Japan Underestimated Tsunami Threat, Says IAEA Preliminary Report
Written by NucNet
Wednesday, 01 June 2011
Japanese authorities underestimated the tsunami hazard for several nuclear sites, but their response to the March 2011 nuclear accident at Fukushima-Daiichi has been “exemplary”, a preliminary report from International Atomic Energy Agency (IAEA) experts has concluded.
The report says tsunami waves overwhelmed defences at Tokyo Electric Power Company’s Fukushima-Daiichi nuclear plant.
Defences were only designed to withstand tsunami waves of a maximum of 5.7 metres high, but the larger waves that hit the plant following an earthquake on 11 March 2011 were estimated to be larger than 14 meters high.
The report says tsunami waves reached areas deep within the units causing the loss of all power sources except for one emergency diesel generator.
There was no other significant power source available on or off the site and little hope of outside assistance.
“The operators were faced with a catastrophic, unprecedented emergency scenario with no power, reactor control or instrumentation, and in addition to this, severely affected communications systems both within and external to the site,” the report concludes.
With no means to control or cool the reactor units, the three units at Fukushima-Daiichi that were operational up to the time of the earthquake quickly heated up due to usual reactor decay heating. Despite the “brave and sometimes novel” attempts of the operational staff to restore control and cool the reactors and spent fuel, severe damage of the fuel and a series of explosions occurred.
Moreover, radiological contamination spread into the environment. However, the report notes that to date no health effects have been reported in any person as a result of radiation exposure from the accident.
Among the preliminary report’s conclusions are that nuclear regulatory systems should address extreme events adequately, including their periodic review, and should ensure that “regulatory independence and clarity of roles are preserved”.
The accident also demonstrates the value of on-site emergency response centres with adequate provision for handling all necessary emergency roles including communications.
The report says: “Japan’s response to the nuclear accident has been exemplary, particularly illustrated by the dedicated, determined and expert staff working under exceptional circumstances.”
Japan's long-term response, including the evacuation of the area around stricken reactors, has been impressive and well organised. “A suitable and timely follow-up programme on public and worker exposures and health monitoring would be beneficial.”
The IAEA team held extensive discussions with officials from Japanese nuclear-related agencies and visited three nuclear sites, including Fukushima-Daiichi.
The team was led by Mike Weightman, the UK’s chief inspector of nuclear installations.
The final report will be delivered to the Ministerial Conference on Nuclear Safety at IAEA headquarters in Vienna from 20 to 24 June 2011.
