Everything you need to know about the Chernobyl disaster

1970

Construction begins on Pripyat, one of nine atomgrad, "atomic cities" designed in the Soviet Union to house the employees of nuclear power plants and their families.

1972

The type of nuclear reactors to be installed at Chernobyl is decided. Viktor Bryukhanov, director of the plant, proposes Pressurised Water Reactors (PWR), warning the Ukrainian Minister of Energy Aleksei Makukhin that the RBMKs (РБМК – Реактор Большой Мощности Канальный, or Reaktor Bolshoi Moshchnosty Kanalny, meaning 'high-power channel-type reactor', a boiling water reactor) emit 40 times more radiation than PWRs. Scientist Alekzandrov objects, arguing that the RBMK-1000 is not only safer, but also produces the cheapest electricity. The decision is made to go with the RBMKs.

1977

The first of Chernobyl's reactors comes into operation, shortly followed by reactor no. 2.

1979

Pripyat is officially inaugurated as a city, and Chernobyl reaches its first 10 billion kWh of electricity produced.

1982

Partial meltdown of the core of reactor no. 1: the true extent of the incident will not be made public until 1985. The reactor is shut down, repaired and returns to operation after a few months.

1983

December. Construction of Chernobyl's reactor no. 4 is completed and the plant becomes operational on 20 December 1983. On 22 December, a day of celebration is declared for all public energy sector workers, with public praise and salary bonuses.

1985

April. Energy Minister Anatoly Mayorets decrees that information concerning damage caused by power plants to employees, residents and the environment may not be used by newspapers, radio or television.

1986

February. Vitali Sklyarov, Minister of Electrification in Ukraine, tells the newspaper Soviet Life about Ukrainian nuclear plants: "The odds of a meltdown are one in 10,000 years."

25 April, Friday

1:00. The test begins. The aim is to determine whether the diesel generators can start up in time to pump cooling water into the reactor in the event of a power reduction. Operators begin to reduce it gradually.

13:05. Twelve hours after the power reduction, the reactor is at 50 per cent. Of the two steam turbine generators, one is taken offline.

14:00. The reactor should be at 30 per cent, but the Ukrainian authorities request that the power not be lowered so as not to draw too much from the grid; the reactor therefore remains at 50 per cent for nine hours longer than planned, pushing the entire test back. However, as scheduled, the auxiliary cooling system is switched off.

26 April, Saturday

00:00. The night shift begins under supervisor Aleksandr Akimov, who replaces Yuri Tregub; Tregub nonetheless decides to stay and observe the test, also because, owing to the delay, the ten electrical engineers summoned for the test are exhausted and the night-shift staff are less experienced and not fully prepared.

00:28. Moderation rods are moved — boron rods coated in graphite that serve to reduce, increase or control the nuclear fission reaction of the fuel inside the core. A sudden drop in reactor power, from 1,500 MWt to 30 MWt, causes consternation. Akimov wants to abort the test; Anatoly Dyatlov insists on continuing. After half an hour, power stabilises at 200 MWt.

1:03. Four auxiliary cooling pumps are activated.

1:19. Operators disable the automatic shutdown systems, triggered by low water levels, for fear they might interrupt the test. Furthermore, to raise power by 7 per cent, they remove all but six moderation rods, in violation of procedures.

1:21. The fuel channel caps blow off. Valeriy Perevozchenko, from level 50 where he was stationed, witnesses caps weighing 350 kilograms flying into the air and crashing back down like meteorites onto the plant's structural elements.

1:21:50. Steam pressure in free fall.

1:23:40. An emergency reinsertion of the moderation rods is attempted: the water temperature is extremely high and the coolants are boiling. Akimov presses the AZ-5 button for a class-5 emergency. The moderation rods are, however, ineffective because they are only submerged two metres rather than the seven required. Akimov tries to disconnect their hooks to let them drop, but they do not move. Noise increasing, vibrations, the control panel indicates an absence of water flow and pump malfunction.

1:23:44. An explosion. The reactor reaches a power level 120 times higher than normal, the nuclear fuel disintegrates, and all the excess steam towards the turbines causes the pipelines to explode.

1:23:45. A second explosion — disaster. The 1,000-tonne cover of the nuclear fuel is hurled into the air, triggering a release of radiation. Air reaches the reactor and the oxygen ignites a terrifying graphite fire. The metal of the pipes reacts with the water, producing hydrogen, which explodes. This explosion was underestimated in Soviet reports for years. Subsequent investigations, such as that of Richard Wilson of Harvard University, have established that this was a small nuclear explosion.

Thirty-five tonnes of nuclear fuel are projected outwards. The column of expelled radioactive elements rose to a height of two kilometres and dispersed over a radius of 1,200 kilometres.

The first casualties among plant personnel, the lights go out, clouds of dust and smoke in the rooms.

1:26:03. Fire alarms are activated. Dyatlov orders the cooling systems to be started, assuming the reactor was intact and that the explosion had only involved the water tanks. Several employees are repeatedly sent to check levels 26 to 35, receiving fatal doses of radiation within minutes.

1:28. The first firefighters arrive, commanded by Volodymyr Pravik.

1:35. Firefighters are already battling the blaze in the turbine rooms. There are radioactive graphite fragments — each capable of delivering a lethal dose of radiation — scattered and smouldering within a radius of hundreds of metres.

1:56. Mayor Telyatnikov arrives on the scene.

2:00. Plant workers go outside to carry out checks, then return indoors. They will begin to feel ill three hours later.

2:15. The Pripyat department of the Ministry of Internal Affairs decides to set up roadblocks to stop vehicles entering the city. The police, however, like the firefighters, have no safety equipment whatsoever — no protective suits, no Geiger counters or dosimeters. Of these 16,500 police officers, 57 will die from radiation exposure, 1,500 will develop respiratory diseases and 4,000 will develop other illnesses.

3:00. Bryukhanov calls Maryin, Deputy Minister of Industry, and the news passes from Frolyshev to Vladimir Dolgikh, who eventually informs Gorbachev and the Politburo. By 4:00, Moscow is informed of the events.

4:30. Akimov's version continues to be followed; he believes the reactor is intact and the damage is confined to the water tanks. People keep being sent to check, their reports being ignored, and they are exposed to lethal doses of radiation. Water continues to be pumped into the reactor, causing further eruptions and short circuits that spread to all four units.

6:35. The 37 fire brigades extinguish all fires except the one at reactor no. 4. There is a risk that the heat and the fire will spread to unit 3, then 2, and then 1; moreover, an explosion in the turbine hall could lead to the destruction of all four reactors, with apocalyptic, unimaginable consequences. To avert this, they think at the last minute of injecting liquid nitrogen. It works. Of the eight firefighters who carried out the operation, four die from radiation shortly afterwards.

8:00. The morning-shift workers from all four units arrive. 286 labourers continue construction work on the fifth and sixth reactors.

20:00. A governmental emergency committee is formed, headed by Valery Legasov. Pieces of graphite are noticed all over the area. The residents of Pripyat and their families gather on the railway bridge, which offers an excellent view of the plant, to admire the multicoloured flames — like a rainbow, typical of a graphite fire — rising even above the smoke itself. Tragically, what they did not know was that the light breeze was carrying radiation of 500 roentgens (exposure to 750 roentgens/h is lethal). None of the people on the bridge that night survived.

27 April, Sunday

0:00. Convoys of buses arrive in Pripyat. They are awaiting the order to evacuate the city. They will remain on standby throughout the night.

1:13. Chernobyl reactors 1 and 2 are shut down. Twenty-four hours have passed since the accident at unit 4.

7:00. General Vladimir Pikalov, an expert in biological, nuclear and chemical warfare, decides to approach the plant in a truck equipped with radiation-shielding panels. He sees with his own eyes that it is the graphite in the core that is burning, releasing monstrous quantities of radiation and heat. At this moment, they have officially understood what was happening.

10:00. Helicopters begin to fly. From this point until 1 May, 1,800 helicopters deposit 5,000 tonnes of sand, lead, clay and boron — which in theory has the capacity to absorb neutrons — into the ruptured reactor. Unfortunately, it is now known that almost none of the materials dropped from above reached the core, and indeed they contributed to keeping it hot.

14:00. The evacuation of Pripyat begins. Residents are given two hours to gather their belongings. 43,000 people board 1,200 buses from Kyiv in great haste: they are told to take what they need for two or three days, some food and their documents. They will never return. Dosimeters and Geiger counters are confiscated.

28 April, Monday

9:30. An engineer at the nuclear energy plant at Forsmark, in Sweden, discovers that the soles of his shoes are radioactive. Air measurements also trigger an alarm. A leak from the plant is feared, but all checks prove negative. They discover radioactivity in the surrounding woodland, which suggests a worldwide event has occurred somewhere unknown.

21:00. State television announces that an accident has occurred at Chernobyl.

23:00. A nuclear research laboratory in Denmark reports a Maximum Credible Accident (MCA) localised at Chernobyl. For the first time, mention is made of a core meltdown and the consequent release of all the radioactivity.

29 April, Tuesday

As the sixth item on the USSR news programme Vremya, it is reported that two people died during the accident at Chernobyl, that part of the reactor is destroyed, and that the city has been evacuated. An American satellite transmits to Washington the shocking images of Chernobyl: the roof completely collapsed, masses of luminescent smoke.

30 April, Wednesday

Moscow denies the reports circulating in the Western world about the scale of the accident and repeats that there are two victims and 197 people in hospital, with radiation levels decreasing.

1 May, Thursday

The wind changes direction and blows towards Kyiv. The planned annual May Day parades are not cancelled either in Kyiv or in Minsk, so as not to cause panic. State bureaucrats, however, withdraw their own children and move away.
Playgrounds in southern Germany are closed.

2 May, Friday

Two cooling pools full of radioactive water risk triggering a third explosion, due to temperatures of over 1,200°C in the graphite and fuel, which upon contact with the water would create radioactive lava — corium. Three volunteer engineers, Alexei Ananenko, Valeri Bezpalov and Boris Baranov, dive in to open the valves and drain them. They succeed, but will die a few days later from radiation exposure.

6 May

Schools in Kyiv close. Radio warns the Ukrainian population not to eat broad-leaved vegetables.

7 May

Firefighters complete the draining of radioactive water from beneath the core, pumping out 20,000 tonnes. They receive as compensation 1,000 roubles each (approximately €2,000 in today's money).

8 May

This disaster "is unprecedented". So states, in an interview with Izvestia, Yevgeny Velikhov, vice-president of the Soviet Academy of Sciences.

10 May

Aleksandr Akimov dies — the supervisor on duty at the Chernobyl nuclear plant on the night of 26 April. He had radiation burns over 100% of his body.

23 May

Iodine tablets are distributed throughout the USSR. At this point, however, they are of no value, because all the radioactive iodine has already been absorbed by the thyroid glands of the exposed population during the first ten days.

15 June

The entire management staff of the Chernobyl plant is dismissed for "irresponsibility and lack of oversight". Among them are director Victor Bryukanov and his deputy Nikolai Fomin. The trial involved five other defendants. Director V.P. Bryukhanov had experience with coal-fired plants, as did Nikolai Fomin, who had experience with conventional plants; Anatoliy Dyatlov, too, had only limited experience with nuclear reactors, mostly of the small type used in submarine applications.

3 July

The Politburo sentences Viktor Bryukanov to ten years in prison for "serious errors and lack of foresight that led to the Chernobyl disaster and its consequences". He is also expelled from the Communist Party. For health reasons, due to radiation poisoning, Bryukhanov will serve only 5 years in prison, until 1991. Anatoly Dyatlov was also released after five years of imprisonment. He wrote a book arguing that the primary cause of the accident lay in the faulty design of the plant, not in the behaviour of the operators, and in 1995 he died as a result of the exposure of 5.5 Sievert (550 REM) he received during the accident. According to the WHO, 240,000 workers from 1986 to 1987 and at least 600,000 liquidators between 1986 and 1992 were exposed to unsafe working conditions.

20 August

An extensive report by the International Atomic Energy Agency (Iaea) speaks of "an extraordinary sequence of negligence, mismanagement and violation of safety rules that led to the accident".

29 September

Reactor no. 1 at Chernobyl is restarted.

10 October

Construction work