Japan underestimated the hazard posed by tsunamis to nuclear plants, the UN atomic watchdog has said, while praising its response to the 11 March disaster as 'exemplary'.
The International Atomic Energy Agency also stressed the importance of 'regulatory independence and clarity of roles', touching on the fact that Japan's nuclear watchdog is part of the ministry of trade and industry, which promotes atomic power.
Japan's magnitude 9.0 quake and tsunami caused the world's worst nuclear disaster since Chernobyl 25 years ago.
Radiation has since leaked into the air, ground and sea around the Fukushima Daiichi plant.
The IAEA sent an 18-member team of its own experts and specialists from 12 countries, including the US, China, Russia and South Korea, on a fact-finding mission to Japan.
'The tsunami hazard for several sites was underestimated,' said the IAEA team in the preliminary report it handed to Japan's government, ahead of a full report to be presented in Vienna later this month.
'Nuclear designers and operators should appropriately evaluate and protect against the risks of all natural hazards, and should periodically update those assessments and assessment methodologies,' it said.
The 14m wave that slammed into the plant knocked out reactor cooling systems and backup power generators, causing partial reactor meltdowns and forcing emergency crews to douse reactors with water since then.
The embattled operator Tokyo Electric Power Company (TEPCO) has said it hopes to bring the plant to a stable state of 'cold shutdown', with low pressure and temperatures, some time between October and January.
The IAEA report on lessons learnt from the disaster said that nuclear plants should be designed to withstand 'extreme external events, particularly those with common mode implications such as extreme floods'.
'Severe long-term combinations of external events should be adequately covered in design, operations, resourcing and emergency arrangements,' it said.
The IAEA report summary also said that at nuclear plants 'simple effective robust equipment should be available to restore essential safety functions in a timely way for severe accident conditions'.
The team, which visited three nuclear plants, said Japan's government, plant operators and agencies had been 'extremely open in sharing information' and praised the country's initial response to the disaster.
'The response on the site by dedicated, determined and expert staff, under extremely arduous conditions has been exemplary and resulted in the best approach to securing safety given the exceptional circumstances,' it said.
'The Japanese government's longer term response to protect the public, including evacuation, has been impressive and extremely well organised.'
Japan has evacuated tens of thousands of people from a 20km zone around the stricken plant and from some areas beyond which have received high doses of aerial radiation.
The IAEA said that 'a suitable and timely follow-up programme on public and worker exposures and health monitoring would be beneficial'.
The team will present its full report at a ministerial meeting on nuclear safety at IAEA headquarters in Vienna from 20 to 24 June.