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Lessons-Learned from Severe Accident in Fukushima Dai-ichi NPP and Ensuring Nuclear Safety Akira Yamaguchi Department of Energy and Environment Osaka University,

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Presentation on theme: "Lessons-Learned from Severe Accident in Fukushima Dai-ichi NPP and Ensuring Nuclear Safety Akira Yamaguchi Department of Energy and Environment Osaka University,"— Presentation transcript:

1 Lessons-Learned from Severe Accident in Fukushima Dai-ichi NPP and Ensuring Nuclear Safety Akira Yamaguchi Department of Energy and Environment Osaka University, Osaka, Japan 1 Lec. 6 (Seminar on “Sharing Experience on Nuclear Power for Development” in Vietnam)

2 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 2

3 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 3

4 Beginning of Disaster Earthquake at 14:46, March 11, 2011 The first tsunami attack at 15:27 The second at 15:35, which exceeded the site elevation – Diesel generators failed at 15:37-41 Station Blackout with no power provision nor recovery – Seawater system failed Loss of Ultimate Heat Sink – DC batteries failed No plant parameter information visible (pressure, water level, injection rate) 4

5 5 M9.0 3/11 14:46 Onagawa #1 524MW, 1984 #2 825MW, 1995 #3 825MW, 2002 Onagawa #1 524MW, 1984 #2 825MW, 1995 #3 825MW, 2002 Fukushima #1 460MW, 1971 Dai-ichi #2 784MW, 1974 #3 784MW, 1976 #4 784MW, 1978 #5 784MW, 1978 #6 1,100MW, 1979 Fukushima #1 1,100MW, 1982 Dai-ni #2 1,100MW, 1984 #3 1,100MW, 1985 #4 1,100MW, 1987 Fukushima #1 1,100MW, 1982 Dai-ni #2 1,100MW, 1984 #3 1,100MW, 1985 #4 1,100MW, 1987 Tokai Dai-ni 1,100MW, 1978 Earthquake Source and NPPs

6 Earthquake (Observed PGA/Design Ss ground motion ) 6 Fukushima Dai-ichi (Units 1-6) Fukushima Dai-ni (Units 1-4) Onagawa (Units 1-3) Source: NISA Homepage Tokai Dai-niHigashidori S-N E-W U-D

7 Tsunami Flood Height 7 Onagawa Fukushima Dai-ichi Fukushima Dai-ni Tokai Dai-ni Unit 1-4Unit 5&6 Flood Level Run-up ( South of Unit 1) Higashidori ( 2.6m 以下) Design Site Elv.

8 Summary of Fukushima Dai-ichi Accident Radiation Exposure of Fukushima Residents (for 4 months after the accident) – 99 out of 25,520 exceed 10mSv (public), maximum 25.1mSv – 48 out of 147 exceed 10mSv (workers in nuclear facilities) Confusion in evacuation, food and water control Contamination of land is significant If emergency preparedness works, they should have been mitigated – Exposure limitation in emergency situation – Intake control of food and drinks – Mitigation measure of significant radioactive release 8

9 9 15:42, Mar.11 15:43, Mar.11 15:46, Mar.11 15:57, Mar.11

10 10

11 The Tsunami Caused Station Blackout (SBO) and Loss of Ultimate Heat Sink 11 Unit 1Unit 2Unit 3Unit 4Unit 5Unit 6 Off site powerX (0/6/7) Emergency DGXXX-(X) X X O Metal clad (6.9kV)XXXXX(X) XXXXXO XX Power center (480V) XOX-XO XOXOXO XXO DC batteryXXOXOO XXOXOO Ultimate heat sinkXXXXXX

12 Crossroad in Fukushima Dai-ichi Accident Vicious Circle Earthquake - Practically no damage on safety functions Tsunami - Loss of multi-functions (not only safety but logistics) – Station blackout (SBO) – Loss of ultimate heat sink (LUHS) – Loss of instrumentation and control (I&C) – Loss of communication and information (lighting, computer, mobile phone, paging) – Loss of off-site external assistance – Fear on aftershock and another tsunami Hydrogen Explosion on Unit 1 at 15:36, March 12 – Loss of accident management – Loss of accessibility – Loss of habitability – Fear on another explosion (units 2 and 3) 12

13 Crossroad in Fukushima Dai-ichi Accident Recovery from Disaster The staff always considered priority; to select the best action on the worst unit Knowledge-base management – Mobile equipment – Car batteries Information is helpful for good decision making – Helicopter flight confirmed water in the spent fuel pool on March 16 External support started (Self-Defense Force, Fire Management Agency, etc.) – Core cooling using fire engines – Spent fuel pool cooling using concrete pump vehicles 13

14 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 14

15 Lessons from Japanese Government Report The most important basic principle in securing nuclear safety is “defense in depth” – Strengthen preventive measures against a severe accident (8) – Enhancement of response measures against severe accident (7) – Enhancement of nuclear emergency responses (7) – Reinforcement of safety infrastructure (5) – Thoroughly instill a safety culture (1) 15 Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety, The Accident at TEPCO's Fukushima Nuclear Power Stations (June 2011)

16 Recommendations for Enhancing Reactor Safety in the 21st Century Clarifying the Regulatory Framework (1) Ensuring Protection (2) Enhancing Mitigation (5) Strengthening Emergency Preparedness (3) Improving the Efficiency of NRC Programs (1) 16 The Near-Term Task Force Review of Insight from the Fukushima Dai-ichi Accident, JULY 12, 2011

17 Initiated by a natural disaster A severe accident with damage to boundaries Multiple reactors units A mid- to long-term initiative is needed A long-term evacuation of many residents in vicinity A major impact on industrial activities Many aspects different from the accidents in the past Fukushima NPS Accident

18 Emergency response activities in the seriously deteriorated social infrastructure The occurrence of aftershocks frequently and tsunami alarm This accident led to a severe accident, shook the trust of the public, and warned those engaged in nuclear energy Characterization of the Accident

19 1. Strengthen measures against earthquakes and tsunamis 2. Ensure power supplies 3. Ensure robust cooling functions of reactors and PCVs 4. Ensure robust cooling functions of spent fuel pools 5. Thorough accident management (AM) measure 6. Response to issues concerning the siting with more than one reactor 7. Consideration of NPS arrangement in basic designs 8. Ensuring the water tightness of essential equipment facilities Category 1: Strengthen preventive measures against a severe accident

20 9. Enhancement of measures to prevent hydrogen explosions 10. Enhancement of containment venting system 11. Improvements to the accident response environment 12. Enhancement of the radiation exposure management system at the time of the accident 13. Enhancement of training responding to severe accidents 14. Enhancement of instrumentation to identify the status of the reactors and PCVs 15. Central control of emergency supplies and equipment and setting up rescue team Category 2: Enhancement of response measures against severe accidents

21 16. Responses to combined emergencies of both large-scale natural disasters and prolonged nuclear accident 17. Reinforcement of environmental monitoring 18. Establishment of a clear division of labor between relevant central and local organizations 19. Enhancement of communication relevant to the accident 20. Enhancement of responses to assistance from other countries and communication to the international community 21. Adequate identification and forecasting of the effect of released radioactive materials 22. Clear definition of widespread evacuation areas and radiological protection guidelines in nuclear emergency Category 3: Enhancement of nuclear emergency responses

22 23. Reinforcement of safety regulatory bodies 24. Establishment and reinforcement of legal structures, criteria and guidelines 25. Human resources for nuclear safety and nuclear emergency preparedness and responses 26. Ensuring the independence and diversity of safety systems 27. Effective use of probabilistic safety assessment (PSA) in risk management Category 4: Reinforcement of safety infrastructure

23 28. Thoroughly instill a safety culture – “Nuclear safety culture” is stated as “A safety culture that governs the attitudes and behavior in relation to safety of all organizations and individuals concerned must be integrated in the management system.” (IAEA, Fundamental Safety Principles, SF-1, 3.13) – Licensees have prime responsibility for securing safety Review every knowledge and finding, and confirm whether or not they indicate a vulnerability of a plant Take appropriate measures for improving safety, when they consider uncertainties exist in terms of risks concerning the public safety – Japan will establish safety by: Looking at the basis, namely defense-in-depth Constantly learning professional knowledge on safety Maintaining an attitude to identify weaknesses and vulnerability Category 5: Thoroughly instill a safety culture

24 Lessons from Nuclear and Industrial Safety Agency (NISA) Report Accident sequences have been investigated Accident prevention strategies have been identified Emergency countermeasures has been selected – To control the accident within design basis – To prevent the beyond design basis event progress to SA – To mitigate the SA consequence so that source term emission is limited Japanese Regulatory Authority Requires 30 Provisions – Baseline for ensuring safety for Fukushima scenario but more general – Station Blackout (SBO) and Loss of Ultimate Heat Sink (LUHS) scenarios are protected 24 Technical Knowledge of the Accident at Fukushima Dai-ichi Nuclear Power Station of Tokyo Electric Power Co. Inc., Nuclear and Industrial Safety Agency, March 2012

25

26 External Power (Offsite) Supply (4) On-site power Supply (7) Core Cooling / Injection (6) Primary Containment Vessel and Hydrogen Explosion Control (7) Command, Communication and Instrumentation & Control (6) Technical Knowledge and 30 Provisions

27 Lessons Learned - General We can manage situations, even if they are serious Flexibility, knowledge-base and imagination really work in beyond design basis event Prior agreement with society is important to make emergency response practicable Justification of nuclear benefit and acceptance of risk are inseparable Risk is uncertainty– PRA deals with uncertainties and identifies vulnerabilities 27

28 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 28

29 How to Ensure Safety… Risk Model - in Mathematical Form To ensure safety, suppress the risk to low level – Reduce Frequency – Mitigate Consequence It works only If we know the frequency precisely and we control the consequence 29 Risk = Frequency Consequence ×

30 Frequency and Consequence; Which Is More Important? 30 Frequency Consequence Large Risk Small Risk Low Consequence Low Frequency Lack of knowledge and recognition Large risk means large uncertainty

31 Approach to Ensure Safety – Limit the Risk Risk is not “frequency” times “consequence” – Risk comes from uncertainty which we cannot be free from – We must be prepared for uncertainty and overcome ignorance Approach to prepare for uncertainty and to limit the risk is : Defense-in-Depth What causes the risk? – Source term or radioactive material: fission product causes the risk Who sustains the risk? – Public health and safety and environment sustain the risk 31

32 Defense in Depth Keep Public Distant from Hazard Identify Hazard Source – Radioactive materials Define Safety Objective – Health and property of public and environment Keep Hazard and Public Separate 32 Hazardous Material Public health and property, environment Management

33 3 rd Defense : Objective Is the Goal To protect public is most important Emergency response is scenario-less – Scenario is unpredictable – 1 st defense depends on scenario Flexibility and knowledge- base action works – Management system – Drill and education – Safety culture 33

34 2 nd Defense Is Flexible and Broad 34 Hazard (Fission Product) Hazard (Fission Product) Health and safety, and property of public 1 st Defence Prevention 3 rd Defence Emergency preparedness No Severe Accident Respond to Emergency Barrier / Distance / Time 2 nd Defence Mitigation Contain Fission Product Management Boundaries of the 2 nd defense are ambiguous and overlapped

35 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 35

36 “Absolutely Unlikely” Is Impossible Black Swan / White Raven 36 Black Swan (N. Taleb, 2007)White Ravens (Hempel's Ravens)

37 Four Categories of Undesirable Event 37 Recognition Unknown known Unknown unknown Knowledge Known known Known unknown Two Beyond-Design-Basis Type: Unlikely Event and Rare Event Unlikely Rare Unknown

38 Precursor is Messenger of Safety White Snake (Iwakuni City, Yamaguchi, Japan) Appearance of unknowns is messenger of safety 38

39 Known Unknown Becomes Reality 39 Earthquake and tsunami in the Indian Ocean off Sumatra : Kalpakkam NPP Flooding: Le Blayais NPP, France Fort Calhoun NPP: Missouri River Flooding in 2011, USA

40 Prepare for Unknown High Consequence Event Low Frequency High Consequence Event ( Rare ) – We recognize the event but it is rare. So we refrain from collecting knowledge – Even though frequency is very low, we should have at least knowledge on: Cliff edge, weak link, safety margins, practical management Low Likelihood High Consequence Event ( Unlikely ) – Event is unlikely and we disregard the risk – We should delineate unknown scenario and investigate every possibility Accident sequence precursors (Empirical / Factual) Probabilistic Risk Assessment (Deductive / Eliciting) 40

41 Identification and Preparation for Unknowns “Known Known” is already prevented – Design basis “Unknown” becomes “Known” – PRA find out sequences (Unknown Known) BWR containment vessel failure (SBO scenario): hardened vent – Unexpected event becomes reality (Known Unknown) Small LOCA and human error (TMI) SBO+LUHS (Fukushima Daiichi) by tsunami “Unknown knowns” are investigated – Stress test (comprehensive safety evaluation) “Known unknowns” are protected – Emergency provisions 41

42 Role of Stress Test, PRA and Provisions 42 PRA Stress test Design Basis Urgent Provisions Known unknown Recognition level Known known Unknown known Unknown unknown Knowledge level Beyond recognition Beyond knowledge Residual Risk The 1 st Defense The 2 nd Defense How to deal with the residual? The 3 rd Defense

43 Do We Accept Residual Risk? 43 正義の女神、ユースティティア( Justitia ) Justification is the action of declaring or making righteous in the sight of God (Oxford Dictionary of English) Be ready to accept risk under justification But continue to reduce / optimize risk IAEA Fundamental Safety Principles Facilities and activities that give rise to radiation risks must yield an overall benefit.

44 Contents Fukushima Dai-ichi Accident Lessons-Learned How to Ensure Safety Unknowns – Black Swan and White Raven….White Snake Comprehensive Risk Assessment Conclusions 44

45 Probabilistic Risk Assessment Lessons learned #27 (Japanese government) – Effective Use of PSA in Risk Management – Utilize PSA in improving safety measures as well as accident management Recommendation 1 (USNRC) – The Task Force recommends establishing a logical, systematic, and coherent regulatory framework for adequate protection that appropriately balances defense- in-depth and risk considerations. 45

46 Atomic Energy Society of Japan Organizations for PRA Standards 46 Standard Committee Risk Assessment Technical Committee Risk Informed Regulation Level 1 PRA Level 1 PRA Shutdown PRA PRA Parameters Level 2 PRA Level 3 PRA Seismic PRA Internal Flooding PRA Tsunami PRA Fire PRA PRA Qualification Risk Standard Steering Task System Safety Technical Committee Nuclear Fuel Cycle Technical Committee Advanced and Fundamental Systems Technical Committee Subcommittees Committees

47 Atomic Energy Society of Japan PRA Standard Line-up 47

48 Direct Effect of Earthquake and Tsunami on March 11 48

49 External Events Coupling 49 Seismic experience of secondary failure – Onagawa NPP: a fire induced by the earthquake in a transformer in 2011 earthquake off the Pacific coast of Tohoku – Kashiwazaki-Kariwa NPP, the Tyuetsu Offshore Earthquake triggered a fire of a station service transformer of in 2007 – Spent fuel pool sloshing Some external events may induce multiple failures and/or another external event – Earthquake and tsunami – Seismic-induced fire and seismic-induced internal flooding of especially non-safety grade equipment are probable

50 Other Unresolved Issues Level 2 PRA and Level 3 PRA Tsunami PRA standard deals with level 1 PRA for reactors in power operation AESJ has developed level 2 PRA standard and level 3 PRA standard for internal events – They can be applied to external event as well – Necessity to develop standards for level 2 PRA and level 3 PRA for external events – Consideration if an external event has specific features in terms of the fission product release, containment performance and the emergency responses 50

51 Other Unresolved Issues Shutdown PRA 51 In Fukushima Dai-ichi Accident: – Units 4, 5 and 6 are in shutdown for refueling. – In unit 4, all the fuel assemblies are unloaded and transferred to the spent fuel pool to replace the reactor vessel shroud In shutdown condition – Some safety systems may be out of service – Water-tight doors and openings may not be closed Practical and efficient tsunami protections is to keep the safety-related SSCs away from the tsunami water – Dry site concept, water-tight structure, water-resist and water-proof SSCs – During shutdown situations, they may not work Is shutdown PRA for external events necessary? Yes, probably.

52 Other Unresolved Issues Risk Assessment of Spent Fuel Pool 52 Spent fuel pool risk is an safety concern Inventory of the fuel and fission products may be larger than in the reactor core Decay heat in the spent fuel pool is quite small and the maintenance of water level is enough for cooling Do we need to estimate the spent fuel pool risk in the framework of the PRA? In the Fukushima Dai-ichi accident – Difficulty in spent fuel pool cooling comes from hydrogen explosion – Accessibility were extremely deteriorated – Spent fuel pool risk may be well controlled by management Technological requirement, quantification methodology, and data are not enough to complete spent fuel pool PRA but the risk is controllable

53 Other Unresolved Issues Comprehensive External Event PRA 53 External event PRA – Necessary if occurrence frequency and/or consequence significant – Internal flooding PRA and fire PRA standards are under development – Other external events? Concurrency of multiple external events – Priority on combined PRA of earthquake and tsunami Seismic failure of anti-tsunami SSC, loss of function or deterioration of infrastructure such as power supply system and communication system Deterioration of the accessibility for the operators and workers is to be considered in case that the human recovery actions are taken into account. – Combinations of earthquake and internal flooding and fire – Other combinations?

54 Comprehensive External Event PRA Impact Based Approach Nuclear power plant should be designed safe for postulated events, that are “likely” and “influential” All possible “influential” events are considered in PRA regardless of the likelihood – We cannot tell an event is likely or unlikely precisely Provisions for influential but unpostulated events have varieties according to event characteristics Selection of influential unpostulated event is based on impact and tolerability – Frequency of event, capacity of plant, isolation capability – Practical and effective metrics is “Risk” 54

55 Procedure for External Hazard and Risk Assessment Comprehensive survey of external hazard Risk potential of external hazard Risk-significance of external event Available quantitative evaluation methodology Protection and management against external hazard 55 Identify external hazard Natural event / Man-made event Single event / Multiple event Categorize external hazard Frequency Distance Grace period Influence Select risk assessment approach Screening Bounding analysis Margin analysis DSA PRA

56 Identification of Possible Natural Hazard Japanese Diet Report – Natural disaster for – Secondary disaster included ASME/ANS standard – Natural event – man-made event IAEA NS-R-3 – Natural event – man-made event 56

57 List of Potential External Hazard 57 External hazardPhenomena Natural hazard Earthquake and tsunami disaster Earthquake / Subsidence / Ground uplift / Ground crack / Soil discharge / Liquefaction / Landslide / Debris flow / Mountain slide / Cliff failure / Flooding / Tsunami / Fire Volcano disaster Volcanic bomb / Volcanic lapillus / Pyroclastic flow / Lava flow / Debris flow / pyroclastic surge / Blast wave / Ash fall / Flooding / Tsunami / Forest fire / Volcanic gas stagnation / Cold weather by volcanic gas / Boiling water / Earthquake / Mountain collapse Meteorological calamity Storm(wind / fire / avalanche / sandblasting) / Wind wave/Tidal wave / Abnormal elevation of sea level / Intense rainfall (immersion / flooding / Debris flow / flash flood / mountain slide / landslide / cliff failure / High tide water / Seiche / Wind wave / Fog and mist / Heavy snow (dead weight / avalanche) / Snowstorm / Heavy snow (flooding) / Snowmelt (mountain slide / landslide) / Lighting strike (current / fire) / Hail / Frost / Tornado / River blockage by ocean water / Water level declination in lake and river / Drought / High temperature / Low temperature (freeze) / Abnormal change in ocean current Others Forest fire / Coastal erosion / Biological event / Meteor / High tide / Toxic gas / River channel change Man-made event Airplane crash / Artificial satellite / Transport accident / Ship impingement / Turbine missile / Industrial or military facility accident / Pipeline accident / Abnormal gas eruption cased by boring / Oil spill / Chemical substance release from onsite storage facilities / Flooding and wave by failure of flood control structure Underlined hazards typed in red are from ASME/ANS Standard, IAEA NS-R-3

58 Selection Criteria of External Event Yes / No / Uncertain C-1: Current PRAs practice cover the external event? C-2: Licensing evaluations cover the event and show it is not influential and dominant C-3: Occurrence frequency is definitely small C-4: Distance of hazard and NPP is kept large enough C-5: Hazard progression (time scale) is slow enough C-6: Influence on NPP is small enough If one or more of C-2 to C-6 is yes, PRA is not required If one or more of C-2 to C-6 is uncertain, appropriate risk assessment method is selected and applied If all of C-2 to C-6 is no, PRA is required 58

59 Concept of Event Selection 59 Occurrence of natural hazard Physical Distance Large grace time Criterion 3Criterion 4Criterion 5Criterion 6 FrequencySeparationConsequence -Volcano -Geological change -Drought Little impact -Fog -Frost Criterion 1 Criterion 2 Safety evaluation by PRA or Design Hazard Impact Separation Barrier Separation Barrier

60 Conclusions Good management and knowledge lead us to the right way at crossroads Preparation for uncertainty (unpostulated scenarios) – Defense-in depth – Unlikely event and rare event Being prepared for three types of unknowns – Stress test covers rare event : unknown known – Appropriate back-fit prepares for : known unknown – PRA deal with unlikely event : unknown unknown External PRA standard development at AESJ 60


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