A UN report says Japanese nuclear regulators failed to review and approve steps taken after 2002 to protect against tsunamis at the Fukushima plant.
And these proved insufficient to prevent the disaster three months ago.
The detailed assessment by an expert team from the International Atomic Energy Agency was the first outside review of Japan's nuclear crisis.
It said the country underestimated the threat from tsunamis to the Fukushima plant and urged sweeping changes to its regulatory system.
It suggested several shortcomings both before and after a tidal wave crippled the power station.