It also repudiated the widely-held belief that a hydrogen explosion might have occurred at its No. 2 reactor.
The Tokyo Electric Power Co's in-house report indicated the utility prepared the manual with a view to dealing with nuclear plant accidents including severe incidents on the assumption that emergency power generators, including diesel generators, would work properly to keep reactor cooling systems functioning.
In fact, none of the backup generators worked after the March 11 earthquake and tsunami hit the plant located on the Pacific coast.
According to the report compiled by an intra-company investigative committee, the plant operator first recognized that large explosions had been heard at the No. 2 and No. 4 reactors past 6 a.m. on March 15.
The utility then confirmed that the air pressure in an area near the containment vessel of the No. 2 unit was falling and also that the upper part of the building housing the No. 4 unit had been seriously damaged.
Subsequent analysis of the data led the company to conclude that an explosion had occurred at the No. 4 reactor, but it "erroneously recognized" that something akin to an explosion had possibly taken place at the No. 2 unit, according to the report.
In the accident at the Fukushima plant, the buildings housing the No. 1 and No. 3 reactors were damaged due to hydrogen explosions while that of the No. 4 unit, which was idled for a regular inspection at the time of the natural disasters, was also destroyed. The building of the No. 2 reactor still stands.