ROOT CAUSES OF THE FUKUSHIMA DAI-ICHI NUCLEAR PLANT ACCIDENCTS AND STUDY OF ITS CLIFF-EDGE

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1 Proceedings of the International Symposium on Engineering Lessons Learned from the 0 Great East Japan Earthquake, March -4, 0, Tokyo, Japan ROOT CAUSES OF THE FUKUSHIMA DAI-ICHI NUCLEAR PLANT ACCIDENCTS AND STUDY OF ITS CLIFF-EDGE Hiroshi MIYANO and Takao NAKAMURA Professor, System Design, Hosei University, Tokyo, Japan, hiroshi.miyano.77@hosei.ac.jp Professor, Sustainable Energy and Environmental Eng., Osaka University, Japan, nakamura@see.eng.osaka-u.ac.jp ABSTRACT: The Fukushima Daiichi Nuclear Power Plant has been stupendously affected by the massive earthquake and tsunami of the Great East Japan Earthquake occurred on March, 0, where the cooling system for reactors and spent fuel storage pools became uncontrollable. As a result, reactor cores were damaged at unit -3. Furthermore, the cooling function of the spent fuel storage pools of unit -4 were also lost, and some fuels may have been damaged. The Cliff Edge factors are DC power loss, the loss of cooling system, and the loss of the ultimate heat sink. Key Words: Nuclear Power Plant, East Japan earthquake, tsunami engineering, cliff-edge, nuclear safety INTRODUCTION Since Japan is a highly earthquake-prone country, many years have been spent considering how to ensure integrity of the nuclear facility against earthquakes, and particularly the maintenance of three reactor safety functions: containing nuclear reaction, assuring cooling of the fuels, and containing radioactive materials. As a result, the methodology for seismic assessment was established and used to create the currently effective codes and standards. Based on these codes and standards, concrete measures have been adopted by all Japanese Nuclear Power Plants. Therefore, the operating units in Fukushima Dai-ichi even after being hit by the Tohoku-Pacific Ocean Earthquake were successfully shut down and maintained cooling until Tsunami came, as designed. This means the safety functions successfully worked and were protected against the earthquake. The direct causes of the accident at Fukushima Dai-chi Nuclear Power Plant arise from the loss of multiple safety functions of systems/components vital to safety; the quake-hit plant was no longer able to withstand further crisis beyond design basis. The height of tsunami triggered by the Tohoku Pacific Ocean Earthquake yielded no allowance. Why, then, the progress of the event led to a nuclear crisis? Various factors cross our minds, such as the way each cause influenced one another. What can we do to prevent the recurrence of a similar accident? Contemplating when to expect the unimaginable natural disaster leads us to our utmost concern: prevention of a similar accident, with the right focus. The following is a summary of the accident, with the content limited to tsunami-related technical issues. It is based on the extent of disaster damage and handling of the crisis. 570

2 NORMAL SHUTDOWN PROCEDURE Reactor will automatically shut down normally under an earthquake;, the automatic isolation valve containment will close, followed by depressurization and cooling, and flooding of the fuel will be maintained.. In this case, if all of the equipments are operating properly, the reactor will reach a cold shutdown through the following procedure. (reference to Fig.) Reactor cooling water is injected into the reactor by Isolation Cooling (IC) system or Reactor Core Isolation Cooling (RCIC) system, thereby maintaining reactor water level. Reactor cooling water is injected into the reactor by Emergency Core (ECCS), there by maintaining reactor water level. (Ensuring the heat sink) continuing nuclear reactor cooling by (RHR) Residual Heat Removal system. If the above-mentioned equipment and /or the system is dysfunctional, alternative water injection measure should be taken to inject water, continue cool down, and maintain stable water level.. There is hardly an alternative for a seawater pump motor lost by the tsunami. The dysfunctional heat removal system impacted greatly on the progress of the accident.., The core meltdown was due to lack of water required to cool the reactor core, and the water level response to ensure the furnace is important. Transmission Line Emergency Power Reactor Core Isolation Cooling (RCIC) Containment Vessel RPV Main Steam Line Condenser Stack Water Piping Core Fuel Water Tank RHRS Pump Suppression Chamber Suppression Chamber Emergency Diesel Generator RHR Pump Heat Exchanger Sea Water Fig. Nuclear Power Plant System (BWR) CONSIDERATION OF THE "CLIFF EDGE" FACTOR DETERMINING STABILITY AND ACCIDENT What factors determine the equipment and/or structure damage the loss of function or keeping performance - under an earthquake? It is possible to evaluate fragility of the system under hazard situation with this assessment. With the tsunami case,, the equipment is subject to a serious situation as losing all functions under certain conditions, the so-called Cliff edge effect - factors which lead to a disastrous accident. Therefore, identifying the Cliff Edge factors of the event is very important.. 57

3 Evaluation of Spacing As shown below, to evaluate the tsunami reached if the range in power plant has been expanded gradually. In other words, the system lists affected by the tsunami, the equipment, at each stage of the tsunami reached, to assess the extent to which function can be maintained. (reference to Fig.) Assuming the damage of the intake facility with submerged intake facilities. Assuming the outdoor installation of equipment damage by power plant site flooding. 3 Assuming equipment damage in the turbine building. 4 Assuming the damage of equipment in the service building auxiliary facilities have been installed. 5 Assuming the damage in the reactor building equipment. Service Building Turbine Building Battery Outdoor Installation equipment Cooling Water Intake EDG RHRS Pump Fig. Evaluation the Elevation of Equipment BWR Nuclear Power Plant System (for example) Evaluation of Event Sequence Assessment at each stage of the progress of the sequence of events triggered by large tsunami, As shown in the following examples, the assessment is conducted step-by-step for the sequence of events triggered by large tsunami. The affected systems and the equipment are listed, and their functions are evaluated in each step. [BWR] (reference to Fig.3) a) Step : Earthquake occurrence (The assumed result is a loss of off-site power, EDG startup.) b) Step : Large tsunami event (The assumed result is a loss of function for all equipment in the intake, flooding of power supply in the turbine building.) c) Step 3: SBO, which is a loss of off-site power, EDG operation failure (The assumed results is a loss of the cooling systems.) d) Step 4: Loss of ultimate heat sink (The assumed result is temperature increase of water in the suppression chamber, failure of PCV venting ) e) Step 5: Loss of fresh water source, seawater injection (The assumed result is dry out of the reactor/ fuel damaged) f) Step 6: Insufficient injection of seawater (The assumed result is further fuel damaged, hydrogen generation, and then hydrogen venting for avoiding explosions) [PWR] a) Step : Earthquake occurrence (The assumed result is a loss of off-site power, EDG startup) b) Step : Large tsunami event (The assumed result is a loss of function of all equipment in the intake, 57

4 flooding of power supply in the turbine building) c) Step 3: SBO, which is a loss of off-site power, EDG startup failure (The assumed result is a loss of the cooling systems/ loss of heat removal by the steam generators.) d) Step 4: Loss of fresh water source, seawater injection (The assumed result is temperature increase of water in the reactor/fuel damaged.) e) Step 5: Insufficient injection of seawater (The assumed result is further fuel damaged, hydrogen generation.) In this assessment, a comparison can be made between the assessment results before and after the countermeasures. This comparison clarifies the effectiveness of these countermeasures. プラント名泊発電所, 号機 Plant : Sample Plant of PWR : 稼動 In Operation 想定津波高さ 5m : 待機 Height of Tsunami Wave : 5 m On Stand-by : 使用済燃料ピットに関係する項目 Fuel Pool 緊急安全対策以前 Off-Site 外電備考地震による原子炉自動停止により発電停止地震による外部電源喪失応急復旧はいずれか 系列津波による海水浸水または海水ポンプ機能喪失により停止 約 5 時間で枯渇 By restored () 余熱除去系 A 系 非常用 DGの停止により起動不能 Circulation Off-site Power 循環型冷却 () 余熱除去系 RCIC B 系 外部電源復旧後は 系列運転 約 8 時間で水源枯渇 ( 補助給水タンク ) 非常用 DGの停止によりポンプ停止 Time Dependent 時間 Transition 注水型冷却 Water Injection 到達 Power 復旧 () 所内電源 In-Site Power () 外部電源 (75kV 泊幹線 ) Off-Site Power (3) 外部電源 (75kV 後志幹線 ) x Cliff-Edge Power (4) 外部電源 (66kV 泊支線 ) 電源 (5) 非常用 EDG DG A 系 (6) 非常用 DG B 系 津波 Quake 地震 Tsunami Hour 時間 () タービン動補助給水ポンプ () 電動補助給水ポンプ ECCS A 系 5 Hs 時間 8 8 Hs 時間 Day 日 3 Days 3 日 Week 週間 7 Ws 7 週間 (7) A 系直流バッテリー DC Battery (8) 直流バッテリー B 系 ( 淡水 Cooling ) System (3) 電動補助給水ポンプ Alternate Injection B 系 Pump 注水型冷却 ( 海水 ) Condition of 状態 the Reactoer なし原子炉 Core 使用済燃料ピット Spent Fuel Pool Hot 安定冷却 Shut-Down CoreMelt 炉心溶融 Cooling = Temp. Increasing 温度上昇 蒸散 ( Water Level Decreasing) FuelMelt 燃料損傷 約 40 日で使用済燃料が露出 Fig.3 Progress of Event after SBO at the Seismic and Tsunami-Hit Plants Cliff Edge Points (a) Recovery of external power with the aid of power supply vehicles that continuously supply power even after the SBO Time Dependent Transition Quake Tsunami hr. 5 hr. 8 hr. day 3 days week months Power. On-Site Power X. Off-Site Power X SBO 3. Off-Site Power (backup) X 4. EDG X 5. DC Battery 6. Power Vehicle SBO Condition of Reactor Core Stability Sustained (b) Recovery of residual heat removal system by continuous water injection through alternate injection pumps Loss of Ultimate Heat Sink Seawater injection by backup electric motor or alternate pumps Time Dependent Transition Circulation. RCIC. Turbine-driven Alternate Injection Pump Water Injection. Electricity-driven Alternate Injection Pump 3. Fire Engines Quake Tsunami X hr. 5 hr. 8 hr. day 3 days week months Freshwater Injection Seawater Injection Water Injection (seawater). Fire Engines Condition of Reactor Core Stability Sustained Fig.4 Progress of Event after SBO at the Seismic and Tsunami-Hit Plants (after countermeasure) Study In the event of a SBO that does not directly lead to core melt, the loss of DC power supply and EDG are important Cliff Edge factors. Depending on external events, Cliff Edge effects are important factors 573

5 and branching points that determine an accident or maintenance of stability.. Core damage may still be prevented with continuous cooling by power supply vehicles as a countermeasure. On the other hand, even where the power is supplied, the loss of circulating cooling system as the ultimate heat sink would lead to the core damage. Yet, it can be easily prevented by injection of fresh water and sea water by fire engines. This means that the fundamentally important Cliff-Edge factors are power loss, the loss of cooling system, and the loss of the ultimate heat sink as shown in Fig.3. Evaluation of these factors and accident countermeasures of the plant may be judgmental criteria for the assessment of the plant s tolerance against external events, on whether it will remain stable or strained by a crisis, or not.. By securing the battery, the core cooling system such as IC or RCIC is activated as shown in Fig.4. And then, it is necessary to continue the cooling of the reactor core cooling system by securing on an ongoing basis, an alternative pump or fire-engine. Finally, as the ultimate heat sink, an alternative heat exchanger, filtered venting system, or a portable pump and etc., will be prepared. CONCLUSIONS If the Nuclear Power Plant is met by major natural disasters such as the tsunami, in order to ensure safety of nuclear power plants, Cliff Edge must be eliminated. There are three magor Cliff Edge. One is the DC power loss, one is the loss of cooling system, and the other is the loss of the ultimate heat sink. ACKNOWLEDGMENTS The study was performed by the members of professional and their knowledge belong to the special committee of the Japan Society of Maintenology (JSM). The authors would like to express heartfelt gratitude to the members and the chair-person of the committee for their kind support... REFERENCES Guideline for Assessing Large Tsunami Countermeasures in Japanese Nuclear Power Plants Japan Society of Maintenology, December 0 574