On 26 April 1986, reactor number four at the Chernobyl plant blew up, spewing out a radioactive cloud and contaminating much of Europe. It was the world's worst civilian nuclear disaster.
Between 15,000 and 30,000 people are estimated to have died in the aftermath.
Published on the internet site of the Ukrainian secret service - SBU, the documents, which cover the period from 1971 to 1988, mostly concern events surrounding the catastrophe of 26 April, 1986.
A report from 1984 noted the poor quality of the equipment sent to the plant from Yugoslav factories, as well as 'deficiencies' in the third and fourth reactors.
Certain documents also refer to an earlier accident at Chernobyl, on 9 September, 1982, which released strong doses of radiation into the surrounding region.
The archives contain a letter sent to Moscow on the day of the 1986 disaster, in which the Ukrainian KGB chief, General Mukha, sent assurances that his men had 'taken control of the situation in the plant and the immediate area'.
The three other reactors at the plant continued in service until one of them was shut down in 1991 following a fire and another was taken out of service in 1996.
The plant was finally shut down in December 2000 under a $2.3-billion deal with the world's richest nations.
The documents can be viewed at www.sbu.gov.ua
