Idiomas:
The report looks at the causes and consequences of the 11 March 2011 accident at the Fukushima Daiichi Nuclear Power Plant in Japan, triggered by the tsunami that followed a massive earthquake. It was the worst emergency at a nuclear power plant since the Chernobyl disaster in 1986. The report aims to provide 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.
On 11 March 2011 a sudden release of energy at the interface where the Pacific tectonic plate forces its way under the North American plate, caused a massive earthquake with a magnitude of 9.0. The earthquake’s epicentre was off the eastern coastline of Japan, generating a tsunami which struck a wide area of coastal Japan, particularly its north-eastern coast, where several waves reached heights of more than ten metres. 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 ofwriting of this report, around 2500 people were still reported to be missing.
An extensive international framework for emergency preparedness and response was in place at the time of the accident, comprising international legal instruments, IAEA safety standards and operational arrangements.
However, 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 national legislation and guidance in Japan addressed measures to be taken for the protection of emergency workers, but only in general terms and not in sufficient detail.
The Prime Minister declared a nuclear emergency on the evening of 11 March but the consequences of the earthquake and tsunami, as well as the 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.
The activation of the Off-site Centre, located 5 km from the Fukushima Daiichi nuclear power plant, was difficult because of extensive infrastructure damage caused by the earthquake and tsunami. Within a few days, it became necessary to evacuate the emergency center due to adverse radiological conditions.
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.
At the Fukushima Daiichi nuclear power plant (NPP), the earthquake damaged the electric power supply lines and the tsunami caused substantial destruction of the operational and safety infrastructure on the site. The combined effect led to the loss of off-site and on-site electrical power and to 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 Units 1–3 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.
The four other nuclear power plants along the coast were also affected to different degrees by the earthquake and tsunami.
In response to the accident, the initial decisions on protective actions towards the public were made on the basis of plant conditions and included: evacuation; sheltering; restrictions on the consumption of food and drinking water; relocation; and 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. There were difficulties in evacuation including of patients from hospitals and nursing homes due to the communication and transportation problems.
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 received via hotlines and counselling services identified the need for easily understandable information and supporting material.
Shortly after the accident the area in which people were ordered to shelter was extended to within 20–30 km until 25 March, when the national Government recommended voluntary evacuation.
Medical management of emergency workers was also severely affected, and major efforts were required to meet the needs of on-site emergency workers.
Implementation of the arrangements for ensuring the protection of workers against radiation exposure was severely affected by the extreme conditions at the site and as there were no arrangements in place to integrate into the response those emergency workers 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.
National authorities issued guidance on the type of activities that members of the public, referred to as ‘helpers’, volunteered to assist in the off-site emergency response could carry out and on measures to be taken for their protection.
Radioactive materials were released to the atmosphere and were deposited on land and on the ocean.
Most of the atmospheric releases were blown eastward by the prevailing winds, depositing onto and dispersing within the North Pacific Ocean. Uncertainties in estimations of the amount and composition of the radioactive substances were difficult to resolve for reasons that included the lack of monitored data on the deposition of the atmospheric releases on the ocean. A number of oceanic transport models have been used to estimate the oceanic dispersion.
A relatively small part of the atmospheric releases were deposited on land, mostly in a north-westerly direction from the nuclear power plant. The measured activity of radionuclides decreases over time due to physical decay, environmental transport processes and cleanup activities.
In addition to radionuclides entering the ocean from the atmospheric deposition, there were also liquid releases and discharges at the nuclear site directly into the sea.
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 generally comparable with the range of effective doses incurred due to global levels of natural background radiation. The earlier assessments of radiation doses used environmental monitoring and dose estimation models, resulting in some overestimations.
In the short term, the most significant contributors to the exposure of the public were:
In the long term, the most important contributor to the exposure of the public will be external radiation from the deposited caesium-137.
Following the accident, the Japanese authorities applied conservative reference levels of dose included in the recent ICRP recommendations. The application of some of the protective measures and actions proved to be difficult for the implementing authorities and very demanding for the people affected. Indeed there were some differences between the national and international criteria and guidance for controlling drinking water, food and non-edible consumer products in the longer term aftermath of the accident, once the emergency phase had passed.
No early radiation-induced health effects were observed among workers or members of the public that could be attributed to the accident.
For children, the reported thyroid equivalent doses 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. The early and highly sensitive detection and treatment of screening of children’s thyroid glands is taking place as part of the Fukushima Health Management Survey implemented to monitor the health of the affected population. Because of the low reported thyroid doses attributable to the accident, an increase in childhood thyroid cancer attributable to the accident is deemed unlikely. However, uncertainties remained concerning the thyroid equivalent doses incurred by children immediately after the accident. Prenatal radiation effects have not been observed and are not expected to occur, given that the reported doses are well below the threshold at which these effects might take place. 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, by December 2011, 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 established by the Japanese authority.
Some shortcomings occurred in the implementation of occupational radiation protection requirements including during the early monitoring and recording of radiation doses of emergency workers, in the availability and use of some protective equipment, and in associated training.
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 among an exposed population by observations a few years after exposure. However, given the low levels of doses reported among members of the public, the conclusions of this report are in agreement with those of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to the United Nations General Assembly.
More globally, UNSCEAR found that “no discernible increased incidence of radiation-related health effects are expected among exposed members of the public and their descendants”. Even for the group of workers who received effective doses of 100 mSv or more, UNSCEAR concluded that “an increased risk of cancer would be expected in the future. However, any increased incidence of cancer in this group is expected to be indiscernible because of the difficulty of confirming such a small incidence against the normal statistical fluctuations in cancer incidence.”
Concerning the possibility of parents’ exposures resulting in hereditary effects in their descendants, UNSCEAR concluded that, in general, “although demonstrated in animal studies, an increase in the incidence of hereditary effects in human populations cannot at present be attributed to radiation exposure”.
UNSCEAR estimated that “The most important health effect from the accident is on mental and social wellbeing, related to the enormous impact of the earthquake, tsunami and nuclear accident, and the fear and stigma related to the perceived risk of exposure to ionizing radiation. » Indeed, some psychological conditions in some vulnerable groups of the affected population affected by the nuclear accident were observed, such as increases in anxiety and post-traumatic stress disorders. Since a number of these people had suffered the combined impacts of a major earthquake and a devastating tsunami as well as the accident, it is difficult to assess to what extent these effects could be attributed to the nuclear accident alone.
No observations of direct radiation induced effects in plants and animals have been reported, although limited observational studies were conducted in the period immediately after the accident. There are limitations in the available methodologies for assessing radiological consequences, but based on previous experience and the levels of radionuclides present in the environment, it is unlikely that there would be any major radiological consequences for biota populations or ecosystems as a consequence of the accident.
Knowledge of the impacts of radiation exposure on organisms other than humans needs to be strengthened by improving the assessment methodology and understanding of radiation induced effects on biota populations and ecosystems.
At the time of the accident, the IAEA had four roles in the response to a nuclear or radiological emergency:
The IAEA liaised with the official contact point in Japan, shared information on the emergency as it developed, and kept informed States, relevant international organizations, and the public
Communication with the official contact point in Japan in the early phase of the emergency response was difficult. The IAEA Director General’s visit to Japan, and the subsequent deployment of liaison officers to Tokyo, improved communication between the IAEA and the contact point. The IAEA also dispatched several missions to Japan, including an international fact finding mission and peer review missions on decommissioning and remediation.
In June 2011, a Ministerial Declaration on Nuclear Safety outlined a number of measures to further improve nuclear safety, emergency preparedness and radiation protection of people and the environment worldwide and expressed the firm commitment of IAEA Member States to ensure that these measures were taken.
The Ministerial Declaration also requested the Director General to prepare a draft IAEA Action Plan on Nuclear Safety, in consultation with Member States. which defined a programme of work to strengthen the global nuclear safety framework.
The IAEA also undertook cooperative activities in Fukushima Daiichi, which provided the basis for cooperation on radiation monitoring and remediation, human health, and emergency preparedness and response.
The design bases were derived using a range of postulated hazards, however, external hazards such as tsunamis were evaluated mainly on the basis of historical seismic records and evidence of recent tsunamis in Japan. This made that the means of protection intended to provide the fourth level of the so called « defence in depth » concept, that is prevention of the progression of severe accidents and mitigation of their consequences, were not available to restore the reactor cooling and to maintain the integrity of the containment.
The earthquake caused vibratory ground motions, which shooked the plant structures, systems and components. There were no indications that the main safety features of the plant were affected by the vibratory ground motions generated by the earthquake but it was followed by a series of tsunami waves, one of which inundated the site, and significantly exceeded the assumptions of hazards that had been made.
The flooding resulting from the tsunami simultaneously challenged the first three protective levels of the « defence in depth » concept resulting in common cause failures of equipment and systems in plant conditions at each of these three levels that were not envisaged in the design:
By consequence, the complete loss of power, the lack of information on relevant safety parameters due to the unavailability of the necessary instruments, the loss of control devices and the insufficiency of operating procedures made it impossible to arrest the progression of the accident and to limit its consequences.
The failure to provide sufficient means of protection at each level of « defence in depth » resulted in severe reactor damage in Units 1, 2 and 3 and in significant radioactive releases from these units.
Three fundamental safety functions are important for ensuring the safety of a nuclear plant:
Following the earthquake, the control of reactivity was fulfilled in all six units at the Fukushima Daiichi nuclear power plant.
However, the removing heat from the reactor core and the spent fuel pool could not be maintained because the operators were deprived of almost all means of control over the reactors and loss of most of the electrical systems. This was, in part, due to the failure to implement alternative water injection which led to overheating and melting of the fuel in the reactors.
The confinement of radioactive material function was also lost as a result of the loss of AC and DC power, which rendered the cooling systems unavailable and made it difficult for the operators to use the containment venting system necessary to relieve pressure and prevent its failure. The primary containment vessels for Units 1, 2 and 3 eventually failed and this resulted in radioactive releases into the environment.
The accident at the Fukushima Daiichi nuclear power plant showed that the methodological method was not fully appropriate. To better identify plant vulnerabilities, says the report, « it is necessary to take an integrated approach that takes account of the complex interactions between people, organizations and technology ».
Safety analyses conducted during the licensing process of the Fukushima Daiichi nuclear power plant and during its operation, did indeed not fully address the possibility of a complex sequence of events that could lead to severe reactor core damage. In particular, the safety analyses failed to identify the vulnerability of the plant to flooding and weaknesses in operating procedures and accident management guidelines. The probabilistic safety assessments did not address the possibility of internal flooding, and the assumptions regarding human performance for accident management were optimistic. Furthermore, the regulatory body had imposed only limited requirements for operators to consider the possibility of severe accidents.
Although TEPCO, the company which was owing and managing the Daiichi Fukushima nuclear power plant had developed severe accident management guidelines, they did not cover this unlikely combination multi-unit loss of power and the loss of cooling caused by the combined events. By consequence, the operators were not fully prepared, they had not taken part in relevant severe accident exercises, and the equipment available to them was not adequate in the degraded plant conditions.
More globally, it appeared also that the regulation of nuclear safety in Japan and the guidelines and procedures in place at the time of the accident were not fully in line with international practice in some key areas, most notably in relation to periodic safety reviews, reevaluation of hazards, severe accident management and safety culture.
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. At the time of the accident, there were no regulatory requirements in Japan for such reassessments, and relevant domestic and international operating experience was not adequately considered in the existing regulations and guidelines. The regulatory guidelines in Japan on methods for dealing with the effects of events associated with earthquakes, such as tsunamis, were generic and brief, and did not provide specific criteria or detailed guidance.
Nevertheless, before the accident, the operator had conducted some reassessments of extreme tsunami flood levels, using a consensus based methodology developed in Japan in 2002, which had resulted in values higher than the original design basis estimates. Based on the results, some compensatory measures were taken, but they proved to be insufficient at the time of the accident.
In addition, a number of trial calculations were performed by the operator before the accident, using wave source models or methodologies that went beyond the consensus based methodology. At the time of the accident, further evaluations were being conducted, but in the meantime, no additional compensatory measures were implemented.
There were no policies and strategies for post-accident remediation in place in Japan, and a remediation policy was enacted by the Government of Japan in August 2011. Specific policies, guidelines, criteria and arrangements for the transition from the emergency phase to the recovery phase developed only after the Fukushima Daiichi accident. In developing these arrangements, the Japanese authorities applied the latest recommendations of the International Commission on Radiological Protection (ICRP).
At the time of writing, safety functions had been re-established and structures, systems and components were in place to reliably maintain stable conditions. However, there was a continuing need for control of ingress of groundwater to the damaged and contaminated reactor buildings. There were also difficulties in establishing locations to store the large amounts of contaminated material arising from off-site remediation activities. Several hundred temporary storage facilities had been established in local communities and efforts to establish an interim storage facility were continuing.
Following stabilization of the conditions of the reactors, work to prepare for their eventual decommissioning began. Efforts towards the recovery of the areas affected by the accident, including remediation and revitalization of communities and infrastructure, began in 2011. The long term goal of post-accident recovery is to re-establish an acceptable basis for a fully functioning society in the affected areas. Consideration needs to be given to remediation of the areas affected by the accident in order to reduce radiation doses, consistent with adopted reference levels. 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.
The authorities adopted a ‘reference level’ consistent with the lower end of the range specified in international guidance, as a target level of dose for the overall remediation strategy. 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 sequences arising from radiological accidents being sought.
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. The remediation strategy focused on decontamination activities to reduce the levels of radiocaesium deposited on the ground and other surfaces in priority areas, the main pathway of exposure. Internal doses continue to be controlled by restrictions on food, as well as through remediation activities on agricultural land. Japanese authorities have estimated that the timescale for completing decommissioning activities is likely to be in the range of 30–40 years.
The application of a low reference level has effect on the quantity of contaminated materials generated in remediation activities, and thereby on the costs and the demands on limited resources.
Two categories of contaminated areas were defined on the basis of additional annual doses estimated in the autumn of 2011. The national Government was assigned responsibility for formulating and implementing remediation plans in the first area (the ‘Special Decontamination Area’) — within a radius of 20 km of the Fukushima Daiichi site and in areas where additional annual doses arising from contamination on the ground were projected to exceed 20 mSv in the first year after the accident. The municipalities were given responsibility for implementing remediation activities in the other area (the ‘Intensive Contamination Survey Area’), where the additional annual doses were projected to exceed 1 mSv but to remain below 20 mSv. Specific dose reduction goals were set, including a long term goal of achieving an additional annual dose of 1 mSv or less.
Decisions regarding the final conditions of the plant and site will be the subject of further analysis and discussions. The report gives more details of the remediation strategy and subsequent plans adopted.
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 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, which 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:
Also, the experience obtained in Japan could be used in developing practical guidance on the application of international safety standards in post-accident recovery situations.
The « defence in depth » concept remains valid, but worldwide operating experience has indeed shown instances where natural hazards have exceeded the design basis for a nuclear power plant. In particular, the experience from some of these events demonstrated the vulnerability of safety systems to flooding. The safety of nuclear power plants needs to be re-evaluated on a periodic basis to consider advances in knowledge, and necessary corrective actions or compensatory measures need to be implemented promptly.
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.
The main practical improvements identified in the report include two sectors that are briefly summarised in table 1 (safety analysis and preparedness) and table 2 (arrangements to manage a better public protection from nuclear risks).
| 1. The necessity to adopt an integrated approach that takes account of the complex interactions between people, organizations and technology in order to better identify plant vulnerabilities through the entire life cycle of nuclear installations; | 11. Arrangements in place to integrate into the response those emergency workers and helpers who volunteer to assist in the emergency response who had not been designated prior to the emergency; |
| 2. Comprehensive, well designed and up to date accident management provisions for accidents that affect several units at a multi-unit plant.derived on the basis of a comprehensive set of initiating events and plant conditions; | 12. Strengthened implementation of international arrangements for notification and assistance; |
| 3. Perform comprehensive probabilistic and deterministic safety analyses to provide a high degree of confidence in the robustness of the plant design and to confirm the capability of a plant to withstand applicable beyond design basis accidents; | 13. Improved consultation and sharing of information among States on protective actions and other response action; |
| 4. Training, exercises and drills need to include postulated severe accident conditions to ensure that operators are as well prepared as possible. They need to include the simulated use of actual equipment that would be deployed in the management of a severe accident; | 14. Prompt quantification and characterization of the amount and composition of the release in case of release of radioactive substances to the environment; |
| 5. Ensure effective regulatory oversight of the safety of nuclear installations by is independent regulatory body possessing legal authority, technical competence and a strong safety culture; | 15. For significant releases, a comprehensive and coordinated programme of long term environmental monitoring to determine the nature and extent of the radiological impact on the environment at the local, regional and global levels. |
| 6. Continuous challenge or re-examination of the prevailing assumptions about nuclear safety and the implications of decisions and actions that could affect nuclear safety | 16. Development by relevant international bodies of explanations of the principles and criteria for radiation protection that are understandable for non-specialists with a better communication strategy to convey the justification for such measures and actions to all stakeholders, including the public. |
| 7. Consideration of 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; | 17. Recognize the socioeconomic consequences of any nuclear accident and of the subsequent protective actions, and to develop revitalization and reconstruction projects that address issues such as reconstruction of infrastructure, community revitalization and compensation |
| 8. Clearly defined roles and responsibilities in the emergency management system for the operating organization and for local and national authorities ; | 18. An effective recovery programme requires the trust and the involvement of the affected population that should be invoved population in the decision making processes. |
| 9. Regularly testing in exercises of the system, including the interactions between the operating organization and the authorities; | 19. Pre-accident planning and national strategies and measures for post-accident recovery is necessary to improve decision making under pressure in the immediate post-accident situationan enable an effective and appropriate overall recovery programme in case of a nuclear accident. |
| 10. Establish a strategic plan flexible and readily adaptable to changing conditions for maintaining long term stable conditions and for the decommissioning of accident damaged facilities. Essential for on-site recovery | 20. 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. |
| 1. Allow decisions to be made on the implementation of predetermined, urgent protective actions for the public, based on predefined plant conditions ; | 10. Timely analyse an emergency and the response tthat was given to it, drawing lessons and identifying possible improvements and enhances emergency arrangements. |
| 2. Enable urgent protective actions to be extended or modified in response to developing plant conditions or monitoring results ; | 11. In a prolonged exposure situation, consistency among international standards, and between international and national standards, would be beneficial, particularly those associated with drinking water, food, non-edible consumer products and deposition activity on land. |
| 3. Enable early protective actions to be initiated on the basis of monitoring results ; | 12. Personal radiation monitoring of representative groups of members of the public provides invaluable information for reliable estimates of radiation doses and needs to be used together with environmental measurements and appropriate dose estimation models for assessing public dose. |
| 4. Ensure that protective actions and other response actions in a nuclear emergency do more good than harm. A comprehensive approach to decision making needs to be in place to ensure that this balance is achieved ; | 13. Restrict the consumption of fresh milk from grazing cows even if dairy products were not the main pathway for the ingestion of radioiodine in Japan but is the most important method of limiting thyroid doses, especially to children ; |
| 5. 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 ; | 14. Present clearly to stakeholders the risks of radiation exposure and the attribution of health effects to radiation, making it unambiguous that any increases in the occurrence of health effects in populations are not attributable to exposure to radiation if levels of exposure are similar to the global average background levels of radiation |
| 6. Address public concerns locally, nationally and internationally ; | 15. Health surveys are very important and useful to provide information to support medical assistance to the affected population, but should not be interpreted as epidemiological studies. |
| 7. Develop termination of protective actions and other response actions at the preparedness stage, and for transition to the recovery phase ; | 16. Provide radiological protection guidance to address the psychological consequences to members of the affected populations in the aftermath of radiological accidents. |
| 8. Implement rigorous testing and controls on food is necessary to prevent or minimize ingestion doses. | 17. Implement further international guidance on the practical application of safety standards for radiation protection in post-accident recovery situations. |
| 9. Develop a robust system for monitoring and recording occupational radiation doses, via all relevant pathways, particularly those due to internal exposure that may be incurred by workers during severe accident management activities | 18. Suitable and sufficient personal protective equipment should be available for limiting the exposure of workers during emergency response activities and workers be sufficiently trained in its use. |
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