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.
Shaking recorded at the site was around the maximum that the plant was designed to cope with and still maintain nuclear safety but walk-down checks by plant staff showed no indication of significant damage to coolant systems.
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.
This situation was noted as the 'direct cause' of the accident. The lack of emergency power and the inability to restore it eventually led to the loss of four reactor units and a significant release of radioactivity.
The report was based in part on analysis of observable factors at the site - such as high-water marks and physical damage - in addition to reams of hard data produced by reactor control systems up to the time they lost power. This report was an interim publication, running to hundreds of pages, and was released for comment while the investigating commission promised to report again as more information comes to light.
Researched and written
by World Nuclear News