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Context - What are the causes and consequences of the 11 March 2011 accident at the Fukushima Daiichi Nuclear Power Plant in Japan, which was triggered by a tsunami that followed a massive earthquake ?
This is a faithful synthesis and summary of several scientific consensus reports. For the full list of sources, see the references.
The report assesses the causes and consequences of the 11 March 2011 accident at the Fukushima Daiichi Nuclear Power Plant in Japan, which was triggered by a tsunami that followed a massive earthquake.
The earthquake and tsunami caused great loss of life and considerable damage to buildings and infrastructure. More than 15 000 people were killed, over 6000 were injured, and at the time of this report around 2500 people are still reported to be missing. It was the worst emergency at a nuclear power plant since the Chernobyl disaster in 1986.
The report aims at providing an understanding of what happened and why, so that the necessary lessons learned can be acted upon by governments, regulators and nuclear power plant operators throughout the world.
At the time of the accident, there were no coordinated arrangements at the national and local levels for responding to a nuclear emergency and a natural disaster occurring simultaneously.
The Prime Minister declared a nuclear emergency on the evening of 11 March but the consequences of the earthquake and tsunami, together with increased radiation levels made the on-site response extremely difficult, and meant that many mitigatory actions could not be carried out in a timely manner.
Once radionuclides were detected in the environment, arrangements were made regarding protective actions in the agricultural area and restrictions on the consumption and distribution of food and consumption of drinking water. In addition, a certification system for food and other products intended for export was established.
The earthquake led to the loss of off-site and on-site electrical power which caused the loss of the cooling function at the three operating reactor units as well as at the spent fuel pools.
All operating reactor units at these plants were safely shut down but despite the efforts, the reactor cores in two Units overheated, the nuclear fuel melted, and the three containment vessels were breached. Explosions inside the reactor buildings damaged structures and equipment, and injured personnel. The four other nuclearpower plants along the coast were also affected to different degrees by the earthquake and tsunami.
The initial decisions on protective actions towards the public included: evacuation, sheltering, restrictions on the consumption of food and drinking water, relocation, and the provision of information. Administration of stable iodine for iodine thyroid blocking was not implemented uniformly, primarily due to the lack of detailed arrangements.
The evacuation of people from the vicinity of the Fukushima Daiichi nuclear power plant gradually extended from a radius of 2 km of the plant to 3 km, and by the evening of 12 March, it had been extended to 20 km.
The protection of workers against radiation exposure was severely affected by the extreme conditions at the site, and there were no arrangements in place to integrate into the response the additionnal emergency personnel who had not been designated prior to the accident. In order to maintain an acceptable level of protection for on-site emergency workers, a range of impromptu measures was implemented.
Most of the atmospheric releases were blown eastward towards the North Pacific Ocean, and there were also liquid discharges to the sea directly. A relatively small part of the atmospheric releases were deposited on land.
Radionuclides such as iodine-131, caesium-134 and caesium-137 were released and found in drinking water, food and some non-edible items. Restrictions were placed on the distribution and consumption of food and the consumption of drinking water. At the time of writing the report, many people were still living outside the areas from which they were evacuated.
For the members of the public at large, the estimates used indicate that the effective doses incurred were low, and no early radiation induced health effects were observed.
For children, the reported thyroid equivalent doses were low because their intake of iodine-131 was limited, partly due to the restrictions placed on drinking water and food. So far, prenatal radiation effects have not been observed, and unwanted terminations of pregnancy attributable to the radiological situation have not been reported.
For the around 23 000 emergency workers involved in the emergency operations, the effective doses incurred by most were below the occupational dose limits in Japan. Of this number, 174 exceeded the original criterion for emergency workers, and 6 emergency workers exceeded the temporarily revised effective dose criterion in an emergency situation.
Although UNSCEAR estimated that no detectable health effects of radiations is expected, it will take time to see the full effects since the latency for health effects of radiation can be decades. The UNSCEAR organisation found that “no discernible increased incidence of radiation-related health effects are expected among exposed members of the public and their descendants”. The most important health effect is on mental and social wellbeing, related to the enormous impact of the earthquake, tsunami and nuclear accident.
In plants and animals, no observations of direct radiation-induced effects have been reported, however, only limited observational studies were conducted in the period immediately after the accident.
In the planning and construction of the power plant, external hazards such as tsunamis were evaluated mainly on the basis of historical seismic records and evidence of recent tsunamis in Japan. However, the methodological method applied was not fully appropriate and did not fully explore the possible scenarios that could lead to severe reactor core damage.
The flooding created by the tsunami simultaneously challenged the key protective levels of plant equipment and systems resulting in common cause failures that were not anticipated in the design.
By consequence, the complete loss of power led to the failure of the three fundamental functions important for ensuring the safety of a nuclear plant :
Also, the vulnerability of the Fukushima Daiichi nuclear power plant to external hazards had not been reassessed in a systematic and comprehensive manner during its lifetime.
There were no policies and strategies for post-accident remediation in place in Japan at the time of the accident. A comprehensive high level strategic plan for stabilization and decommissioning of the damaged nuclear power plant was developed jointly by TEPCO and the relevant Japanese Government agencies. At the time of writing, safety functions had been re-established and structures, systems and components were in place to reliably maintain stable conditions.
Japanese authorities have estimated that the timescale for completing decommissioning activities is likely to be in the range of 30–40 years.
Because of the basic assumption that nuclear power plants in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety and this resulted in a situation where safety improvements were not introduced promptly.
Based on the lessons of the accident, the Contracting Parties to the Convention on Nuclear Safety, adopted the Vienna Declaration on Nuclear Safety. This declaration includes principles to prevent accidents with radiological consequences and to mitigate such consequences, should they occur after having reported on the implementation of safety upgrades, including 6 main axes:
In particular, the assessment of natural hazards needs to consider the potential for their occurrence in combination, either simultaneously or sequentially, and their combined effects on multiple units of a nuclear power plant.
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