UN inspectors have criticised Japan for under-estimating the threat of a tsunami on its crippled nuclear plant, but praised its overall response to the crisis as exemplary.
The preliminary report by a team from the International Atomic Energy Agency (IAEA) also said the tsunami hazard was under-estimated at several other nuclear facilities in Japan, and called for experts worldwide to learn from the disaster to avert future accidents.
The IAEA team of international experts from 12 countries, which spent a week in Japan conferring with officials and inspecting the plant, will submit its full report at a high-level IAEA conference in Vienna from June 20-24.
"Japan's response to the nuclear accident has been exemplary, particularly (as) illustrated by the dedicated, determined and expert staff working under exceptional conditions," the report said. It also praised the evacuation of those living near the plant as "impressive and well-organised".
The Fukushima Daiichi facility was crippled when a huge tsunami generated by a magnitude 9.0 earthquake hit Japan on March 11. It suffered explosions, fires and meltdowns in the days after the tsunami. More than 80,000 people have been evacuated from its vicinity.
The report said the earthquake and tsunami were the direct cause of the power outages and communications blackouts which ensued. It said the potential size of the tsunami that hit the plant, estimated at as high as 49ft, was not sufficiently planned for and "overwhelmed" the plant's defences.
"The tsunami hazard for several sites was under-estimated," it said. "Nuclear plant 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 did not go into further detail.
The report also said Japan's crisis underscored the value of disaster-proof emergency centres onsite to protect workers and provide a safe command headquarters for the management of the response.
A crisis centre was built at Fukushima Daiichi after an earthquake hit a different part of Japan in 2007. The centre still serves as the crisis headquarters at the plant, and the IAEA report suggested it has functioned well.
Mike Weightman, the IAEA team leader and the United Kingdom's chief inspector of nuclear installations, said the IAEA team focused on finding lessons from the crisis which can be applied around the world.
"You can make nuclear plants safe against natural events, but you have to understand those events," he said.
The team examined the technical processes more closely than political or managerial issues, which he said would probably be a topic raised at the IAEA meeting in Vienna.
He said Japanese officials offered their full co-operation, and his team was given access to the plant and officials and answers to their questions.
"I think there are many constructive suggestions in this report," said Goshi Hosono, director of the government's nuclear crisis taskforce. "We will read it thoroughly."