Safety Implication for Gen-IV SFR based on the Lesson Learned from the Fukushima Dai-ichi NPPs Accident. Ryodai NAKAI Japan Atomic Energy Agency

Transcription

1 Safety Implication for Gen-IV SFR based on the Lesson Learned from the Fukushima Dai-ichi NPPs Accident Ryodai NAKAI Japan Atomic Energy Agency

2 Contents Introduction Japanese Government Report to the IAEA Safety Implication for Gen-IV SFR Lesson learned from the Fukushima Dai-ichi NPPs accident for the safety of the Gen-IV SFR system Approach to Backfit the Countermeasures for Existing Reactor Summary 1

3 1. Introduction For the safety of Gen-IV reactor systems Simple and robust structures Enhanced capability of inspection and maintenance Restriction the plant transient - not to lead the accident Restriction of the effect of accident within the plant Design consideration on the Severe Accidents defined as the DiD- Level 4 Prevention and Mitigation Reliable Active Safety System + Robust Passive Safety Diversity to eliminate the common cause failure Functional capability under the loss of all AC power supplies 2

4 1. Introduction First sights on the lesson learned from the accident Wide area earthquake, nearly as the level of the Design Basis Tsunamis over the Design Basis resulted in long-term loss of all AC power supplies in multi-units NPP For the Gen-IV reactor system Prevent the significant radioactive material release from a plant, even under the extreme external events as in Design Extension Condition (Beyond Design Basis) For the SFR, the feed-and-breed AM cannot be utilized due to that sodium is used as the coolant; hence, appropriate provisions for prevention and mitigation of the severe accident are keys to achieve the high level safety. 3

5 2. Japanese Government Report to the IAEA 28 Key Points in 5 Groups as Lesson Learns from the Accident Group 1: Strengthen preventive measures against a severe accident 1. Strengthen measures against earthquakes and tsunamis 2. Secure power supply 3. Secure a firm cooling function of a reactor and a RCV 4. Secure a firm cooling function of spent fuel pools 5. Thorough accident management (AM) measures 6. Response to issues in concentrated siting of reactors 7. Consideration on basic design such as placement of NPS, etc. 8. Ensuring the water-tightness of important equipment facilities Group 2: Enhancement of measures against severe accidents 9. Enhancement of prevention of hydrogen explosion 10. Enhancement of containment vent system 11. Enhancement of accident response environment 12. Enhancement of the radiation exposure management system at accident 13. Enhancement of training responding to severe accident 14. Enhancement of instrumentation reactors and PCVs 15. Central control of emergency supplies and equipment and rescue team in place 4

6 2. Japanese Government Report to the IAEA 28 Key Points in 5 Groups as Lesson Learns from the Accident Group 3: Enhancement of nuclear emergency response 16. Response to combined emergency of both large-scale natural disaster and nuclear accident 17. Reinforcement of environment monitoring 18. Segregation of duties between relevant central and local organizations, etc. 19. Enhancement of communication on the accident 20. Enhancement of response to support from overseas and communication to the international community 21. Adequate identification and forecast of effect of released radioactive materials 22. Clear definition of widespread evacuation area and radiological protection guideline in nuclear emergency Group 4: Reinforcement of safety infrastructure 23. Reinforcement of safety regulatory bodies 24. Establishment and reinforcement of legal structure, criteria and guidelines 25. Human resources for nuclear safety and nuclear emergency preparedness 26. Securing independency and diversity of safety system 27. Effective use of Probabilistic Safety Assessments (PSA) in risk management Group 5: Raise awareness of safety culture 28. Raise awareness of safety culture 5

7 3. Safety Implication for Gen-IV SFR 28 Key Points in 5 Groups as Lesson Learns from the Accident Group 1: Strengthen preventive measures against a severe accident 1. Strengthen measures against earthquakes and tsunamis 8. Ensuring the water-tightness of important equipment facilities For the External hazards Due consideration of loss of all AC power supplies following the extreme external hazards. Seismic events may be accompanied by subsequent events 2. Secure power supply For the Emergency power supply Diversity to the extent practicable and redundancy for suppressing common cause failure including external events. 6

8 3. Safety Implication for Gen-IV SFR Group 1: Strengthen preventive measures against a severe accident 3. Secure a firm cooling function of a reactor and a RCV For the Decay heat removal Decay heat removal systems for reactor cooling even under loss of all AC power supply. Utilization of passive heat removal capability for DEC. Diversity of ultimate heat sinks for decay heat removal. 4. Secure a firm cooling function of spent fuel pools For the Fuel storage systems Heat removal & status monitoring even under loss of all AC power supplies. 7. Consideration on basic design such as placement of NPS, etc. For the Design extension conditions Designs for Prevention and Mitigation of the severe accident consequences 7

9 3. Safety Implication for Gen-IV SFR Group 2: Enhancement of measures against severe accidents 9. Enhancement of prevention of hydrogen explosion For the Control of containment conditions Prevention/Mitigation of the sodium fire and sodium-concrete reaction, and Due consideration of the challenges on the integrity of containment. 12. Enhancement of the radiation exposure management system at accident For the Means of radiation monitoring Adequate radiation monitoring in DEC 14. Enhancement of instrumentation reactors and PCVs For the Fuel storage systems Adequate heat removal and status monitoring even under DEC including the loss of all AC power supplies. 8

10 3. Safety Implication for Gen-IV SFR Group 4: Reinforcement of safety infrastructure 26. Securing independency and diversity of safety system For the Decay Heat Removal Diversity to the extent practicable and redundancy for suppressing common cause failure including external events. For the Ultimate heat sink Diversity of the ultimate heat sinks for the decay heat transfer 9

11 4. Approach to Backfit the Countermeasures for Existing Reactor 10 Need to Backfit the Add-on Countermeasures for Existing Reactor taking into account of Safety Implication from Lessons Learned from Fukushima Daiichi NPP Stress Test are being performed for Existing Reactors including Monju Safety Margin for Extreme External Events (Earthquake and Tsunami) Safety Margin for Station Blackout Safety Margin for Loss of Ultimate Heat Sink Safety Margin for a Combination of Severe Events Severe Accident Management

12 Measures against station blackout (SBO) by tsunami After reactor shutdown, decay heat is removed by natural convection during SBO. Heat is emitted to atmosphere Air cooler Reactor vessel IHX around 17m Heat sink (Air cooler) Air Heat source (Core) Center of reactor around 7m Power supply Power-supply vehicle T. P. +0m T. P m 6. 4m T. P. +21m Diesel building Spent fuel pool Reactor building Ex-vessel fuel storage tank Reactor auxiliary building T. P m T. P. +31m Intake Breakwater T. P m Curtain wall Screen pump room Important facilities, including sodium systems and spent fuel storage facility are located at 21m above sea level. 11

13 Measures against Extreme Sequences (Summary) Securing emergency power supply Securing final heat removal functions in an emergency Ensuring the cooling of the spent fuel storage tank and pool Additional measures against severe accidents Emergency Safety Measures Power supply vehicle (500 kva -> 300 kva X 2) Prepare rules and manuals and training for power supply vehicle Operator training for SBO Confirm systems and procedures to establish natural convection cooling under SBO <Cooling of Ex-vessel Storage Tank (EVST) and water pool for SF> EM pump powered by power supply car Training using fire engines to feed water to fuel pool Revise manuals of EVST for SBO Confirm natural convection capability of EVST Secure ventilation and air-condition system of main control room Prepare emergency equipment (handy phones, transceivers, satellite phones, flash lights, personal dosimeters, etc. ) Prepare radiation-shielding suits of tungsten Future Plans Introduce an alternative air-cooled power supply system with sufficient capacity <Recovery of emergency diesel generator> Prepare of spare seawater pump and motor Provision to facilitate emergency operator actions, including modification of thermal insulator for easy access to valves <Recovery of forced convection cooling function> Prepare spare seawater pump and motor Continued detailed analysis of decay heat removal capability under various SA conditions. <Cooling of EVST> Prepare an alternative air-cooled power supply system with sufficient capacity Continued detailed analysis of decay heat removal capability under various SA conditions Measures against hydrogen explosion, even though remote, are (Study for blowout measures) Deploy a wheel-loader for rubble clean-up 12

14 5. Summary Lesson learned from the Fukushima Dai-ichi NPPs accident for the safety of the Gen-IV SFR system External hazards Emergency power supply Decay heat removal Fuel storage systems Design extension conditions Control of containment conditions Means of radiation monitoring Ultimate heat sink Need to Backfit the Add-on Countermeasures for Existing Reactors as well 13