On 4 October 2009 the official report about Sayano–Shushenskaya hydro accident was published by the Federal Environmental, Technological and Atomic Supervisory Service (Rostekhnadzor) on its website.[2] However, later the report and the press release on the report were removed from the website.
Names of people killed and those who bear responsibility for the accident, and other data including a historical and technical review about the plant and plans for its future, are given in the report. The report states that the accident was primarily caused by the turbine vibrations which led to the fatigue damage of the mountings of the turbine 2, including the cover of the turbine. It was also found that at the moment of accident at least six nuts were missing from the bolts securing the turbine cover. After the accident 49 recovered bolts were investigated from which 41 had fatigue cracks. On 8 bolts, the fatigue damaged area exceeded 90% of the total cross-sectional area.[2]
According to this report, on 17 August 2009 at 1:20AM (local time) there was a fire at the hydroelectric power station of Bratsk which broke both communications and the automatic driving systems of other power plants in the region, including Sayano–Shushenskaya. The situation was recovered on 17 August 2009 at 15:03. At 8:12AM local time, turbine 2's output power was reduced by the turbine regulator and it entered into the non-recommended powerband II. Shortly after this, the bolts keeping the turbine cover in place were broken, and under water pressure of about 20 bars (2,000 kPa), the spinning turbine with its cover, rotor, and upper parts started to move up, destroying machinery hall installations. At the same time, pressurized water flooded the rooms and continued damaging plant constructions.[2]
According to Rostekhnadzor, the automatic shutdown system of the water intake pipes' gates failed after failure of the turbine 2.[13][14] This accusation was dismissed by Rakurs, the company which designed the automated safety system for the plant.[15]