The March 11 earthquake was not a major factor in the accident at the Fukushima Daiichi nuclear plant in Japan, according to a report from the Fukushima Nuclear Accident Investigation Commission. The commission found that the tsunami was the “direct cause” of the disaster.

After the magnitude 9.0 earthquake struck, a safe emergency shutdown was achieved and control rods were fully inserted within seconds, and all 13 diesel generators started as per design. Cooling systems and instrumentation were working correctly.

Shaking recorded at the site was around the maximum that the plant was designed to handle and still maintain nuclear safety but walk-down checks by plant staff showed no indication of significant damage to coolantsystems, the report said.

However, tsunami waves inundated the plant with up to 9 meters (29 ft) of water, which flooded six of the generators and ruined the supporting equipment of another six within an hour of the earthquake, the report said. The remaining unit was used alternately to maintain essential systems at units 5 and 6, using only one of three power distribution panels. Some 36 other distribution panels were rendered useless by the water, according to the report.

The lack of emergency power and the inability to restore it eventually led to the loss of four reactor units and a 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.

The commission is comprised of experts independent of plant owner Tokyo Electric Power Co.

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