Ukraine has declassified previously secret Soviet documents on the explosion at the Chernobyl nuclear plant on 26 April 1986, making them public on Monday.
The archives, published in a book edited by the Ukrainian secret service, SBU, give an overview of the various construction errors, accidents and emergency interruptions at the plant between 1971 and the day of the disaster.
After Reactor 4 of the Soviet nuclear plant at Chernobyl exploded, a no-go zone was set up around the area due to the risk of radiation.
The biggest nuclear disaster in History caused the deaths of thousands of people, while tens of thousands had to be displaced.
On Sunday, a new fire occurred in the red zone around the plant, which is about three hectares in area. The Ukrainian authorities said on Monday that the fire had, for the most part, been put out.
In early April, a forest fire fed by gusty winds and unusually dry weather had broken out around the abandoned nuclear plant.
By the time it was brought under control in mid-May, about 11,500 hectares had been destroyed.