It remains uncertain whether the information on the course of the accident is completely reliable. In 1987, five possible courses of events leading up to the accident were put forward. However, the following account is the one generally accepted.

The test
One of the tests incompletely carried out before the reactor becoming operational was on the functioning of the turbine in the case of a defect. [ more]

25 April (Friday)
13.05 hours (times are local times - Moscow times where similar then) -Preparations for the turbine test begin. For this test, the plant's capacity must be reduced and for this reason one turbine is turned off.
14.00 hours - The controller of the Ukraine electricity network requests that the test be delayed. All electricity from Unit 4 is necessary. It is not clear why it was not predictable beforehand that work would have to continue all through Friday afternoon in order to achieve the production planned for April.
16.00 hours - The day shift leaves. The members of this shift have been given information about the test during the previous days, and know about the entire procedure. A special team of electronic engineers is present.
23.10 hours - Preparations for the test start again. The ten hour delay has a large number of consequences. Firstly, the team of engineers is tired. Secondly, during the test, the evening shift is replaced by the night shift. This shift has fewer experienced operators, besides which they were not prepared for the test. Achier Razachkov, - Yuri Tregub and A. Uskov are the operators who were responsible for carrying out the test earlier in the day: later in interviews they declared that test procedures were only explained to the day and evening shifts. Yuri Tregub decides to stay and help the night shift.

26 April (Saturday)
01.00 hours - During preparations for the test, the operators have difficulty keeping the capacity of the nuclear plant stable. While doing this they make six important mistakes.
1. The control rods which can stop the reactor are raised higher than regulations permit. Operator Uskov of the day shift said later that he would have done the same. He said: "We often don't see the need to follow the instructions to the letter, because rules are often infringed all around us." As well as this, he pointed to the fact that during training it was repeated over and over again that "a nuclear power plant cannot explode". Operator Kazachkov said: "We have often had fewer control rods than were required, and nothing ever happened. No explosion, everything just went on as normal."
2. The plant's capacity decreases to below the safe level. Because of this the core becomes unstable. Preparations for the test should have been stopped by now. It should have been obvious that all attention should be given to measures for regaining the plant's stability.
3. In order to raise the capacity, an extra circulation pump is turned on. Because of the strong cooling down, the pressure falls, thus reducing the reactor's capacity rather than increasing it. Normally at this stage the scram system should start working, but in order to still be able to carry out the test, this system is turned off.
4. The automatic emergency shut-down system is turned off in order to prevent the reactor stopping itself.
5. The systems to prevent the' water level decreasing too much and the temperature of the fuel elements becoming too high are also turned off.
6. Finally, the emergency cooling system is turned off to prevent it working during the test.
1.23.04 hours - The real test now begins. The power plant's capacity suddenly increases unexpectedly.
1.23.40 hours - Leonid Toptunov, responsible for the control rods, presses a special button for an emergency shutdown. The test has been going on for 36 seconds.
1.23.44 hours - The control rods start to descend, but shocks can be felt. The operators see that the control rods have become stuck. The fuel tubes have become deformed because of the large increase in the steam pressure.
1.24.00 hours - The test has now been going on for 56 seconds. Pressure in the reactor is now so high that the fuel elements burst and small particles land in the cooling water. The cooling water turns into steam and pressure in the tubes increases: they burst.
The 1000 tonne lid above the fuel elements is lifted: the first explosion. The release of radiation starts. Air gets into the reactor. There is enough oxygen to start a graphite fire. The metal of the fuel tubes reacts to the water. This is a chemical reaction which produces hydrogen, and this hydrogen explodes: the second explosion. Burning debris flies into the air and lands on the roof of Chernobyl Unit 3. (There was barely any attention paid to this hydrogen explosion in the Soviet report about the accident. In studies commissioned by the US government, however, it was concluded that the second explosion was of great significance, and that the original explanation of the accident was incorrect. Richard Wilson of the Harvard University in the US said this second explosion was a small nuclear explosion.)
The head of the night shift, Alexander Akinhov, and the engineer responsible for industrial management, Anatoly Diatlov, do not believe that an accident has taken place. When somebody claims the core has exploded, they send out operators to examine the core. These people are killed by radiation. On hearing the report that the reactor has been destroyed Akimov cries out, "The reactor is OK, we have no problems."
Akimov and Diatlov, assisted by manager Bryukhanov and engineer N.Fomin, keep ordering the operators to add more cooling water. They remain convinced that there is nothing wrong. Akimov and Toptunov, who was responsible for the control rods, both died of radiation illness. Diatlov and Fomin were both sentenced to ten years imprisonment for infringement of the safety regulations. However, at the end of 1990 they were both released.

Unit 4 of the Nuclear power plant at Chernobyl explodes. Debris flies into the air and lands on the roof of Unit 3 which is right next to the exploded Unit 4. The units share a communal machine turbine hall with a roof of bitumen, a flammable material. Thirty fires develop. The fact that the accident happens at night has one great advantage: in the daytime, 2000 people are working on the construction of Chernobyl Units 5 and 6. These people are now at home.

01.25 hours - The fire alarm rings at the local fire station. Meanwhile more people are killed: The nuclear plant's fire fighters arrive with three fire engines. The leader, Lieutenant Pravik, quickly realizes that his team is too small and asks the fire brigades from Pripyat, the town of Chernobyl and the entire area of Kiev for their assistance. Pravik and his team climb onto the roof of the machine hail and start their attempts to extinguish the fire. The fire brigade, from Pripyat arrives minutes later and fights the fires in the reactor building. Pravik and several firemen from Pripyat die later of radiation illness.
01.45 hours - New teams of fire fighters from the area arrive. They know nothing about the danger of radiation, have no protective clothing or dosimeters. One of the fire engine drivers, Grigory Khmel said later: "We arrived at ten minutes to two in the morning. We saw graphite lying everywhere. I kicked a bit of it. Another fireman picked up a piece and said 'hot'. Neither of us had any idea of radiation. My colleagues Kolya, Pravik and others all went up the ladder to the roof of the reactor. I never saw them again."
02.15 hours - The Pripyat department of the Ministry of Home Affairs calls a crisis meeting. It is decided to organize a road block in order to prevent cars from entering or leaving the town. Police assistance is requested. Thousands of police arrive; and, as with the fire fighters, they have no knowledge of radiation, no dosimeters or protective clothing. Later, in 1988, it is admitted that a total of 16,500 police were deployed. Of those, 57 people developed chronic radiation illness, 1500 of them suffer from chronic respiratory problems and 4000 suffer from other symptoms.
03.12 hours - An alarm signal goes off at the army headquarters in the central area of the Soviet Union at 03.12 hours. General Pikalov decides to send in troops to help. They arrive in Kiev at 14.00 hours. These are the first people to arrive well prepared for their task. About the same time, the responsible authorities such as the Energy Minister, A. Mayorets, hear that an accident has occurred, but are led to believe it is a small defect.
05.00 hours - In spite of the fires, Chernobyl Unit 3 is not closed down until five o'clock am.
06.35 hours - No fewer than 37 fire brigades, with a total of 186 fire fighters, have been called in to extinguish all the fires; the fire in the reactor could not actually be extinguished. The importance of the deployment of these fire fighters cannot be emphasized enough. The roof of Unit 3 caught fire immediately, which meant that this reactor could have been seriously damaged as well. The nuclear plants' machine hell is also connected to Units 1 and 2. An explosion in the machine hall could have led to the destruction of all four Chernobyl reactors. An explosion was only averted by spraying nitrogen at the last minute. Four of the eight people who did this died shortly afterwards.
20.00 hours - A government committee is established, led by Valery Legasov; at eight o'clock in the evening the committee arrives in the area. They are surprised by the bits of graphite they see lying around. None of them suspect a graphite fire.

26 April to 4 May 1986: Most of the radiation is released in the first ten days. At first, southernly and southeasternly winds predominate. The first radioactive cloud went high into the atmosphere and winds blew it northwest away from Ukraine toward Sweden. It was Kiev's good fortune that the wind carried the radioactive cloud away at first rather than directly to the Ukrainian capital and its 3 million population as it did several days later. At the end of April the wind switches to the north and northwest. There are frequent but local showers. This results in a very varied regional and local distribution of the radiation. An estimated 75% of total released radiation contaminates Belarus.[see maps]

27 April (Sunday)
A radius of 10 km around the plant (cities of Pripyat and Yanov) evacuated (“for three days” they are told) (50.000 people) to the town of Poliske (50 km west – coincidently -?- wind is blowing in that direction too). Dosismeters are confiscated.
01.13 hours - The operation of Units 1 and 2 had already been stopped at 01.13 and 02.13 hours, twenty-four hours after the start of the accident at Block 4
07.00 hours - General Pikalov sets out in a truck fitted out with radiation apparatus. He rams through the closed gates and stops at the plant to measure the radiation. He establishes that the graphite in the reactor is burning, and that an enormous amount of radiation and heat is being given off. Shortly afterwards - the government in Moscow is warned. [ more]

28 April (Monday)
Forsmark NPP Sweden (times are Chernobyl-times)
09:00 am An alarm was sent from Reactor 1, where a routine check revealed that the soles of the shoes worn by a radiological safety engineer were radioactive.
Lars Wahlström, radiology supervisor at Forsmark, has given this summary of the events:
"Something indicated that radioactivity had leaked out from one of the blocks at Forsmark. Rumors about the activity circulated between noon and 14hours and people said 'Now let's leave here.' At the same time news arrived that radioactivity had been detected in Finland. I said, I want evidence. Among other things I called Studsviks Energiteknik AB, where management was sitting in a crisis meeting and where they said 'We think it's coming from one of our laboratories.' But that wasn't so. Soon I also began to have doubts that there was anything wrong in any of the Forsmark reactors, which I told the National Institute of Radiation Protection. We had even been inside the chimney and checked. Then the Institute said the fallout had come from somewhere in the east, and by around 15.30 it was determined that the fallout definitely did not come from Forsmark." [ more]
20:00 Radio Moscow broadcasts a Tass’ statement that there has been an accident at the Chernobyl nuclear power station and that there have been casualties. “Measures are being taken to eliminate consequences of the accident. Aid is being given to those affected. A government  commission has been set up” according to Tass. From about 30 minutes later west-european newsagencies are reporting an “incident in a Ukrainian nuclear reactor”
23:00 A Danish nuclear research laboratory announces that an MCA (maximum credible accident) has occurred in the Chernobyl nuclear reactor. They mention a complete meltdown of one of the reactors and that all radioactivity has been released.

29 April (Tuesday)
- The sixth item on the main television evening news program, Vremya, says that 2 people died during the accident, a portion of the reactor building was destroyed, and residents of Pripyat and three nearby towns were evacuated.
- The first real information in the western world came on Tuesday morning, when a powerful American reconnaissance satellite provided Washington analysts with photos of Chernobyl. They were shocked to see the roof blown off above the reactor and the glowing mass still smoking. The first Soviet photos of the Chernobyl accident were censored by removal of the smoke before being printed in the newspapers.
- The first official statement by German authorities: Minister of the Interior Zimmermann states there is no danger for the German public: “danger only exists in a radius of 30-50 km of the reactor”.
- Polish authorities decide to distribute iodine tablets in the north-east of the country to infants and children to protect them from thyroid cancer.

30 April (Wednesday)
- Tass carries a government statement denying western reports on mass casualties. The statement repeats the earlier assertion that only two people died during the accident and that 197 have been hospitalized and levels of radiation are decreasing
- Pressreports on fire in second unit: scientist see second fire on satellite images, claims are later denied
17.00 hours: The reactor fire seems to be extinguished.