Version 1.0, May 2015 - PowerPoint Presentation

Version 1.0, May 2015 BASIC PROFESSIONAL TRAINING COURSEModule III Basic principles of nuclear safety This material was prepared by the IAEA and co-funded by the European Union. 

WHAT IS NUCLEAR SAFETY? Learning objectivesAfter completing this chapter, the trainee will be able to:Describe the basic goal of nuclear safety. Define the fundamental safety objective according to IAEA Safety Fundamentals SF-1. Describe the relation between nuclear safety and safety culture. 2

WHAT IS NUCLEAR SAFETY? There are numerous applications of nuclear phenomena, ranging from generation of power to uses in medicine, industry and agriculture.Energy production inevitably involves some risk.One specific risk is the release of radioactive material.Therefore the use of nuclear technology must be subject to rigorous safety standards.In 2006, the IAEA published SF-1. 3

WHAT IS NUCLEAR SAFETY? – cont. SF-1 Fundamental Safety Principles.The fundamental safety objective is to protect people and the environment from harmful effects of ionizing radiation.Nuclear safety is more than just a technical concept – safety culture is an important part of nuclear safety.Out of all nuclear facilities, NPPs have a greatest potential for the release of radioactive materials.Therefore no complacency is allowed in managing NPPs. Radiation risk does not recognize borders – nuclear safety is a global concept.4

SAFETY FUNDAMENTALS Learning objectivesAfter completing this chapter, the trainee will be able to:List the main safety objectives and principles as defined in the IAEA SF-1 Safety Fundamentals document.Describe legislative and regulatory framework in a country. Describe the basic principles of safety management. List the important engineering aspects to be taken into consideration throughout the lifetime of a nuclear installation. Describe the basic principles of safety verification. 5

IAEA fundamental safety principles Article III of the IAEA Statute “development of safety standards…”SF – 1 Fundamental Safety Principles:Fundamental Safety Objective,10 Safety Principles.Safety includes:Safety of nuclear installations,Radiation safety,Radioactive waste management , Safety in transport of radioactive material .6

Fundamental safety objective The fundamental safety objective is to protect people and the environment from harmful effects of ionizing radiation. From this fundamental safety objective, the above 10 principles were derived.This fundamental safety objective should be fulfilled without unduly limiting the operation of facility or activity.This fundamental safety objective applies through entire lifetime of all facilities and activities from planning to decommissioning. 7

Fundamental safety objective – cont. To ensure that the highest standards of safety are achieved, measures have to be taken:To control the radiation exposure of people and the release of radioactive material to the environment;To restrict the likelihood of events that might lead to a loss of control over a nuclear reactor core, nuclear chain reaction, radioactive source or any other source of radiation;To mitigate the consequences of such events if they were to occur.8

Fundamental safety objective – cont. PRINCIPLE ONE: Responsibility for safetyThe prime responsibility for safety must rest with the person or organization responsible for facilities and activities that give rise to radiation risks. Responsibility cannot be delegated.Licensee – holds the authorization to operate a facility. 9

Fundamental safety objective – cont. PRINCIPLE ONE: Responsibility for safetyThe licensee is responsible for:Establishing and maintaining the necessary competencies;Providing adequate training and information;Establishing procedures and arrangements to maintain safety under all conditions;Verifying appropriate design and adequate quality of facilities and activities and their associated equipment;Ensuring safe control of all radioactive material that is used, produced, stored or transported;Ensuring safe control of all radioactive waste that is generated.Radioactive waste management extends over several generations. 10

Fundamental safety objective – cont. PRINCIPLE TWO: Role of GovernmentAn effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained. Framework for regulation.Government responsibility for:Adopting laws,Regulations,Standards,Establishment of an independent RB,Emergency arrangements,Monitoring radioactive releases, Disposing radioactive waste. 11

Fundamental safety objective – cont. PRINCIPLE TWO: Role of GovernmentThe regulatory body must:Have adequate legal authority, competencies and resources to fulfil its responsibilities;Be effectively independent so that it is free from any undue pressure from interested parties;Set up appropriate means of providing information about the safety, health and environmental aspects of facilities and activities, and about regulatory processes;Consult parties in the vicinity, the public and other interested parties, as appropriate, in an open and inclusive process.12

Fundamental safety objective – cont. PRINCIPLE THREE: Leadership and Management for SafetyEffective leadership and management for safety must be established and sustained in organizations concerned with, and facilities and activities that give rise to, radiation risks.Leadership must be demonstrated at the highest levels.Safety must be achieved and maintained by an effective management system.Management system promotes strong safety culture, including:Individual and collective commitment to safety by the leadership, management and personnel at all levels;Accountability of organizations and individuals at all levels for safety; Measures to encourage a questioning and learning attitude and discourage complacency with regard to safety. 13

Fundamental safety objective – cont. PRINCIPLE THREE: Leadership and Management for SafetyThe management system must ensure:Regular assessment of safety performance including:Systematic analysis of normal operation,The ways in which failures might occur,The consequences of such failures,Safety measures to control hazards.Assessment of the design, engineered safety features and operator actions.Safety demonstration before construction and commissioning. Reassessment as required. Processes are put in place for the feedback of operating experience.14

Fundamental safety objective – cont. PRINCIPLE FOUR: Justification of Facilities and ActivitiesFacilities and activities that give rise to radiation risks must yield an overall benefit.For facilities and activities to be considered justified, the benefits that they yield must outweigh the risks to which they give rise.All significant consequences of operating facilities or conducting activities must be taken into account. 15

Fundamental safety objective – cont. PRINCIPLE FIVE: Optimization of protectionProtection must be optimized to provide the highest level of safety that can reasonably be achieved.All risks must be assessed:From normal operation, From abnormal and accidental conditions.Risk as low as reasonable achievable (ALARA).Risks should be regularly reassessed.Inevitable uncertainties in knowledge should be taken into account. 16

Fundamental safety objective – cont. PRINCIPLE FIVE: Optimization of protectionFactors considered in the optimization of risk include:The number of people who may be exposed to radiation;The likelihood of their incurring exposure;The magnitude and distribution of radiation doses received;Radiation risks arising from foreseeable events;Economic, social and environmental factors.Optimization means also the use of good practices.Resources devoted should be commensurate with risk. 17

Fundamental safety objective – cont. PRINCIPLE SIX: Limitation of Risks to IndividualsMeasures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm. Limits to doses and risk must be established.18

Fundamental safety objective – cont. PRINCIPLE SEVEN: Protection of Present and Future GenerationsPeople and the environment, present and future, must be protected against radiation risks.Radiation risks may transcend borders and persist for long times.The possible consequences now and in the future must be taken into account. In particular:Safety standards apply not only to local populations but also to populations remote from facilities and activities;Where effects could span generations, subsequent generations must be adequately protected without any need for them to take significant protective actions. Radioactive waste must not be a burden for future generations. 19

Fundamental safety objective – cont. PRINCIPLE EIGHT: Prevention of AccidentsAll practical efforts must be made to prevent and mitigate nuclear or radiation accidents. To ensure that likelihood of an accident is low, the following measures should be taken:Prevent the occurrence of failures or abnormal conditions (including breaches of security) that could lead to a loss of control;Prevent escalation of any such failures or abnormal conditions that do occur;Prevent loss of, or loss of control over a radioactive source or other source of radiation.Defence-in-depth (DiD ) Is the primary means of preventing and mitigating accidents. 20

Fundamental safety objective – cont. PRINCIPLE EIGHT: Prevention of AccidentsDefence-in-depth:Consecutive and independent barriers,Effective management system and strong safety culture,Adequate site selection,Good design and engineering features providing safety margins, diversity and redundancy,Comprehensive operational procedures and practices,Accident management procedures.21

Fundamental safety objective – cont. PRINCIPLE NINE: Emergency Preparedness and ResponseArrangements must be made for emergency preparedness and response in case of nuclear or radiation incidents.The primary goal of emergency preparedness and response are:Ensure that arrangements are in place for an effective response at the scene and, as appropriate, at the local, regional, national and international levels;Ensure that, for reasonably foreseeable incidents, radiation risks would be minor;Take practical measures to mitigate any consequences for human life and health and the environment for any incidents that do occur.22

Fundamental safety objective – cont. PRINCIPLE NINE: Emergency Preparedness and ResponseThe licensee, employer, regulator and government must in advance establish arrangements for emergency preparedness and response.Criteria when to take different protective actions should be determined.Emergency plans should be exercised periodically.23

Fundamental safety objective – cont. PRINCIPLE TEN: Protective Actions to Reduce Existing or Unregulated Radiation RisksProtective actions to reduce existing or unregulated radiation risks must be justified and optimized. Radiation risks outside facilities and activities which are being regulated include:Mitigation of exposure from natural sources of radiation; Exposure arising from human activities conducted in the past that were never subject to regulatory control (or an earlier, less rigorous control), such as residue from mining operations; Remediation measures following an uncontrolled release of radionuclides to the environment. 24

Fundamental safety objective – cont. PRINCIPLE TEN: Protective Actions to Reduce Existing or Unregulated Radiation RisksProtective actions for these situations are justified only if they yield sufficient benefit.Protective actions should be optimized – greatest benefit in relation to cost.25

Legislative and regulatory framework Government is responsible for providing legislation which:Defines that the responsibility for safety rests with the licenseeEstablishes the regulatory body (RB) with responsibility for:Licensing,Regulatory control,Enforcement of regulation.RB must be independent RB must have: Adequate authority,Competence,Resources.26

Management of safety Safety management is the set of measures which ensure that an adequate level of safety is maintained throughout the lifetime of an installation. Managers should establish safety policies and ensure that:There is a clear division of responsibilities;Staff is adequately educated, trained and retrained;Adequate procedures are developed and adhered to;Safety matters are regularly reviewed, monitored and audited. 27

Management of safety – cont. Organizations should establish and implement a sound safety management programme. It should span throughout the entire lifetime.Capabilities and limitations of human behaviour should be recognized.Emergency plans should be prepared and exercised. 28

Management of safety – cont. The main goal of the management system is to achieve and enhance safety by:Bringing together in a coherent manner all the requirements for managing the organization;Describing the planned and systematic actions necessary to provide adequate confidence that all these requirements are satisfied;Ensuring that health, environmental, security, quality and economic requirements are not considered separately from safety requirements, to help preclude their possible negative impact on safety.29

Safety considerations during the various phases of the installation In all stages of the lifetime of the nuclear installation various safety aspects need to be taken into account.During siting:Man-made and natural hazards need to be identified;Impact of the installation on the environment must be evaluated;Feasibility of carrying out emergency plans must be assured;All needs to be performed by the utility and reviewed by the RB.30

Safety considerations during the various phases of the installation – cont. During the design and construction: It should be assured that potential radioactive exposures are limited;Prevention and mitigation of accidents are assured by DiD;Technologies used must be proven.31

Safety considerations during the various phases of the installation – cont. In the commissioning stage:A specific approval by the RB is necessary;This approval must be based on safety analysis and a commissioning plan;Consistency with the design and safety requirements must be verified;Operating procedures must be validated.32

Safety considerations during the various phases of the installation – cont. For the operation phase:A set of operating limits and conditions need to be established;These are derived from safety analyses;Each time the design is modified safety analyses and derived operating limits and conditions need to be revised;Installation must be regularly inspected, tested and maintained to ensure that SSC are available and operate as intended;Engineering and technical support must be ensured throughout the lifetime of the installation;Procedures need to be developed for:Normal operation,EOPs,SAMGs;Feedback of Operating Experience (FOE) programme must be established. 33

Safety considerations during the various phases of the installation – cont. During operation radioactive waste is generated:It should be kept to a minimum;Waste treatment and interim storage must be strictly controlled.Decommissioning programme must be developed and approved by the RB.34

Verification of safety Verification of safety should be performed regularly.It includes many activities such as:Review of site related factors;Independent assessment of the design;Review of tests during construction and commissioning;Continued monitoring and inspection of the installation during operation;Continuous monitoring of the environment;Assessment and control of modifications. safety verification also includes investigation of incidents , determination of root causes, lessons learned and implementation of corrective actions. Periodic reassessment of safety should be done every 10 years.35

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Questions Define in simple words “nuclear safety”!What is the fundamental safety objective as defined in the IAEA Safety Fundamentals document SF-1?How many safety principles are defined in SF-1?What are the basic principles of safety management?List few important engineering aspects to be taken into account throughout the lifetime of a nuclear installation!What are the basic principles of safety verification? 37

FUNDAMENTAL SAFETY FUNCTIONS Learning objectivesAfter completing this chapter, the trainee will be able to:Define the three fundamental safety functions.Describe the role of reactivity control. Describe the role of accumulation of fission products. Describe the role of decay heat. 38

Three fundamental safety functions IAEA Safety Requirement SSR-2/1 on Design of nuclear power plants identifies the following three fundamental safety functions that shall be ensured for all plant states:Control of reactivity;Removal of heat from the reactor and from the fuel pool;Confinement of radioactive material, shielding against radiation and control of planned radioactive releases, as well as limitation of accidental radioactive releases. 39

Three fundamental safety functions Systematic approach to identify structures, systems and components that are necessary to fulfil these functions at all times. Establish means for monitoring the plant status to ensure that the required fundamental safety functions are fulfilled.The main purpose of the fundamental safety functions is to reduce the likelihood of releases of fission products and radionuclides into the environment. 40

Three fundamental safety functions 41

Reactivity Control 42The reactivity control system must fulfil the following functions:Control the reactor power level in operation and provide for shutdown under normal and abnormal conditions;Provide for rapid shutdown and maintain the reactor subcritical, including in accident conditions (control rods, boric acid injection into the coolant);Negative reactivity feedbacks limit the reactor power:Negative moderator temperature effect; Negative fuel temperature effect; Negative coolant void effect; Negative power effect.

Removal of Heat 43Decay heat is produced by the decay of radioactive fission products after reactor shutdown: Principal reason of safety concern in LWRs;Adequate cooling must be maintained at all times to remove decay heat and prevent cladding failure in the reactor or in spent fuel storage. Decay heat curve for a typical LWR, that has been operating for a long time (given in % of full power P)

Confinement of Radioactive Material 44Unique hazard associated with a nuclear reactor: inventory of radioactive material that accumulates in the core.Sources of radionuclides include: fission event , about two fission fragments per fission; neutron absorption in structural materials, which produces various radioactive products such as cobalt-60; neutron absorption in fertile material (primarily U-238), to produce transuranic elements, which are important to the long-term radiation hazard from spent fuel .

Confinement of Radioactive Material 45Characteristics of the isotopes that are of most concern:Chemical volatility (facilitates release in case of accidents);Chemical affinity for the human body;High energy gamma decay (shielding needed); Long half-life.   Isotopes of particular interest:noble gases, strontium-90, iodine-131, cesium-137 .

Confinement of Radioactive Material 46Uncontrolled release of radioactive materials must be prevented by confinement as close as possible to the point of their origin or their intended location. This is achieved by physical barriers that are in direct contact or very close to the radioactive material . In principle, these barriers have to be passive.

Confinement of Radioactive Material 47In a typical PWR or BWR the confinement is provided by:Fuel matrix (sintered UO2) retains solid fission products; Zirconium cladding of the fuel rods retains fission gases and volatile fission products;Reactor coolant pressure boundary retains fission products that may have leaked through the cladding and dissolved activation products.

Confinement of Radioactive Material 48In the spent fuel pit the confinement is provided by the: Fuel matrix; Zirconium cladding of the fuel rods;Spent fuel pit stainless steel cladding and the filtered cooling system.

Confinement of Radioactive Material 49In the radioactive waste storage and repository the confinement is provided by the:Waste form (usually it is in some way solidified);Container.

DEFENCE-IN-DEPTH Learning objectivesAfter completing this chapter, the trainee will be able to:Define 5 levels of defence in depth.Define 4 barriers in preventing spread of radioactive materials. 50

The defence-in-depth concept DiD concept developed in 1960is in the US.Approach links prevention, monitoring and mitigation actions.DiD concept consists of several levels and several barriers, each level preventing degradation into the next level. INSAG-10 Defence in Depth in Nuclear Power Plants presents:the history of the concept, how it is currently applied,Indicates application to the next generation of reactors.DiD concept from INSG-10 comprises 5 levels . 51

The defence-in-depth concept – cont. First Level – prevention of abnormal operation and failuresInstallation must be designed utilizing conservative provisions.For each major SSC possible failure mechanisms should be determined.Operating transients and various shutdown conditions are considered.SSC are then constructed, installed, checked, tested and operated in such a way as to allow adequate margins. Systems dealing with abnormal situations are dedicated and do not need to be actuated daily.52

The defence-in-depth concept – cont. First Level – prevention of abnormal operation and failuresExternal events should be identified and it should be demonstrated that the NPP can withstand such conditions. On this basis it is possible to determine:Reference seismic level,Extreme meteorological conditions like:Wind speed,Weight of snow,Maximum overpressure wave,Temperature range,Etc.Human-System interface provision should be determined and time allowances for manual intervention determined. 53

The defence-in-depth concept – cont. First Level – prevention of abnormal operation and failuresIn order to guaranty quality and reliability of equipment, sets of rules and codes should be defined for:Design,Supply,Manufacture,Construction,Checking,Initial and periodic testing, Operation, Preventive maintenance, Selection of staff, their training and the overall organization should be established.Systematic use of FOE should be established. 54

The defence-in-depth concept – cont. Second level – control of abnormal operation and detection of failuresReliable regulation, control and protection systems must inhibit any abnormal development.Temperature, pressure and nuclear and thermal power control systems should be installed.Systems for monitoring radioactive levels should be installed.Protection systems, most important of which is the emergency shutdown system should be installed.Periodic equipment surveillance program should be in place. 55

The defence-in-depth concept – cont. Third level – control of accidents within the design basisDespite of first two levels of protection a complete set of incidents and accidents is postulated.These are the PIEs in Deterministic Safety Analyses.Engineered safety features are put in place to cope with such events.They have no function during the normal operation.Their actuation should be automatic and human intervention should be required only after a certain time. They need to be included in the original design as retrofitting might be extremely difficult.56

The defence-in-depth concept – cont. Fourth level: control of severe plant conditions including prevention of accident progression and mitigation of severe accident consequences.TMI accident showed the need to consider multiple failures and further protection.There is a need to control plant situations when the first three levels of DiD have been bypassed.Such situations can lead to core melt.Procedures should be developed to cope with such situations and to prevent the core melt – severe accident management guidelines need to be established – SAMGs. Containment function needs to be maintained. 57

The defence-in-depth concept – cont. Fifth level: mitigation of radiological consequences of significant off-site releases of radioactive materials.In case four levels fail, population protection measures are needed:Evacuation,Confinement indoors, with doors and windows closed,Distribution of stable iodine tablets,Restrictions on certain foodstuffs,Etc.Evacuation and confinement are the responsibility of public authorities. Short and long term measures are included in the external emergency plans. Periodic drills are necessary in this area.58

The defence-in-depth concept – cont. Elements common to different levelsCare needs to be taken at each level of DiD.Conservative approach is suitable for the first three levels, severe accidents require the use of more realistic assessments.Strong safety culture is required from all involved:Operators,Constructors,Contractors,Members of safety organizations. 59

The defence-in-depth concept – cont. 60DID from INSAG-10

The defence-in-depth concept – cont. General commentsLevels must be independent as much as possible.The same event or failure should not be able to affect several levels simultaneously.Safety system reliability must be adequate.Special design, layout and maintenance rules apply to safety systems.61

The defence-in-depth concept – cont. Quality controlQuality control in all activities must be ensured:Design,Supply,Manufacture,Construction,Tests and inspections,Operating preparations,Actual operation. This implies appropriate organizational procedures.Quality assurance is very hard to implement in severe accident conditions. 62

The defence-in-depth concept – cont. DiD implementation in operationLevel 1: Prevention Plant organization, staff selection and training. Normal operation procedures.Implementation of the technical specifications.Level 2: SurveillancePeriodic testing programme.Preventive maintenance programme.Incident detection and analysis. 63

The defence-in-depth concept – cont. DiD implementation in operationLevel 3: MitigationIncident and accident procedures – Emergency Operating Procedures (EOPs).Level 4: Accident managementDesign extension conditions accident procedure/guidelines – Severe Accident Management Guidelines (SAMGs).Internal emergency plan (links with external emergency plan).Level 5: Emergency response External emergency plan. 64

The role of successive barriers in preventing spread of radioactive materialsIn design three physical barriers are engineered:Fuel cladding,Pressure boundary of the primary coolant system,Low-leakage containment building.Fuel matrix represents a fourth, innermost barrier, containing many fission product nuclides.Each barrier is subject to different: Challenges, Surveillance requirements, Leakage specifications.In safety analyses the performance of each barrier is examined. 65

The role of successive barriers in preventing spread of radioactive materials 66MAIN PWR BARRIERS

The role of successive barriers in preventing spread of radioactive materials The fuel matrixMost power reactors are fuelled with a low-enriched uranium dioxide (UO2).Fuel fabricated as pellets.Fission products and transuranic elements remain in the fuel.Under steady state most remain in the fuel matrix. Some fission products and their daughters are gases and others are vaporised at normal operating temperatures – hence fuel matrix is only a partial barrier.Nobel gases, krypton and xenon plus tritium migrate from matrix to fission gas plenum within the cladding. 67

The role of successive barriers in preventing spread of radioactive materials The fuel matrix – cont.Volatile fission products, iodine and cesium will also migrate in the fuel cladding gap. Changes in fuel temperature (due to power change for example) result in the release of fission gases trapped within the microstructure of the fuel.This fission gas release is postulated to result in high mechanical loading of the cladding and possible cladding failure.Fission products and transuranic elements (apart from fission gases and volatile fission products) are kept within the matrix unless near-melting temperatures are reached.68

The role of successive barriers in preventing spread of radioactive materials The claddingThe UO2 fuel pellets are contained within a metal cladding tube.In current water cooled reactors cladding is of Zircalloy.The Zirconium alloy is chosen for its good structural and corrosion properties and low neutron absorption . The cladding tube is closed by welded end caps – hermetic seal.69

The role of successive barriers in preventing spread of radioactive materials The cladding – cont.Failures can occur in the welded caps or in the cladding tube itself.Such failures are relatively rare compared to the number of fuel rods.Failure mechanisms can be:Pellet-cladding mechanical interaction,High pressure due to fission gas releases in transients, Flow induced or mechanical vibrations, Excessive cladding corrosion. Cladding failures can be detected promptly by fission products or delayed neutrons in the coolant.The technical specifications (TS) may allow for certain fraction of failed fuel, ALARA demands that failed fuel be removed at the earliest practical time. 70

The role of successive barriers in preventing spread of radioactive materials The primary coolant systemThe boundary of the primary coolant is clearly defined in the reactor building but branches out in a complex manner in the auxiliary buildings.In PWR the primary coolant system pressure boundary consists of:Reactor pressure vessel,Coolant piping,Steam generators,Main coolant pumps, Pressurizer, Some auxiliary systems like: Chemical and volume control system(CVCS),Emergency core cooling system,Residual heat removal system. 71

The role of successive barriers in preventing spread of radioactive materials The primary coolant system – cont.An important class of DBAs are related to the loss of integrity of the primary boundary:LB LOCA,SB LOCA.Failure modes of the primary coolant pressure boundary include:Piping leaks,Piping breaks,Pump seal failures,SGTR,Valve failures or misalignment,Pressure vessel failure. 72

The role of successive barriers in preventing spread of radioactive materials Piping leaksLeaks can occur in many ways.Such leaks can be detected by increased radiation readings or loss of coolant inventory.Detection of leaks is extremely important – can develop into breaks.Concept of “leak-before-break” argues that double-ended “guillotine” breaks don’t need be considered as design basis accident.LB LOCA is still analysed as a design basis accident in many regulatory regimes. 73

The role of successive barriers in preventing spread of radioactive materials Piping breaksBreaks in the primary system piping is a major class of DBAs.LB LOCA used for the design of Emergency Core Cooling System and the Containment.SB LOCA studied extensively since the TMI-2 accident.The Design requirements are to show that: Cladding temperature, Cladding oxidation, Containment pressure.remain within acceptable limits, so that fuel integrity and containment function are maintained.74

The role of successive barriers in preventing spread of radioactive materials Pump seal failuresPump seals have small controlled leakage flow.A failure of a pump seal is equivalent to a SB LOCA.Probabilistic safety assessment (PSA) shows that pump seal failures can be a significant contributor to CDF. 75

The role of successive barriers in preventing spread of radioactive materials SGTRSteam Generator Tube Rupture (SGTR) in PWRs results in containment bypass.Strict limits on the amount of leakage and the number of leaking tubes is imposed.SG tubes are frequently inspected and plugged or repaired as necessary.76

The role of successive barriers in preventing spread of radioactive materials Valve failure or misalignmentValve failures by themselves are not a new class of accidents.Most valve failures are characterized as SB LOCAs.Valves that separate high pressure primary system from low pressure auxiliary systems are of special importance.Such example is residual heat removal system . Failure of these valves constitute “interfacing system LOCA”.PSA shows that their contribution to the CDF can be significant. 77

The role of successive barriers in preventing spread of radioactive materials Pressure vessel failurePressure vessel failure is not considered in DBA.The material of the vessel is subject to neutron irradiation and potential embrittlement.Pressurized thermal shock is a phenomena appearing when the vessel is subject to influx of cold water at operating pressure.If vessel temperature during the cooling transient approach null-ductility temperature mitigating measures are necessary.In extreme cases annealing the vessel is done to restore ductility. It has been done in some older Soviet-designed PWRs.78

The role of successive barriers in preventing spread of radioactive materials The containment buildingContainment building is the final barrier against spread of radionuclides.Leak rate typically in the order of 0.1 % per day.Periodic tests are performed.Many different types:Large dry buildings – PWR,Pressure suppression types – ice condensers in BWR, Sub-atmospheric buildings. Design basis for most containments is LB LOCA.Containments shown to be robust against short-term failures.The biggest threat is long-term pressurization. 79

Mitigation of radiological consequences of significant release Mitigation is the fifth level of DiD.It is not considered to be a part of the design.However the goal of modern designs is to consider design extension conditions to include severe accidents and practically eliminate the radiological consequences.Mitigation measures include:On-site emergency plans, Of-site emergency plans. On-site and remote emergency control centres are needed for coordination of emergency response.80

Emergency response On siteWell organized and tested on-site emergency plan are needed:Definition of the decision-making process and the people responsible for making emergency decisions;Criteria for declaring various levels of alert or emergency situation;Notification of appropriate company, local, state, and national authorities of the occurrence, depending on the severity of the situation; Activation of an on-site or near-site emergency control centre, with appropriate staff, communications, and support, including public communications personnel; Activation of emergency response teams as required by the nature of the situation; If necessary, activation of control room habitability features or a remote reactor control room;Evacuation of non-essential personnel from the site. 81

Emergency response On site – cont.On-site emergency organization needs sufficient information about the event to assess the need to activate off-site emergency plan.Local, state and national regulatory and emergency organizations also require information.Appropriate communication arrangements must be included in the emergency plan.82

Emergency response Off site Clearly defined organization and decision making process needed.Generally three possible actions:Sheltering,Chemical protection (iodine tablets),Evacuation.For small off-site releases sheltering sufficient.Stable iodine pills to prevent radioactive iodine into the thyroid (children specially at risk). Evacuation of population is the most extreme measure. 83

Defence-in-depth SummaryFive levels of defence in depth and four physical barriers for the confinement of radioactive material are defined in INSAG-10 as follows:Barrier 1: Fuel Matrix,Barrier 2: Fuel cladding,Barrier 3: Primary circuit boundary.

Defence-in-depth Summary – cont.Level 1: the aim is to prevent the occurrence of abnormal operation and failures. This is done by producing a conservative design and ensuring a high quality of construction and operation.Level 2: the aim is to control abnormal operation and detect failures if they should occur. This is done by incorporating control and surveillance systems. Level 3 : the aim is to control accidents within the design basis if they should occur. This is done by incorporating engineered safety features and developing emergency operating procedures. 85

Defence-in-depth Summary – cont.Level 4: the aim is to control severe plant conditions if they should occur which requires the prevention of accident progression and the mitigation of the consequences of beyond design basis accidents. This is done by incorporating severe accident management measures.Level 5: the aim is to mitigate the radiological consequences of significant releases of radioactive material from the plant. This is done by developing off-site emergency response measures.86

Questions Define 5 levels of defence in depth.Name barriers for prevention of spread of radioactive materials used in the defence in depth concept.87

THE INTERNATIONAL NUCLEAR SAFETY REGIME Learning objectivesAfter completing this chapter, the trainee will be able to:Describe the underlying principles governing the review process under the Nuclear Safety Convention.Describe the main purpose of the Code of Conduct for research reactors. Describe the main elements of the IAEA Safety Standards Series publications. Describe the main elements of the EC Nuclear Safety Directive. 88

THE INTERNATIONAL NUCLEAR SAFETY REGIME The IAEA serves as secretariat for:Legally binding conventions,Non-binding codes of conduct and safety standards.89

Conventions and codes of conduct Since 1986 five conventions come into force in nuclear, radiation, transport and waste safety:The Convention on Nuclear Safety,The Convention on Physical Protection of Nuclear Material,The Convention on Early Notification of a Nuclear Accident,The Convention on Assistance in the Case of a Nuclear Accident or Radiological Emergency,The Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management.In addition two non-legally binding Codes of Conduct have been adopted:The Code of Conduct on the Safety of Research Reactors, The Code of Conduct on the Safety and Security of Radioactive Sources. 90

Conventions and codes of conduct The convention on nuclear safetyThe objectives of the Convention on Nuclear Safety are:To achieve and maintain a high level of nuclear safety worldwide through the enhancement of national measures and international co-operation including, where appropriate, safety-related technical co-operation;To establish and maintain effective defences in nuclear installations against potential radiological hazards in order to protect individuals, society and the environment from harmful effects of ionizing radiation from such installations;To prevent accidents with radiological consequences and to mitigate such consequences if they occur.91

Conventions and codes of conduct The convention on nuclear safety – cont.The Convention applies to the safety of land-based civil NPPs including such storage, handling and treatment facilities for radioactive materials which are on the same site and directly related to the operation of the NPP.Obligation are based on the IAEA Safety Series 110 The Safety of Nuclear Installations, now superseded by SF-1. 92

Conventions and codes of conduct Legislation and regulation General safety considerations Safety of installations Legislation and regulatory framework Priority to safety Siting: effect of environment on the NPP Safety requirements and regulations Financing for safety Siting: effect of NPP on the environment System of licensing Competence of staff Siting: re-evaluation/consulting Regulatory inspection and assessment Human performance Design: defence in depth Enforcement Quality assurance Design: proven technology Regulator with authority Safety assessment Easily manageable operation Independent regulator Verification: analysis and survey Initial authorization and commissioning 93 Obligations under NSC

Conventions and codes of conduct Operator’s responsibility Radiation protection Operational limits and conditions Emergency preparedness Emergency operating procedures Emergency operating procedures Engineering and technical support Incident reporting Operating experience feedback Waste management 94 Obligations under NSC

Conventions and codes of conduct Implemeting measures under NSCEach contracting party shall submit for review National Report.The National Report should demonstrate that:The appropriate steps are taken to ensure that the safety of nuclear installations is reviewed and to ensure that all reasonably practicable improvements are made as a matter of urgency to upgrade the safety;The legislative and regulatory framework is established and maintained;The appropriate steps are taken to ensure an effective separation between the Regulatory Body and those concerned, with the promotion or utilization of nuclear energy; A Regulatory Body entrusted with the implementation of the legislative and regulatory framework is established; Prime responsibility for the safety of a nuclear installation rests with the holder of the relevant license and that the appropriate steps are taken to ensure that each such licence holder meets its responsibility. 95

Conventions and codes of conduct Implementing measures under NSC – cont.The Convention is an incentive instrument.No control or sanctions.Regular meetings held every 3 years where National Reports are peer reviewed.The convention entered into force on 24 October 1996.Review meetings held in April of 1999, 2002, 2005, 2008, 2011 and 2014. In August 2012 an Extraordinary meeting held to address the impact of the Fukushima accident.96

Conventions and codes of conduct The code of conduct on the safety of research reactorsThe objective of the Code of Conduct on the Safety of RR is to achieve and maintain high level of safety worldwide.The Code is non-binding international legal instrument.It is based on:Safety Series 110, The Safety of Nuclear Installations (now superseded),GS-R-1, Legal and Governmental Infrastructure for Nuclear, Radiation, Radioactive Waste and Transport Safety,NS-R-4, Safety of Research Reactors, WS-R-2, Predisposal Management of Radioactive Waste, Including Decommissioning.97

Conventions and codes of conduct The code of conduct on the safety of research reactors – cont.Different designs and power rates of RRs, hence different risk.States should adopt a graded approach commensurate with the risk.Guidance provided to:The Sate,The Regulatory Body,The Operating organization. 98

Conventions and codes of conduct The code of conduct on the safety of research reactors – cont.The State:Responsible for establishing legislative and regulatory framework;Responsible for establishing an effectively independent RB;Responsible for providing the RB with authority and resources;Should ensure that the operating organization has a financial system for safe operation, extended shutdown and decommissioning;If reactor is in extended shutdown and there is no longer operating organization, State should make arrangements for safe management;Should ensure adequate legal and infrastructure arrangements for decommissioning. 99

Conventions and codes of conduct The code of conduct on the safety of research reactors – cont.The regulatory body:Issues the license,Performs inspections,Performs assessment of compliance,Enforces regulations and authorizations,Reviews and assesses regulatory submissions,Makes available information on its regulatory requirements and decisions.Code of Conduct offers guidance for the regulatory body in most areas.100

Conventions and codes of conduct The code of conduct on the safety of research reactors – cont.The operating organization:Established its own policies that give safety the highest priority and promote strong safety culture.Carries out the safety assessment and prepares the safety analysis report before construction and commissioning.Carry out safety reviews at appropriate intervals including:Modifications,Changes in utilization,Experiments having safety significance,For management of aging.Operating organization should ensure effective financial system for operation, extended shutdown and decommissioning. 101

Conventions and codes of conduct Guidance topic State Regulatory body Operating organization Legal and governmental infrastructure √     Regulatory process √ √   Management of safety   √ √ Assessment and verification of safety √ √ √ Financial and human resources √ √ √ Quality assurance   √ √ Human factors   √ √ Radiation protection   √ √ Emergency preparedness √ √ √ Siting   √ √ Design, construction, commissioning   √ √ Operation, maintenance, modification, utilization   √ √ Extended shutdown √     Decommissioning √     102 Guidance topics covered by the code of conduct of RR

IAEA safety standards Historical development – NUSS programmeThe development of safety standards is a statutory function of the IAEA.Statute expressly authorizes the IAEA to:Establish standards of safety,Provide for the application of these standards.Nuclear Safety Standards – NUSS Programme started in 1970is. Within this programme, 5 Codes of Practice and about 60 Guides were produced. 103

IAEA safety standards 104 Example of NUSS publications

IAEA safety standards Safety standards seriesThe Safety Standards Series includes three levels of documents:Safety Fundamentals;Safety Requirements; andSafety Guides.105

IAEA safety standards 106Example of safety standards series publications

IAEA safety standards Safety standards seriesSafety Fundamentals:It is a policy document;It states basic objectives, concepts and principles involved in ensuring protection and safety;SF – 1 Fundamental Safety Principles is the only safety fundamentals document.107

IAEA safety standards Safety standards series – cont.Safety Requirements:They set forth the basic requirements which must be met in order to ensure safety;These requirements are governed by the basic objectives, concept and principles presented in SF- 1;Written with “shell” statements, so that MS can adopt them in their national regulations.108

IAEA safety standards Safety standards series – cont.The Safety Guides:Contain recommendations based on international experience and best practices on how to fulfil the safety requirements.Written with “should” statements and can be adopted by MS as national regulatory guidance material.109

IAEA safety standards Safety standards series – cont.All IAEA Safety Standards are adopted by consensus.Member States (MS) can adopt them taking into account local conditions and governmental practices.They are not binding for the MS but nevertheless very useful as they discuss key issues and present possible solutions.If there is a large deviation in national practice compared to the internationally accepted safety level, special consideration should be given to these issues. 110

IAEA safety standards Safety standards series – cont.Single Safety Fundamental document SF – 1.Each Safety Requirements document is followed by several Safety Guides.They are:Thematic safety standards,Facility and Activity specific safety standards.Every 5 years safety standards are reviewed and if necessary revised. 111

IAEA safety standards Safety standards series – cont.Thematic Safety StandardsLegal and governmental infrastructure;Emergency preparedness and response;Management systems;Assessment and verification;Site evaluation;Radiation protection;Radioactive waste management;Decommissioning;Remediation of contaminated areas;Transport safety. 112

IAEA safety standards Safety standards series – cont.Facility and activity specific safety standardsNuclear power plant: design;Nuclear power plant: operation;Research reactors;Fuel cycle facilities;Radiation-related facilities;Waste treatment and disposal facilities.113

IAEA safety standards 114Bodies for the endorsement of safety standards

IAEA safety standards 115The new development of safety standards

National and international institutions for standardization In addition to the IAEA Safety Standards a number of national and international industrial standards exist:International Organization for Standardization (ISO),International Electrotechnical Commission (IEC).To avoid duplication cooperation is established through:Memorandum of Understanding IAEA/ISO,Written agreement IAEA/IEC. Examples of national institutions are:American Society of Mechanical Engineers (ASME),German Nuclear Safety Standards Commission (KTA),Deutsche Institut fur Normung (DIN), Association Francaise de Normalization (AFNOR). 116

Questions What is the process used to verify the safety status in Member States under the Nuclear Safety Convention?How often are the Review meetings held under the Nuclear Safety Convention?Describe main elements of the Code of Conduct for research reactors.How many levels of documents are present in the IAEA Safety Standards Series? Name them.Name all Bodies for the endorsement of the IAEA safety standards. 117

NUCLEAR SAFETY AND SECURITY INTERFACE Learning objectivesAfter completing this chapter, the trainee will be able to:Describe the synergy between safety and security. Describe responsibilities for safety and security at different levels. Explain the concepts of safety and security culture. 118

NUCLEAR SAFETY AND SECURITY INTERFACE 3 S – Safety, Security, Safeguards:Safety preventing accidents;Security preventing intentional acts;Safeguards preventing the diversion of nuclear material for nuclear weapons.In simple terms:Safety protects people from the installation;Security protects installation from people.119

NUCLEAR SAFETY AND SECURITY INTERFACE – cont. Both safety and Security have common objective to protect people and the environment, hence many action taken serve both.For example Containment serves both purposes.However some action that enhance security might have a negative impact on safety.For example strict control access might hinder prompt action necessary in a nuclear safety event.Therefore a coordinated approach to safety and security is necessary. 120

NUCLEAR SAFETY AND SECURITY INTERFACE – cont. DefinitionsNuclear safety:”The achievement of proper operating conditions, prevention of accidents or mitigation of accident sequences, resulting in protection of workers, the public and the environment from undue radiation hazards”.Nuclear security:“The prevention and detection of, and response to, theft, sabotage, unauthorized access, illegal transfer or other malicious acts involving nuclear material, other radioactive substances or their associated facilities”. 121

NUCLEAR SAFETY AND SECURITY INTERFACE – cont. Both have in common the fact that they rely on DiD:The first priority is prevention;If it fails, the second stage is early detection and prompt actions;The third layer is mitigation;The fourth layer is emergency planning.122

Responsibility for safety and security State responsibility:Appropriate legislation and regulatory framework put in place;Regulatory authorities established in safety and security fields;If not within one authority, proper coordination needs to be established;Even though the main responsibility for both rests with the operator, State support to the operator in terms of intelligence information from specialized State agencies might be necessary;123

Responsibility for safety and security – cont. Responsibility of the regulatory body:The main task is to define the requirements to be fulfilled;Another prime responsibility is to put in place an effective inspection and enforcement system;Adequate emergency response system also needs to be put in place on all levels.124

Responsibility for safety and security – cont. Responsibility of the operating organization:The prime responsibility for safety and security rests with the licensee;National police and military might be asked to help in security issues;National intelligence agencies might also be asked;Plant vulnerabilities are however best identified by the operating organization.125

Safety and security at nuclear installations Safety and security cultureSafety culture definition: “that assembly of characteristics and attitudes in organizations and individuals which establishes that, as an overriding priority, nuclear safety issues receive the attention warranted by their significance”.Definition of nuclear security culture is the same, just the focus is on security issues.However there are some differences. For example:Safety culture asks for transparency and cooperation in exchanging information on safety issues,The same does not apply to security issues. 126

Safety and security at nuclear installations Emergency preparedness and responseEmergency plans are developed at different levels:State level,Municipal level,Plant level.Safety and security plans must be compatible and complementary.Joint exercises are necessary. 127

Safety and security at nuclear installations Safety and security considerationsSafety and security considerations are needed during:Siting – possible vulnerabilities identified;Design – DiD principles to be applied;Construction – large number of subcontractors;Operation – during outages and modifications also large number of subcontractors on site. 128

Questions Describe synergy between nuclear safety and security.Give an example when nuclear safety and security measures can be in conflict.What is the difference between safety and security culture?129

HISTORY OF ACCIDENTS IN NUCLEAR INDUSTRY Learning objectivesAfter completing this chapter, the trainee will be able to:List three major accidents in nuclear industry. Describe the root causes of these three accidents. Describe the courses of these three accidents. Describe the consequences of these three accidents. 130

HISTORY OF ACCIDENTS IN NUCLEAR INDUSTRY 131Accidents are a major source of lessons learned and profoundly influence the understanding of nuclear safety.Source: World Nuclear Association

Three Mile Island accident 132Three Mile Island Nuclear Generating StationMiddletown, Pennsylvania, USA PWR Babcock&Wilcox 852 M W e Severe accident on March 28, 1979 Source: Wikipedia

Three Mile Island accident 133Schematics of TMI-2 reactor Failure in the main feedwater system; Turbine-generator and reactor shutdown; Pressure in the primary system increased; Pressurizer relief valve opened ; Pressurizer relief valve should have closed when the pressure fell, but stuck open ; Instruments in the control room indicated valve closed ; Operators unaware of primary coolant loss because other instruments provided inadequate information; There was no instrument that showed water level in the core. Accident sequence:

Three Mile Island accident 134Schematics of TMI-2 reactor Operators assumed that as long as the pressurizer water level was high, the core was covered; D id not realize that the plant was experiencing a loss-of-coolant accident ; They took a series of wrong actions: Low primary pressure caused cavitation of reactor coolant pumps; P ressurizer filling up; Pumps turned off to prevent vibrations; Emergency cooling water flow reduced; Reactor core started overheating . Accident sequence cont.:

Three Mile Island accident 135Fuel overheated, cladding ruptured, fuel pellets began to melt;About half of the core melted during the early stages of the accident due to loss of coolant; Chemical reaction between steam and the zirconium cladding created a hydrogen bubble in the dome of the pressure vessel; Concern: hydrogen bubble might explode and rupture the pressure vessel; C risis ended: experts determined that hydrogen could not explode due to absence of oxygen in reactor. Accident sequence cont . : Source: Wikipedia

Three Mile Island accident 136Consequences outside the plant minimal; TMI-2's containment intact and held all radioactive material;Health effects: an average 0,01mSv above background to 2 million people around TMI-2, maximum 1mSv at the site boundary;Presently rated INES 5;  Turning point in the global development of nuclear power.

Chernobyl accident 137Graphite moderated pressure tube type reactor (RBMK-1000) 2% enriched uranium dioxide fuel; zirconium alloy cladding; Vertical pressure tubes; Positive void coefficient; Reactor coolant: light water boiled in the pressure tubes and provided the steam to drive the turbines.

Chernobyl accident 138Accident sequence:26. April 1986, run down test of the turbine generator (for powering of the main circulating pumps in an emergency);Preceded by a series of operator actions, including the disabling of automatic shutdown mechanisms; By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. Design feature of the control rods caused a dramatic power surge as they were inserted into the reactor .

Chernobyl accident 139Accident sequence cont.:Interaction of the very hot fuel with the cooling water: Fuel fragmentation; Rapid steam production;Increase in pressure, 1000 t cover plate of the reactor detached, rupturing the fuel channels and jamming all the control rods; Steam explosion and release of fission products to the atmosphere;About two seconds later, a second explosion (probably caused by the hydrogen from zirconium-steam reactions ) threw out fragments from the fuel channels and hot graphite.

Chernobyl accident 140Accident sequence cont.:At least 5 % of the radioactivity in the core released into the atmosphere due to graphite fire and steam explosion;The plume of smoke and fission products rose up to about 1 km into the air and were blown by the prevailing wind to the northwest of the plant;Contamination was detected over all of Europe (except Iberian peninsula). Chernobyl accident is rated 7 on the INES scale.

Chernobyl accident 141Severe immediate radiation effects: Of 600 workers present on the site during the early morning of 26 April, 134 received 0.8-16 Gy and suffered from radiation sickness. 28 died in the first three months and another 19 died in 1987-2004 of causes not necessarily associated with radiation exposure. According to the UNSCEAR 2008 Report, the majority of the 530,000 registered recovery operation workers received doses 0.02 Gy - 0.5 Gy between 1986 and 1990 . That cohort is still at potential risk of late consequences such and their health will be followed closely .  

Chernobyl accident 142Thyroid cancer in children: Doses in the first few months after the accident were particularly high in children and adolescents in Belarus, Ukraine and some Russian regions due to the consumption of milk with high levels of radioactive iodine; More than 6,000 thyroid cancer cases b y 2005 in this group;9 children died, the others were cured;It is expected that the increase in thyroid cancer incidence will continue for many more years, although the long-term increase is difficult to quantify precisely.

Chernobyl accident 143No clearly demonstrated increase in the incidence of solid cancers or leukaemia due to radiation in the exposed populations;No proof of other non-malignant disorders related to ionizing radiation; Widespread psychological reactions due to fear of the radiation, not to the actual radiation doses. In addition, thousands of individuals were forced to leave their homes due to the contamination;The vast majority of the population need not live in fear of serious health consequences due as they were exposed to radiation comparable to or a few times higher than natural background ; Future exposures continue to slowly diminish as the radionuclides decay.

Fukushima accident 144Fukushima Daiichi Nuclear Power PlantGeneral Electric boiling water reactors (BWR) of an early design; Supplied by: General Electric; Toshiba; Hitachi.Mark I containment; Reactors 1-3 came into commercial operation 1971-75. Source: Wikipedia

Fukushima accident 145Great East Japan Earthquake Friday, 11 March 2011, 2:46 pm Magnitude 9.0

Fukushima accident 146Accident sequence:Automatic shutdown of the reactors due to earthquake; No significant damage to the plant; External power supply sources lost due to earthquake; E mergency diesel generators to run the Residual Heat Removal system (RHR ) pumps and equipment was available as designed.

Fukushima accident 147 Accident sequence contd.: 15 m tsunami 41 minutes after the earthquake inundated/disabled: Seawater pumps for the condensers; RHR cooling system ; Diesel generators, the electrical switchgear and 125-volt DC batteries in the basements of the turbine buildings. Station blackout ; Loss of ultimate heat sink. Source: Wikipedia Source: chong.zxg.net

Fukushima accident 148 Accident sequence in Unit 1.: Decay heat (some 1.5 % of nominal thermal power) produced steam in the reactor pressure vessel (RPV); Steam and later hydrogen (from the reaction with the zirconium cladding) was released into the dry primary containment through safety valves; By early Saturday, water injection was provided to the RPV utilizing fire pumps. Fukushima Daiichi Unit 1, s implified representation

Fukushima accident 149 Accident sequence in Unit 1 cont.: Fuel melting 8 hours after the trip; 16 hours after the trip (7 am Saturday) the corium had fallen into the water at the bottom of the RPV; RPV temperatures decreased; Attempts to vent steam, noble gases and hydrogen from the containment resulted in a hydrogen explosion on the service floor of the building above unit 1 reactor containment. Fukushima Daiichi Unit 1, s implified representation

Fukushima accident 150Accident sequence in Unit 2:Water injection (steam-driven pump) failed on Monday 14th;After cooling was lost, the fuel melted and most likely fell into the water at the bottom of the RPV about 100 hours after the scram ; Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen; After 6 hours a fire pump started injecting seawater into the RPV. Pressure was vented and the blowout panel near the top of the building was opened to avoid a repetition of unit 1 hydrogen explosion; On Tuesday 15th, the drywell containment pressure inside dropped and the primary containment vessel ( PCV ) developed a leak. Most of the releases from the site appeared to come from unit 2.

Fukushima accident 151Accident sequence in Unit 3:The beginning of the sequence was similar as in Unit 2;Fuel melted on the morning of Sunday 13th and possibly fell into the water at the bottom of the RPV;  On Monday 14th PCV venting was repeated, and this backflowed to the service floor of the building;At 11:00 am a very large hydrogen explosion occurred in the unit 3 reactor containment and blew off much of the roof, walls and top part of the building, creating radioactive debris on the ground near unit 3.

Fukushima accident 152Accident sequence in Unit 4:Reactor defueled at the time of the accident; On Tuesday 15 March an explosion destroyed the top of the building due to backflow of hydrogen from unit 3 through shared ducts.

Fukushima accident 153Spent fuel ponds:Located adjacent to the top of all reactor buildings;Shielding and cooling of spent fuel by pumping water through external heat exchangers; Water level in the fuel ponds after the accident was low because of loss of circulation through heat exchangers, leading to elevated temperatures and probably boiling: Replenishing with fire pumps unsuccessful;C oncrete pump with a high boom successful. The ponds survived the earthquake, tsunami and hydrogen explosions without significant damage to the fuel or significant radiological release.

Fukushima accident 154Radioactive releases to air:Major air releases of radionuclides mainly in mid-March; Evacuation within a 20km radius;Considerable work to reduce the amount of radioactive debris on site and to stabilise dust; Hydrogen explosion in the suppression chamber of Unit 2 on 15th March was the main source of radioactive releases; Radioactive releases in mid-August 2011 had reduced to 5 GBq/hr, and dose rate from these at the plant boundary was 1.7 mSv/yr, less than natural background.

Fukushima accident 155Radiation exposure of workers on site:No radiation casualties (acute radiation syndrome);Higher than normal doses accumulated by several hundred workers;High radiation levels in the three reactor buildings hindered access to the site into 2012; Six workers received radiation doses over 250 mSv.

Fukushima accident 156 The Fukushima accident is rated 7 on the INES scale Source: Wikimedia Radiation exposure beyond the plant site: As of now, no harmful effects on local people , nor any doses approaching harmful levels; 160,000 people evacuated , in 2012 limited return; In October 2013, 81,000 evacuees remained displaced due to government concern about radiological effects from the accident. The views expressed in this document do not necessarily reflect the views of the European Commission.