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Welcome to the Portal on Observations and Lessons from the Fukushima Daiichi Accident

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The assessment of natural hazards needs to be sufficiently conservative. The consideration of mainly historical data in the establishment of the design basis of NPPs is not sufficient to characterize the risks of extreme natural hazards. Even when comprehensive data are available, due to the relatively short observation periods, large uncertainties remain in the prediction of natural hazards.
Extreme natural events that have a very low probability of occurrence can result in significant consequences, and the prediction of extreme natural hazards remains difficult and controversial due to the existence of uncertainties. Additionally, such predictions may change during the life of an NPP as more information becomes available and methods of analysis improve. It is therefore necessary to use all relevant available data, both domestic and international, to ensure a reliable prediction of hazards, to define a reliable and realistic design basis against natural extreme events, and to design NPPs with sufficient safety margins.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
The safety of NPPs 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.
The periodic safety review programme at the Fukushima Daiichi NPP did not lead to safety upgrades based on regulatory requirements. TEPCO performed the re-evaluation on a voluntary basis considering advances in knowledge, including new information and data. When faced with a revised estimate of a hazard that exceeds previous predictions, it is important to ensure the safety of the installation by implementing interim corrective actions against the new hazard estimate while the accuracy of the revised value is being evaluated. If the accuracy of a new hazard estimate is confirmed, the operating organization and regulatory authority need to agree on a schedule and comprehensive action plan to promptly address the method of coping with such higher hazards to ensure plant safety.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
The assessment of natural hazards needs to consider the potential for their occurrence in combination, either simultaneously or sequentially, and their combined effects on an NPP. The assessment of natural hazards also needs to consider their effects on multiple units at an NPP.
The Fukushima Daiichi accident demonstrated the need to fully investigate the potential for a combination of natural hazards affecting multiple units at an NPP. The complex scenarios resulting from the occurrence of a combination of natural hazards need to be taken into account when considering accident mitigation measures and recovery actions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
 
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.
The operating experience evaluation programme at the Fukushima Daiichi NPP did not lead to design changes that took account of domestic or international experience involving flooding. The review of operating experience needs to be a standard part of plant oversight processes, with account taken of relevant sources such as the Incident Reporting System of the IAEA and the OECD Nuclear Energy Agency. Regulatory bodies need to perform independent reviews of national and international operating experience to confirm that operating organizations are taking concrete actions to improve safety.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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 flooding resulting from the tsunami simultaneously challenged the first three protective levels of defence in depth, resulting in common cause failures of equipment and systems. Even when faced with this situation, operators were able to apply effective, albeit delayed, mitigation strategies. 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 application of the defence in depth concept needs to be periodically re-examined over the lifetime of an NPP to ensure that any change in vulnerability to external events is understood and that appropriate changes to the design are made and implemented. There is a need for extreme external hazards to be addressed in periodic safety reviews, because such hazards can result in common cause failures that may simultaneously jeopardize several levels of defence in depth.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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.
The loss of instrumentation and control during the accident at the Fukushima Daiichi NPP left operators with little indication of actual plant conditions. The loss of instrumentation and control systems had a serious impact on efforts to prevent a severe accident or to mitigate its consequences. The extent and nature of the necessary instrumentation and control systems need to be defined with care, according to the characteristics of the design of the plant, including spent fuel pools. Systems need to be protected to ensure they are available when needed. This also demonstrated the need to improve strategies to allow for manual control of vital equipment.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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.
At the Fukushima Daiichi NPP, the operators were eventually, after some delay, able to deploy portable equipment to inject water into the reactors. Cooling systems based either on installed or portable equipment need to be qualified and tested to ensure that they function and can be deployed by operators when needed.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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.
At the Fukushima Daiichi NPP, the failure of venting the containment, and the subsequent failure of the reactor building due to the hydrogen explosion, led to a significant release of radioactive material to the environment. The confinement function needs to be assessed to ensure that all possible hazards are considered in the design of equipment intended to maintain the integrity of the confinement system.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Comprehensive probabilistic and deterministic safety analyses need to be performed to confirm the capability of a plant to withstand applicable beyond design basis accidents and to provide a high degree of confidence in the robustness of the plant design.
Safety analyses can be used both to evaluate and to develop response strategies for beyond design basis accidents and may include the use of both deterministic and probabilistic methods. The probabilistic safety assessment studies conducted for the Fukushima Daiichi NPP were of limited scope and did not consider the possibility of flooding from internal or external sources. The limitations in these studies contributed to the limited scope of accident management procedures available to the operators.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Accident management provisions need to be comprehensive, well designed and up to date. They need to be derived on the basis of a comprehensive set of initiating events and plant conditions and also need to provide for accidents that affect several units at a multi-unit plant.
The accident management procedures available to the operators at the Fukushima Daiichi NPP did not consider the possibility of a multi-unit accident, nor did they provide guidance for the complete loss of electrical power. Accident management provisions need to be based on a plant specific analysis performed by using a combination of deterministic and probabilistic methods. Accident management guidance and procedures need to consider the possibility of events taking place in several units simultaneously and in spent fuel pools. They also need to take into account the possibility of disrupted regional infrastructure, including serious deficiencies in communication, transport and utilities. Accident management provisions should also take into consideration the best available guidance from the international community and be periodically updated to account for new information.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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.
Operators at the Fukushima Daiichi NPP had not been specifically trained on how to manually operate systems such as the Unit 1 isolation condenser and fire trucks as an alternative source for low pressure water injection. Special attention is needed in personnel training to perform actions under conditions of prolonged loss of all power, with limited information about the plant status and no information on important safety parameters. Staff training, exercises and drills need to realistically simulate the progression of severe accidents, including the simultaneous occurrence of accidents in several units at the same site. Training, exercises and drills need to involve not only on-site accident management personnel but all off-site responders at the operating organization, local, regional and national levels.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
In order to ensure effective regulatory oversight of the safety of nuclear installations, it is essential that the regulatory body is independent and possesses legal authority, technical competence and a strong safety culture.
NISA did not have sufficient authority to take necessary actions, including inspections at regulated facilities. It is essential that the regulatory body is able to make independent decisions on safety over the lifetime of installations. To ensure such independent decision making, the regulatory body must be competent and must possess sufficient human resources, adequate legal authority — including the right to suspend operation and/or to impose improvements in safety on operating organizations — and adequate financial resources. The regulatory body needs the authority to adapt its inspection programme in the light of new safety information. It must also be able to ensure that national regulatory requirements and corresponding guidelines for assessing the safety of nuclear installations are revised periodically in accordance with scientific and technical developments, operational experience, and international standards and practices.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
In order to promote and strengthen safety culture, individuals and organizations need to continuously challenge or re-examine the prevailing assumptions about nuclear safety and the implications of decisions and actions that could affect nuclear safety.
This can be achieved by individuals and organizations embracing a questioning attitude to identify the nature, boundaries and potential threats of their shared assumptions about nuclear safety. The institutionalization of a continuous dialogue within organizations, and among different organizations, on issues related to nuclear safety, and their significance and impact on decisions and actions, is essential. Periodic assessments of safety culture can help to foster reflection and dialogue on basic assumptions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
A systemic approach to safety needs to consider the interactions between human, organizational and technical factors. This approach needs to be taken through the entire life cycle of nuclear installations.
The accident at the Fukushima Daiichi NPP showed that it is difficult to identify vulnerabilities in systems that involve complex interactions between people, organizations and technology because basic assumption regarding nuclear safety can remain undetected. A systemic approach that includes human, technological and organizational considerations is necessary to understand how the components of the overall system function and interact in both normal operation and accident conditions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
In preparing for the response to a possible nuclear emergency, it is necessary to 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.
Consideration needs to be given to the possibility of a severe nuclear accident, irrespective of the cause, possibly involving more than one unit at a site and occurring simultaneously with a natural disaster, which could result in disruption at the site and of the local infrastructure. Systems, communications and monitoring equipment for providing essential information for both on-site and off-site responses need to be able to function under such circumstances. Facilities where the response will be managed (e.g. on-site and off-site emergency response centres) need to be selected or designed to be operational under a full range of emergency conditions (radiological, working and environmental conditions), and need to be suitably located and/or protected so as to ensure their operability and habitability under such conditions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
The emergency management system for response to a nuclear emergency needs to include clearly defined roles and responsibilities for the operating organization and for local and national authorities. The system, including the interactions between the operating organization and the authorities, needs to be regularly tested in exercises.
Arrangements are needed that integrate the response to a nuclear emergency with the response to natural disasters and human-made disasters (e.g. earthquakes, floods and fires). The on-site response needs to be managed by personnel located at the site who have knowledge of the plant and of the situation. The on-site and off-site responses need to be coordinated based on pre-planned arrangements.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Emergency workers need to be designated, assigned clearly specified duties, regardless of which organization 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 prior to the emergency, and helpers who volunteer to assist in the emergency response.
The practical arrangements for the protection of emergency workers need to be addressed in a consistent manner and in adequate detail in relevant plans and procedures. Account needs to be taken of those who may not have been designated as emergency workers at the preparedness stage. Dose criteria for emergency workers need to be set in advance and applied in a consistent manner for the assigned emergency duties. Arrangements for ensuring the well-being of emergency workers (including contact with their families) need to be in place. In addition, arrangements need to be pre-planned for members of the public (referred to as ‘helpers’) who volunteer to assist in response actions to be integrated into the emergency response organization and to be afforded an adequate level of radiation protection.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Arrangements need to be in place to allow decisions to be made on the implementation of predetermined urgent protective actions for the public, based on predefined plant conditions.
These arrangements are necessary because decision support systems, including those using computer models, may not be able to predict the size and timing of a radioactive release (the ‘source term’), the movements of plumes, deposition levels or resulting doses sufficiently quickly or accurately in an emergency to be able to provide the sole basis for deciding on initial urgent protective actions. At the preparedness stage, there is a need to develop an emergency classification system based on observable conditions and measurable criteria (emergency action levels). This system enables the declaration of an emergency shortly after the detection of conditions at a plant that indicate actual or projected damage to the fuel and initiation of predetermined, urgent protective actions for the public (in the predefined zones) promptly following classification of the emergency by the operator. This emergency classification system needs to cover a full range of abnormal plant conditions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Arrangements need to be in place to enable urgent protective actions to be extended or modified in response to developing plant conditions or monitoring results. Arrangements are also needed to enable early protective actions to be initiated on the basis of monitoring results.
At the preparedness stage, there is a need to establish arrangements to, among other things: (1) define emergency planning zones and areas; (2) establish dose and operational criteria (levels of measurable quantities) for taking urgent protective actions and other response actions, including dealing with special population groups within emergency zones (e.g. patients in hospitals); (3) enable urgent protective actions to be taken before or shortly after a release of radioactive material; (4) enable prompt establishment of access controls in areas where urgent protective actions are in place; (5) extend protective actions beyond the established emergency planning zones and areas if necessary; (6) establish dose and operational criteria for taking early protective actions and other response actions (e.g. relocation and food restrictions) that are justified and optimized, taking into account a range of factors such as radiological and non-radiological consequences, including economic, social and psychological consequences; and (7) establish arrangements for revision of operational criteria for taking early protective actions on the basis of the prevailing conditions.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
Arrangements need to be in place to 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.
These arrangements need to be developed with a clear understanding of the full range of possible health hazards presented in a nuclear emergency and of the potential radiological and non-radiological consequences of any protective actions. Protective actions need to be taken in a timely and safe manner, taking into account possible unfavourable conditions (e.g. severe weather or damage to infrastructure). Preparations in advance are necessary to ensure the safe evacuation of special facilities, such as hospitals and nursing homes; continuing care or supervision must be provided for those who need it.
Reference Document: Director General’s Report on the Fukushima Daiichi Accident
Link to Reference Document:
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