Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

ROOT CAUSES OF THE FUKUSHIMA DAI-ICHINUCLEAR PLANT ACCIDENCTS AND STUDYOF ITS CLIFF-EDGE
Hiroshi MIYANO1 and Takao NAKAMURA2
1 Professor, System Design, Hosei University,Tokyo, Japan, hiroshi.miyano.77@hosei.ac.jp

2 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 themassive earthquake and tsunami of the Great East Japan Earthquake occurred on March 11, 2011,
where the cooling system for reactors and spent fuel storage pools became uncontrollable. As aresult, reactor cores were damaged at unit 1-3. Furthermore, the cooling function of the spent fuelstorage pools of unit 1-4 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
INTRODUCTIONSince 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 safetyfunctions: 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 protectedagainst 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 towithstand 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 toprevent 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 isbased on the extent of disaster damage and handling of the crisis.

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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 bemaintained..
In this case, if all of the equipments are operating properly, the reactor will reach a cold shutdown throughthe following procedure. (reference to Fig.1)
・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 Cooling System (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 heatremoval system impacted greatly on the progress of the accident.. , The core meltdown was due to lackof water required to cool the reactor core, and the water level response to ensure the furnace is important.
Reactor Building
Emergency
Power Supply
Transmission Reactor Core
Isolation CoolingLine(RCIC)

Containment Vessel
Main Steam Line
RPV
Stack

Condenser
Water Supply Piping
CoreFuel
Water
SupplyTank

Heat Exchanger
Suppression Chamber
Suppression Chamber
Emergency Diesel Generator

RHRS Pump

RHR Pump

Sea Water

Fig. 1 Nuclear Power Plant System (BWR)

CONSIDERATION OF THE "CLIFF EDGE" FACTOR DETERMINING
AND ACCIDENT

STABILITY

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 adisastrous accident. Therefore, identifying the “Cliff Edge” factors of the event is very important..

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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.2)① Assuming the damage of the intake facility with submerged intake facilities.
② 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.

Reactor Building

Service Building
Turbine Building
BatteryOutdoor Installationequipment

EDG

Cooling
Water
IntakeRHRS Pump

Reactor Building

Fig. 2 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 areevaluated in each step.
[BWR] (reference to Fig.3)a) Step 1: Earthquake occurrence(The assumed result is a loss of off-site power, EDG startup.)b) Step 2: 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 hydrogenventing for avoiding explosions)[PWR]a) Step 1: Earthquake occurrence (The assumed result is a loss of off-site power, EDG startup)b) Step 2: Large tsunami event (The assumed result is a loss of function of all equipment in the intake,1572

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 ofthe 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 increaseof water in the reactor/fuel damaged.)e) Step 5: Insufficient injection of seawater (The assumed result is further fuel damaged, hydrogengeneration.)In this assessment, a comparison can be made between the assessment results before and after thecountermeasures. This comparison clarifies the effectiveness of these countermeasures.
１２

プラント名
泊発電所1,2号機
Plant : Sample Plantof PWR
想定津波高さ15m

Height of Tsunami Wave : 15 m

：稼動
In Operation：待機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】
時
間Time Dependent
Transition

Power
Supply電源

Circulation
循環型冷却Cooling System

Condition ofthe Reactoer
状態

1時間
1 Hour

1日
1 Day

3日
3 Days

1週間 7 Ws
7週間
1 Week外電Off-Site
復旧
Power

１x Cliff-Edge
１１

備考

１
１××××２１１２

約5時間で枯渇１２

非常用DGの停止により起動不能
Off-site Power
外部電源復旧後は1系列運転
Supply×By restored

約8時間で水源枯渇（補助給水タンク）××２

地震による外部電源喪失
応急復旧はいずれか1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××２

２8 8時間Hs

地震による原子炉自動停止により発電停止

２(1) 余熱除去系 A系
RCIC(2) 余熱除去系
B系

55時間
Hs××××２(1) タービン動補助給水ポンプ
ECCS
Water Injection (2) 電動補助給水ポンプ
注水型冷却A系
Alternate Injection（淡水）
B系Pump
Cooling System (3) 電動補助給水ポンプ
注水型冷却（海水）津波

地震 Tsunami
Quake
到達

１２

(1) 所内電源
In-Site Power(2) 外部電源（275kV泊幹線）
Off-Site Power(3) 外部電源（275kV後志幹線）(4) 外部電源（66kV泊支線）
EDG(5) 非常用DG
A系(6) 非常用DG B系
DC Battery A系(7) 直流バッテリー(8) 直流バッテリー B系

２

非常用DGの停止によりポンプ停止

なし

Core
原子炉
Spent Fuel Pool
使用済燃料ピットHot
Shut-DownCoreMelt
安定冷却
炉心溶融
温度上昇・蒸散
Cooling = Temp. Increasing( Water Level Decreasing)

燃料損傷
FuelMelt

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

PowerSupplyTime Dependent Transition

Quake1. On-Site Power
2. Off-Site Power
3. Off-Site Power (backup)
4. EDG
5. DC Battery
6. Power Supply Vehicle

XXX

1 hr.

Tsunami

5 hr.

8 hr.

1 day

3 days1 weekmonths

SBO

SBO

X

Condition of Reactor Core

Stability Sustained(b) Recovery of residual heat removal system by continuous water injection through alternate injection pumps

Seawater injection by backup electric motor
or alternate pumps

Loss of Ultimate Heat Sink
Time Dependent Transition
Circulation
1. RCIC
Cooling System

Quake

Tsunami

1 hr.

5 hr.

8 hr.

1 day

3 days1 weekmonthsXFreshwater Injection

Seawater Injection

1. Turbine-driven Alternate Injection Pump
Water Injection
2. Electricity-driven Alternate Injection Pump
Cooling System
3. Fire Engines
Water Injection
Cooling System(seawater)

1. 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 areimportant “Cliff Edge” factors . Depending on external events, “Cliff Edge” effects are important factors

1573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 acountermeasure. On the other hand, even where the power is supplied, the loss of circulating coolingsystem as the ultimate heat sink would lead to the core damage. Yet, it can be easily prevented by injectionof 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 theassessment of the plant’s tolerance against external events, on whether it will remain stable or strainedby 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 ongoingbasis, 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 safetyof nuclear power plants, “Cliff Edge” must be eliminated. There are three magor “Cliff Edge”. One is theDC 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 2011

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