A team of UN inspectors has concluded that Japan underestimated the threat that a tsunami could pose to its nuclear reactors in advance of the crisis it faced in Fukushima.
A preliminary report from the International Atomic Energy Agency says Japan must do more to protect its reactors from "all natural hazards," while ensuring that its preventative measures are effective and up to date.
When a massive earthquake and subsequent tsunami struck Japan in mid-March, the Fukushima Dai-ichi nuclear plant was rendered unstable and suffered meltdowns, fires and explosions in the days that followed.
The preliminary IAEA report says the natural disasters were the direct cause of the problems at the Dai-ichi facility, which was not sufficiently protected from the incoming tsunami waves that "overwhelmed" its defences.
The Fukushima crisis proved the utility of having disaster-proof emergency centres onsite, which protected plant workers and allowed them to respond to events at hand, the inspectors said in their report.
A crisis centre had been in place at the Dai-ichi plant for several years before the March tsunami. The report said it is still being used as the crisis headquarters at the plant and has functioned well in that role.
In their report, the inspectors also gave credit to Japan for its response to the Fukushima crisis, as well as the way it handled the mass evacuations in areas surrounding the plant.
"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.
Mike Weightman, the IAEA team leader, said the inspectors focused on finding lessons from the crisis that could be applied around the world.
"You can make nuclear plants safe against natural events, but you have to understand those events," he said.
Japanese officials offered full co-operation to the IAEA inspectors and welcomed the results of the preliminary report.
"I think there are many constructive suggestions in this report," said Goshi Hosono, director of the government's nuclear crisis task force.
"We will read it thoroughly."
A spokesperson for the Tokyo Electric Power Company said the organization hopes the results of the IAEA report "will be shared as a common international asset and used in a way that will contribute to nuclear safety around the world."
The inspectors are due to submit a full version of their report at an IAEA conference in Vienna later this month.
With files from The Associated Press
