Japan under-estimated Fukushima tsunami risk: IAEA

Tokyo: Japanese regulatory authorities and the Tokyo Electric Power Company (Tepco), owner of the Fukushima nuclear plant, failed to implement adequate safeguards at the plant before the 2011 tsunami despite being aware of the risks, the International Atomic Energy Agency has said.

IAEA concluded in its report on the accident -- obtained and made public on Monday by Kyodo news agency -- that "the Fukushima Daiichi NPP had some weaknesses which were not fully evaluated by a probabilistic safety assessment, as recommended by the IAEA safety standards".

The 240-page report, prepared by 180 experts from 42 countries, will be presented to the annual IAEA meeting in September, if approved by its board of directors in June.

According to the report, several analyses carried out between 2007 and 2009 predicted the possibility of an 8.3-magnitude earthquake on the coast of Fukushima, which could result in the plant being hit by a tsunami of around 15 metres.

Despite this assessment, Tepco and Japanese regulatory authorities (the now defunct Nuclear and Industrial Safety Agency or NISA) delayed responding to this possibility as they felt "further studies and investigations were needed".

"Tepco did not take interim compensatory measures in response to these increased estimates of tsunami height, nor did NISA require Tepco to act promptly on these results," reads the text.

Tepco relied heavily "on the basic assumption in Japan, reinforced over many decades, that the robustness of the technical design of the nuclear plants would provide sufficient protection against postulated risks".

It failed to take sufficient measures to protect the electrical system and the emergency generators at the plant.

As a result of the tsunami, a loss of power triggered a train of events -- a loss of cooling to the reactors, a partial meltdown of which led to an emission of radioactive gas -- generating millions of tonnes of contaminated water, part of which seeped into the sea.

This report, analysing the causes and consequences of the disaster, will serve as a reference in the field of nuclear safety in the years to come.

In July 2012, two reports produced by Japanese experts and the Japanese government reached the same conclusions, and accused the authorities and the power company of over-estimating the capacity of safety measures in place at the plant.