A report published by the International Atomic Energy Agency assesses the causes and consequences of the March 2011 accident at the Fukushima-Daiichi nuclear station in Japan. The report considers human, organisational and technical factors and aims to provide an understanding of what happened, and why, so that lessons learned can be acted upon by governments, regulators and nuclear power plant operators. In this special summary, NucNet highlights the report’s key findings, conclusions and recommendations.
On 11 March 2011 an earthquake caused damage to the electric power supply lines to the Fukushima-Daiichi nuclear site on the northeast coast of Japan, and a tsunami caused substantial destruction of the operational and safety infrastructure on the site. The combined effect led to the loss of offsite and onsite electrical power.
This resulted in the loss of the cooling function at the three operating reactor units as well as at the spent fuel pools. Despite the efforts of the operators to maintain control, the reactor cores in Units 1, 2 and 3 overheated, the nuclear fuel melted and the three containment vessels were breached.
Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in Units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean. There were also direct releases into the sea.
Nuclear Safety Considerations
Both the recorded ground motions from the earthquake and the heights of the tsunami waves significantly exceeded the assumptions of hazards that had been made when the plant was originally designed. The earthquake and the associated tsunami impacted on multiple units.
The seismic hazard and tsunami waves considered in the original design were evaluated mainly on the basis of historical seismic records and evidence of recent tsunamis in Japan. This original evaluation did not sufficiently consider tectonic-geological criteria, and no re-evaluation using such criteria was performed.
The assessment of natural hazards needs to be sufficiently conservative. The consideration of mainly historical data in the establishment of the design basis of nuclear power plants is not sufficient to characterise the risks of extreme natural hazards.
The safety of nuclear power plants needs to be re-evaluated regularly to consider advances in knowledge. Corrective actions or compensatory measures need to be implemented promptly.
The assessment of natural hazards needs to consider the potential for their occurrence in combination – either simultaneously or sequentially – and their combined effects on a nuclear plant. The assessment of natural hazards also needs to consider their effects on multiple units at a nuclear power plant.
Operating experience programmes need to include experience from both national and international sources. Safety improvements identified through operating experience programmes need to be implemented promptly. The use of operating experience needs to be evaluated periodically and independently.
All layers of defence in depth associated with both prevention and mitigation of accidents should be strengthened by adequate independence, redundancy, diversity and protection so that they are not simultaneously challenged by an external or internal hazard and are not prone to common cause failure.
The defence in depth concept remains valid, but implementation of the concept needs to be strengthened at all levels by adequate independence, redundancy, diversity and protection against internal and external hazards. There is a need to focus not only on accident prevention, but also on improving mitigation measures.
The loss of instrumentation and control during the Fukushima-Daiichi accident left operators with little indication of actual plant conditions. Instrumentation and control systems that are necessary during beyond design basis accidents need to remain operable in order to monitor essential plant safety parameters and to facilitate plant operations
Robust and reliable cooling systems that can function for both design basis and beyond design basis conditions need to be provided for the removal of residual heat.
There is a need to ensure a reliable confinement function for beyond design basis accidents to prevent significant release of radioactive material to the environment.
Emergency Preparedness And Response
At the time of the accident, separate arrangements were in place to respond to nuclear emergencies and natural disasters at national and local levels. There were no coordinated arrangements for responding to a nuclear emergency and a natural disaster occurring simultaneously.
There were difficulties in evacuation due to the damage caused by the earthquake and tsunami and resulting communication and transportation problems. There were also significant difficulties encountered when evacuating patients from hospitals and nursing homes within the 20 km evacuation zone.
In preparing for the response to a possible nuclear emergency, authorities must consider emergencies that could involve severe damage to nuclear fuel in the reactor core or to spent fuel on the site, including those involving several units at a multi-unit plant possibly occurring at the same time as a natural disaster.
Emergency workers need to be designated, assigned clearly specified duties, regardless of which organisation they work for, be given adequate training and be properly protected during an emergency.
Arrangements need to be in place to integrate into the response those emergency workers who had not been designated before the emergency, and helpers who volunteer to assist in the emergency response.
The emergency management system for response to a nuclear emergency needs to include clearly defined roles and responsibilities for the operating organisation and for local and national authorities. The system, including the interactions between the operating organisation and the authorities, needs to be regularly tested in exercises.
Several channels were used to keep the public informed and to respond to people’s concerns during the emergency, including television, radio, the Internet and telephone hotlines. Feedback from the public identified the need for easily understandable information and supporting material.
Arrangements need to be in place to assist decision makers, the public and others (e.g. medical staff) to gain an understanding of radiological health hazards in a nuclear emergency in order to make informed decisions on protective actions. Arrangements also need to be in place to address public concerns locally, nationally and internationally.
No early radiation induced health effects have been seen among workers or members of the public that could be attributed to the accident. The latency time for late radiation health effects can be decades, and therefore it is not possible to discount the potential occurrence of such effects a few years after exposure. However, given the low levels of doses reported among members of the public, the conclusions of the IAEA report are in agreement with those of the United Nations Scientific Committee on the Effects of Atomic Radiation (Unscear) that “no discernible increased incidence of radiation-related health effects are expected among exposed members of the public and their descendants”.
The accident resulted in the release of radionuclides to the environment. Estimates indicate that the effective doses incurred by members of the public were low and generally comparable with the range of effective doses incurred due to global levels of natural background radiation.
Radionuclides such as iodine-131, caesium-134 and caesium-137 were released and found in drinking water, food and some non-edible items. Restrictions to prevent the consumption of these products were established by the Japanese authorities in response to the accident.
In the short term, the most significant contributors to the exposure of the public were external exposure from radionuclides in the plume and deposited on the ground; and internal exposure of the thyroid gland, due to the intake of iodine-131, and internal exposure of other organs and tissues, mainly due to the intake of caesium-134 and caesium-137.
In the long term, the most important contributor to the exposure of the public will be external radiation from the deposited caesium-137.
In the aftermath of a nuclear accident involving releases of iodine-131 and its intake by children, the uptake and subsequent doses to their thyroid glands are a particular concern. Following the Fukushima-Daiichi accident, the reported thyroid equivalent doses of children were low because their intake of iodine-131 was limited, partly due to restrictions placed on drinking water and food, including leafy vegetables and fresh milk.
In case of an accidental release of radioactive substances to the environment, the prompt quantification and characterisation of the amount and composition of the release is needed. For significant releases, a comprehensive and coordinated programme of long term environmental monitoring is necessary to determine the nature and extent of the radiological impact on the environment at the local, regional and global levels.
International bodies need to develop explanations of the principles and criteria for radiation protection that are understandable for non-specialists in order to make their application clearer for decision makers and the public, the report says. Some protracted protection measures were disruptive for the affected people, and a better communication strategy is needed to convey the justification for such measures and actions to all stakeholders, including the public.
There is a need for simple explanations of a number of radiation protection issues, including differences between the concepts of dose limits and reference levels; criteria for the justification of protective measures and actions aimed at averting radiation doses in the long term; and specific situations relating to the radiation protection of workers in an emergency.
The long term goal of post-accident recovery is to re-establish an acceptable basis for a fully functioning society in the affected areas. In preparing for the return of evacuees, factors such as the restoration of infrastructure and the viability and sustainable economic activity of the community need to be considered. A remediation strategy has been developed and implementation begun. The strategy specifies that priority areas for remediation are residential areas, including buildings and gardens, farmland, roads and infrastructure, with emphasis on the reduction of external exposures.
Pre-accident planning for post-accident recovery is necessary to improve decision making under pressure in the immediate post-accident situation. National strategies and measures for post-accident recovery need to be prepared in advance in order to enable an effective recovery programme to be put in place in case of a nuclear accident.
These strategies and measures need to include the establishment of a legal and regulatory framework; generic remediation strategies and criteria for residual radiation doses and contamination levels; a plan for stabilisation and decommissioning of damaged nuclear facilities; and a generic strategy for managing large quantities of contaminated material and radioactive waste.
As part of the remediation strategy, the implementation of rigorous testing and controls on food is necessary to prevent or minimise ingestion doses.
Further international guidance is needed on the practical application of safety standards for radiation protection in post-accident recovery situations.
Following an accident, a strategic plan for maintaining long-term stable conditions and for the decommissioning of accident damaged facilities is essential for onsite recovery. The plan needs to be flexible and readily adaptable to changing conditions and new information.
Retrieving damaged fuel and characterising and removing fuel debris require solutions that are specific to the accident, and special methods and tools may need to be developed.
National strategies and measures for post-accident recovery need to include the development of a generic strategy for managing contaminated liquid and solid material and radioactive waste, supported by generic safety assessments for discharge, storage and disposal.
IAEA director-general Yukiya Amano says in his foreword to the report that a major factor that contributed to the accident was the widespread assumption in Japan that its nuclear power plants were so safe that an accident of this magnitude was simply unthinkable. This assumption was accepted by nuclear power plant operators and was not challenged by regulators or by the government. As a result, Japan was not sufficiently prepared for a severe nuclear accident.
The Fukushima-Daiichi accident exposed certain weaknesses in Japan’s regulatory framework. Responsibilities were divided among a number of bodies, and it was not always clear where authority lay.
There were also certain weaknesses in plant design, in emergency preparedness and response arrangements and in planning for the management of a severe accident. There was an assumption that there would never be a loss of all electrical power at a nuclear power plant for more than a short period. The possibility of several reactors at the same facility suffering a crisis at the same time was not considered. And insufficient provision was made for the possibility of a nuclear accident occurring at the same time as a major natural disaster.
Since the Accident: Progress And Changes
Since the accident, Japan has reformed its regulatory system to better meet international standards. It gave regulators clearer responsibilities and greater authority. The new regulatory framework will be reviewed by international experts through an IAEA Integrated Regulatory Review Service mission. Emergency preparedness and response arrangements have also been strengthened.
Other countries responded to the accident with measures that included carrying out ‘stress tests’ to reassess the design of nuclear power plants against site specific extreme natural hazards, installing additional backup sources of electrical power and supplies of water, and strengthening the protection of plants against extreme external events.
Mr Amano said IAEA peer reviews have a key role to play in global nuclear safety, enabling countries to benefit from the independent insights of leading international experts, based on the common reference frame of the IAEA safety standards. They address issues such as operational safety at nuclear power plants, the effectiveness of nuclear regulators and the design of nuclear power plant sites against specific hazards. He said the IAEA has strengthened its peer review programme since the accident and will continue to do so.