WEC2011_Wen_2_Nariai

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Facts of and Lessons Learned from the Fukushima Daiichi Nuclear Power Plant Accident WEC2011 Special Session Fukushima Facts and Consequences September 7, 2011 Hideki Nariai Professor Emeritus of the University of Tsukuba Former President of the Japan Nuclear Energy Safety Organization

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Contents Earthquake and Tsunami on March 11,2011 Nuclear Power Plants in Japan and in Tohoku area Fukushima Daiichi Nuclear Power Station Accident Sequence at Fukushima Daiichi NPS Sequence of Other Plants Road map for the settlement of the accident Lessons learned from the accident Actions regarding to the Lessons Learned 2

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Earthquake and Tsunami on March 11, 2011 On March 11, 2011, at 14:46 (Japanese standard time) Earthquake with Mw 9.0 at the Pacific Ocean of Tohoku From 40 minutes to 1 hour later Several waves of large tsunami attacked the coast of Tohoku-Kanto area Iwate, Miyagi, Fukushima, Ibaraki and Chiba Pref. Peoples, buildings, thermal and nuclear power plants and every structures on the coast were suffered More than 15,000 persons: died Almost 5,000 persons: missing 3

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Cited from Japanese government report to IAEA, June 7 4

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Nuclear Power Plants in Japan and in Tohoku Area Operating Nuclear Power Plants in Japan Total 54 plants , BWR 30 and PWR 24 Tohoku and Kanto Area Total 15 BWR Plants Higashidori NPS (Tohoku EPCO): 1 plant Onagawa NPS (Tohoku EPCO): 3 plants Fukushima Daiichi NPS (TEPCO): 6 plants Fukushima Daini NPS (TEPCO): 4 plants Tokai Daini NPS (JAPCO): 1 plant 5

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Location of NPSs in the Tohoku-Kanto District Higashidori NPS Onagawa NPS Fukushima Daiichi NPS Fukushima Daini NPS Tokai Daini NPS Tokyo 6

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Fukushima Daiichi Nuclear Power Station Fukushima Daiichi NPS: Japan's Oldest NPS Unit 1 (BWR3) : constructed by GE commercial operation from Jan. 1971 Unit 6 (BWR5) : constructed by Toshiba commercial operation from Oct. 1979 At the Earthquake and Tsunami Unit 1,2,3 plants were under normal operation Unit 4,5,6 plants were under periodic inspection 7

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Generation Facilities at the Fukushima Daiichi NPS 8 Cited from Japanese government report to IAEA, June 7

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Accident sequence at Fukushima Daiichi NPS By the earthquake Operating 3 plants: automatically shut down All off-site power was lost Emergency diesel generator power systems started Emergency Cooling systems started Isolation Condenser ( IC ):Unit 1 Reactor Core Isolation Cooling System ( RCIC ): Unit 2 &3 High Pressure Injection Cooling System ( HPIC ):Unit 3 9

Accident sequence at Fukushima Daiichi NPS : 

Accident sequence at Fukushima Daiichi NPS By the tsunami of almost 15 m high All Emergency Diesel Generator Power Supply was lost for Unit 1 to 4 Loss of all AC power Loss of all terminal heat sink One emergency diesel (air cooled) generator was alive It could supply power for Unit 5 and 6

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Cited from Japanese government report to IAEA, June 7 11

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Sequence at Fukushima Daiichi Unit 1 After earthquake at 14:46 and Tsunami at around 15:40 Loss of all electric power Isolation condenser was stopped at the attack of Tsunami. Loss of cooling water for reactor fuels (14 hours with no cooling) Loss of all terminal heat sink Nuclear fuels are estimated to start melting at 17:00 . (one hour and 20 min. later of the Tsunami attack.) Melted fuel moved to the bottom of reactor vessel and possibly some of the fuels dropped on the dry well floor . Hydrogen generated by the Zirconium-Water reaction. Containment Pressure increased . Wet well venting at 14:30 on March 12, and hydrogen explosion at 15:36 on March 12. Destruction of reactor building 12

Sequence at Fukushima Daiichi Unit 2: 

Sequence at Fukushima Daiichi Unit 2 After earthquake at 14:46 and Tsunami at around 15:40 Loss of all electric power RCIC was continued to inject the water to the fuel. Loss of all terminal heat sink : feed and bleed operation RCIC stopped at 13:25 on March 14 . (Loss of cooling water for 6 hour and 30min., until sea water injected.) Nuclear fuels are estimated to start melting at 18:00 on March 14. Melted fuel moved to the bottom of reactor vessel . Containment pressure increased . Wet well venting at 11:00 on March 13. Sound of explosion in the torus room of containment vessel at 6:00 on March 15. No visible damage for reactor building.

Sequence at Fukushima Daiichi Unit 3: 

Sequence at Fukushima Daiichi Unit 3 After earthquake at 14:46 and Tsunami at around 15:40 Loss of all electric power RCIC was started to inject the water at 16:03 on March 11. Loss of all terminal heat sink : feed and bleed operation RCIC stopped at 11:36 on March 12. HPIC started at 12:35 on March 12 and stopped at 2:42 on March 13 . (Loss of cooling water for 6 hour and 40 min., until sea water injected.) Nuclear fuels are estimated to start melting at 8:00 on March 13 . Melted fuel moved to the bottom of reactor vessel . Wet well venting at 5:20 on March 14, and hydrogen explosion at 11:01 on March 14. Destruction of reactor building

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Sequence at Fukushima Daiichi Unit 4 Unit 4 was shut down for periodic inspection all of the fuels were transferred to the spent fuel pool External power was lost by the Earthquake and emergency diesel generator stopped by the Tsunami: The function to cool the spent fuel pool and to feed the water was lost At around 6:00 on March 15, explosion of No.4 building occurred. Attention was paid for spent fuel pool and sea water and later pure water was poured Cause of the explosion is estimated to be the inflow of Hydrogen through the piping from the Unit 3 15

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Cited from TEPCO homepage Unit 1 Unit 4 Unit 2 Unit 3 16

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Trench for sea water pipes at Unit 3 (plain view) Cited from Japanese government report to IAEA, June 7 17

Sequence of Other 11 Plants : 

Sequence of Other 11 Plants Important Items for the plants (1) External power supply : alive or lost (2) Emergency diesel generator : alive or lost (3) RHR (Residual Heat Removal system) : alive or lost (4) Plant status : in operation or under periodic inspection (1) External AC power supply alive : Operating 6 plants. (1-1) RHR/auxiliary cooling system alive , and RCIC et al. alive : 3 plants. Onagawa Unit 2 and 3, and Fukushima Daini Unit 3. Cold shutdown status was reached on March 12 . (1-2) RHR system lost . Later Temporary Cable installed for RHR : 3 plants. Fukushima Daini Units 1,2 and 4. Cold shutdown status was reached on March 14 or 15 .

Sequence of Other 11 Plants: 

Sequence of Other 11 Plants (2) External AC power supply lost : Operating 2 plants. (2-1) Emergency Diesel Generator on , and RHR alive : 2 plants. Onagawa Unit 1 and Tokai Daini Unit. Cold shutdown status was reached on March 12 and 15 . (3) External AC power supply lost : Under periodic inspection, 3 plants. (3-1) Emergency Diesel Generator on , and RHR lost, Later it recovered . 2 plants. Fukushima Daiichi Unit 5 and 6. Cold shutdown status was reached on March 20 . (3-2) Emergency Diesel Generator on . Fuels were outside reactor : 1 plant. Higashidori Unit. Either External AC Power or Emergency diesel Generator was alive,then plants can be reached to the cold shutdown status

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Road map for the settlement of the accident TEPCO announced on April 17 Basic Policy Reactors and spent fuel pools: stable cooling condition Mitigating the release of radioactive materials Every effort to enable evacuees to return to their home All citizens to secure a sound life Targets: two steps Step 1: Time line is Middle of July Radiation dose in steady decline Step 2: Time line is 3 to 6 months after step 1 Release of radioactive materials under control Radiation doses being significantly suppressed 20

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Cited from TEPCO homepage 21

Evaluation of Step 1 and Target of Step 2 : 

Evaluation of Step 1 and Target of Step 2 Evaluation of Step 1 (1) Stable cooling was established for reactors and fuel pools. The temperature at the bottom of reactor pressure vessel are keeping about 100 to 120 ℃. (2)The radiation dose has been declining during the period of Step 1 and confirmed that the exposure dose at the site boundary is approximately 1.7 mSv/year at the most (Cs134, 137). Target of Step 2 (1)Continue the circulating cooling and bring cold shutdown status . (2)Release of radioactive materials under control and radiation dose is significantly held in.

Circulating Water Cooling System: 

Circulating Water Cooling System

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Lessons learned from the accident (5categories and 28 items) Category I: Strengthen measures against a severe accident measures against earthquakes and Tsunami et al. Categoruy 2: Enhancement of response measures against severe accident: measures to prevent hydrogen explosion, containment venting et al Category 3: Enhancement of nuclear emergency responses Combined emergencies of both large-scale natural disaster and prolonged nuclear accident et al Category 4: Reinforcement of safety infrastructure Reinforcement of safety regulatory bodies, reinforcement of legal structure, criteria and guidance et al. Category 5: Thoroughly instill a safety culture 24

Actions Regarding to the Lessons Learned: 

Actions Regarding to the Lessons Learned (1) Overall Safety Evaluation Evaluate the robustness of plants for external events (2) Revision of Safety Guideline Seismic safety and severe accident (3) Revise of the Regulatory Bodies NSC, NISA and a part of MEXT will become one agency (4) Restart Condition of the Stopping Plants (5) Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of TEPCO Comprehensive investigation for fundamental cause of the accident

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