capital Kiev was supposed to be interrupted, but as there occured a lack of power in the grid, new orders were received to continue the power supply and return to normal production. When the effect again suddenly increased, the core lacked of certain neutron absorbing isotopes that slow down the effect. The rapid and unplanned increased effect could thus happen at a much faster pace than during normal circumstances.The cooling water in the reactor vaporized, which further increased the effect and the result was an explosion that destroyed the very reactor vessel and lit the moderating graphite.
The upper part of the reactor building had a thin roof and thin walls and the lower parts of the building's walls were made of concrete. The steam explosions and the intense heat lifted the 2000 tons heavy reactor lid and dislocated it, which made it possible for radioactive particles to spread over a large area.
.
The reactor contained many radioactive elements with varying half life - from seconds to weeks and when these decay, they produce heat and due to the extreme temperatures it became very difficult to put out the graphite on fire. Water could not be used. If water would have been used in the extinguishing process, the river Pripyat, close to the nuclear power plant, would have transfered radioactive substance to other towns by the river. Instead they tried smothering the fire from the sky...Helicopters flew back and forth over the reactor, carrying soldiers who threw down sand bags into the broken chimney (later the sand was replaced by lead), thus exposing themselves to lethal levels of radiation. A pilot would take several turns a day and there are witnesses' reports of pilots throwing up and fainting during flights.
The extingushing process went on for nine days and during this time, a large amount of radioactive dust was spread through the air and the fire was the main cause of this.
But what about the personnel of Reactor 4 that morning? Why didn't they do anything? The answer is simple - they couldn't do anything. Only the day shift personnel had gone through training and simulations but those operating at night, had not.
Two workers were killed by the first explosion. In the following three months, 28 fire fighters and liquidators would die from acute radiation sickness.
On the 27th of April, 1986, the evacuation of Pripyat began, and this is the alarm that went out to the almost 50 000 inhabitants.
Note: all of the photos in this post are official photo matters of the Ukrainian government, but I have no information about the exact dates they were taken. The mid right image is said to be a part of the control room of Reactor 4. 
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