May - December 1986
1 May - The accident did not interfere with the May Day parades held on the 1st of May in the Ukrainian capital Kiev and the Belarussian capital Minsk. Apparently the government wanted to emphasize that all was "normal" although the reactor was still burning and invisible, deadly radioactivity was pouring into the air. However, the Soviet Communists bureaucrats and the nomenklatura immediately after the accident removed their children from Kiev and other threatened areas while assuring others that everything was normal until several days later
- The authorities claim the situation is stable. But the amount of radiation released is still enormous, besides which, the wind has changed direction and is now blowing in the direction of Kiev. The material thrown onto the plant does not completely extinguish the fire and in fact generates a rise in temperature. Scientists and engineers become aware of a new danger. The hot reactor core could melt into the cement and end up in the water reservoir underneath. A steam explosion would follow, even more powerful than the first explosion.
2 May - More and more radioactivity is released into the area. Fire fighters start pumping the water out of the storage reservoir underneath the reactor, a long and dangerous task, not completed until 8 May. As a reward, the fire fighters receive 1000 rubles each (approximately 2000 US dollars according to the official rate of exchange).
- Politburo members Ryzhkov and Ligachev visit Chernobyl. Ukrainian party leader Volodymyr Shcherbitsky visits the area also. Shcherbitsky survived the Chernobyl crisis and was not criticized in the Western press as was Gorbachev for his long 18 day delay in speaking publicly about Chernobyl
- A 30 kilometer zone around the reactor is designated for evacuation (90.000 people).
- According to the Russian permanent representative at the IAEA, chain–reaction inside the reactor has stopped
4 May - The first film footage, shot from a helicopter, is shown on Vremya. The commentator says the film disproves Western reports of massive destruction
- A second step taken to prevent a steam explosion is that of making holes in the earth under the reactor. Fluid nitrogen is pumped into them to freeze the earth.
- Radioactive cloud reaches Japan (8-9,000 km from Chernobyl)
5 May - A government report says an embankment is being constructed on the Pripyat River to prevent it from being contaminated
- To start with, there is a great deal of radioactivity released, nearly as much as on 26 April. However, the release later stops almost entirely. No acceptable explanation has yet been found for this fact. According to Grigory Medvedev, who was a member of the government committee, the fire was extinguished because the graphite had burnt up.
- Canada: health officials found that Ottawa rains carried six times as much radioactive iodine as is considered acceptable for drinking-water
- Increased radiation levels are measured in the USA, too
- Hans Blix, director-general, and a IAEA delegation arrives in Moscow. Unsure if the can visit the area
6 May  - The first extensive report on the situation appears in Pravda.
- schools in Gomel and Kiev closed, all children are sent elsewhere.This brings total number of people forced to leave: 500.000. 140.000 of which are not allowed to return
- Kiev radio finally, eleven days late, warned its audience not to eat leafy vegetables and to stay indoors as much as possible. The Soviet government was very slow to warn its citizens of the precautions they should take: keep children and pregnant women indoors, avoid fresh vegetables and milk, don't drink rainwater, and wash your clothes and your shoes every time you come in.
7 May - Tass reports that many Kiev residents are trying to leave the city and that additional trains and flights have been scheduled. The (Russian) media drops its insistence that everything is under control.
- Bavarian Environmental minister Alfred Dick critisices maximum radiation levels for vegetables and meat of the (german) Radiation Protestion Agency. He says: “If we now start to have maxumim levels for Ceasium too, we will not even be able to eat meat shortly!”
8 May - In an interview with Izvestiya, Academician Yevgeny Velikhov, vice-president of the Soviet Academy of Sciences and chief scientist sent to Chernobyl, says the disaster is “without precedent”.
9 May - IAEA states that Moscow started to encapsulate the reactor, especially pouring concrete under the reactor, preventing it from reaching groundwater
10 May - Accordig to the IAEA the fire is extinguished, but temperature in reactor is still rather high. Meanwhile Ukrainian goverment official states: reactor is still burning and fire-fighters are contuiniously trying to put the fire out.
11 May - three local officials  in charge of the transport combine at the plant, are expelled from the party, or reprimanded for mistakes concerning evacuations
14 May - Gorbachev speaks for the first time publicly about the accident on Vremya.  He insisted there was no cover-up:  “The moment we received reliable data we gave it to the Soviet people and sent it abroad”. He declared his desire for "serious cooperation" with the IAEA, with respect to four specific proposals:
1. The creation of an international regime for safe development of nuclear energy involving close cooperation among all nuclear energy-using states;
2. A highly authoritative special international conference in Vienna under the aegis of the IAEA to discuss these "complex questions";
3. An increased role and scope for IAEA;
4. Safe development of "peaceful nuclear activities," involving the United Nations and its specialized departments, such as the World Health Organization (WHO) and the United Nations Environmental Program (UNEP)
These proposals suggested that Gorbachev was broadening the scope of the accident to one of international concern, but at the same time he was implying that such accidents were common enough to warrant the establishment of a global regime to deal with them.
15 to 16 May - New fires break out and more radiation is released.
22 May - Russian First Deputy Health Minister denies popular believe that vodka (& red wine) is a good cure for radiation exposure.
23 May - A Soviet government committee orders the distribution of iodine preparations. At this point, such prophylaxis is of no medical value. Radioactive iodine is only active for ten days, and will already have accumulated in the thyroid glands of the inhabitants of the contaminated territories.
27 May - A month after the accident the danger is not yet over,. A concrete foundation will be made, the idea of the sarcophagus is born
30 May - An unprecedented concert took place in Moscow’s Olympic Stadium. The popconcert was organised by leading Soviet rock bands to raise funds for the Chernobyl victims

Soviet authorities try to hush up the scale of the tragedy, admitting reluctantly that about 30 people had died in the first few weeks after the blast. Hundreds of thousands of people (many military reservists) from all over the Soviet Union, now popularly known as "liquidators," are mobilised by the Communist Party to clean up the disaster.
The ‘Liquidators’ are those people who were recruited or forced to assist in the cleanup or the "liquidation" of the consequences of the accident. As a totalitarian government the Soviet Union forced many young soldiers to assist in the cleanup of the Chernobyl accident, apparently without sufficient protective clothing and insufficient explanation of the danger involved. Over 650,000 liquidators helped in the cleanup in the first year. The total number is estimated to be over 1 million. Many of those who worked as liquidators became ill and according to some estimates about 8,000 to 10,000 have died in the first few years after the accident from the radioactive dose they received. This group apparently includes those who built the containment building over the destroyed reactor No. 4 which is called the sarcophagus. More to follow in the time-line.

June: Overview of remedial actions taken in various countries in the first weeks [pdf, 50 kb]
9 June - ‘By accident’ a foundation of lead was established under the reactor. Tonnes of lead thrown on the burning reactor, melted and leaked under the reactor. When the temperature decreased it solidified.
15 June - Almost the complete management team of the reactor has been dismissed for ‘irresponsibility and lack of control’, Pravda announces. Amongst them Chernobyl Director Victor Bryukanov and deputies (senior engineer) Nikolai Fomin who will be brought on trial a year later.
20 July - Report (which will be published in full later) of the Government commission of inquiry found that human error caused the disaster
20 August - The full report on the cause of the accident was submitted (in Russian) to the IAEA. It states there was an extraordinary sequence of carelessness, mismanagement and violations of safety codes leading to the accident
26 August.- Estonian press tell of strikes and demonstrations by Estonian military reservists forcibly conscripted Chernobyl for clean-up labour. In November reports claim 12 people were executed.
20 September - The Soviet Union paid already US$3 billion, mainly for relocation, compensation and loss of power.
29 September - Block 1 of the Chernobyl NPP restarts again, and connects to the grid on Oct. 1
10 October - Construction-work on Block 5 & 6 is resumed.
9 November - Block 2 restarts
14 December - A concrete roof ("sarcophagus") is completed over the fourth reactor. It is built to protect the environment from radiation for at least 30 years. 300,000 tons of concrete and 6,000 tons of metal constructions were utilised.