The tsunami of 11 March was the ‘direct cause’ of the accident at the Fukushima Daiichi nuclear power plant, concluded an official investigation report. It dismissed the idea that earthquake damage was a major factor in the accident.
A safe emergency shutdown was achieved within seconds of the magnitude 9.0 earthquake, said the Fukushima Nuclear Accident Investigation Commission composed of experts independent of plant owner Tokyo Electric Power Company. Control rods were fully inserted within seconds and all 13 diesel generators started as per design when tremors disconnected the grid connection. Instrumentation was working correctly, as were cooling systems.
What was the cause, then?
Within an hour of the earthquake, however, almost the entire site was submerged to a depth of up to nine metres by a series of tsunami waves. Over about ten minutes these flooded six of the diesels and ruined the supporting equipment of another six. Only one diesel unit survived and this was used alternately to maintain essential systems at units 5 and 6 – using one of only three power distribution panels that had not been submerged. Some 36 other distribution panels throughout the emergency diesel generator system were made useless by water.
