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
How did the Japanese authorities react at the moment of the accident?
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
What were the immediate consequences for the Fukushima Daiichi nuclear plants?
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.
How was the public at large and the emergency workers protected?
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.
Where were the radioactive materials from the Fukushima Daiichi plant released?
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.
Were radiation effects already observed among workers, the general public or the environment?
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
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.
Why did the Fukushima Daiichi plant not resist to the earthquake and the tsunami?
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 :
- The control of reactivity in the nuclear fuel;
- The removal of heat from the reactor core and spent fuel pool;
- The confinement of radioactive
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.
What is the remediation strategy adopted for the post accident recovery?
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
Japanese authorities have estimated that the timescale for completing
decommissioning activities is likely to be in the range of 30–40 years.
What main lessons are learned from the Fukushima-Daiichi accident?
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:
- improvement of severe accident management provisions and guidelines ;
- re-evaluation of site specific external natural hazards and multi-unit
- enhancement of power systems ;
- additional means to withstand prolonged loss of power and cooling for
the removal of residual heat;
- strengthening of measures to preserve containment integrity
- improvements of on-site and off-site emergency control
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