Examination of Accident at Tokyo
Electric Power Co., Inc.’s Fukushima
Daiichi Nuclear Power Station and
Proposal of Countermeasures(Draft)This report has become final through to chapter 2.
But subsequent chapters are still being checked to
improve the quality of the translation.
It will become the final version after completingcorrection, such as unifying a term.

October 2011

Japan Nuclear Technology InstituteExamination Committee on Accident at Fukushima Daiichi Nuclear
Power Station

Revision History
Date of Revision
Month Day, 2011

Content of Revision
Newly createdNoteIntroduction
We, the Examination Committee on Accident at Fukushima Daiichi Nuclear Power Station,
recognize that the accident at Tokyo Electric Power Co., Inc.’s Fukushima Daiichi Nuclear
Power Station (hereafter called Fukushima Daiichi) caused by the Great East Japan
Earthquake has shaken the foundation of the Japanese nuclear industry, and damaged
the credibility of Japanese nuclear engineers. It has therefore made people deeply
skeptical of the morale of the engineers who could not prevent the accident.
It is widely known that the direct cause of the accident was the tsunami which was far larger
than had been anticipated when the power station was designed. The assumptions made at
the time of designing the plant had been based on scientific data that were then the latest.
However, it is true that the plant had insufficiently prepared for the worst case in which atsunami far larger than anticipated might occur. As a result, the cores of Unit 1 to Unit 3
melted one after the other and radioactive materials were emitted into the environment to adegree as high as level 7 of the International Nuclear Event Scale (INES). We are very sorrythat the accident caused a great deal of anxiety to the people living around the power
stations that had understood and supported the operation of the power stations but were
now forced to evacuate and live in unspeakable hardship.
The situation at Fukushima Daiichi has been stabilized for now thanks to the intensive workof the Self-Defense Forces, police, firefighters and local governments; desperate struggle ofthe power station staff and the employees of cooperative companies; the cooperation ofeach electric power company in neighboring monitoring; and various support and equipment
provided by plant makers in Japan and overseas. Nevertheless, we still have to expect that it
will be quite a long time until we can restore the surrounding environment.
Meanwhile, there is growing concern about the safety of nuclear power stations whichwere not directly affected by the Great East Japan Earthquake too, and stations located in
populated areas. Therefore, the power stations that were stopped for periodical inspection
or to investigate the causes of trouble or implement countermeasures still have notrestarted even though the prescribed inspection or construction has finished. This is in part
because the general public does not sufficiently understand the explanation of the situation
given by the nation or electric companies. Therefore, the supply and demand balance ofelectricity in the summer was tight in many areas throughout the country, though this was
narrowly overcome thanks to the efforts made by companies and individuals to saveelectricity, and the substitute use of thermal power stations.
A stable energy supply is the basic premise of our national economy and, in Japan whereenergy resources are scarce, the necessity of a stable power supply provided by nuclear
power stations will not change at all in the future.
We think that the most overriding priority for the nuclear power industry is to calmly analyze
the cause of the accident by returning to its origin, taking as many lessons as possible fromthe analysis. It must then use those lessons to improve the safety of Japanese nuclear
power stations and, by doing so, make the efforts of the industry known to society.
In the case of Japanese nuclear power stations, emergency measures have been
implemented twice at the initiative of supervising governmental bodies. However, somepeople living near nuclear power stations say that the correlation between the measurestaken as the emergency measures and the cause of the accident or the passage of1events seen at Fukushima Daiichi is not clear. And they are not convinced that suchmeasures can prevent an accident that would threaten the lives of neighboring people
like that of the Fukushima Daiichi Nuclear Power Station did.
Under the circumstances mentioned above, detailed data on the plant behavior or analysis
results were publicized by Tokyo Electric Power Co., Inc. during this period, which was
almost half a year after the accident. In addition, the Japan Nuclear Technology Institute(JANTI) has established the “Accident Examination Committee” within the institute,convening many experts from inside and outside the nuclear power industry in addition to
the experts of the JANTI as if collecting all of the power of the industry. It aims to clarify theview of the industry, analyze the sequence of events noticed at the accident site and thecause of the accident, extract lessons to be learned and, based on the lessons obtained put
together measures which are likely to enhance the safety of nuclear power stations as thecommittee’s proposal.
We, the Accident Examination Committee, asked the professionals of the “Nuclear Power
Safety Professional Committee, Technology Analysis Subcommittee” within the Atomic
Energy Society of Japan, which is the only academy to totally deal with nuclear powerissues, to review our proposal. It will check if there were elements we overlooked or
unreasonable points which were inconsistent with the measures to be taken.
The examination conducted this time is limited to the events which happened at thepower station site during approximately the 5 initial days including the occurrence of theearthquake, the coming of the tsunami and the following core melting and hydrogen
explosion. The purpose is to find a way to prevent core melting and the following
emission of radioactive materials to the environment. We are of the opinion that if eachcompany seriously takes up the measures described in this report, much stronger nuclear
power stations can be built with multiple safety measures which can resist tsunami which
are much bigger than the design grade, like the one that hit the Fukushima Daiichi
Nuclear Power Station. However, we know that there are some points that have still notbeen clarified among the events that happened at Fukushima Daiichi Nuclear Power
Station. Hence, we believe we need to review this report and add new findings whenevernew information comes to light from now on. Furthermore, concerning the behavior ofradioactive materials around power stations and the reaction of people concerned too,
we need to analyze the lessons to be learned and study proposals for improvement.
The biggest lesson learned from this accident is that we must always review the safetymeasures imagining what will happen when there are events that exceed the conditions
considered in designing or operating plants, and what measures are to be taken to ease
the impact of such events. And we believe that the first step in restoring the credibility of
nuclear power stations or of engineers working for nuclear power, i.e., the credibility lost in
this accident, is to seriously continue to implement such measures and make the situation
known not only to professionals but also to the general public.
We would greatly appreciate any opinions or comments you may have on this report.
October 2011
Examination Committee on Accident at Fukushima Daiichi Nuclear Power Station
Chief Examiner, Takashi Dodo

2

Members of the Examination Committee
Deliberation Members of the Examination Committee on Accident at
Fukushima Daiichi Nuclear Power Station(Title omitted, position at the time of deliberation)
Chief
examiner

Japan Nuclear Technology Institute, Executive Director
Hokkaido Electric Power Co., Inc., Nuclear Power Division
Nuclear Fuel Control Office, Nuclear Power Technology
Group Leader
Tohoku Electric Power Co., Inc., Thermal Nuclear Power
Headquarters
Nuclear Power Division (Nuclear Power Technology),
Section Manager
Chubu Electric Power Co., Inc., Nuclear
PowerHeadquarters, Nuclear Powder Division
Safety Technology Group Manager
Hokuriku Electric Power Co., Inc., Nuclear Power
Headquarters, Nuclear Powder Division
Vice Manager (in charge of Technology and Quality
Assurance)
Kansai Electric Power Co., Inc., Nuclear Power Business
Headquarters
Nuclear Power Technology Division Manager
Chugoku Electric Power Co., Inc., Electric Power Supply
Business Headquarters(in charge of Nuclear Power Construction Electric
Design) Manager
Shikoku Electric Power Co., Inc., Nuclear Power
Headquarters
Nuclear Power Division, Operation Group Leader
Kyushu Electric Power Co., Inc.
Nuclear Power Generation Headquarters, Safety
Technology Group Manager
Japan Atomic Power Co., Inc., Power Generation
Management Office
Technology, Safety Group Manager
Japan Nuclear Fuel Limited, Safety Technology Office
Safety Technology Division, Safety Technology GroupElectric Power Development Co., Ltd.
Nuclear Power Business Headquarters, Nuclear Power
Construction Division, Assistant Manager
Toshiba Corporation
Nuclear Power Fukushima Restoration Technology
Division Manager
Hitachi-GE Nuclear Energy, Ltd.
Engineering Manager
Mitsubishi Heavy Industries Co., Ltd.
Nuclear Power Business Headquarters, Safety
Enhancement Measures Promotion Office, Office
Manager

3

Takashi Dodo

Tomohiro Kodaira

Tsunehiro Tada

Kouhiro Takeyama

Toshihiko Takahashi

Hirohiko Yoshida

Yuichi Ida

Masahiko Sato
Kazuaki Umada

Satoru Fukuyama
Tadashi Makihara
Sadayuki Kuramoto

Mamoru Hatazawa

Kumiaki Moriya
Yoshihiko Kato

Report Study Members of the Examination Committee on Accident
at Fukushima Daiichi Nuclear Power Station(Title omitted, position at the time of deliberation)
Hokkaido Electric Power Co., Inc.
Nuclear Power Division, Nuclear Power Facility Group
Katsumi Dazai
Nuclear Power Division, Nuclear Fuel Control Office, Nuclear Shinichiro Yasui
Power Technology Group
Tohoku Electric Power Co., Inc.
Thermal Nuclear Power Headquarters, Nuclear Power Division Kouji Tadakuma(in charge of Safety Enhancement) Section Manager
Thermal Nuclear Power Headquarters, Nuclear Power Division Daisuke Sato
(Nuclear Power Technology) Vice Manager
Thermal Nuclear Power Headquarters, Nuclear Power Division Jun Takaishi(Nuclear Power Technology)
Chubu Electric Power Co., Ltd.
Nuclear Power Headquarters, Nuclear Power Division,
Tetsuya Watanabe
Operation Group, Section Manager
Nuclear Power Headquarters, Nuclear Power Division, Safety Kazuyuki Matsumoto
Technology Group, Section Manager
Nuclear Power Headquarters, Nuclear Power Division, Safety Akihiro Urano
Technology Group
Hokuriku Electric Power Co., Inc.
Nuclear Power Headquarters, Nuclear Power Division, Nuclear Seishi Arakawa
Fuel Technology Team
Nuclear Power Headquarters, Nuclear Power Division, Nuclear Junichi Nishii
Power Facility Management Team
Nuclear Power Headquarters, Nuclear Power Division, Nuclear Akira Ohbata
Power Generation Operation Team
Kansai Electric Power Co., Inc.
Nuclear Power Business Headquarters, Plant MaintenanceToshihiko Tanaka
Technology Group Manager
Nuclear Power Business Headquarters Manager
Kensuke Yoshihara
Chugoku Electric Power Co., Inc.
Electric Power Supply Business Headquarters (in charge of
Takatsugu Takatori
Nuclear Power Construction Electricity Design) Sub Manager
Electric Power Supply Business Headquarters (in charge of
Takeo Makino
Nuclear Power Construction Safety) Sub Manager
Shikoku Electric Power Co., Inc.
Nuclear Power Headquarters, Nuclear Power Division,
Daisuke Mizobuchi
Operation Group Sub Leader
Kyushu Electric Power Co., Inc.
Nuclear Power Generation Headquarters, Power Generation
Tsuyoshi Muto
Management Group
Japan Atomic Power Co., Inc.
Power Generation Management Office, Plant Management
Masashi Nachi
Group, Sub Manager
Power Generation Management Office, Technology, Safety
Masaru Yamanaka
Group Chief
Japan Nuclear Fuel Limited
Safety Technology Office, Safety Technology Division, Safety
Tadashi Makihara
Technology GroupElectric Power Development Co., Ltd.
Nuclear Power Business Headquarters, Nuclear PowerKouzou Miyao
Construction Division, Facility Technology Group4

Toshiba Corporation
Power Systems Company, Nuclear Power System Design
Division
Hitachi-GE Nuclear Energy, Ltd.
Nuclear Power Planning Division, Nuclear Reactor PlanningGroup, Chief Engineer
Mitsubishi Heavy Industries Co., Ltd.
Nuclear Reactor Safety Technology Division Manager
Nuclear Reactor Safety Technology Division, Safety
Technology Control Group Manager
Nuclear Reactor Safety Technology Division, Safety
Technology Control Group
Japan Nuclear Technology Institute
Director, Operating Experience Analysis Division Manager
ObserverTokyo Electric Power Co., Inc., Nuclear Power Operation
Management Division Manager
Federation of Electric Power Companies of Japan, Nuclear
Power Division Sub Manager

5

Hirohide Oikawa

Kohei Hisamochi

Shigemitsu Umesawa
Hisanaga Takahashi
Mitsuhiro Kano

Masuhiro Nakano
Takeshi Takahashi
Tai Furuta

Report Review Team of the Examination Committee on Accident at
Fukushima Daiichi Nuclear Power Station(Japan Nuclear Technology Institute)
Board Members
Director, General Affairs Division Manager

Akio Fukuda

Director, Safety Culture Division Manager

Etsuji Obu

Director, Codes and Standards Division Manager

Hiroyuki Ito

Consultant

Tamihei Nakamura
Hiroaki Kawashima

Planning OfficeNobuyuki Kitamura
Toshiki Nagasawa

General Affairs DivisionNobuyuki Onishi

Operating Experience Analysis Division

Ichiro Murakami
Tomoyuki Yamazaki
Kazuki Takada

6Abbreviations
ADS
AM
AOvalve
APD
ASW
BAF
BWR
CCS
CRD
CS
CST
CWP
D/D FP
DG
D/W
DWC
ECCS
EECW
FCS
FP
FPC
HPCI
HPCS
HPCW
IA
IC
ITV
M/C
MCC
MCR
MOvalve
MP
MSIV
MUWC
MUWP
O.P.
P/C
PCIS
PCV
PSA
PWR
R/B
RCIC
RCW
RHR
RHRC

Automatic Depressurization System
Accident Management
Air Operated Valve
Alarm Pocket Dosimeter
Auxiliary Sea Water System
Bottom of Active Fuel
Boiling Water Reactor
Containment Cooling System
Control Rod Drive
Core Spray system
Condensate Storage Tank
Circulating Water Pump
Diesel Driven Fire Pump
Diesel Generator
Drywell
Drywell Cooling System
Emergency Core Cooling System
Emergency Equipment Cooling Water system
Flammability Control System
Fire Protection system
Fuel Pool Cooling and Filtering system
High Pressure Coolant Injection System
High Pressure Core Spray System
HPCS Closed Cooling Sea Water System
Instrument Air-System
Isolation Condenser
Industrial Television
Metal-Clad Switch Gear
Motor Control Center
Main Control RoomMotor Operated Valve
Monitoring Post
Main Steam Isolation Valve
Makeup Water System (Condensated)
Makeup Water system (Purified)
Onahama Pile
Power Center
Primary Containment Isolation System
Primary Containment Vessel
Probabilistic Safety Analysis
Pressurized Water Reactor
Reactor Building
Reactor Core Isolation Cooling System
Reactor Building Closed Cooling Water System
Residual Heat Removal System
RHR Cooling Water System
7

RHRS
RPV
RSW
S/C
S/P
SA
SBO
SFP
SGTS
SHC
SLC
SRV
T/B
TAF
TSW
UHS

RHR Sea Water System
Reactor Pressure Vessel
Reactor Building Closed Cooling Sea Water System
Suppression Chamber
Suppression Pool
Severe Accident
Station Black Out
Spent Fuel Pit (Cooling System)
Stand-By Gas Treatment System
Shutdown Cooling System
Stand-by Liquid Control
Safety Relief Valve
Turbine Building
Top of Active Fuel
Turbine Building Closed Cooling Water System
Ultimate Heat Sink

8

Glossary
Accident Management(Measures against severe accidents) AM

Measures which are taken by effectively utilizing the safety allowance and the expectedfunctions of the safety design in the existing design, other functions than the duly expectedfunctions and devices which have been newly set up for such accidents in order to prevent a
beyond design-basis accident which could lead to major core damage from expanding to asevere accident or to mitigate its impact in the event that the accident has developed to asevere accident.
Air operated valve
AO valve
Valves which operate with compressed air
Alarm Pocket Dosimeter
APD
A personal radiation monitor with an alarm using a semiconducting detector. It can record
the name and time of the operation the carrier is engaged in.
Alternative containment vessel spray
It refers to the spray function for the containment vessel, effectively utilizing the existing
condensate water makeup system and the water source and pumps of the fire service’s
water system.
Alternative control rod insertion
To shut down the reactor by automatically inserting the control rods after detecting anabnormality (high pressure, low water level in the reactor) by the instrumentation and control
system, which is installed aside from the existing emergency shutdown system.
Alternative reactivity control
To conduct the recirculation pump trip and insert control rods by water level and pressure of
the reactor with the use of an RPS signal.
Alternative water injection
It sprays water and removes heat when the emergency core cooling system does not
function for some reason. In accordance with the original function, there is water injection to
the high-pressure reactor and cooling of the containment vessel. A control rod drive
hydraulic control system and reactor coolant clean-up system are used to conduct
alternative water injection to the high pressure reactor, and the condensate make-up water
system, the fire protection system, dry well cooler and the containment & vessel coolingwater system are used to cool the containment vessel.
Automatic depressurization systemADS
One of the emergency core cooling systems, it refers to the back-end unit of the highpressure core spray system or the high pressure coolant injection. Its purpose is to lower thereactor pressure and facilitate the water injection of the low pressure injection system by
opening the main steam relief safety valve installed in the main steam line.
Auxiliary seawater system
ASW
A system for feeding seawater to the heat exchangers for the facilities which feed cyclicallycooling fresh water to the process heat exchangers, bearing coolers and air-conditioners in
the plant.
B equipment
Equipment which has to be worn when entering zones that could be contaminated withradioactive substances.
Back wash valve pit
A place where valves are installed for reversing the flow of the seawater in the tube to clean
the condenser tubes.

9Boiling water reactor

BWR

It mainly uses enriched uranium as nuclear fuel and water as a moderator and coolant.
Steam is sent directly to the steam turbine without going to the heat exchangers. The steamwhich contains radioactive substances is sent to the steam turbine.
Bottom of active fuel

BAF

Bottom of the fuel assembly.
Charcoal filter
A filter filled with granular activated charcoal to remove the radioactive iodine. While iodine
is removed by activated charcoal by the effect of physical absorption, it is often the case thatsome chemical material is added to the activated charcoal in order to absorb iodine
compounds that are difficult to absorb such as methyl iodide.
Circulating water pump
CWP
Steam that has been used in the main turbine is cooled down and condensed in the maincondenser. Seawater is used to cool down the steam and is fed to the seawater system(circulating water system) by this pump.
Cold shutdown
It refers to the condition where the temperature of the core water is less than 100°C and thereactor mode switch is placed in “Start-up,” “Shutdown” or “Refueling.”
Condensate and feed water system
A system for pressurizing and heating condensate steam condensed by the condenser andfeeding it to the reactor.
Condensate storage tank
CST
A tank for storing the water of the condensate water system. It is used for feeding
condensate, storing surplus condensate and make-up water and others. In BWR, it is used
as water source for the emergency core cooling system.
Condenser
Seawater cooler for condensing the steam which has acted in the steam turbine. It reduces
the final pressure of the turbine drive steam by the obtained high vacuum and expands theheat drop resulting in the improvement of the turbine efficiency.
Constant rated electric power operation
An operating mode to maintain a steady level of electric power output.
Constant rated thermal power operation
An operating mode to maintain a steady level of thermal power, in which the electric power
output fluctuates depending on the environmental conditions such as the seawatertemperature.
Containment Cooling system
CCS
A device to cool down the energy of the coolant leakage and the decay heat of the fuel byspraying water in the containment vessel in order to prevent the pressure and temperatureof the inside the containment vessel from exceeding the maximum operating temperature at
the time of a loss of coolant accident. At the same time, it helps eliminate iodine in thecontainment vessel and reduce iodine leakage from the containment vessel.
Control rod drive
CRD
A device for taking control rods into and out of the reactor core. Hydraulically-powered
drives are generally adopted for BWRs. (For advanced BWRs, electrically powered drives
are also used concurrently.)

10

Control room air handling and ventilation
system
A system for maintaining a clean atmosphere in the control room by automatically isolating
the room from the external air and by recirculating the air in the room, when a leakage ofradioactive material takes place.
Core
It refers to the area where nuclear fuel is located and the chain reaction of nuclear fission
takes place in the reactor. It consists of nuclear fuel and a moderator, among which coolant
is traveling.
Core spray systemCS

A system which is part of the emergency core cooling system of a BWR. It sprays water tothe upper part of the fuel to cool it down at the time of a loss of coolant accident.
Decay heat
Heat generated when the atomic nucleus of a radioactive substance spontaneouslytransforms to another atomic nucleus.
Demineralized water tank
Tank for storing the demineralized water which is obtained by demineralizing the water fromrivers or dams with the use of a demineralizer. It is used for the system that needs water ofwhich quality is controlled.
Diesel driven fire pump
D/D FP
Diesel driven pumps installed in the fire protection system. They are automatically to start,
when the pressure in the fire protection system becomes low or electric-motor-driven firepump cannot operate.
Diesel generator
DG
A generator which feeds power to facilities necessary to safely shut down the power plant at
the time of loss of normal power of the plant. It is driven by a diesel engine.
Disconnect switch
A device to disconnect the circuit for the safe checking work. Its capability to disconnect isoriginally low and basically it cannot switch the load current. It is equipped with an interlock
so that equipment operation is not possible unless the breaker is open.
Dry well

D/WSpace other than the suppression chamber in the containment vessel.
Dry well cooler
DWC
Facility which cools the dry well while the reactor is operating and also cools during theperiodical inspection to avoid a severe temperature rise in the containment vessel.
Duct
It serves as path for air or as a flow channel for water and gas.
Dust extractor
A device for extracting dust contained in the obtained seawater.
Earthquake-proof classClasses according to the importance of the facility, determined in the importanceclassification concerning the aseismic design.
Electric magnetic valve
A valve which is opened and closed by the electromagnetic power.

11

Emergency core cooling system

ECCS

Engineered safety features intended to effectively cool down the reactor core even when aloss of coolant accident takes place in the reactor. It has a volume sufficient to cool down thereactor core, regardless of the size of the pipe rupture on the primary cooling system for thereactor. It is comprised of a high pressure core spray system, high pressure core injection
system, low pressure core spray system, low pressure core injection system and automatic
depressurization system. (In the case of advanced BWR, the reactor core isolation coolingsystem is included in the ECCS.)
Emergency equipment cooling water system

EECW

A facility to feed the cooling fresh water to emergency diesel generators and the coolers of
emergency ventilation equipment so that every piece of emergency equipment can maintainits required function at the time of a loss of coolant accidents. (It also feeds the cooling waterto residual heat removal pump motor.)
Engineered safety features
It refers to the facilities designed to have functions to contain or prevent the release of
considerable radioactive material caused by a fuel failure or such like in the reactor, which
are brought about by some damages or failure to the reactor facilities.
Enhanced pressure resistant ventA containment vessel vent line of high pressure resistance, which was prepared asmeasures against severe accidents. There are two vent lines for D/W and S/C, each ofwhich has big and small valves of AO valve. After the confluence of two lines, the MO valve
and the rupture disk are placed and the converged line further connected to the vent stack.
The word “containment vessel vent” in this report refers to the vent from this enhanced
pressure resistant vent line.
Fail-safe
To ensure safety even when a failure occurs. It means that devices are designed to keep
safety without endangering the original function even when part of the device fails or a
failure takes place in the function of the safety protection device.
Filtered water
Water from rivers or dams is used after processing as the water for the plant. It is used in thesystems which do not need high quality water.
Fire protection system
FP
Fire protection system in a power plant. Besides the normal fire hydrants, carbon dioxide
extinguishing systems for oil fires and others are available.
Fire-resistant clothes
Clothes which are difficult to burn.
Flashing
To wash radioactive substances away from piping with clean water to reduce the dose.
Fuel Pool Cooling and Filtering system
FPC
Fuel assemblies taken out of the reactor need to be cooled down in the fuel pool, because
the fission products they contain emit heat and radiation. It is a purification system forremoving the impurities and maintaining the water quality, while cooling down the water ofthe pool.
Fuel cladding tube
A thin-walled circular pipe used as cladding material for the fuel rod. A zirconium alloy or astainless steel pipe is adopted. Fuel cladding tubes are placed between fuel and coolant andplay an important role in maintaining the integrity of the fuel. It is one of the five layer barriersand dissolved when reaching a temperature of 1800 degrees.

12Fuel day tankLight fuel oil, which is fuel for emergency diesel generators, is transferred from the light oil
tank outside to the fuel day tank located in the building where the emergency diesel
generators are found. A necessary stockpile for each tank according to the operation time isdecided in safety regulations.
Full-face mask
One of the purification respiratory protection tools. It covers all of the face.
HPCS Closed Cooling Sea Water System
HPCW
A system which feeds seawater to the heat exchanger of the facilities which cyclically feed
freshwater to the cooler for the motors of the high pressure core spray system, bearings andoil cooler.
Heat sink
A cooling source for securing the heat removal function (heat release).
High pressure coolant injection system
HPCI
One of the emergency core cooling systems, which, with the use of steam-turbine-driven
high pressure pump, injects cooling water into the reactor core in the time of the accident
where the break of the piping is relatively small and the reactor pressure does not rapidly
fall. The flow (i.e., capacity) of the pump is approximately 10 times larger than that of thereactor core isolation cooling system, but smaller than that of the reactor shutdown coolingsystem and the residual heat removal cooling system ( approximately 1800 m3/h, for Units 2
to 5 of the Fukushima Daiichi Nuclear Power Station). It is installed in Units 1 to 5 of the
Fukushima Daiichi Nuclear Power Station.
High pressure core cooling function
High pressure core cooling function including the high pressure core spray system, the highpressure core injection system and others.
High pressure core spray system

HPCS

One of the emergency core cooling systems, which has an independent power source(diesel generator) in the time of the accident in which the reactor pressure does not rapidly
fall and sprays with the use of an electric-motor-driven high pressure pump to the reactor
core for cooling.
In-core instrumentation piping
Instrumentation and equipment for measuring in the reactor the process volume needed formonitoring the control, safety and conditions of the reactor. It is a general name for an
in-core neutron flux monitoring system, coolant flow monitoring system, control rod position
monitoring system and others.
Independence
To design a system for operation and system for safety separately so that a failure on one
system does not affect the other.
Industrial television

ITV

TV camera installed for reducing the exposure of the plant worker, monitoring the operationand leakage of radioactive fluid, monitoring the alarm of the local control panel, monitoring
the situation of the water intake structure in the winter time and others. Cameras installed forsite monitoring in the industry are generally called ITV.
Inflammability limit
Breaking point of the gas concentration of hydrogen and oxygen generated by metal-water
reactions or radiolysis of water in the time of the loss of coolant accident, above which
combustion occurs.
Instrument air systemIA13

A system which feeds the clean and dry compressed air for the air operated valves,
air-regulated devices and measuring instruments.
Iodine preparation
Since the thyroid has a function to absorb and accumulate iodine, radioactive iodine isinstantly combined with the thyroid hormone in the thyroid, which keep emitting the radiation
inside the body, once the radioactive iodine in the atmosphere is absorbed in the humanbody. As a result, disorder of the thyroid gland caused by the radiation leads to developmentof goitrous tumor or hypothyroidism as late radiation injury.
In order to prevent this disorder, people need to take iodine without radiation before being
exposed in order to saturate thyroid with iodine without radiation so that the thyroid withradiation is not be taken into the thyroid, even if exposed internally. Thus a preventive effectcan be expected.
Isolation condenserIC
A heat removing device for the reactor at the time of the isolation of the core in the boilingwater reactor. It cools down the reactor steam with the water of the secondary system andreturns the condensate to the reactor by natural circulation. (Only installed for the
Fukushima Daiichi Nuclear Power Station and Unit 1 in the Tsuruga Power Station.)
Leak
Leakage
Loss of coolant accident
One of the credible accidents of the reactor. In this accident coolant in the reactor pressure
vessel is lost because of the damage of piping or other reasons. Cooling of the reactor is notpossible because the coolant is lost from the reactor pressure vessel.
Low pressure core cooling system
Low pressure emergency core cooling systems. They include a low pressure core injection
system, low pressure flooder system, low pressure core spray system and others.
Lower plenum
Space found in the lower part of a reactor core. During normal operation, water which flows
down between the inside wall of the reactor pressure vessel and the core shroud or along
the primary loop recirculation system to this space, where it turns back into the core to cool
down. In the lower plenum, monitoring guide tubes and control rod guide tubes are found.
Main control room
MCR
A room where a main control board, on which monitoring and operating devices necessary
for the operation of the main system of the plant are collected, is placed and plant workers
carry out the monitoring, control and operation in a centralized manner.
Main steam isolation valve
MSIV
Valves installed in the main steam line, which close up when it is needed to isolate thereactor from the turbine facility.
Makeup Water System (Condensated)
MUWC
A system for feeding the various types water (The water source is the condensate storagetank. The water is the purified water which has been used in the reactor. Though it contains
some amount of radioactive substance, its dose is low.) necessary for operation of the plantby using pumps (condensate water transfer pump). It is not for emergency, but it is injectedto the reactor in the implementation of the accident management. The flow volume of thepumps is smaller than those of the reactor core isolation cooling system (approximately 70m3/h).
Makeup Water System (Purified)
MUWP
A system feeding to the equipment, piping, valves and others installed in buildings, auxiliary
facilities and others the make-up water with necessary volume and pressure, which isneeded for the smooth operation and maintenance of a power plant.
14

Maximum response acceleration
Maximum value of acceleration of the vibration (response) of the structure when the seismic
motion acts on that structure.
Melted fuel
Fuel assembly melted by high temperature, which became a lump.
Metal-clad switch gear
M/C
A power panel used for high-voltage circuit in the plant, in which the electromagnetic
breakers, vacuum breakers, protective relay and auxiliary instruments are compactly
housed. It consists of regular, common and emergency switches.
Metal-water reaction
When zirconium used for the fuel cladding is heated, it gradually reacts with the surrounding
coolant, which is water and becomes oxidized. This reaction produces hydrogen gas.
Monitoring post
MP
They are placed in several places in the plant and monitor the rate of atmospheric gamma
dose. The vehicle which can monitor is called monitoring car.
Motor control center

MCC

A power panel used for low-volume low-voltage circuits in the plant, in which line breakers,
magnetic contactors and protective relay are compactly housed. It consists of regular,
common and emergency switches.
Motor operated valve

MO valve

A valve whose driving parts for opening and closing are operated by a motor.
Multiplicity
To prepare more than two systems or pieces of equipment of the same function and thesame quality.
Nonessential
A system used normally
NuclideA term used to refer to types of atom or nucleus.
Nuclide analysis
To identify radionuclide from sample
Off-site Center
Facilities predesignated by the competent minister provided in Paragraph 1, Article 12 of theSpecial Law of Emergency Preparedness for Nuclear Disaster in order that the related
personnel from the national, prefectural and municipal governments and other organizations
meet together and that the local nuclear emergency response headquarters of the national
government and the disaster countermeasures headquarters of the prefectural governments
share the information in the time of nuclear disasters with a view to conducting concerted
emergency measures and promoting the coordinated and smooth implementation of nuclear
disaster countermeasures.
Its legally official name is the Emergency Response Operation Facilities.
On-site power
AC power distributed to equipment and others in a power plant.
Onahama Pile
O.P.
Datum level 0.727 m lower than the Tokyo Pile.
Paging15

A facility of in-house communication consisting of handset stations and speakers placed inmany places in the plant. It is easy to use and enables clear broadcasting and telephone
calls in a very noisy environment.
PelletA highly compacted small cylindrical substance containing the fissionable material. It is oneof the five layer barriers. It is generally made by compacting the oxidation product with highpressure, sintering it and making it into ceramic form. A fuel rod is made by putting them into
the fuel rod cladding.
Pier
A part of the port facilities in the plant. It is the place for unloading the cargo carried by ships.
Pool gate
Gates for separating the spent fuel pool, the reactor well and the steam-water separator
pool. During the periodical inspection, the in-core structures such as steam-water separator
are transferred to the steam-water separator pool and the loaded fuel is transferred to thespent fuel pool after removing the lid of the reactor pressure vessel. Since those pieces ofequipment or devices have a very high dose, they are transferred from one pool to another
under the water, securing a biological shield wall.
Power center
P/C
A facility for controlling electromotor loads, motor control center loads and others ofintermediate capacity less than 600 V source voltage at one place in a centralized manner. It
is comprised by arranging in one board units into which air circuit breakers and protection
equipment.
Pressurized Water Reactor

PWR

Type of nuclear reactor in which water, which is used as moderator and coolant, is highlypressurized to suppress boiling of water. Primary cooling system that is to retrieve thethermal energy generated in the core and secondary cooling system that is to generatesteam is completely separated by the heat exchanger (steam generator).
Primary Containment Vessel Cooling
Seawater System
A system supplying seawater to the components of the containment cooling system
Primary Containment Vessel Vent
PCV Vent
A device to decrease the pressure inside the PCV by partially releasing the gas containing
radioactive materials (mostly nitrogen) from the PCV in order to prevent the pressure fromincreasing abnormally and to protect the PCV.
Primary Containment Vessel
PCV
A vessel housing reactor-related components and piping including reactor pressure vessel.
This vessel needs to have a capability to resist a transient pressure and temperature which
occur in the time of pipe rupture accident of reactor-related piping (what is called a Loss of
Coolant Accident) and to maintain the post-accident integrity and has the leak-tightness forminimizing the leakage of the radioactive material from the vessel as low as possible. This is
one of the five-layer barriers.
Primary containment isolation system
PCISThis system operates so that the pressure vessel is isolated in the time of the fuel damage
accidents, that the isolation valves between the reactor pressure vessel and the damaged
part close to prevent radioactive materials and coolant from leaking in the time of a break
accident on the primary system outside of the containment vessel and that the release pathof radioactive materials is shut to contain them in the containment vessel in the time of abreak accident on the primary system inside the containment vessel.
Probabilistic Safety Analysis
PSA
To assess the safety in consideration of the probability of various possible events
16

Process computer
A computer for performing the process control, monitoring and controlling the process
volume and arithmetic processing. The connection with the plant process volume isperformed through the process input-output units and generally highly reliable computers
are used, because high operational availability and effectiveness are required. In the
nuclear plant they are installed for monitoring the process volume, calculating the core
performance and the plant performance and often applied as systems having the plant
operation supporting function. They are installed with the diagnosis function.
RHR cooling water system

RHRC

A system which feeds cooling freshwater to the heat exchangers and the pumps of theresidual heat removal system, mechanical seal coolers of the pumps of the low pressure
core spray system and others.
Rapid depressurization
An operation to decrease the reactor pressure by opening the main steam safety releasevalve manually in order to supply water from low-pressure emergency core cooling system
into the reactor.
Reactor building

R/B

A building housing a reactor and its related facilities. This is one of the five-layer barriers.
Reactor building closed cooling seawater
RSW
systemCooling water of the reactor building closed cooling water system cools through a heatexchanger. This system is feeding seawater for cooling down the cooling water of thereactor building closed cooling water system.
Reactor building closed cooling water system RCW
One of the component cooling systems. Cooling system for reactor-related non-safety
components or cooling system for reactor-related non-safety and safety components.
Reactor coolant pressure boundaries
It refers to the zone where during the normal operation, the reactor coolant is included andthe same pressure condition as the reactor. It forms a pressure barrier for the primarycooling system, breakage of which leads to the loss of coolant accident. It usually includes
the reactor pressure vessel, primary piping and others, but it does not refer to the part that is
isolated in the time of the loss of coolant accident.
Reactor core isolation cooling system
RCIC
During the normal operation, when the main condenser somehow cannot be used due to theclose of the main steam isolation valve or other reasons, the turbine-driven pump should be
operated by steam of the reactor to feed the cooling water to the reactor in order to remove
decay heat of the fuel and reduce pressure. When the water feeding system is out of order,
the system is used as auxiliary water feed pump to maintain water level of the reactor. Since
the source of power is steam generated by the reactor, this system cannot be operated
without a certain level of the reactor pressure. The source of water is either the condensate
storage tank or the suppression pool. Since steam is discharged to the suppression pool
after the start of the turbine, the temperature in the suppression chamber and the
suppression pool rises during the operation of this system. Therefore the operation of thissystem needs to be coordinated with the residual heat removal system to prevent thetemperature from rising.
Reactor mode switch
A switch for selecting the interlock according to the situation of the reactor. Modes include“Operation,” “Start-up,” “Shutdown” and “Refueling.”

17

Reactor pressure vessel

RPV

A vessel in which a reactor core, in-furnace structure, primary coolant and others arehoused and steam are generated by nuclear reaction of the fuel.
(Reactor) Subcritical
To stop the chain reaction of nuclear fission by totally inserting control rods, which is a
shutdown function at the time of the reactor scram. It is possible to shut down the reactor
safely by the subcritical operation.
Receiving circuit breaker
Device for isolating the troubled circuit when an accident occurs on the transmission
network.
Reference surface
Water is held to take a reference pressure for measuring the water level inside the reactor
pressure vessel.
Residual heat removal seawater system
RHRS
Cooling water of the residual heat removal system cools through a heat exchanger. Thissystem feeds seawater for cooling down the cooling water of the residual heat removal
system.
Residual heat removal system
RHR
A system for removing and cooling the decay heat and the sensible heat generated by thecore after the shutdown of the reactor. It is operated in the below-mentioned operation
modes according to the composition of valves: Shutdown cooling system, Low pressure
coolant injection system, Containment vessel spray system, Suppression pool coolingsystem and Spent fuel storage pool cooling system.
It’s often the case that the alternative water injection as a countermeasure against severe
accidents injects water into the reactor or the containment vessel with use of the piping of
the residual heat removal system.
Rupture disk
Quaquaversal metal board which is intended for preventing the air-tight devices such as thepressure vessel, rotating devices, piping and ducts from breaking because of excessive or
negative pressure. It is a safety device, which is to break at a designed pressure, to release
the abnormal pressure in the facilities by rupturing.
Safety relief valve

SRV

A valve which discharges steam to the suppression pool automatically or in a manualmanner from the control room for protection of the reactor pressure vessel when the reactor
pressure rises abnormally (discharged heat is cooled down and condensed in the
suppression pool), and additionally has function of ADS (Automatic Depressurization
System) of ECCS (Emergency Core Cooling System).
Scram
It refers to a prompt shutdown of the reactor by automatically inserting negative reactivity to
the reactor, when the signals of the detectors installed in the reactor exceed the limit of therange of operational conditions. Normally it is caused automatically by the safety devices of
the reactor. Setting items of the predetermined conditions for scram, which is called scram
conditions, include abnormal increase of output of the reactor, seismic acceleration andturbine trip. It is a generic term for emergency shutdowns.
Sea Water Heat Exchanger Building
Building housing pumps supplying seawater to heat exchangers of various types of fresh
water cooling system, heat exchangers and others.

18

Self-air set
One of the respiratory protection tools, which feeds air from a portable cylinder. It is used forpreventing the holder, who is in the atmosphere of high concentration of radioactive
materials, from inhaling radioactive materials in the air.
Cesium iodide
An inorganic compound whose composition formula is CsI. It is the metal halogen
compound which consists of cesium, which is an alkali metal, and iodine, which is a metal
halogen. For scientific purpose, it is used as an X-ray fluoric-magnifying pipe and as single
crystal for detecting gamma rays. It is also used in the simple radiation measuring device“Hakarukun.” Radioactive iodine generated in the reactor is emitted from the core to thecontainment vessel as Csl and most of it is absorbed in water.
Severe accident
An event, in which sufficient cooling of the reactor core and control of the reactivity cannot
be realized by the measures assumed by the assessment of the safety design, considerably
exceeding the design basis event and resulting in significant damages of the reactor core.
The significance of a severe event depends on the degree of damages or the degree of lossof integrity of the containment facility.
Shared auxiliary operational facilities
Shared spent fuel pool in the Fukushima Daiichi Nuclear Power Station and buildings
housing the emergency diesel generators for units 2 and4 of the Fukushima Daiichi Nuclear
Power Station.
ShroudA cylindrical structure housing fuel assemblies and control rods constituting the core part.
Shutdown cooling system
SHC
Shutdown cooling mode
Facility for removing decay heat by cooling the coolant (reactor water) with the use of pumps
and the heat exchanger after the shutdown of the reactor. It has capability to implement thecold shutdown. The pump flow is large and the heat exchanger is highly capable. (All units
other than Unit 1 in the Fukushima Daiichi Nuclear Power Station have the cooling system“Shutdown cooling mode” in RHR system.)Significant building with base isolation
A building constructed to set up the disaster countermeasure headquarters when natural
disasters such as earthquakes take place. It is made with the reinforced concrete structure
and base isolation and consists of meeting rooms, communication facilities, power supply
system, air-conditioners and others. It is designed so that the first break after the earthquake
can performed without any problems even when the earthquake of 7th seismic intensity.
Simulator training
Training on how to shut down safely a reactor by simulating transients and severe accidents
with the use of computers.
Skimmer level
Water level of skimmer surge tank. Supernatant fluid of the spent fuel pool flows over theskimmer weir and is guided to the skimmer surge tank. It is needed to maintain the suction
pressure of the pumps of the spent fuel pool cooling and purification system, and water
quality of the pool by removing the floating objects from the pool surface.
Special Law for Nuclear Emergency
An abbreviated name for Special Law of Emergency Preparedness for Nuclear Disaster.
Spectrum
It generally refers to an array of components laid out in quantitative order of somecharacteristics after breaking down an object of complex compound into components.

19

Spent fuel pit

SFP

A pool for storing fuel taken out from the reactor. Besides spent fuel, fuel taken out for theperiodical examination, neutron source, damaged fuel and others are stored there. Water
plays a role of biological shield and removes decay heat. Quality of water is maintained by
the spent fuel storage pool and filtering system.
Stack
Facilities intended to discharge and diffuse the radioactive gaseous waste into the air. The
discharge of radioactive gaseous waste is regulated by the stipulation of the law down to thepredetermined volume, and it is discharged from stacks into the atmosphere.
Stand-by gas treatment system

SGTS

A facility, which is one of the engineered safety features, to automatically shut down regularventilation systems and reduce the external emission of the radioactive iodine andradioactive material in particulate form while keeping the negative pressure inside thebuildings, when a radioactive leakage accident takes place.
Stand-by liquid control
SLC
A back-end system of the control rod drive system, which shuts down the reactor by feeding
sodium quatroborate solution which can absorb a lot of neutron when it is impossible toinsert control rods for some reason.
Standard earthquake ground motion (Ss)An earthquake ground motion employed in the seismic design of a nuclear power plant. It is
the earthquake ground motion which is properly assumed to have a probability to, thoughextremely rarely, occur during the period of the operation and a significant impact on thefacilities from the viewpoint of seismology and earthquake engineering considering
geological condition and structure and earthquake activity in the vicinity of the site. Bothhypocenter-specified and hypocenter-not-specified standard earthquake ground motions on
the free bed-rock surface in the site are to be prepared in horizontal and vertical directions
respectively. Safety functions of the aseismatically important facilities need to be maintainedin the face of seismic force of these standard earthquake ground motions Ss.
Station black out

SBO

A loss of AC power, which is a necessary source of power for the power plant, for somereasons. Source of AC power includes external power supplies and emergency diesel
generators.
Suppression Chamber, Suppression Pool

S/C, S/PThe systems, adopted only in Boiling Water Reactors (BWR), constantly hold water ofapproximately 4,000 m3 (in case of units 2 to 4 of The Fukushima Daini Nuclear Power
Station). In case that cooling water in the reactor pressure vessel is reduced by some
accident and the steam pressure is increased, these systems decrease the pressure inside
the pressure vessels by guiding the steam through the vent pipes to the pressure chamber
to cool it down. They are also utilized as a source of water for the emergency core coolingsystem.
SurveyTo explore the presence of radioactive material and measure its dose.
Switching Station
Staging station set up for transmitting electric power generated by electric power stations to
the power supply system and for bringing it from power system into the plant. Switching of
the power supply system is made by switches.
Temperature falling rate
Fall rate of temperature per hour.

20Thermocouple
A sensor for detecting the temperature difference. When two different types of metal areconnected, voltages according to the different temperature are generated at the twoconnecting points due to the different thermopower of each type of metal and an electric
current flows in a certain direction. This is a temperature sensor using a phenomenon(Seebeck effect) in which thermoelectromotive force arises because of the temperature
difference between the two connecting points of different types of metal.
Top of active fuel
TAF
Zero point of the fuel level instrument. It refers to the top of the pellet of the fuel assembly.
Torus room
A room which houses the doughnut-shaped tunnel (suppression chamber) containing water
source for the emergency core cooling system. Since the shape of this tunnel is called torus,
the room housing is called torus room. In the torus room piping other than the suppression
chamber are also installed. The torus room is placed at the lower part of the containment
vessel, surrounding it.
Total black out
A loss of both AC and DC power
Transient Phenomenon record Server
A sever which ceaselessly records data of main parameters of the plant operation,
automatically stores the data before and after the event when their values exceed or fall
below the manually or previously set ranges, and supports the event analysis afterwards.
TrenchA tunnel intended to lay cables between buildings.
Turbine Building Closed Cooling Water
TSW
System
A system for feeding seawater to the heat exchanger of the turbine auxiliary cooling water
system, which cools down turbine auxiliaries bearing, oil coolers, air conditioners and otherswith circulating fresh water.
Turbine building
T/B
A building for housing main turbines, generators, main condensers, reactor feed water
pumps, turbine auxiliaries and others.
Ultimate heat sink

UHS

A final disposal place which removes and releases the heat collected from the fuel (decayheat) and operating machines. Usually the heat is removed through heat exchangers anddisposed in the sea.
Vent line nap
Composition of system for vent.
Ventilation system of Primary Containment
Vessel= drywell cooler
Void
Air bubbles generated when boiling.
Water radiolysis
Water is decomposed by irradiating ionizing radiation into hydrogen and oxygen.
Zirconium-water reaction
Same as metal-water reaction. When zirconium used for the fuel cladding is heated, it
gradually reacts with the surrounding coolant, which is water, and becomes oxidized.
Oxidized zirconium cladding becomes brittle and generates hydrogen.
21

Table of Contents
Introduction .................................................................................................................... 1
Members of the Examination Committee...................................................................... 3
Deliberation Members of the Examination Committee on Accident at Fukushima
Daiichi Nuclear Power Station ................................................................................... 3
Report Study Members of the Examination Committee on Accident at Fukushima
Daiichi Nuclear Power Station ................................................................................... 4
Report Review Team of the Examination Committee on Accident at Fukushima
Daiichi Nuclear Power Station ................................................................................... 6
Abbreviations .............................................................................................................. 7
Glossary ....................................................................................................................... 9
Chapter 1 Purpose...................................................................................................1-1
Chapter 2 Fukushima Daiichi Nuclear Power Station Accident Development....2-1
2.1 Complete overview of Fukushima Daiichi Nuclear Power Station andaccident.....................................................................................................................2-1
2.1.1 Overview of Fukushima Daiicihi Nuclear Power Station .......................2-1
2.1.2 Overview of Great East Japan Earthquake and Tsunami......................2-3
2.1.3 Overview of accident due to Great East Japan Earthquake...................2-6
2.1.4 Impact of earthquake.............................................................................. 2-11
2.1.5 Impact of tsunami ...................................................................................2-14
2.2 Course of accident at Unit 1 .........................................................................2-18
2.2.1 Conditions from earthquake to arrival of tsunami................................2-18
2.2.2 Situation of nuclear reactor building from tsunami to hydrogen explosion
2-19
2.2.3 Condition of nuclear reactor building after hydrogen explosion ..........2-24
2.2.4 Main events after accident .....................................................................2-25
2.2.5 Status of spent fuel pool .........................................................................2-25
2.3 Course of accident at Unit 2 .........................................................................2-37
2.3.1 From occurrence of earthquake to arrival of tsunami...........................2-37
2.3.2 From arrival of tsunami to malfunctioning of pressure suppression
chamber...............................................................................................................2-37
2.3.3 Major events thereafter ..........................................................................2-42
2.3.4 Condition of spent fuel pool ....................................................................2-43
2.4 Course of accident at Unit 3 .........................................................................2-54
2.4.1 From occurrence of earthquake to arrival of tsunami...........................2-54
2.4.2 From arrival of tsunami to hydrogen explosion of the reactor building
2-54
2.4.3 After hydrogen gas explosion in the R/B................................................2-58
2.4.4 Major events thereafter ..........................................................................2-58
2.4.5 Condition of spent fuel pool ....................................................................2-59
2.5 Course of accident at Unit 4 .........................................................................2-69
2.6 Course of accident at Unit 5 .........................................................................2-74
2.7 Course of accident at Unit 6 .........................................................................2-78
2.8 Status of Dose dose Rate rate Around around Power power Stationstation
2-80
2.9 Evaluation of Core core Conditions conditions of Unit 1 to Unit 3 at the Timetime of Accident accident .......................................................................................2-81
2.9.1 Evaluation of Core core Conditions conditions of Unit 1 at tthe Time of

TOC-1

Accidentaccident.................................................................................................2-81
2.9.2 Evaluation of Core core Condition condition of Unit 2 at the Time of
Accidentaccident.................................................................................................2-82
2.9.3 Evaluation of Core core Conditions conditions of Unit 3 at the Time of
Accidentaccident.................................................................................................2-83
Chapter 3 Analysis of Causes for Accident Events and Extraction of Problems..3-1
3.1 Flow of Progress of Accident Events...............................................................3-1
3.2 Extraction of subject from accident event progress.......................................3-4
3.2.1 Cause analysis from accident event progress (event tree) ......................3-4
3.3 Arrangement of problems confirmed from a functional viewpoint .............3-15
3.3.1 Causal analysis of the accident in unit 1 to 3 and management of issues
3-15
3.3.1.1 Problems by function ........................................................................3-22
3.3.1.2 Problems related to common factors/failures ..................................3-28
3.4 Causal analysis of the accident in unit 4 and management of issues ........3-31
3.5 Management of the events in units 5 and 6.................................................3-34
3.6 Comparison of progress of accidents of Fukushima Daiichi Nuclear Power
Station with that of other stations ........................................................................3-38
3.7 Causal analysis .............................................................................................3-42
Chapter 4 Lessons Learned and Measures To Take ..............................................4-1
4.1 Countermeasures against natural hazards ...................................................4-1
4.2 Securing of power supply ................................................................................4-1
4.3 Measures against loss of heat sink systems ..................................................4-2
4.4 Measures against hydrogen leakage ..............................................................4-2
4.5 Preparation for emergencies (training/drills) ................................................4-3
4.6 Countermeasures against earthquakes/tsunamis (example measures) .......4-4
4.6.1 Assumption of earthquake/tsunami strength..........................................4-4
4.6.2 Protection of the site from tsunami..........................................................4-4
4.6.3 Protection of buildings..............................................................................4-5
4.7 Preparation for power supply (example measures) .......................................4-6
4.7.1 SBO and loss of DC power sources...........................................................4-6
4.8 Examples of measures addressing the heat sink loss ...................................4-8
4.8.1 Water Injection to the reactors.................................................................4-8
4.8.2 Loss of seawater cooling .........................................................................4-10
4.8.3 Containment vent ................................................................................... 4-11
4.9 Hydrogen measures ......................................................................................4-12
4.10 Emergency preparedness (Especially training)........................................4-13
4.10.1 Training................................................................................................4-13
4.10.2 Air conditioning and shield at Central control room..........................4-14
4.10.3 Measurement during accidents...........................................................4-14
4.10.4 Emergency Operation Facility ............................................................4-16
4.10.5 Radiation control/working control.......................................................4-17
4.10.6 Organization/command or order .........................................................4-19
4.10.7 Communication....................................................................................4-19
4.10.8 Environmental monitoring..................................................................4-20
4.10.9 Preparing disaster countermeasures (heavy equipment/rescue),emergency cooperative framework ....................................................................4-21
4.11 Security of interigty of the spent fuel .......................................................4-23
4.12 Summary of the countermeasures ............................................................4-23
TOC-2

Chapter 5 Timeline of the Accident to the Present................................................5-1
Chapter 6 Conclusion..............................................................................................6-1
Appendix-1
A Comparison of the Development of the Accident at the Fukushima
Daiichi Nuclear Power Station and at Other Stations (Detailed Version)......... App1-1
Appendix-2 Agenda for Future Review ............................................................. App2-1
Appendix-3 Comparison with Government Report / NRC Task Team Report App3-1
Appendix -4 MARK-I Containment................................................................... App4-1
Reference -1 Plant Overview ................................................................................ Ref-1

TOC-3

Chapter 1 Purpose
The accident at the Fukushima Daiichi Nuclear Power Station that happened due tothe major earthquake that took place on March 11, 2011, made us realize once again,
how severe the consequences can be in the case of an accident at a nuclear power
plant. It also destroyed the public’s trust in nuclear technology, which had been built
up till now by that industry. It is very important that we seriously consider why the
nuclear power industry could not prevent such an accident and ensure that nuclear
power plants are even safer so that serious accidents can be prevented.
Hence, we conducted an investigation by setting up the Fukushima Daiichi Nuclear
Power Station Accident Investigation Committee in the Japan Nuclear Technology
Institute. Our aim was to summarize the necessary countermeasures by gathering
the lessons learned from the Fukushima Daiichi Nuclear Power Station accident andfocusing all the efforts of the Japan nuclear power industry.
The investigation committee is formed of the Japan Nuclear Technology Institute,Tokyo Electric Power Corporation and manufacturers, and has proposed various
necessary countermeasures by drawing lessons from this disaster and keeping a
focus on the analysis of reasons. It has done so based on the information related toplant design and information accumulated from its experience operating power
plants.
Presently, work to recover from the accident is going on in Fukushima Daiichi
Nuclear Power Station, but at the same time, it is also necessary to executecountermeasures so as to ensure even more safety in the nuclear power plants
functioning within the country. Likewise, the adequate countermeasures should betaken for the halted power plants.
The scope of the investigation covers from the occurrence of the earthquake to thetsunami and up to the release of radioactive material. It was based on all theinformation related to the Fukushima Daiichi Nuclear Power Station accident
published by Tokyo Electric Power Corporation. The main purpose of theinvestigation was to analyze the reasons for the accident and investigate
countermeasures with a view to ensuring policies to prevent accidents due to thetsunami, preventing such accident from escalating, and reducing their effects.
Based on the facts related to the development of the accident the analysis of themain cause for widespread of the accident was analyzed according to the event tree
and topics were picked up. Moreover, the reason why the safety system could notwork was organized in matrix format from the [stop], [cool] function screens ensuringsafety and topics were picked up in such a way that no topic is missed.
Based on all these topics, lessons were extracted and a variety of concrete
examples of countermeasures summarized.
Further, we are continuing to make investigations whenever we find new topics andcontinue to design countermeasures by giving importance to issues that should betackled by the industrial world. We also decided that in the case of issues which need
to be analyzed with the government or self -governing bodies, such as Bousai, we
will continue investigations regarding how to deal with them as an enterprise.
Hereafter, we will keep proposing additional countermeasures so as to improvesafety after conducting investigations and revising this report; if we identify new facts
and find important topics.

1-1

Chapter 2 Fukushima Daiichi Nuclear Power Station Accident
Development
2.1 Complete overview of Fukushima Daiichi Nuclear Power Station
and accident
2.1.1

Overview of Fukushima Daiicihi Nuclear Power Station

Fukushima Daiichi is located in the north-east of Japan (latitude 37° North,longitude 141° East), It is located in Okuma and Futaba City of Fukushima
Prefecture, which is approximately 225 km to the north of Tokyo and is facing the
Pacific Ocean. (Population of Okuma city: approximately 11,500, Futaba City:
approximately 6,900.)

Fukushima
DaiichiNuclearPower
StationThe site area of the power plant is approximately 3,500,000 m2, in a semicircularshape stretching 1.5 km east-west and 3 km north-south. It is 35 m above sea level
and is leveled at a height of approximately 10 to 13 m facing a flat beach andconsists of 6 boiling water reactors (hereafter called BWR).
The arrangement of power plant premises is as shown below, units 1 to 4 are locatedin Okuma City and Unit 5 and Unit 6 are in Futaba City. The main office is located onelevated ground.

Unit 6
Unit 5

Unit 1
Unit 2

Unit 3
Unit 42-1

Each unit of the power plant units started operating in 1970 and the total power
output is 4,696 MW. The salient features of each unit are described below.
Electric
Unit

power

Operation start

Reactor modelday

Container model

Reactor supplier

(MW)
1

460

March 26, 1971

BWR-3

Mark I

GE

2

784

July 18, 1974

BWR-4

Mark I

GE, Toshiba

3784

March 27, 1976

BWR-4

Mark I

Toshiba4784

October 12, 1978

BWR-4

Mark I

Hitachi

5784

April 18, 1978

BWR-4

Mark I

Toshiba61100

October 24, 1979

BWR-5

Mark II

GE, Toshiba2-2

2.1.2

Overview of Great East Japan Earthquake and Tsunami

The earthquake took place at around 2:46 p.m. on March 11, 2011 with theepicenter located approximately 130 km to the east -south -east of Sanriku,
Oshika Peninsula (latitude 38.1°N, longitude 142.9 °N)• Scale: Moment Magnitude Mw 9.0• Depth of epicenter: 24 km• Aftershocks: 6 with a magnitude of more than M7.0, 93 with a magnitude ofmore than M6.0 (announcement on September 8 by meteorologicalagency).
• Maximum slippage: Approximately 30 m
• Rupture: Length approximately 450 km, width approximately 150 km• Time of continuous destruction: Approximately 170 seconds
Earthquake of the coast of Sanriku at around 2.46 pm on March 11, 2011
Earthquake Distribution Diagram

LegendMagnitude 7
Magnitude
Higher 5 level
Magnitude
lower 5 level
Magnitude
Higher 5 level
Magnitude
lower 5 level
Magnitude 4
Magnitude 3
Magnitude 2

Epicenter

Magnitude 1

Source: Meteorological Agency(About Sanriku earthquake which occurred at 14:46 on March 11, 2011)

2-3

Seismic source time function

Seconds
Destruction starting point of this earthquake
Epicenters of the earthquakes of magnitude 7
or greater after March 9
Epicenters of the earthquakes of magnitude 5
or greater within a day after the occurrence of
this earthquake
Center points of each small fault
Observation points used in the analysis

Amount of slip (m)Contour spacing is 4m

Source: Meteorological Agency (March 2011, earthquake, volcano monthly report)

It is presumed that the tsunami occurred when the sea floor rose roughly above
the epicenter by approximately 3 m. The maximum height was about 35 m to thenorth of Miyako City. Also, the height of flood to the north of Miyako City has
exceeded 25 m. The flood area has become 58 km2 in Iwate Prefecture, 327 km2
in Miyagi Prefecture, 112 km2 in Fukushima Prefecture and 23 km2 in Ibaraki
Prefecture.

2-4

[Legend]

Traces of tsunami

Tohoku_Flooded area
AomoriPrefecture

Tohoku_Run-up height

Hachinohe

Miyako

Iwate
Prefecture
Rikuzentakada

Sendai

MiyagiPrefecture

Fukushima
Prefecture

IbarakiPrefecture

Flooded area and run-up
Sea embankment
Flooding

Run-upHeight of tide when thetsunami hit the shore
Averagesea-level
Tohoku (T.P.)

in

Flooding: Height from the tide level when the tsunami reached the shore till traces of tsunami
Run-up: Height from tide level till the tsunami ran up

Source: Excerpts from First Special Investigation Committee Meeting related to earthquake and tsunami
study of Great East Japan Earthquake

As of September 26, the damage due to the earthquake and tsunami is as
serious as 15,811 deaths, with 4,305 people missing, and complete destructionof 117,542 buildings and partial destruction of 177,192 buildings.
2-5

2.1.3

Overview of accident due to Great East Japan Earthquake

When the Great East Japan Earthquake occurred on March 11, Units 1, 2 and 3of the Fukushima Daiichi Nuclear Power Station were in operation, whereas
units 4, 5 and 6 were not in operation; the spent fuel pool in Unit 4 was in a used
up condition (hereafter termed as SFP) as its shroud was being replaced.
When the earthquake of magnitude 9 occurred at 14:46 on March 11, Units 1, 2
and 3, which were in operation, automatically stopped due to a [PeakEarthquake Acceleration] signal. The outside electric power supply wascompletely lost as the outside electric power supply facilities, circuit breakers,
cables, steel power lines and such like got damaged or were destroyed due tothis earthquake. Hence, all the units except Unit 4, which was undergoing aperiodic inspection, automatically started to use a diesel engine generator(hereafter called “emergency use diesel generator”) and the cooling functionalityof the nuclear reactor as well as SFP was maintained.
After that, all the emergency diesel generators, except the emergency DG 1 of
the air cooling system of Unit 6, stopped operating as the emergency diesel
generators, seawater pump and power supply equipment were all submerged inwater due to a large tsunami. AC power supply to Units 1 to 5 was completely
lost.
This large tsunami swept away heavy machines, tanks and such like in the flood
water within the power plant, and a great deal of rubble was scattered on theroads; moreover, it became completely dark inside the premises, almost all themeans of communication had stopped. In this situation, two workers who wereconducting post-earthquake investigations in the turbine building (hereafter
called “T/B”) of Unit 4, were lost and later confirmed to be dead.
In Fukushima Daiichi Nuclear Power Station, at 15:42 on March 11, the Station
concluded that the situation corresponded to a particular phenomenon (complete
loss of AC power supply) as per Article 10 of the Special Law of Emergency
Preparedness for Nuclear Disaster (hereafter called “Special Law for Nuclear
Emergency”) and the Station announce it to the national and local government
bodies. Also, the power supply to measurement displays was also lost andhence it was not possible to determine the water level of the nuclear reactor or
the flood conditions. Hence, the Station concluded that the situation
corresponded to a particular phenomenon in which emergency core coolingequipment cannot inject water and announced this to the national and local
government bodies at 16:45.
The summary of how the accident spread for all units is mentioned below.
After automatic shutdown of Unit 1, on March 11 at 14:47, the main steam
isolation valve (MSIV) (hereafter called “MSIV”) stopped due to a loss of external
power supply, the pressure of reactor pressure vessel (hereafter called “RPV”)
rose, the emergency condenser (hereafter called “IC”) automatically started at
14:52. As per the operation manual, workers performed a control operation byrepeating the manual operations (isolation valve open-close) of IC, so that therate of decrease in RPV temperature did not increase above 55°C/h.
After this, all the AC power was lost and simultaneously the DC power was also
lost due to a large tsunami that came at 15:37 on March 11. Hence, it was notpossible to pump out the water in the nuclear reactor or confirm the status of theparameters. Hence, workers started preparing to use alternative water pumping
methods. When the water level was restored at 21:19 on March 11, due to the
2-6temporary recovery of the electricity supply, it was confirmed that the water level
was above the available fuel level (hereafter called “TAF”). The radiation level of
T/B rose around 11:00 on March 11. At 0:06 on March 12, workers were
instructed to prepare for PCV venting by the nuclear power plant manager, as
there was a possibility that the PCV pressure had exceeded the maximum
allowed pressure. At around 5:46 on March 12, alternative pumping of water
started. At 7:11 on March 12, the prime minister visited the power plant to makeobservations and returned to Tokyo at 8:04. The evacuation of residents of
Okuma City (Kumachi-ku) was confirmed at 9:03 on March 12. At around 9:15,
25% of the monitor valve (hereafter called “MO valve”), which is one of the twovalves in series in the PCV vent line, opened manually; after that we went
towards the premises to open the other air operation valve (hereafter called “AO
valve”), but it was not possible to do so due to high level of radiation. When thevalves were opened from the main control room (hereafter called “MCR”), it
could not produce a sufficient result and hence, a temporary air compressor was
set up and the AO valve was opened. Thus, the PCV pressure reduced ataround 14:30. After this, at around 15:36 on March 12, there was a hydrogen
explosion (hereafter called “hydrogen explosion”) in the upper part of the nuclear
reactor building; the roof and outer walls were damaged. The injection ofseawater into the nuclear reactor started at around 19:04 on March 12.
In the case of Unit 2, after it automatically shut down at 14:47 on March 11, MSIV
stopped and the RPV pressure rose due to a loss of external power supply, but
the pressure was controlled due to main steam safety relief valve (hereafter
called “SRV”). The nuclear reactor water level control was done by the reactor
core isolation cooling system (hereafter called “RCIC”) and after repeated
operations of manual start by the workers and auto -stop according to [nuclear
reactor water level], once again RCIC was started manually at 15:39 on March
11. Immediately after this, at 15:41, the AC power was lost completely and at thesame time, the DC power was also lost; hence, it was not possible to confirm thestatus of the water injection into the nuclear reactor or that of the parameters..
When the nuclear reactor water level was restored due to the temporary
recovery of a power supply at 21:50, it was confirmed that the nuclear reactor
water level had been maintained. The workers confirmed that the RCIC wasfunctional when they checked the field instrument at around 2:00 on March 12.
The power supply recovery operation of the alternate water injection system wascontinued, but it was interrupted as the power supply cables were damaged dueto the hydrogen explosion that occurred in Unit 1 at 15:36 on March 12.
Moreover, the workers continued to prepare to inject seawater and they had laida hose or fire engine, but it was damaged due to the hydrogen explosion in Unit
3 which occurred at 11:01 on March 14 and hence became unusable. Moreover,preparation work for a PCV vent, which was also in progress at the same time,
was significantly affected by the explosion.
On March 14, at around 13:25, the Station concluded that the situation in the
nuclear reactor was as per Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster, due to a possibility that the RCIC had
stopped due to the reduced water level. When radiation caused by the explosionin Unit 3 was spreading, the injection of seawater and preparation work for a
PCV vent were started again. On March 14, at around 18:00, the pressure in the
nuclear reactor began to be reduced due to SRV and at around 19:54 on March
14, the seawater injection started with a fire engine. The dry well (hereafter
2-7

called “D/W”) pressure exceeded the maximum operating pressure at 22:50 on
March 14. At around 6:00 on March 15, there was a sound of an impact andalmost at the same time, the pressure inside the pressure control room(hereafter called “S/C”) fell below 0 MPa [abs].The pressure of D/W also fell
below 155 KPa [abs] at 11:25 on March 15.
After automatic shutdown of Unit 3 at 14:48 on March 11, MSIV stopped due tothe loss of external power supply, the RPV pressure rose but the pressure wascontrolled by the SRV. At 15:05 on March 11, the RCIC was started manually tocontrol the nuclear reactor water level and the RCIC was stopped after that,
corresponding to an indication [reactor high water level] which was due to a risein the nuclear reactor water level at 15:25.
After that, the AC power supply was lost completely at 15:38 on March 11, due toa large tsunami. However, the RCIC and high pressure injection system(hereafter called “HPCI”) operation was possible as the flood was avoided due tothe DC power supply. The operation was continued by restarting the RCIC at
16:03 on March 11, but it automatically stopped at 11:36 on March 12. This
resulted in a lower water level, HPCI started automatically at 12:35,
corresponding to [reactor low water level] indication and recovered the previous
water level. However, at 2:42 on March 13, the HPCI also stopped and RCIC
also could not be restarted. Due to this, the Station concluded at 5:10 at March
13 that the situation corresponded to Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster. At 9:08 on March 13, the SRV
was manually started using an automobile battery, the nuclear reactor pressure
was lowered and injection of fresh water containing acid started at around 9:25.
After this, as the fresh water supply was exhausted, the workers switched toseawater at 13:12 on March 13.
On one side, the work to prepare the PCV vent line was also in progress at thesame time, and both the valves (MO valve and AO valve) of the PCV vent line
were opened at 8:41 on March 13. At 9:24 on March 13, the workers noticed thatthe pressure of the D/W had fallen. After this, the AO valve of the vent line closed
due to a reduction in the pressure of the compressed gas cylinder, hence thisvalve was opened by replacing the cylinder and setting up a temporarycompressor. At around 11:01 on March 14, an explosion that was assumed to bea hydrogen explosion occurred at the upper part of the nuclear reactor building
and the roof and walls were damaged.
The injection of seawater was temporary interrupted as the fire engine and hosewhich were being used for the injection of seawater was damaged by this impact.
After this, the hose was drawn out from the pier and the injection of seawater
was started again by the fire engine at around 16:30 on March 14.
Unit 4 was undergoing a periodic inspection and all fuel assemblies had been
removed from the reactor to the spent fuel pool due to the shroud replacing work.
Therefore, fuel with relatively high decay heat for one full core was stored in the
SFP. A total of 1,535 pieces of spent fuel assemblies were stored there, whichamounted to 97% of its storage capacity of 1,590 pieces. Due to the loss of theexternal power supply and subsequent loss of AC power supply, the electricmotor operated pump and seawater system pump became nonfunctional andthe cooling function of the SFP and make-up water function was lost. At 4:08 on
March 14, the SFP water temperature rose to 84°C.At around 6:00 on March 15,
an explosion that was assumed to be a hydrogen explosion occurred in thereactor building, and it was confirmed that the upper part of the building
2-8

collapsed. Furthermore, at 9:38 on March 15, a fire was identified in thenorthwest part of the third floor of the reactor building.
On March 16, when an SFP inspection was done by the Self-Defense Forces
helicopter, they stated that the fuel was not exposed by observing the water
surface. Water was injected in the SFP by the Self-Defense Forces on March 20
and by a U.S. armed forces high-pressure water truck on March 21. Moreover,the injection of water was carried out by a concrete pump motor after March 22.
The cause of the hydrogen explosion which occurred in Unit 4 was not clearly
identified initially. Later, it was assumed that it must have been because the fuelin the SFP was exposed because of the low water level in the SFP due toevaporation and hence, the water from the nuclear reactor side flowed into the
SFP through the pool gateway. Moreover, it could be considered after analyzingof nuclides from the water that there was no damage to the fuel rods and hence,
it was thought that the main cause of the hydrogen explosion was not the fuelwithin the SFP, but one possible reason was the hydrogen gas, which was
released by the PCV vent line of Unit 3 and flowed through the exhaust duct.
Unit 5 was undergoing a periodic inspection, and RPV pressure leakage tests
were being conducted with fuel loaded in the reactor. There was enough water inthe RPV and all the control rods were inserted.
Due to the loss of external power supply and large tsunami after that, the AC
power supply was also completely lost, but the flood water was prevented fromentering the building because of the DC power supply and the plant could be
used. The nuclear reactor pressure rose due to decay heat, but because theSRV (safety valve function) was opened and closed from around 1:40 on March
12, the pressure dropped below the maximum operating pressure. At 6:06 on
March 12, the upper vent valve of the RPV was manually opened from thecontrol room and the pressure was lowered below the atmospheric pressurelevel, in order to carry out an alternative injection of water in the nuclear reactor.
At 20:54 on March 13, a temporary cable was laid from Unit 6 to the condensewater makeup system (hereafter called “MUWC”) and the MUWC pump wasstarted manually. Due to the effect of decay heat, the pressure within the nuclear
reactor rose once again. To combat this, the control was recovered for the SRV
system, which had been made nonfunctional in order to perform the Leak andHydrostatic test, and after 5:00 on March 14, the pressure of RPV was loweredintermittently. After 4:30 on March 14, the alternative water injection to the
nuclear reactor was started by MUWC and after that the water level of thenuclear reactor was checked by performing water injection intermittently.
Moreover, the supply of water to the SFP was started at 9:27. At 1:55 on March
19, a temporary RHR seawater pump was started and after that, the RHR pump
was manually started at around 5:00 on March 19, and SFP cooling was started.
At 12:25 on March 20, the cooling of nuclear reactor was started by changing theoperation mode of the RHR system and it was stopped at 14:30 on March 20.
Unit 6 was undergoing a periodic inspection and the nuclear reactor and was incold shutdown condition (all control rods (CR) were in an inserted state) with thefuel being loaded.
Due to the loss of external power supply and subsequent large tsunami, twoemergency DGs (seawater type) halted and the remaining one emergency DG(air cooled type) could supply electricity via an emergency generating line
because the flood water did not enter and the power supply was also available.
2-9

At 13:20 on March 13, alternative injection of water to the nuclear reactor wasstarted by manually starting the MUWC pump at 13:01. After that, the injection ofwater to the nuclear reactor was continued intermittently. Then, water to the SFP
was continuously supplied from 14:13 on March 14.
In response to the rise in pressure of the nuclear reactor due to the effect of
decay heat, SRV was manually opened from the control room and the reactor
pressure was lowered continuously . At 19:07 on March 18, the Station
confirmed that the emergency DG seawater pump (6A), which had been flooded
by the tsunami, was in good condition and hence the same seawater pump wasstarted and at the emergency DG (6A) which had stopped at 4:22, was started.
At 21:26 on March 19, the temporary RHR seawater pump was started and at
22:14 on March 19, the RHR pump was started and the SFP cooling was started.
At 18:48 on March 20, the operation mode of RHR system was switched andcooling of nuclear reactor was started and it was stopped at 19:27 on March 20.

2-10

2.1.4

Impact of earthquake

On March 11, 2011, an earthquake occurred at 14:46, with the epicenter located
off the coast of Miyagi Prefecture
The epicenter depth was 24 km and the moment magnitude was 9.0, the rupture
length was 450 km and the rupture width was approximately 150 km (all values
estimated). It is believed that it occurred due to the destruction of plate edges
because of an accumulation of strain at the plate boundaries. The rupture length
increased as multiple epicenters were joined together.
As shown in Figure 2.1-1, the peak acceleration values obtained at the lowest
basement of the nuclear reactor buildings at the time of this earthquake
exceeded the values specified in one section of the table corresponding to basic
earthquake ground motion Ss.
The acceleration time history waveform and response spectrum observed for thebasic nuclear reactor Unit 2, is displayed as an example in Figure 2.1-1. In
general, the response spectrum exceeds the basic earthquake ground motion
similar to what is shown by a specific period in this record.
Due to this earthquake, power supply was taken from the emergency DG,
seawater system or IC, RCIC and such like were functioning and the engineered
safety functions of the emergency core cooling system (hereafter called “ECCS”)(including seawater system) were in good condition till the tsunami arrived. Also,it was confirmed that the normal make-up water system (MUWC) and diesel
driven fire protection pump (hereafter called “D/D-FP”) of Unit 1 and 3 wereoperational even after the earthquake.
Based on indications from the country, Tokyo Electric Power Corporation
conducted earthquake response analysis which was based on observations
related to nuclear reactor buildings, instruments important to earthquake safety
and piping system, for Unit 2 and 4 on June 17 and that for Unit 1 and 3 on July
28. It was concluded that all the instruments important to earthquake safety had
maintained their safety functionalities before and after the earthquake.
Before the earthquake, the external power supply for Unit 1 and 2 was from
Okuma line 1 and line 2 (275 kV) and Okuma line 3 and 4 (275 kV) for Unit 3 and
4 (however, the start-stop equipment of Okuma line 3 was under construction);for Unit 5 and 6, the Yonomori line 1 and 2 (66 kV) was connected to the new
Fukushima transformer substation. Other than this, the TEPCO nuclear powerline (66 kV) from the north-east Tomioka transformer substation was connected
as a spare line to Unit 1.
Due to this earthquake, the circuit breakers in the switchyard of Unit 1 and 2
were damaged; the cables of the TEPCO nuclear line from the north-east power
supply also got damaged. One power line connecting to the switchyard of Unit 5
and 6 was also destroyed. As a result, all the external power supply to units 1 toUnit 6 was lost.
Moreover, from a comparison between the peculiar period of steel tower (0.3 to 1sec) and the earthquake motion acceleration response spectrum (peak response
acceleration 2,000 gal) and from the status of destruction, it is assessed that thesteel tower was not destroyed due to the earthquake motion but collapsed
because the level of the soil fell.

2-11

Table 2.1-1 Maximum acceleration from basement of reactor buildings
Observed data
Observation point(basement of reactor
buildings)
Unit 1

Maximum response acceleration
against basic earthquake groundmotion Ss (Gal)
North-South East-West Vertical
Direction
Direction
Direction
487
489
412

Unit 2

348

550

302

441

438

420

Unit 3

322

507

231

449

441

429

Unit 4

281

319

200

447

445

422

Unit 5

311

548

256

452

452

427

Unit 6

298

444

244

445

448

415

Speed of acceleration (Gal)

Speed of acceleration (Gal)Fukushima
DaiichiMaximum response acceleration(Gal)
North-South East-West
VerticalDirection
Direction
Direction
460
447
258

Observed data
Basic earthquake groundmotion Ss-1
Basic earthquake groundmotion Ss-2
Basic earthquake groundmotion Ss-3

Time (s)Cycle (s)(Note: The recording equipment was interrupted due to the earthquake)
Figure 2.1-1 Ripple figure of instant acceleration history of reactor building andresponse spectrum (east-west direction)

2-12

Shin-Fukushima
Under
construction

Toden
Genshiryoku
line
power
receivingfacility

Unit 1/2
switchyard

Yonomori line 4 L

Receiving circuit
breaker anddisconnector
damageUnderconstruction

Unit 3/4
switchyard

Yonomori line 2 L

Collapse

Yonomori line 1 L

Receiving
circuit breakerdamageIncoming line
structure tilted

Yonomori line 3 L(Tohoku electric power)
66kV Toden Gensiryoku
line

Yonomori line 2 L

Yonomori line 1 L

overhead earth-wiredisconnected

Yonomori line
#27 tower collapse
Under
construction
66kVswithyard

Cable
troubleInoperable as it is filledwith water due to Tsunami
Common highvoltage switchboard(M/C)

Common highvoltage switchboard(M/C)

Emergency highvoltage switchboard(M/C)

Emergency highvoltage switchboard(M/C)

Common high voltage
switchboard(M/C)
Emergency highvoltage switchboard(M/C)

Emergency highvoltageswitchboard(M/C)

Common high voltage
switchboard(M/C)
Emergency highvoltage switchboard(M/C)

Emergency highvoltage switchboard(M/C)

Under inspection/
construction

Emergency DG or other related units
are inoperable as they are filled withwater due to Tsumami

Fulishima Daiichi Nuclear Power Station
Shutdown because it is under inspection/
construction
Water spray or submerged due to Tsunami
Trip (locations where transmission of electric poweris stopped

Figure 2.1-2 Outline figure of external power supply control system (after earthquake,
before tsunami)

2-132.1.5

Impact of tsunami

Due to the earthquake, at 15:27, the first tsunami came from the ocean which ison the front side of the reactor site and the second tsunami appeared at 15:35.
Among these, the second was higher than the reactor height and almost thewhole area of the main building of the site was flooded with water.
Based on the tsunami measurement methods of the Society of Construction
Engineers, the design as revised in 2002 was for a height of tsunami of about
O.P. +5.7 m (Fukushima Daiichi Unit 6). In contrast to this, the height of thetsunami which actually came was, O.P. approximately +11.5 to approximately+15.5 (depth of flood water approximately 1.5 to approximately 5.5.m) from themain building setup area (Fukushima Daiichi 1 to 4 unit side, site height O.P. +10m) and O.P. approximately +13 to approximately +14.5 m (depth of flood water
approximately below 1.5 m) from the main building setup area (Fukushima
Daiichi 5 to 6 unit side, site height O.P. +13 m) (without including changes to theground due to the earthquake).
The height of the tsunami was much larger than the tsunami heightmeasurements that had been made so far, because of the activity of multiple
territories which until now used to be thought of as single territory activity andhence a major earthquake of magnitude 9.0 occurred. The length of thedislocation was 400 km and width was 200 km, which was something that had
never been seen before, and the slippage of the dislocation was extremely large
at more than 20 m, which was something that had never been considered.
It is thought that the surrounding area of the main building. which is on a site of
O.P. +10 m and O.P. +13 m, was almost flooded by the tsunami. However, nosignificant damage was confirmed to the construction of the outer wall or pillarsof the main building. However, mainly considering the east side (sea side) of theT/B of Unit 1 to 4, parts of the door or shutter and such like installed at theentrance, were damaged by the tsunami. Parts of the entrance above the groundof the main building (entrance and exit of the main building and the entrance formoving the instruments (hatch), exhaust outlet (louver) or entrance connecting
to the trench or ducts laid underground on the site (the penetration entrance forcables or plumbing) came in the path of the flood water inside the building andthe flood water flowed through the passages or the stairs and flooded a large
underground area.
A large number of emergency DGs and switchboards, which were installed in thebasement (height 0 m to 5.8 m) of the reactor building or T/B, were damaged by
the tsunami, all the units except Unit 6 lost the emergency electric supply. Out of
the three emergency DGs in Unit 6, the DG installed in the reactor building did
not stop and could receive an emergency electric power supply. The main DC
busbar of units 1, 2, and 4 was under the flood water and that of 3, 5, 6 was notunder the flood water. The extent of flooding of the emergency electric supply,
emergency diesel power generation device and DC busbar and the impact is
described in Table 2.1-2.
All the auxiliary cooling seawater pumps were also flooded by the tsunami.
However, except the pumps which had been taken out for inspection, all thepumps were present at the installation location even after tsunami and the pumpbody was not washed away. Destruction of the crane used for equipment
installation, damage to pumps or ancillary equipment when they collided with

2-14

flotsam and the mixing of seawater in electric motor bearing lubricating oil was
identified.
It seems that the important safety instruments must also have been damaged
due to the seawater flood within the reactor building, but because of constraints
due to high radiation, investigation was not done. It was confirmed that during
this the MUWC pumps of Units 1 to 4 were flooded and even after temporarily
recovering the electricity, their functions could not be restored.

2-15

Table 2.1-2 Impact on in -power source system in the plant due to flood water oftsunami

2-16

Sea
area

Assumedhighest waterlevel of tsunami
O.P. + 5.7m

Site height O.P.+

side

Measurescompletedto 5.7m

Mainbuilding
installation area
Flooding height O.P + 11m – 15m

Reactor buildingup
Turbine building

Sea water
pump
Site height O.P.+10m(Units 1-4)

Base level
O.P. 0 m
Breakwater* Site height of Units 5-6: O.P.+13m

Water intake

Figure 2.1-3 Situation of tsunami

Figure 2.1-4 Extent of flooding within the power plant2-17

2.2 Course of accident at Unit 1
2.2.1

Conditions from earthquake to arrival of tsunami

While Unit 1 was operating at a fixed rated electric output (460 MWe), it
automatically stopped in response to the signal [Peak Earthquake Acceleration]
which was due to the Great East Japan Earthquake which occurred at 14:46 on
March 11, 2011 and all the control rods (CR) were inserted.
Due to the impact of the earthquake, the circuit breaker of Okuma line 1L, 2L
was damaged and hence the outside power supply was completely lost andelectricity from the emergency busbar was also lost. Hence, immediately two
emergency DGs (1A, 1B) started automatically and electricity to the emergency
busbar was recovered.
At 14:47 on March 11, due to the loss of electricity to the emergency busbar, theelectricity was lost for the reactor protection system and hence, MSIV was
shutdown automatically by a fail -safe system. The nuclear reactor pressure rosedue to the MSIC shutdown and at 14:52 on March 11, the 2 ICs started
automatically in response to the signal [Reactor Pressure High (7.13 MPa[gage]). The reduction of pressure and cooling of nuclear reactor was started
and the nuclear reactor pressure suddenly started declining.
The nuclear reactor water level became very low for a moment, due to asmashing of a void immediately after the automatic shutdown of nuclear reactor;
however, the feed water to the nuclear reactor continued till the nuclear reactor
feed water pump was tripped after the loss of external electricity. The water level
did not reach a low level when the HPCI started automatically and was restored
and changed to an almost normal water level.
The workers concluded that the decrease in nuclear reactor pressure that
occurred when the IC started was fast and the rate of decrease in temperature of
the nuclear reactor cooling material was not observed to be 55°C/h, as decided
by the operation manual. Hence, at 15:03, March 11, for regulating of nuclear
reactor pressure, 2 IC devices were manually stopped (IC piping isolation
reverse valve MO-3A,3B [close] operation). Thereafter, startup and shutdown of
IC series 1 (A) started (IC piping isolation reverse valve MO-3A [start], [close]
operation) was repeated and nuclear reactor pressure was regulated toapproximately 6 to 7 MPa [gage].
On one side, MSIV stopped automatically; hence, in order to execute cooling of
S/C, the workers manually started the containment cooling system (hereafter
called “CCS”) and thus started cooling the S/C from 15:07 to 15:10 on March 11.
As mentioned above, the normal loss of external power supply meant that
support after scram operation of reactor was carried out during the time from theoccurrence of the earthquake to the arrival of the tsunami.

2-18

2.2.2Situation of nuclear reactor building from tsunami to hydrogen
explosion

On March 11, approximately 51 minutes after the earthquake, at 15:37, due tothe tsunami, power connection to the seawater cooling pump flooded. As a resultof it being submerged, the two emergency DGs stopped and resulted in acomplete power loss. As a result, the Station decided that the situation
corresponded to Special Law for Nuclear Emergency Article 10 (Complete lossof AC power supply).
Accompanying this Complete Loss of AC Power supply, while the MCR
illumination, signal light had gone out gradually, a warning sound also ceased.
The illumination of Unit 1 was emergency lights only. Unit 2 was in complete
darkness.
The equipment operating on DC power supply were IC and HPCI. But after
examining the situation, the workers were unsure whether the IC had an open or
closed valve and the HPCI also had its indicator lit up and did not start and wasturned off on the control panel.
As the power supply of the instrument had been lost, the reactor water level had
been uncertain since 15:30 on March 11. At 16:36 on March 11 , as the flood
water situation in the reactor was unknown, the Station decided that the situation
corresponded to Article 15 of the Act on SSpecial Law of Emergency
Preparedness for Nuclear Disaster (Non-emergency core cooling systempriming). After that, at 16:45 on March 11, the situation recovered from theSpecial Law for Nuclear Emergency Article 15 as the reactor water was restored,
but at 17:07 on March 11 the Station decided that the situation corresponded to
Article 15 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster as the reactor water level was unknown.
At 17:12 on March 11 , to securely inject water to the reactor, the power plant
manager (Power Station Emergency Division) instructed the workers toinvestigate the extinguish fire system (hereafter called “FP”), MUWC, CCS andalternate injection at a fire engine as one of the accident management
measures.
Then, simultaneously with the alternate injection, an open process investigation
started at the containment (called “PCV”) vent because there was no power.
The support status of PCV vent and alternate injection is separately described
below.
Support status of alternative injection
At Headquarters of Power plant emergency response (hereafter called“headquarters for the power plant”), it was decided to use D/D-FP, instead of FP
line using core spray system (hereafter called “CS”) as an alternative toalternate injection. At 17:30 on March 11 D/D-FP was started.
Due to the loss of power on valve operation of the alternate injection line, thetasks to be performed during the operation could not be performed. Because ofthis, the nuclear reactor building (hereafter called “R/B”) was in total darkness,and the CS and other valves were opened manually. As there was a possibility

2-19

that the reactor had been flooded after depressuring (below 0.69 MPag), time
was required to perform difficult tasks.
The power plant restoration team from the headquarters investigated the degree
to which the power panel (high voltage power supply board and power center)had been submerged and damaged, and applied insulation resistance material.
The power panel of Unit 1 was found to be out of service, one part (P/C-2C,
P/C-2D) of the power center of the adjoining Unit 2 was confirmed to beavailable. Hence, workers investigated the possibility of restoring power by using
the Unit 2 power center so as to make it possible to use the high pressure
injection system (hereinafter called SLC).
At 17:00 on March 11, electricity distribution Tokyo head office instructed all its
branches to secure high and low voltage power cars and confirm the rounting toFukushima Daiichi and requested Tohoku Electric Power to dispatch a highvoltage power supply vehicle. Although power supply cars of all bases headed
toward Fukushima, due to the damaged roads and congestion they were notadvancing as expected. Apart from that, air transport of power supply vehicles
from the self -defense force, and the US armed forces was investigated but theidea was abandoned as the vehicles were too heavy.
As the temporary DC power supply system to the IC was restored, the supply
line isolation valve of IC (A system) MO-2A and return piping isolation valve
MO-3A was indicated as being in closed by the indicator lamp. Hence at 18:18
on March 11, the MO-2A valve and MO-3A valve of IC (A system) were opened
and it was confirmed that steam was coming out from the IC system ventpipe(condenser steam emission pipe). After that, at 18:25 the piping isolation
valve MO-3A was closed.
Meanwhile, the condition of 4 isolation valves (MO-1A, MO-1B, MO-4A, MO-4B)
between IC (B System) and PCV was uncertain as the electric power to thedisplay lamp had been lost.
In the central operating room, due to the loss of electrical power, the indicatedvalue of the monitoring instruments was unclear, and so the worker entered thereactor building (R/B), which was in total darkness, to examine the pressuregauge. As a result, at 20:07 on March 11, on the atomic pressure gauge thepressure was found to be 6.9 MPa [gage].
To restore the lighting and monitoring instruments in the central operating room,
the headquarters of the power plant restoration team accumulated thenecessary batteries and cables. On At 20:49 on March 11, temporary lighting
was used in the central operating room using a small generator. Also by
connecting a temporary battery to the monitoring instrument, it was identified
that the nuclear reactor water level (fuel range) was +200 mm from TAF on themeter. (Note)
It was confirmed that steam was in the IC vent pipe once again at 21:30 on
March 11 after opening the return line Isolation valve MO-3A. After that, as theradiation dose was raised in the R/B, at 21:51 on March 11 entering in the R/B
was banned. At 22:00 on March 11, an alarm pocket dosimeter (hereafter called“APD”) in the R/B showed 0.8 mSv after a short time and that radiation dose at
the site had increased was reported to the headquarters of the power plant.
2-20Again at 23:00 on March 11 radiation doses on the north side double door of
T/B1 floor (R/B entrance) were 1.2 mSv/h, and 0.5 mSv/h on the south side
double door.
At 01:48 on March 12, it was found that D/D-FP, which was in operational status
for nuclear reactor injection, had stopped , so battery exchange, refuel and such
like were carried out for restoration, but it was not possible to start
D/D-FP.Because of that, the workers investigated using a fire engine and tying
up a water hose for injection. Along with spraying the water from a hydrant, using
the filtered water tank as a source was not found to be possible, hence theStation decided to search for a water source in the earth and use fire
extinguisher tanks.
Out of 3 fire tanks at the power plant, one had been damaged by the tsunami,
one was deployed at units 5 and 6 and was difficult to remove due to roaddamage and rubble from the tsunami. The management planned to deploy theremaining one at Fukushima Daiichi but due to the rubble from the tsunami, aquite bit of time was spent for moving the unit.
On one side, the nuclear reactor at around 2:45 on March 12 was found tohave a reactor pressure that had declined to 0.8 MPa [gage]. At around 3:45 on
March 12, , when the double door of R/B was opened for radiation measurement,a white haze was seen and hence the doors were rapidly closed and the workers
abandoned the idea of measuring the radiation.
At around 4:30 on March 12, an aftershock occurred and a tsunami warning wasissued. As a result, the headquarters of the power plant ordered to stop
fieldwork.
The fire engine pump house connection to Fukushima Daiichi FP line water
output gate was completed. As a result, from 5:46 on March 12 fresh water
injection from a fire tank to the nuclear reactor was started.
At around 5:52 on March 12, the injection volume from the fire engine became
1,000 liters, and after that, at around 9:15 on March 12, the fresh water injection
was 6,000 liters, and on the same day by 9:40 it was 21,000 liters.
Moreover, further arranged fire engines arrived and it became possible totransfer fresh water from the fire tank at Unit 3 to that at Unit 1. However, only
one hose could be connected to the fire tank, so the fresh water supply to thereactor needed to be discontinued.
Thus, by March 12 at 14:53, a total of 80,000 liters of fresh water had been
injected.
Due to the limitations of the fire tank fresh water, concurrent with the fresh water
infusion, seawater infusion was arranged. On March 12 at 14:54 the power
director ordered the infusion of seawater in the reactor. From the situation at thesite, the water source of seawater had been deemed to have flowed back to theunit 3 backlash valve. To ensure lightweight, a 3 fire engines was connected inseries to Unit 3 and an injection line for the reactor was configured.
Alternately using a fire pump engine and concurrent infusion, the SLC power
restoration also advanced. High voltage power supply cars and the Unit 2 power
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center, which was not impacted by the tsunami, were used and a cable wasconnected to the low voltage power supply board of Unit 1. Thus, at around
15:30 on March 12 the SLC power supply was restored and SLC injection
preparation was completed
Note: In the Instrument display, water level at the core is shown as ensured
level but considering damage to the core and such like there was a possibility
that the display of the instrument was inaccurate.
Support status at PCV Vent
On March 11 in the evening, the workers progressed to study the operatingprocedure of the PCV vent line valve in a situation where there was no power.
They found that in a small valve of the S/C vent valve (AO valve), there was asmall handle for manual operation so the workers confirmed that manual
opening operation is possible and they continued preparing to conduct PCV
vent.
On top of the lack of power supply to instruments due to the tsunami, the D/Wpressure also was uncertain. On March 11 at 23:50, when operating a small
generator that had been set up for temporary lighting in the central operating
room used to supply power to the D/W pressure gauge, after comparing theindicated value the workers found it to be 600 kPa [abs], the maximum allowed
pressure for D/W was 528 kPa [abs] (427 kPa [gage]). Thus, they found it to be
excessive during temporary lighting operation using a small generator. Hence, at
0:06 on March 12, the plant manager ordered workers to prepare for the PCV
ventilation, and they went on to confirm the concrete operation method andprocedure for PCV ventilation line valves.
At 0:30 on March 11, complete evacuation of the residents in a 3 km radiusoccurred. (1:45 Reconfirmation & Implementation.)
At 0:49 on March 12, as there was a chance that the D/W pressure would go
beyond 600kPa [abs], the Station decided that the situation corresponded to
Article 15 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster (abnormal increase in the containment vessel pressure).
At 1:30 on March 12, the Plant applied to TEPCO regarding PCV vent
implementation at Unit 1 and 2, and also obtained the consent of the Prime
Minister, Minister of Economy and Nuclear Safety and NISA. (Note: Later, after
understanding that the reactor core isolation cooling system of Unit 2 wasworking, the PCV vent of Unit 1 was implemented as a priority.)
At 2:24 on March 12, the working hours and exposure dose of PCV vent field
operations was evaluated. In the case of an atmospheric dose of 300 mSv/h, theemergency dose limit (100 mSv/h), possible working hours are 17 minutes. In
the case of self-air set equipment, the time is 20 minutes. So the Station
evaluated that workers must receive iodine doses.
At 2:30 on March 12, the D/W pressure increased to 840 kPa [abs]At 3:06 on March 12, a press release was issued about the PCV vent
implementation.

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At 3:45 on March 12, the Station reported to the TEPCO headquarters about the
exposure and results of dose assessment around the power plant during PCV
vent.
At 4:45 on March 12, the APD which was set to 100 mSv and front masks arrived
at the central operating room from the headquarters of the power plant.
At 4:50 on March 12, pollution was seen on the workers returning from theseismic isolation building. Thus, while entering the site from the seismic isolationbuilding equipment [front mask, charcoal filter, B equipment, C equipment or
Cover All] were set.
At 4:55 on March 12, the radiation dose in the power plant increased. (Near themain entrance, it went from 0.069μSv/h at 4:00 to 0.59 μSv/h at 4:23.)
At around 5:00 on March 12, workers were instructed to use (front mask andcharcoal filter and B equipment) even in the central operating room. As theradiation dose toward the Unit 1 side rose in the central operating room, workers
were evacuated to the low radiation dose side of Unit 2 by the shift supervisor.
As the radiation dose in the power plant was rising and the D/W pressure wasfalling, the Station concluded that the power plant had leaked radioactive
material at 5:14 on March 12.
At 5:44 on March 12, the Prime Minister announced that residents staying within
a 10 km of radius of Fukushima Daiichi must evacuate.
At 6:50 on March 12, the Ministry of Economy sent instructions for a PCV ventbased on regulations for nuclear power plants.
In this context, at 7:11 on March 12, the Prime Minister visited Fukushima Daiichi
for inspection and left for Tokyo at 8:40.
At 8:03 on March 12, the power plant manager ordered PCV vent to be
performed at 09:00 and prepared a three -member team system with a shift
supervisor and 2 deputy of duty class workers.
At 8:27 on March 12, the Station understood that the evacuation of someresidents of Okuma City had not been completed. Hence on March 12 at 8:37,
they told Fukushima that the PCV vent was going to be started at 9:00, andadjusted the PCV vent implementation so that it would be conducted after
Okuma City residents had evacuated.
At 9:03 on March 12, it was confirmed that the residents of Okuma City
(Kumachi-ku) had evacuated. An official announcement of PCV vent
implementation was made to Fukushima at 9:05.
At 9:04 on March 12, out of 2 valves in the PCV vent line series, initially to open
the MO valve, Team 1 (2 members) went to the site. At 9:15 the MO valve was
opened about 25% manually as per the procedure.
Subsequently, at 9:24 on March 12, Team 2 (2 members) headed to the torus
chamber of the site to manually open the remaining AO valve (small valve) at thevent line by S/C. But because of a high radiation dose and there was a possibility
that a dose limit exceeded 100 mSv, they abandoned the operation and returnedto the central operating room. Team 3 (2 members) also abandoned theoperation due to the high radiation dose.

2-23

As it was not possible to open the AO valve at vent line from the site by S/C, thepower plant headquarters began to look into the feasibility of connecting atemporary air compressor to the stopped air in the instrument (hereafter called“IA”). And expecting residual pressure in the IA system, the AO valve (small
valve) in the vent line was opened from the central operating room.
On March 12 at 10:17, in the central operating room, the first opening operation
was done by S/C on the AO valve (small valve) on the vent line. In connection
with this, on March 12 at 10:23 (2nd time) and 10:24 (3rd time) the same AO valve
opening operation was done. Note: Confirmation could not be obtained whether
the AO valve was opened or not for the 3rd time.
On March 12 at 10:40, the radiation dose of the main gate and monitoring post
(hereafter called “MP”), was measured and the radiation doses were found tohave gone up temporarily. On March 12 at 11:15 the radiation dose decreased,
so it was assumed that there was a possibility that the PCV vent valve had notbeen open sufficiently.
For that reason, to open the AO valve (large valve), on March 12 from 12:30
onwards the workers continued to prepare a temporary air compressor, andoutside the R/B big equipment hatch, a temporary air compressor was installed
and connected to IA piping connected.
On March 12 from around 14:00 onwards, the IA piping was pressurized, andD/W pressure decreased from 0.75 MPa to 0.58 MPa (at 14:50).
Hence, at 14:30 on March 12, the Station concluded that there had been arelease of radioactive material from the PCV ventilation.
2.2.3

Condition of nuclear reactor building after hydrogen explosion

At 15:30 on March 12, the SLC temporary power had been restored and the SLC
injection preparation had been completed, immediately after that, a hydrogen
explosion occurred at the nuclear reactor building at 15:36. After this explosion,
evacuation from the site was implemented, the injured, 3 TEPCO employees
and 2 collaborator company employees, were rescued and transported.
Seawater injection and SLC injection related preparation work was interrupted till
the condition of the site and safety were confirmed.
The dose was measured (1,015 μSv/h) at the MP of the power plant siteboundary, which exceeded 500 μSv/h. So, on March 12 at 16:27 the Station
decided the situation corresponded to Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster (abnormal increase in radiationdose within the power plant site boundary).
At 18:05 on March 12 the Minister of Economy ordered the workers to fill the
RPV with seawater in accordance with the regulations on nuclear power plants.
This was instructed by the main branch to the power plant.
At around 18:30 on March 12, after a site examination, in a condition where agreat deal of rubble covered the area, a cable, which was constructed as atemporary power supply for the SLC, and a hose to inject seawater weredamaged and found impossible to use. High pressure power supply vehicles
were stopped. Fire engine windows were cracked but not functionally damaged.

2-24

Close to Unit 1, high radiation dose rubble was scattered. Hence, under radiation
control employee’s monitoring, pieces of rubble were cleaned up, hoses forrelaying were gathered from outdoor fire dehydrant, and once again seawater
injection was prepared.
At around 19:04 on March 12, the workers started to inject seawater into the
nuclear reactor using the Unit 3 backlash valve pit as the source, and using an
FP line. Moreover, on March 12 at 20:45 the workers began injecting seawater
mixed with borate into the nuclear reactor.
2.2.4

Main events after accident

Regarding to restoration of power supply, on March 15, TEPCO completed
testing and inspecting access to electricity from the nuclear power lines. On
March 20, the power center was powered and an external power supply was
ensured. Then from March 23, the necessary load cables were constructed,
started to connect from the power center.
Regarding to alternative injection, on March 25, water injection was stopped andfresh water injection using a deionized water tank as the source was resumed.
From March 29, water was injected by using a temporary pump. Furthermore,from April 3, electric power for this pump was changed to the original power
supply.
2.2.5

Status of spent fuel pool

On March 11, 292 spent fuel rods and 100 new fuel rods were stored at Unit 1
SFP.
At 14:46, external power supply was lost in the north-eastern pacific cost ofJapan, and a fuel pool cooling purification valve system (hereafter called “FPC”)
also stopped. Moreover, SFP cooling was possible by using the reactor
shutdown cooling system (hereafter called “SHC”).
Afterwards, due to the tsunami impact, all AC power supply was lost and SFP
cooling functionality and make up water system functionality were lost.
The decay heat of the Unit 1 SFP fuel was evaluated as being approximately
0.18 MWt (on March 11), which was the lowest of all units in Fukushima Daiichi.
From March 31 onwards, using a concrete pump vehicle, fresh water injection
was started.

2-25

Table 2.2-1 Timeline of important events (Unit 1)
March 11, 2011 (Friday)
14:46
Great East Japan Earthquake struck. Automatic reactor scram. Official
announcement of level 3 emergency conditions.
14:47

Automatic shutdown of main turbine, automatic startup of emergency
DG.

14:52

Automatic start of IC.

15:02

Reactor subcritical confirmation.

Around 15:03

In order to control the nuclear reactor pressure by IC, manual
shutdown. Then, the nuclear reactor pressure control was started by
IC.

15:06

Set up emergency disaster countermeasure headquarters in head
office (to understand the damage caused by the earthquake, power
failure recovery etc.)

15:27

Arrival of first tsunami.

15:35

Arrival of second tsunami.

15:37

Complete AC power loss.

15:42

The Station concluded that a specific situation (complete loss of AC
power supply), which corresponds to the Act on Special Law ofEmergency Preparedness for Nuclear Disaster had occurred andreported to the government authorities.

15:42

Official announcement of first emergency organization. Setup of
emergency countermeasures headquarters. (Becomes a joint
headquarters with emergency disaster countermeasure headquarters.)

16:36

Without confirming the water level in the nuclear reactor, the status of
injection of water was unclear, and management concluded that thesituation corresponded to specific conditions based on Paragraph 1 of
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster (Emergency core cooling system (ECCS) water
injection not possible) and reported to the government authorities at
16:45.

16:36

Official announcement of emergency level 2 conditions

16:45

As the workers could confirm the nuclear reactor water level, theStation concluded that a specific situation (Emergency core coolingsystem (ECCS) water injection not possible), based on the provisionsof Paragraph 1 of Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster had come to an end and reported
2-26

to the government authorities at 16:55.
17:07

Again, the workers could not confirm the nuclear reactor water level
and hence, the Station concluded that the situation corresponded to
specific conditions based on Paragraph 1 of Article 15 of the Act on
Special Law of Emergency Preparedness for Nuclear Disaster(Emergency core cooling system (ECCS) water injection not possible)
and reported to the government authorities at 17:12.

17:12

The power plant manager ordered workers to start investigating amethod of injecting water into the nuclear reactor using fire engines
and an FP line, which was established as a countermeasure for severe
accidents.

17:30

Start D/D-FP (standby condition)

18:18

Reverse piping isolation valve (MO-3A) and supply piping isolation
valve (MO-2A) of IC opened, Confirmation that steam was generated.

18:25

Reverse piping isolation valve (MO-3A) of IC closed.

20:49

Temporary illumination inside MCR.

20:50

Evacuation instruction for the citizens of Fukushima Prefecture within aradius of 2 km from Fukushima Daiichi.

21:19

Water level of nuclear reactor became clear, TAF +200 mm (see note).

21:23

Prime Minister instructed people within a radius of 3 km from
Fukushima Daiichi reactor to evacuate, and those within a radius of
3 km to 10 km from the reactor to stay indoors.

21:30

Start operation to open the reverse piping isolation valve (MO-3A) of IC,
Confirmation of steam generation.

21:51

As the radiation level in the nuclear reactor building rose, persons
were prohibited from going inside the reactor building.

22:00

Confirmation that the water level of the nuclear reactor was TAF+550 mm, communicated to the government authorities and such likeat 22:20.

23:00

Results of a survey showed that the radiation level inside T/B (T/B 1st
floor north side before the double door 1.2mSv/h, T/B 1st floor south
side before the double door 0.5 mSv/h) had increased and this wasreported to the government authorities at 23:40.

March 12, 2011 (Saturday)
0:06
Due to a possibility that the D/W pressure had exceeded 600 KPa, theplant manager ordered the main power plant to continue preparations,considering the possibility that containment venting would be
2-27

executed.
0:30

Confirmation that the national evacuees had taken shelter(confirmation that the citizens within a 3 km radius of Futaba City andOkuma City had taken shelter, checked again at 1:45).

0:49

Since there was a possibility that the D/W pressure exceeded 600 kPa
abs, the Station concluded that a specific situation (abnormal rise instorage container pressure) corresponding to the provisions ofParagraph 1 of Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster had arisen and reported to thegovernment authorities at 0:55.

Around 1:30

Obtained consent from and reported to the Prime Minister, Minister of
Economy, Trade and Industry and the Nuclear and Industrial Safety
Agency (NISA) about implementing venting of containment of Unit 1
and Unit 2.

1:48

Confirmed the shutdown of D/D-FP due to improper conditions. Started
investigating whether there was proper connection from the fire engine
to the water outlet of the FP line.

2:47

At 2:30, management told the government authorities that D/Wpressure reached 840 kPa abs.

3:06

Conducted a press interview related to the enforcement of storage
container vent.

4:01

Told the government authorities the results of radiation exposure
evaluation assuming the storage container would be vented.

4:55

Confirmed increase in the radiation level (main entrance vicinity wentfrom 0.069 μSv/h (at 4:00) to 0.59 μSv/h (at 4:23)) inside the power
plant and reported to the government authorities.

5:14

Due to increase in the radiation level inside the power plant andreduction in D/W pressure, management judged that a leakage ofradioactive material to the outside had taken place and reported to thegovernment authorities.

5:44

The Prime Minister instructed the people living within a 10 km radiusfrom Fukushima Daiichi to evacuate.

5:46

Started fresh water injection in nuclear reactor from the FB line using afire engine.

5:52

Completed injecting 1,000 liters of fresh water using a fire engine fromthe FP line.

6:30

Completed injecting 2,000 liters (total) of fresh water using a fireengine from the FP line.
2-28

6:33

Confirmed the evacuation status of the region, and found that theauthorities are considering transferring the evacuees of Okuma City to
the capital.

6:50

Enforcement order based on the laws and ordinances issued by the
Minister of Economy, Trade and Industry for venting (manual venting).

7:11

Prime Minister arrived at Fukushima Daiichi.

7:55

Completed injecting 3,000 liters (total) of fresh water using a fireengine from the FP line.

8:03

The plant manager instructed to carry out the vent operation till 9:00.

8:04

Prime Minister departed from Fukushima Daiichi.

8:15

Completed injecting 4,000 liters (total) of fresh water using a fireengine from the FP line.

8:27

Confirmed that part of Okuma City had not been evacuated.

8:30

Completed injecting 5,000 liters (total) of fresh water using a fireengine from the FP line.

8:37

Informed Fukushima Prefecture that workers were preparing toconduct venting operation at around 9:00. Management decided thatthe venting should be started after residents had evacuated.

9:03

Confirmed that evacuation of Okuma district was complete.

9:04

The person on duty went to the actual site to carry out the venting
operation.

9:05

Issued a press announcement on the venting.

9:15

Completed injecting 6,000 liters (total) of fresh water using a fireengine from the FP line.

Around 9:15

Manually opened the storage container (hereafter called “PCV”) ventvalve (MO valve).

Around 9:30

Tested operation on the site of S/C vent valve (AO valve) small valve
but abandoned testing due to high radiation levels.

9:40

Completed injecting 21,000 liters (total) of fresh water using a fireengine from the FP line.

9:53

Told the government authorities the results of radiation exposure
evaluation assuming that the storage container would be vented.

2-29

10:17

Opened the S/C vent valve (AO valve) small valve of MCR (expected
remaining pressure of pressurized instrumentation air system).

10:40

Confirmed that the radiation level of the main gate and MP had
increased, so management judged there was a high possibility thatradioactive material had been released during the venting.

11:15

Due to a reduction in the radiation level, management confirmed thepossibility that venting had insufficient effects.

11:39

Told the government authorities that one employee of Tokyo ElectricPower Corporation, who had gone inside the nuclear reactor for thevent operation, had been exposed to a radiation level of more than
100 mSv (106.30 mSv).

14:30

To activate the S/C vent valve (AO valve) and large valve, a temporary
air compressor was setup at around 14:00. Management confirmedthat the D/W pressure was reduced and made a judgment that theradioactive material was released due to venting, and reported to thegovernment authorities and such like at 15:18.

14:53

Completed injecting 80 tons (total) of fresh water using a fire engine
from the FP line.

14:54

The plant manager instructed the main power plant to start injecting
seawater.

15:18

The restoring work of SLC was in progress, immediately after thecompletion of preparations, the SLC pump was to be started andinjection of water into nuclear reactor was to be scheduled. Also,
management reported information to the government authorities thatas soon as they completed preparations, they plan to inject seawater
into the nuclear reactor from the FP.

15:36

Power was restored using a power source vehicle, and arrangements
for injecting water into the nuclear reactor using an SLC werecompleted.

15:36

Explosion occurred in the reactor building. The hose, which had been
set up to inject seawater, and the electric supply system for SLC weredamaged, and became unusable.

16:27

MP measured the radiation level as exceeding 500μSv/h (1,015μSv/h).
Hence the Station concluded that a specific condition (abnormal
increase in radiation dose within the site boundary) had occurred
based on Paragraph 1 of Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster and reported to thegovernment authorities.

2-30

Around 17:20

Departed to survey the situation of the fire engines and reactor building

18:05

Headquarters and power plant told there was an order from the
Minister of Economy, Trade and Industry based on laws and ordinances.

18:25

The Prime Minister instructed the people living within a 20 km radius of
the Fukushima Daiichi to evacuate.

Around18:30

The results of analyzing the condition of the fire engine, reactor
building and such like showed that the hose that had been set up forseawater injection and power supply device of SLC had beendamaged and hence were unusable.

19:04

Started injecting seawater using a fire engine from the FP line.

20:45

Started injecting seawater mixed with boric acid into the nuclear
reactor.

Note: The meter indication showed that the water level of the reactor core was being
maintained, but thinking about the period of reactor damage and such like, it waspossible that the meter was not showing a proper indicated value.

2-31

Figure 2.2-1 Course of accident after earthquake at Unit 1 of the Fukushima Daiichi
Nuclear Power Station

2-32

Reactor water level (fuel area) (A)Water level (mm)Reactor water level (fuel area) (B)Time

Figure 2.2-2a Change in reactor water level (Unit 1)

System A Reactor pressure

Pressure (MPa[abs])

System B Reactor pressure

Time
Figure 2.2-2b Changes inreactor pressure (Unit 1)
Figure 2.2-2c Changes in reactor pressure (Unit 1)

2-33

D/W pressure

Pressure (MPa[abs])

S/C pressure

Time

Figure 2.2-2c Changes in containment vessel pressure (Unit 1)

2-34

Reactor water level (fuel area) (A)Water level (mm)Reactor water level (fuel area) (B)Time

Figure 2.2-3a Changes in reactor water level (Unit 1, long term)

System A Reactor pressure

Pressure (MPa[abs])

System B Reactor pressure

Time

Figure 2.2-3b Changes in reactor pressure (Unit 1, long term)

2-35

D/W pressure

Pressure (MPa[abs])

S/C pressure

Time

Figure 2.2-3c Changes in containment vessel pressure (Unit 1, long term)

RPV bellow seal (HVH-12V)
Feed water nozzle (End)

Safety valve exhaust air 203-4A(1)Pressure vessel bottom
D/W HVH return (HCH-12C)
CRD Housing top
CRD Housing bottom
S/C pool water temperature A
S/C pool water temperature B

Fig. 2.2-4c Changes in temperature (Unit 1, long term)

2-36

2.3 Course of accident at Unit 2
2.3.1

From occurrence of earthquake to arrival of tsunami

Unit 2 was in constant thermal output operation at 2,381 WMt. Due to theearthquake, it automatically stalled at 14:47 on March 11 after receiving a ‘large
seismic acceleration’ signal and all control rods were fully inserted into thereactor.
The earthquake damaged various units for the power plant, including powerreceiving circuit breakers for the Okuma No. 1 and 2 lines, causing a total loss ofexternal power supply and loss of power supply for emergency bus conductors.
Then, two DGs (2A and 2B) for emergency use started operating immediately
and automatically to restore power supply to the emergency bus conductors.
Loss of power supply to the emergency bus conductors occurred at around14:47 on March 11 and caused a loss of power supply for the nuclear reactor
protection system, which resulted in an automatic closure of MSIV. This madethe pressure in the nuclear reactor surge, but the main steam relief valve(hereafter called “SRV”) controlled the pressure.
The water level in the nuclear reactor lowered immediately for a time after the
nuclear reactor stopped automatically, due to a void collapse. However, after that,the level was successfully controlled by RCIC without reaching the level at which
HPCI automatically starts (L-2: TAF+2,950 mm). For the RCIC, after the repetitionof manual start by workers and then automatic start by receiving the ‘nuclear
reactor water level high’ signal several times, it was manually started again at 15:39
on March 11, immediately before the massive tsunami hit the power plant.
Automatic opening and closing of the SRV and starting of the RCIC caused thetemperature of the S/C pool water to rise. So, a residual heat removal system(hereafter called “RHR”) was run from 15:00 to 15:07 on March 11 to cool down
the S/C in the torus water cooling mode.
As mentioned above, from the time the earthquake occurred to the arrival of thetsunami, the measures taken were those normally done when external power
supply is lost and there is a scram of a nuclear reactor.
2.3.2

From arrival of tsunami to malfunctioning of pressure suppression
chamber

The first tsunami arrived about 41 minutes after the earthquake hit the area, i.e.,
15:27 on March 11, followed by the second tsunami at 15:35. The two tsunamis
flooded and soaked seawater pumps for cooling and power panels. At 15:41 on
the same day, both emergency DGs (2A and 2B) stopped and a Station BlackOut occurred. D/C power supply was also lost. The management determined at
15:42 on March 11 that the situation corresponded to an event applicable to
Article 10 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster, i.e., a Station Black Out. Due to the station blackout, the RHR and the
CS were unable to function.
Workers investigated the status of RCICs that could be run using D/C powersource, but were unable to detect the status because the indicator lights in thecentral operating room and other lights had all gone out. Moreover, power supply
to the measuring instruments was lost at 15:50 on the same day and the workers
could not measure the water level in the nuclear reactors and D/W pressure. The
2-37

management thus determined at 16:36 on the same day that an event applicable
to Article 15 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster, i.e., cooling water unable to be supplied to emergency core coolingsystem, had occurred.
In order to secure a measure to supply cooling water to the nuclear reactor, theplant manager issued an instruction to examine the possibility of using analternative way of supplying cooling water including FP, MUWC and RHR, which
are installed as one of the measures against severe accidents and fire engines.
In conjunction with examining an alternative cooling water supply, the procedurefor opening and closing the PCV valves without power supply for ventilation of
PCVs was examined.
The following explain how an alternative cooling water supply was used and how
PCV ventilation was made.
Alternative Cooling Water Supply
After examinations, the disaster countermeasures office in the Fukushima
Daiichi Nuclear Power Station determined to construct an alternative coolingwater supply line by way of RHR and construct a system so that cooling water
could be supplied after reducing the pressure in the nuclear reactor (to 0.69 MPa[gauge]) via manually operating valves installed at such places as RHR system
in R/B and T/B in the total darkness.
In order to restore the lights in the central operating room and monitoring
instruments, the restoration team of the Station’s disaster countermeasuresoffice collected batteries and cables required for it, and at 20:49 on the same day,temporary lighting equipment was installed in the central operation room, using a
portable power generator.
The Station still did not know the water level in the nuclear reactor or how coolingwater was supplied to the reactor using RCIC. The management announced
there was a possibility that the water level would reach the TAF level to all
relevant people at 21:02 on the same day. At the same time, relevantorganizations were notified that the estimated time of reaching TAF was 21:40.
After restoring power supply to instruments and other devices using the
temporary power supply system, workers discovered that the level of water in
the nuclear reactor was maintained at TAF+3,400 mm, at 21:50 on the same
day.
Workers investigated how the RCIC was operated on site at around 2:00 on
March 12. Though they were unable to obtain the data, they found that output
pressure from the RCIC pump was higher than the pressure in the nuclear
reactor. Thus, they determined that the RCIC was in operation, which was
informed to the central operating room.
After the report was made, the Station’s disaster countermeasures office
determined that the RCIC was in operation and determined to continue
monitoring parameters for Unit 2, at 2:55 on March 12.
Then, the water level in the condensate storage tank (hereafter called “CST”) fell
from 4:20 to 5:00 on the same day, so workers went on site to manually operate
the valve to switch the water source to RCIC from the CST to the S/C.

2-38

The restoration team of the Station’s disaster countermeasures office
investigated how a power panel of Unit 2 was inundated and how its exterior wasdamaged, and measured its insulation resistance. It was revealed that part of thePower Center (P/C-2C and P/C-2D) was still usable. The control rod drive
system (hereafter called “CRD”) pumps that can inject high-pressure water to thereactor and SLC pumps may be used to supply water to the reactor byconnecting a cable from a power supply vehicle to the power center. The method
to carry it out was examined.
At around 15:30 on March 12, temporary cables were connected from a hightension power supply vehicle to the primary side of the power center for Unit 2
and power connection was completed.
Immediately after the connection, at 15:36 on the same day, an explosion
occurred at the R/B of Unit 1, possibly a hydrogen gas explosion. This damaged
the cables, which had just been laied, and the high tension power supply vehicle
automatically stopped. Due to this explosion, the workers had to evacuate fromthe site temporarily and the restoration work was suspended till the safety of thesite was confirmed.
On March 13, the high tension power supply vehicle that was connected to theUnit 2 power center was switched on again, but its protection system(overcurrent relay) stopped it from working.
The plant manager, in preparation for stoppage of RCIC, instructed the workers to
get it ready for injecting seawater into the reactor, at 12:05 on March 13. In
response to this instruction, fire engines were mobilized to lay down hoses toconstruct a seawater injection system, with a backwash valve pit of Unit 3 as thewater source.
Another hydrogen gas explosion occurred at the R/B of Unit 3 at 11:01 on March
14. This damaged the fire engines and the hoses for seawater injection, whichwere ready to use, and made them unusable.
An on-site investigation started at 13:05 on the same day. After confirming how
debris was scattered over the affected site, fire engines that were still usable
were mobilized and hoses were laid again to start preparing for construction of awater injection system again; but this time, the seawater source was changed
from the Unit 3 backwash valve pit to the cargo unloading station.
On the other hand, the RCIC were in operation, and the water level in the
nuclear reactor was stably moved to over TAF+3,000 mm measured by an active
fuel water level gauge from 22:00 on March 11 to around 12:00 on March 14.
The water level, however, showed a downing trend at around 13:18 on March 14,
which continued thereafter. The management determined at 13:25 on March 14
that the situation corresponded to an event applicable to Article 15 of the Act on
Special Law of Emergency Preparedness for Nuclear Disaster, i.e., loss of
capacity to cool down a nuclear reactor, after RCIC lost its function.
Work to connect seawater injection lines to inject seawater to the reactor fromfire engines using FP lines was completed at 14:43 on March 14.
In order to inject seawater to the reactor from the fire engines, it was necessary
to reduce pressure in the reactor by manually opening the SRVs. However, thetemperature and pressure of the S/C remained high (the temperature and2-39

pressure in the S/C were 149.3 C and 486 kPa [abs] respectively at 12:30 on
March 14). The condition was such that even if the SRVs were opened manually,
it was not likely that steam in the S/C would condensate and the pressure would
be reduced.
So, the Station’s disaster countermeasures office decided to inject seawater
after ventilating the PCV to reduce the S/C pressure then to reduce the pressure
inside the reactor using the SRVs.
However, thereafter, it was likely that it would take time till the PCV valve wasmanually opened. The plant manager, at around 16:00 on March 14, decided toreduce the pressure using the SRVs first and instructed the workers to ventilate
the PCV at the same time. Preparations were made around 16:30 on the same
day so that seawater injection could be started as soon as the fire engine pump
was run and the pressure in the reactor was reduced to an acceptable level.
As the DC power supply to open the SRVs was lost, batteries in the vehicles that
had been parked on high ground and were unaffected by the tsunami werecollected to the central operating room and they were prepared to be used as a
power source for operating the SRVs by connecting the batteries to the power
cable. The pressure in the reactor was reduced and opening of the SRVs started
at 16:34 on March 14. The voltage of the battery was not sufficient. So batteries
were added and the opening operation of more than one SRV continued.
Meanwhile, the water level of the reactor reached TAF at 17:17 on March14.
Pressure reduction by opening SRVs started at around 18:00 on March 14.
However, the high temperature and pressure in the S/C prevented steam fromcondensing and it took time till the pressure was reduced. (The pressure in thereactor was 6.998 MPa [gage] at 16:43, 6.075 MPa [gage] at 18:03, and0.63 MPa [gage] at 19:03 on March 14.)
During this period, at 18:22 on March 14, the water level in the reactor fell to
TAF-3,700 mm and the entire fuel rods were exposed.
The radiation dose at the site was too high to have the operation condition of thefire engines monitored for a long time by the same person, so the workers tookturns in monitoring. At 19:20 on the same day, fire engines waiting to injectseawater into the reactor were found to have stopped at the site due to a fuelshortage.
The fire engines were refueled and seawater injection by them using the FP line
was started at 19:54 on the same day. (One each of the fire engines wasoperated at 19:54 and 19:57 on March 14.)
When the SRV2 valve was manually opened at 21:20 on the same day, thewater level in the reactor showed a slight recovery. (The water level in thereactor was TAF-3,000 mm at 21:30 on March 14.)
PCV Ventilation
Starting from the evening of March 11, the workers examined how to open the
PCV ventilation valves without any power supply, in conjunction with an
examination on alternative methods for water injection.
The power supply to measuring instruments was lost due to the tsunami and theD/W pressure was unknown. However, the measuring instruments were restored2-40

at around 23:25 on March 11 and it was found that the D/W pressure was
0.141 MPa [abs].
As it was found that RCIC for Unit 2 was in operation at 2:55 on March 12, theStation decided to ventilate the PCV of Unit 1 first and to continue monitoring theparameters for Unit 2.
Meanwhile, water injection to the reactor using RCIC was continued and theD/W pressure shifted stably around 200 to 300 kPa [abs]. However, the Station
reasonably assumed that ventilation of the PCV was going to be necessary. The
plant manager issued an instruction at 17:30 on March 12 to start preparing for
PCV ventilation for Unit 2.
In line with the operating manual, at 8:10 on March 13, the workers manually
opened the MO valve at 25%. This is one of the two valves (MO valve and AO
valve) that are serially installed in the PCV ventilation line.
At 10:15 on March 13, the plant manager instructed the workers to open theremaining AO valve. Following this order, at 11:00 on the same day, the workers
opened the AO valve (large valve) by forcibly energizing a solenoid valve using
the power source supplied by a small-sized power generator for temporary lighting
in the central operating room. This completed forming the PCV ventilation line
system excluding a rupture disc. The D/W pressure at this time was still lower than
the working pressure for the rupture disc (i.e., 427 kPa [gage]) and under such alow pressure, ventilation of the PCV could not be done. So the valves in theventilation line were left open and the D/W pressure was continually monitored.
The Station informed the result of evaluating the dose of radiation exposed to theareas neighboring the plant when the PCV ventilation was done to the relevantorganizations at 15:18 on March 13.
An explosion, possibly of hydrogen gas, occurred at the R/B of Unit 3 at 11:01 on
March 14. All workers excluding workers at the central operating room stopped
the entire work and evacuated to the antiseismic building.
The D/W pressure was shifting at around 450 kPa [abs] in a stable manner,which was lower than the working pressure for the rupture disc.
Due to the explosion in Unit 3, the circuit for energizing a solenoid valve for theAO valve (large valve), which was on the ventilation line from S/C, which was
being constructed, was disconnected and closed. After the evacuation order was
lifted, at around 16:00 on March 14, the workers performed an operation to open
the valve again but in vain, due to an insufficient air supply from the aircompressor temporarily installed, or other reasons. (Thereafter, the reason was
assumed to be a malfunctioned solenoid valve.) Due to this, the workers beganbuilding a ventilation line using the AO valve (small valve) installed on theventilation line connecting from the S/C, at around 18:35 on March 14. At around
21:00 on the same day, the AO valve (small valve) was opened and the PCV
ventilation line system was recreated, excluding the rupture disc.
As the D/W pressure exceeded 427 kPa [gage], or the maximum working
pressure, at 22:50 on March 14, the Station determined the situation
corresponded to an event applicable to Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster, i.e., abnormal pressure surge ina primary containment vessel (PCV).

2-41

The D/W pressure tended to go up but the S/C pressure was shifting around 300
to 400 kPa [abs] in a stable manner. This caused an irregular distribution ofpressure in the PCVs.
As the pressure on the S/C side was lower than the working pressure of the
rupture disc, and as the pressure on the D/W side surged, the Station decided
at around 23:35 on March 14 to carry out a measure to initiate PCV ventilation
by opening the AO valve (small valve) on the ventilation line connected from
D/W.
At around 0:02 on March 15, the AO valve (small valve) on the ventilation linefrom the D/W was opened, but several minutes later, it closed again. The D/Wpressure shifted at a high level.
At 3:00 on the same day, the workers tried to reduce the D/W pressure andinject water into the nuclear reactor but the reactor’s pressure could not be
reduced to a sufficiently low level.
At around 6:00 or 6:10 on March 15, a big boom was heard and around thesame time, the pressure of the S/C became 0 MPa [abs].
At 6:50 on the same day, the radiation dose around the main gate showed
583.7 Sv/h, which is higher than 500 Sv/h — the threshold level determined
as an abnormal level of radiation. So the Station determined the situation
corresponded to an event applicable to Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster, i.e., abnormal surge of radiationdose at the boundary of the premises.
At around 7:00 on March 15, the workers in the antiseismic building, except for
those required for emergency restoration work and monitoring, left the plant to
go to Fukushima Daini Nuclear Power Station, hereafter called "Fukushima
Daini" temporarily.
Thereafter, parameters such as D/W pressure were taken by the workers every
few hours at the central operating room. At around 11:25 on March 15, the D/Wpressure showed a decrease (730 kPa [abs] at 7:20 and 155 kPa [abs] at 11:25
on March 15).
2.3.3

Major events thereafter

Inspection and trial recharging of the Station’s access to electricity fromTEPCO’s Nuclear Power Grid owned by Tohoku Electric Co., Ltd. werecompleted on March 15, and then electricity was received by the Power Center
on March 20. Now an external power source had been secured. Then on March
26, the lighting of the MCR was restored.
As an alternative way to inject water into the nuclear reactor, seawater had
been used up to March 26. But on the same day, fresh water supplied from
temporary tanks was used. On the following day, March 27, the pumps usedwere changed from fire pumps to temporary electric pumps. From April 3
onward, power supply to the plant was restored from a temporary power supply
system.

2-42

2.3.4

Condition of spent fuel pool

As of March 11, a total of 587 spent fuel rods and 28 new fuel rods were stored
in the spent fuel pool (SFP) of Unit 2. The Station estimated that the decay heatgenerated by these fuels in the SFP was approximately 0.62 MWt (as of March
11).
The earthquake that occurred at 14:46 on March 11 caused a total loss ofexternal power supply to the plant, and the FPC came to a halt. The massive
tsunami that hit the plant after the earthquake caused a station blackout and thefunctions of cooling and supplying water to the SFP were lost.
The explosion of the R/B of Unit 1 that occurred at 15:36 on March 12 blewaway the blow out panel of R/B in Unit 2 and white smoke was observed
coming out of the blow out panel.
On March 20, seawater was injected to the SFP using FRC pipes already laid.
When injection of seawater to the SFP was done again on March 22, the waterlevel surged in the skimmer surge tank, meaning the SFP was totally filled withwater.
From March 29 onward, the type of water was switched from seawater to fresh
water.
It is considered that these water injections were successful in keeping the waterlevel in the SFP sufficiently high to prevent the fuel rods being exposed to the
air.

2-43

Table 2.3-1 Time-Course Progress (Unit 2)
March 11, 2011 (Fri.)
14:46
14:47
14:50
14:51
15:01
15:02
15:06

15:27
15:28
15:35
15:39
15:41
15:42

15:42
16:36

16:36
17:12

20:49
20:50
21:02

21:13
21:23

22:00

Great East Japan Earthquake occurs. Alert Level 3 automatically issued.
Scram of nuclear reactors automatically occurs and main turbine automaticallystops. Emergency DG automatically activated.
RCIC manually started.
RCIC stops (due to high water level in the reactor).
Subcriticality of the nuclear reactor confirmed.
RCIC manually started.
A disaster countermeasures office established in the TEPCO head office (tounderstand the damage conditions due to the earthquake and restoration fromblackout).
First tsunami arrives.
RCIC stops (due to high water level in the nuclear reactor).
Second tsunami arrives.
RCIC manually started.
Station Black Out occurs.
Determined the occurrence of a special event applicable to Paragraph 1, Article
10 of the Act on Special Law of Emergency Preparedness for Nuclear Disaster(Station Black Out). Notified the same to governmental agencies and otherrelevant organizations.
Alert Level 1 issued. An emergency task force established (as a joint
headquarters with disaster countermeasures office).
Water level in the reactors is unable to be confirmed and how water is injected
is known. So the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
for Nuclear Disaster has occurred (cooling water unable to be supplied toemergency core cooling system), and at 16:45 the same is notified to thegovernmental agencies and other relevant organizations.
Alert Level 2 issued.
The plant manager instructs workers to start an examination to find a way to
inject water into the nuclear reactors using the FP lines installed in preparation
for severe accidents and fire engines.
Temporary lighting installed and used within the MCR.
Fukushima Prefecture directs local residents living within a 2 km radius from the
Fukushima Daiichi Nuclear Power Station to evacuate.
As the water level in the nuclear reactors is unidentified and how water is
injected to them by RCIC cannot be confirmed, the Station sends a notice togovernmental agencies and other relevant organizations saying that the waterlevel could reach TAF.
The time for reaching TAF is estimated as 21:40, which is conveyed togovernmental agencies and other relevant organizations.
The Prime Minister directs local residents living within a 3 km radius from thepower plant to evacuate and those living within a 3 km to 10 km radius to stayindoors.
Water level in the reactors is identified as TAF+3,400 mm. So the Stationassesses that it will take more time before reaching the level of TAF. The Station
notifies governmental agencies and other relevant organizations of this at 22:10
and 22:20.

March 12, 2011 (Sat.)
0:30

Completion of evacuation of the local residents confirmed by the Governmentof Japan (confirmed evacuation of people living within 3 km from the plant inFutaba City and Okuma City, and reconfirmed it at 1:45).

2-44

Around 1:30

2:55
3:06
3:33
4:55

5:44
6:50
7:11
8:04
16:27

17:30
18:25

Ventilation of Unit 1 and Unit 2 requested to the Prime Minister, the Minister of
METI and Nuclear and Industrial Safety Agency, to which an approval is
granted.
Confirmed RCIC is in operation.
Held a press meeting on initiation of the ventilation.
Notified to the governmental agency of the results of radiation exposure
evaluation to be released when the ventilation is done.
Confirmed surge of radiation dose within the plant premises surges (aroundthe main gate: goes from 0.069 Sv/h (at 4:00) to 0.059 Sv/h (at 4:23). The
same is notified to governmental agencies and other relevant organizations.
The Prime Minister directs local residents living within a 10 km radius from
Fukushima Daiichi Nuclear Power Station to evacuate.
The Minister of METI issues an order to initiate ventilation (manual ventilation)
pursuant to the relevant laws.
The Prime Minister arrives at the Fukushima Daiichi Nuclear Power Station.
The Prime Minister leaves the Fukushima Daiichi Nuclear Power Station.
A radiation dose of over 500 Sv/h at the MP i.e., 1,015 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1, Article
15 of the Act on Special Law of Emergency Preparedness for Nuclear Disasterhas occurred (abnormal surge of radiation dose at the boundary of thepremises). The same is notified to governmental agencies and other relevantorganizations.
The plant manager issues an instruction to start preparation for PCV
ventilation.
The Prime Minister directs local residents living within a 20 km radius from the
Fukushima Daiichi Nuclear Power Station to evacuate.

March 13, 2011 (Sun.)
8:10
8:56

10:15
11:00
11:00
11:20
12:05
14:15

15:18

A ventilation valve (MO valve) of primary containment vessels (hereafter called“PCVs”) is opened.
A radiation dose of over 500 Sv/h at the MP i.e., 882 Sv/h observed and theStation determines that a special event applicable to Paragraph 1, Article 15 of
the Act on Special Law of Emergency Preparedness for Nuclear Disaster has
occurred (abnormal surge of radiation dose at the boundary of the premises).
The same is notified to governmental agencies and other relevantorganizations at 9:01.
The plant manager instructs the workers to start the ventilation.
A ventilation line is established, excluding rupture discs.
The Prime Minister directs local residents living within a 20 km to 30 km radiusfrom the Fukushima Daiichi Nuclear Power Station to stay indoors.
Held a press meeting on initiation of PCV ventilation.
The plant manager instructs workers to prepare for using seawater.
A radiation dose of over 500 Sv/h at the MP i.e., 905 Sv/h observed and theStation determines that a special event applicable to Paragraph 1, Article 15 of
the Act on Special Law of Emergency Preparedness for Nuclear Disaster has
occurred (abnormal surge of radiation dose at the boundary of the premises).
The same is notified to governmental agencies and other relevantorganizations at 14:23.
Notified governmental agencies and other relevant organizations of the resultof radiation exposure evaluation to be released when PCV ventilation is done.

March 14, 2011 (Mon.)
2:20

A radiation dose of over 500 Sv/h at around the main gate i.e., 751 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
2-45

2:40

4:00

9:12

11:01

13:05
13:18

13:25

15:28
16:30
16:34

17:17
Around 18:00
18:22

19:20
19:54
Around 21:00
21:20

for Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 4:24.
A radiation dose of over 500 Sv/h at the MP i.e., 650 Sv/h observed and theStation determines that a special event applicable to Paragraph 1, Article 15 of
the Act on Special Law of Emergency Preparedness for Nuclear Disaster has
occurred (abnormal surge of radiation dose at the boundary of the premises).
The same is notified to governmental agencies and other relevantorganizations at 5:37.
A radiation dose of over 500 Sv/h at the MP i.e., 820 Sv/h observed and theStation determines that a special event applicable to Paragraph 1, Article 15 of
the Act on Special Law of Emergency Preparedness for Nuclear Disaster has
occurred (abnormal surge of radiation dose at the boundary of the premises).
The same is notified to governmental agencies and other relevantorganizations at 8:00.
A radiation dose of over 500 Sv/h at the MP i.e., 518.7 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1, Article
15 of the Act on Special Law of Emergency Preparedness for Nuclear Disasterhas occurred (abnormal surge of radiation dose at the boundary of thepremises). The same is notified to governmental agencies and other relevantorganizations at 9:34.
Explosion of R/B of Unit 3 caused the large valve of the S/C ventilation valve(AO valve) closed. Confirmed it is unable to be opened. The completed water
injection line is found unable to be used due to damages to the fire enginesand hoses.
Establishment of a seawater injection line restarted, including fire engines.
As the water level in the reactor tends to be lowering, the Station notifies thegovernmental agencies and other relevant organizations that it will
immediately start preparing to inject seawater into the reactor.
The water level in the reactor keeps decreasing and there is a possibility that
the RCIC is not functioning. The Station determines that a special event
applicable to Paragraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (loss of capacity to cool downa nuclear reactor). The same is notified to governmental agencies and otherrelevant organizations at 13:38.
Time to reach the TAF estimated as 16:30. The same is notified togovernmental agencies and other relevant organizations.
Fire engines start injecting seawater into the reactor.
The Station notifies governmental agencies and other relevant organizations
that it will start reducing the pressure in the nuclear reactor and injecting
seawater from the FP lines.
The water level in the nuclear reactor reached TAF. The same is notified togovernmental agencies and other relevant organizations at 17:25.
The Station starts reducing the pressure in the nuclear reactor (from 5.4 MPa
to 0.63 MPa at 19:03).
The water level in the nuclear reactor reached TAF-3,700 mm. The Station
determined that the whole of the fuel rods have been exposed to the air. The
same is notified to governmental agencies and other relevant organizations at
19:32.
Confirmed that fire engines that are injecting seawater into the nuclear reactor
have stopped because they have run out of fuel.
Started using fire engines to inject seawater into the nuclear reactor from theFP line (one each operated at 19:54 and 19:57).
A small valve of the S/C ventilation valve (AO valve) opened. A ventilation line
excluding rupture discs completed.
Two valves of SRVs opened. Confirmed that the water level in the nuclear
2-46

21:35

22:50

Around 23:35

reactor has improved. The same is notified to governmental agencies andother relevant organizations at 21:34. (As of 21:30, the water level of thenuclear reactor is TAF-3,000 mm.)
A monitoring car recorded a radiation dose of over 500 Sv/h at the MP i.e.,
760 Sv/h and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
for Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 22:35.
The D/W pressure exceeds its maximum working pressure, i.e., 427 kPa[gage] and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal pressure surge in a primarycontainment vessel (PCV)). The same is notified to governmental agencies
and other relevant organizations at 23:39.
The pressure on the S/C side is lower than the pressure required to operate arupture disc, and as the pressure on the D/W side is surging, the plant
determined to initiate ventilation by opening a small valve of the D/Wventilation valve.

March 15, 2011 (Tue.)
0:02

3:00

Around 6:00
to 6:10
6:50

7:00

8:11

16:00

23:05

A small valve of the D/W ventilation valve (AO valve) opened. A ventilation line
excluding rupture discs completed. (Confirmed that the valve is in closed
position a few seconds later.)
The D/W pressure exceeds its design maximum working pressure. The Station
notifies governmental agencies and other relevant organizations at 4:17 that it
is working to reduce the pressure and inject water into the nuclear reactor, but
the pressure still remains high.
The S/C pressure indicates 0 MPa (abs). (Around the same time, a big boom
is heard on the premises.)
A radiation dose of over 500 Sv/h around the main gate i.e., 583.7 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
for Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 7:00.
Notified governmental agencies and other relevant organizations that all
workers, excluding those required to remain in the plant, will evacuate to
Fukushima Daini Nuclear Power Station temporarily.
A radiation dose of over 500 Sv/h around the main gate i.e., 807 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
for Nuclear Disaster has occurred (abnormal release of radioactive
substances due to a fire and an explosion). The same is notified togovernmental agencies and other relevant organizations at 8:36. Confirmed
white smoke (steam or the like) is generated at the 5F of the R/B at 8:25. The
same is notified to governmental agencies and other relevant organizations at
9:18.
A radiation dose of over 500 Sv/h at the main gate i.e., 531.6 Sv/h observed
and the Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster has occurred (abnormal surge of radiation dose at the boundary of
the premises). The same is notified to governmental agencies and otherrelevant organizations at 16:22.
A radiation dose of over 500 Sv/h around the main gate i.e., 4,548 Sv/h2-47

observed and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency Preparedness
for Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 23:20.

2-48

Fig. 2.3-1: Flows of the Accident at Unit 2 of the Fukushima Daiichi Nuclear Power
Station after the Earthquake
2-49

Water Level in the Nuclear
Reactor (Active Fuel) (A)Water Level (mm)Water Level in the Nuclear
Reactor (Active Fuel) (B)Time

Fig. 2.3-2a: Shift of the Water Level in the Nuclear Reactor (Unit 2)

Pressure (MPa[abs])

Pressure in the
Nuclear Reactor (A)Pressure in the
Nuclear Reactor (B)Time

Fig. 2.3-2b: Shift of Pressure in the Nuclear Reactor (Unit 2)

2-50D/W Pressure

Pressure (MPa [abs])

S/C Pressure

Time

Fig. 2.3-2c: Shift of the Pressure in the Primary Containment Vessel (Unit 2)

2-51

Water Level in the Nuclear
Reactor (Active Fuel) (A)Water Level (mm)Water Level in the Nuclear
Reactor (Active Fuel) (B)Time

Fig. 2.3-3a: Shift of the Water Level in the Nuclear Reactor (Unit 2: Long Term)

Pressure in the
Nuclear Reactor (A)Pressure (MPa[abs])

Pressure in the
Nuclear Reactor (B)Time

Fig. 2.3-3b: Shift of Pressure in the Nuclear Reactor (Unit 2: Long Term)

2-52

D/W Pressure

Pressure (MPa [abs])

S/C Pressure

Time

Fig. 2.3-3c: Shift of the Pressure in the Primary Containment Vessel(Unit 2: Long Term)

Safety-Relief Valve Leak Detector(RV-2-71A)
Main Steam Isolation Valve Leak
Detector (2-86A)
Temperature of Water Feed Nozzle(N-4B)

Temperature ( C)

Temperature of Upper Part of the
CRD Housing
Temperature of Lower Part of thePressure Vessel
Temperature of Upper Part of thePressure Vessel Supporting Skirt
Temperature of Upper Part of theDrain Pipe for the Pressure Vessel
Temperature of D/W HVH Return(HVH-16A)
RPV Bellow Seal
Temperature of Gas inside of the
S/CWater Temperature of S/C Pool A
Water Temperature of S/C Pool B

Time

Fig. 2.3-4c: Shift of Temperature (Unit 2: Long Term)

2-53

2.4 Course of accident at Unit 3
2.4.1

From occurrence of earthquake to arrival of tsunami

Unit 3 was in constant thermal output operation at 2,381 WMt. Due to theearthquake, it automatically stalled at 14:47 on March 11 after receiving a ‘large
seismic acceleration’ signal and all control rods were fully inserted into thereactor.
The earthquake damaged various systems including equipment in the New
Fukushima Substation, causing power supply through Okuma No. 4 Line
(Okuma No. 3 Line had been under construction and not used) to be lost. This
resulted in total loss of external power supply to the Staion. At 14:48, the power
supply to the emergency bus conductors was lost. Two DGs for emergency use(3A and 3B) started operating immediately and automatically, and power supply
to the emergency bus conductors was restored.
Loss of power supply to the emergency bus conductors caused loss of power
supply for the nuclear reactor protection system, which resulted in automatic
closure of MSIV. This surged pressure in the nuclear reactor, but the main steam
relief valve (SRV) controlled the pressure.
The water level in the nuclear reactor was once lowered immediately after the
nuclear reactor stopped automatically, due to a void collapse. However, after thatthe level was successfully controlled by RCIC without reaching the level that
HPCI automatically starts (L-2: TAF+2,950 mm). The nuclear reactor remained
stopped automatically after receiving the ‘High Nuclear Reactor Water Level’signal at 15:25 on March 11.
As mentioned above, from the time the earthquake occurred to the arrival of thetsunami, the measures taken by the Station were those normally done when theexternal power supply is lost and there is a scram of a nuclear reactor.
2.4.2

From arrival of tsunami to hydrogen explosion of the reactor building

The first tsunami arrived in about 41 minutes after the earthquake hit the area,i.e., 15:27 on March 11, followed by the second tsunami at 15:35. This caused
inundation and submergion of the seawater pump for cooling and power supply
panel. At 15:38 on the same day, both emergency DGs (3A and 3B) stopped andAC power supply was totally lost. Due to the Station Black Out, the RHR and the
CS were unable to function. The Station determined at 15:42 on March 11 thatthe situation corresponded to an event applicable to Article 10 of the Act on
Special Law of Emergency Preparedness for Nuclear Disaster, i.e., Station BlackOut.
Due to the Station Black Out, only emergency lighting systems were available in
MCR. The ventilation system in MCR also stopped. Unit 4, located next to Unit 3,
was undergoing a regular inspection and all fuel rods were taken out of it. So,
parameters such as water level in the nuclear reactor were checked centered on
Unit 3, using a flashlight.
The DC power supply system in Unit 3 survived the inundation, and RCIC andthe HPCI could be operated by DC power supply. To use the batteries as long as
possible, all loads that were not immediately required were disconnected from
them.

2-54

The RCIC was not in operation, then. As the water level in the nuclear reactor
gradually decreased, it was manually run at 16:03 on March 11, to maintain thewater level while monitoring the operation of the RCIC.
By 21:58 on the same day, a small power generator was installed in MCR andthe temporary lighting system was restored.
From then, the water level in the nuclear reactor was maintained using RCIC.
However, at 11:36 on March 12, the RCIC automatically stopped and the waterlevel started falling again. It reached the low nuclear reactor water level (L-2:
TAF+2,950 mm) at 12:35 on the same day. This automatically started HPCI.
This restored the water level in the nuclear reactor, which was maintained for awhile thanks to the HPCI. At 2:42 on March 13, however, the HPCI automatically
stopped, losing its water injection function to the nuclear reactor.
To reverse the situation, D/D-FP, an alternative water injection method for asevere accident was used, but in vain because the pressure inside the nuclear
reactor surged to approximately 4.1 MPa [gage].
The workers tried to restart the HPCI and the RCIC, but failed to do so because
the battery ran out. So the Station determined at 5:10 on March 13 that thesituation corresponded to an event applicable to Article 15 of the Act on Special
Law of Emergency Preparedness for Nuclear Disaster, i.e., loss of capacity tocool down a nuclear reactor.
The following explain how the alternative cooling water supply was used and
how PCV ventilation was made.
Alternative Cooling Water Supply
The disaster countermeasures office in the Fukushima Daiichi Nuclear Power
Station was examining the use of fire engine as an alternative way of injecting
water into the nuclear reactor. Of the three fire engines held by the Station, one
had already been used for injecting seawater into Unit 1, and the second one
was unusable because of the tsunami. The remaining one, mobilized at the Unit
5 to 6 side was unable to be moved because the earthquake damaged the roadand the tsunami left a lot of debris. So the roads in the Station’s premises were
restored urgently and the remaining third fire engine was successfully moved to
the Unit 1 to 4 side.
Another fire engine was brought in from the Fukushima Daini Nuclear Power
Station and an alternative system to inject water into the nuclear reactor using
RHR from FP was constructed with the pumper as the driving source and awater tank for fire-protection (freshwater) as the water source.
In order to use the alternative water injection method, it was necessary to reduce
the pressure inside the nuclear reactor so that it was lower than the output
pressure of the pumper. For this purpose, batteries had to be collected tomanually open the SRV, but all batteries in the Station had already been used forother purposes including restoration of measuring instruments of Units 1 and 2.
So car batteries were collected from the cars of the plant workers that had beenparked on high ground and thus survived the tsunami, to use them as the powersource for the SRV.

2-55

The SRV was manually opened at 9:08 on March 13 and the pressure inside thenuclear reactor was rapidly reduced.
After the pressure in the nuclear reactor was reduced, the workers were able toinject water using the pumper. At 9:25 on the same day, the workers started to
inject water into the reactor after dissolving boric acid in the freshwater in thewater tank for fire-protection.
At 10:30 on the same day, the plant manager indicated he would consider using
seawater as an alternative water source.
The water tank for fire-protection became empty at 12:20 on the same day. So,
the workers altered the water injection system so that they could use seawater inthe backwash valve pit. Use of seawater started at 13:12 on the same day.
In order to replenish seawater in the backwash valve pit, the Station requested tohave some more fire engines. However, it took a long time before they arrived at
the Station, partly because of the high radiation and contamination in the Stationand partly because of the bad road conditions leading to the Station. For thesereasons, the fire engines were unable to go directly to the Station but had to behanded over to the plant workers at such places as the Off Site Center and
J-Village, and the people there then brought them to the Station. As the seawaterin the backwash valve pit was about to run out, the pumper connected to the FP
stopped once and was moved closer to the backwash valve pit at 1:10 on March
14. Then, after inserting the hose deeper into the pit, the workers started
injecting seawater again at 3:20 on the same day.
The requested fire engines arrived at the Station early in the morning of March
14. The two fire engines were parked around the cargo unloading station to
directly suck seawater from the sea and carry it to the backwash valve pit.
Replenishment of seawater to the backwash valve pit from the cargo unloading
station using the seawater sucking line started at 9:20 on the same day.
A total of 7 water trucks (each 5t) owned by the Self-Defense Forces arrived at
the Station. They were set up at the backwash valve pit and the seawater started
being replenished at 10:53 on the same day.
An explosion occurred at the R/B of Unit 3 at 11:01 on March 14, possibly ahydrogen gas explosion, which severely damaged the R/B and debris was
scattered around the area.
The explosion damaged the fire engines and hoses used for injecting water tothe nuclear reactor, and seawater injection stopped. The backwash valve pit
became unusable due to the debris and seawater replenishment by the water
trucks also stopped.
PCV Ventilation
After the plant manager instructed the workers to start preparing for PCV
ventilation at 17:30 on March 12, procedures for the PCV ventilation were
examined and locations of valves to be operated were confirmed.
At around 4:50 on March 13, in order to open the AO valve (large valve) on theventilation line connected from the S/C, the solenoid valve of the AO valve wasforcibly energized using a power source supplied by a small-sized power
generator for temporary lighting in the central operating room.

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At 5:15 on the same day, the plant manager instructed the workers to construct
a PCV ventilation system, excluding a rupture disc.
A worker checked the opening angle of the AO valve (large valve) in the torus
room and found it was closed. So air cylinders supplying force to open andclose the valve were replaced with new ones. As a result, the valve opened at
5:23 on March 23.
A press meeting was held to announce of starting a PCV ventilation at 5:50 on
March 13 and results of radiation exposure evaluation to the areas neighboring
the Station were notified to the relevant organizations at 7:35.
At 7:39 on the same day, the workers started spraying water to the PCVs using
the line structure normally used when a severe accident has occurred, using
D/D-FP, which was an alternative method of water injection.
Another MO valve on the PCV ventilation line was opened 15% manually on
site, using the method stipulated in the procedure document at around 8:35 on
the same day. At 8:41, a structure for PCV ventilation was completed, excluding
rupture discs. At this point, however, the D/W pressure was lower than theworking pressure of a rupture disc, i.e., 427 kPa [gage], and the workers
continued monitoring the D/W pressure while keeping the valves on the PCV
ventilation line open.
The D/W pressure kept increasing, thereafter. However, it started decreasing(from 0.637 MPa [abs] at 9:10 to 0.540 MPa [abs] at 9:24). So the Station
determined that the PCV ventilation was done.
The pressure of cylinders installed at the AO valve (large valve) on theventilation line connecting from S/C showed a decrease at around 9:28 on
March 13. An investigation revealed that a leakage was found at the joints ofthe cylinders and the joints were additionally tightened. At 11:17 on the sameday, the cylinder pressure fell causing the AO valve (large valve) to close. So
the cylinders were replaced with new ones and the AO valve (large valve) was
opened again at 12:30 on the same day. The workers were unable to perform
work to keep the valve open because the conditions at the site were
unfavorable.
At 14:15 on the same day, the MP indicated 905 Sv/h and the Station
determined the situation corresponded to an event applicable to Article 15 of
the Act on Special Law of Emergency Preparedness for Nuclear Disaster, i.e.,
abnormal surge of radiation dose at the boundary of the premise.
At around 14:31 on March 13, the radiation dose increased by more than
300 mSv/h at the north of the R/B double doors, and by 100 mSv/h at its
southern side and inside the R/B filled with white smoke.
In the central operating room of Unit 3, the radiation dose showed an increaseof 12 mSv/h at 15:28 on the same day, so the shift supervisor evacuated theworkers to the central operating room on the Unit 4 side.
As the IA, the driving source of the AO valve (large valve) on the ventilation linefrom S/C was not in operation, the restoration team of the Station’s disastercountermeasures office installed a tentative compressor at the service entrance
for large items of the T/B and connected it to the IA system at around 17:52 on
the same day.
The D/W pressure fell at around 20:10 on the same day, and the Station
2-57

determined that the AO valve (large valve) was open and the PCV ventilation
was done.
The D/W pressure showed an upward trend at around 2:00 on March 14 (from0.265 MPa [abs] at 2:00 to 0.315 MPa [abs] at 3:00 on March 14) and theStation decided to open another AO valve (small valve) on the ventilation linefrom S/C. At 3:40 on the same day, an operation to open the AO valve started
by forcibly energizing the solenoid valve. The AO valve became open at 6:10.
The radiation dose at the MP recorded 518.7 Sv/h at 9:12 on March 14 andthe Station determined the situation corresponded to an event applicable to
Article 15 of the Act on Special Law of Emergency Preparedness for Nuclear
Disaster, i.e., abnormal surge of radiation dose at the boundary of the premise.
2.4.3

After hydrogen gas explosion in the R/B

An explosion, possibly of hydrogen gas, occurred at the R/B at 11:01 on March
14. It severely damaged the reactor building. All workers excluding workers at
the central operating room stopped all their work and evacuated to theantiseismic building. All restoration work was suspended for a while to checkthe safety of workers, on-site condition and the safety of the site.
Due to debris generated by the explosion, the backwash valve pit was unable tobe used. Fire engines were moved to the area around the cargo unloading
station and hoses were relaid to take seawater directly from the sea to injectwater to the nuclear reactor. Additionally, two fire engines were connected
serially, to form a water feeding system to both Unit 2 and Unit 3. Seawater
injection restarted at around 16:30 on March 14.
On the other hand, for the PCV ventilation, it was difficult to energize thesolenoid valves on the air feed line while maintaining air pressure to drive theAO valve to keep the AO valves on the ventilation line (small and large valves).
An AO valve, even if it was opened, closed instantly. So, for a while the workers
repeated an operation to open the valves a number of times.
AO Valve (large valve)
Mar. 15 at 16:00: Closing confirmed
Mar. 17 at 21:00: Closing confirmed
Mar. 18 at 5:30: Closing confirmed
Mar. 19 at 11:30: Closing confirmed
AO Valve (small valve)
Mar. 15 at 16:00: Closing confirmed
2.4.4

Mar. 15 at 16:05: Opening
operation
Mar. 17 at around 21:30: Opening
operation
Mar. 18 at around 5:30: Opening
operation
Mar. 20 at around 11:25: Opening
operation
Mar. 16 at 1:55: Opening operation

Major events thereafter

To restore power supply to the Station, the workers repaired the transformers in
the New Fukushima Substation and performed bypassing work between the
Yonomori Line 1 and the Okuma Line 3. On March 18, recharging wascompleted up to the portable M/C installed in the plant premises. Lighting in the
2-58

central operating room was restored on March 22.
To build an alternative water injection to the nuclear reactor, the workers
switched the water source to freshwater supplied from deionized water tanks on
March 25. The water source was further changed to a temporary electric motor
pump driven by fire engines on March 28. From April 3 onward, the powersource to the temporary electric motor pump was switched from a temporary
power source to regular power lines.
2.4.5

Condition of spent fuel pool

As of March 11, 514 spent fuel rods and 58 new fuel rods were stored in the
spent fuel pool (SFP) of Unit 3. The Station estimated the decay heat generated
by these fuel rods in the SFP was approximately 0.54 MWt (as of March 11).
The earthquake that occurred at 14:46 on March 11 caused a total loss ofexternal power supply to the plant, and the FPC came to a halt. The massive
tsunami that hit the plant after the earthquake caused Station Blackout and thefunctions of cooling and supplying water to the SFP were lost.
The explosion of the R/B of Unit 3 that occurred at 11:01 on March 14 caused a
massive amount of debris on the SFP.
Seawater was dropped on the R/B using helicopters at around 9:48 on March
17 and generation of steam from the R/B was confirmed. The workers shotwater from water canon trucks starting from 19:05 on the same day. They
repeated the water shooting till March 25 using water canon trucks and towerwater canon vehicles (in most cases using seawater).
From March 27 onward, they used concrete pump trucks for water shooting.
The type of water was switched from seawater to freshwater on March 29.
The Station considers that these water injections were successful in keeping
the water level in the SFP sufficiently high to prevent the fuel rods from being
exposed to the air.

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Table 2.4-1 Time-Course Progress (Unit 3)
March 11, 2011 (Fri.)
14:46
Great East Japan Earthquake occurs. Alert Level 3 automatically issued.
14:47
Scram of nuclear reactors occurs automatically and main turbine
manually stops.
Around 14:48 Emergency DG automatically activated.
14:54
Subcriticality of the nuclear reactor confirmed.
15:05
RCIC manually started.
15:06
A disaster countermeasures office established in the TEPCO head
office. (to understand the damage conditions due to the earthquake andrestoration from blackout)
15:25
RCIC trips (due to high water level in the nuclear reactor).
15:27
First tsunami arrives.
15:35
Second tsunami arrives.
15:38
Station Blackout occurs.
15:42
The Station determined the situation corresponds to a special event
applicable to Paragraph 1, Article 10 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster (Station Blackout).
Notified the same to governmental agencies and other relevantorganizations.
15:42
Alert Level 1 issued. An emergency task force established (as joint
headquarters with disaster countermeasures office).
16:03
RCIC manually started.
16:36
Alert Level 2 issued.
20:50
Fukushima Prefecture directs local residents living within a 2 km radiusfrom the Fukushima Daiichi Nuclear Power Station to evacuate.
21:23
The Prime Minister directs local residents living within a 3 km radius fromthe power plant to evacuate and those living within a 3 km to 10 km
radius to stay indoors.
21:58
Temporary lighting in MCR turns ON.
March 12, 2011 (Sat.)
0:30
Completion of evacuation of the local residents confirmed by theGovernment of Japan (confirmed evacuation of people living within 3 km
from the plant in Futaba City and Okuma City, and reconfirmed it at1:45).
4:55
Confirmed surge of radiation dose within the plant premises surges(around the main gate: 0.069 Sv/h (at 4:00) 0.59 Sv/h (at 4:23)).
The same is notified to governmental agencies and other relevantorganizations.
5:44
7:11
8:04
11:36
12:35
17:30
18:25

The Prime Minister directs local residents living within a 10 km radiusfrom Fukushima Daiichi Nuclear Power Station to evacuate.
The Prime Minister arrives at the Fukushima Daiichi Nuclear Power
Station.
The Prime Minister leaves the Fukushima Daiichi Nuclear Power Station.
RCIC trips.
HPCI automatically runs (due to low water level in the nuclear reactor)
The plant manager instructs the workers to start preparing for PCV
ventilation.
The Prime Minister directs local residents living within a 20 km radiusfrom the Fukushima Daiichi Nuclear Power Station to evacuate.
2-60

March 13, 2011 (Sun.)
2:42
HPCI stops.
5:10
Water injection using RCIC is failed so the Station determines that a
special event applicable to Paragraph 1, Article 15 of the Act on Special
Law of Emergency Preparedness for Nuclear Disaster has occurred(nuclear reactor cooling function lost). The same is notified togovernmental agencies and other relevant organizations at 5:58.
5:15
The plant manager instructs the workers to complete the ventilation line
up excluding a rupture disc.
5:50
Held a press meeting on initiation of PCV ventilation.
6:19
Determined at 4:15 as TAF has been reached. The same is notified togovernmental agencies and other relevant organizations.
7:35
Notified governmental agencies and other relevant organizations of theresult of radiation exposure evaluation to be released when PCV
ventilation is done.
7:39
Started spraying PCVs. The same is notified to governmental agencies
and other relevant organizations at 7:56.
8:35
Ventilation valves (MO valves) of PCVs opened.
8:41
After opening the large valve of the S/C ventilation valves (AO valves),
the ventilation line is completed, excluding a rupture disc. The same isnotified to governmental agencies and other relevant organizations at
8:46.
8:56
A radiation dose of over 500 Sv/h at the MP i.e., 882 Sv/h observed
and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental
agencies and other relevant organizations at 9:01.
Around 9:08 Safety-relief valves used for rapid release of pressure in the nuclear
reactor. Notified the governmental agencies and other relevantorganizations at 9:20 that from then onward water would be injected into
the nuclear reactor using the FP line.
9:25
Started fresh water injection (with boric acid) from the FP line to the
nuclear reactor using fire engines.
9:36
The ventilation operation confirms that the D/W pressure is being
decreased from around 9:20. The Station notifies the governmental
agencies and other relevant organizations that it has started injecting
water into the nuclear reactor using the FP line.
10:30
The plant manager announces that the Station may use seawater
injection as an option.
11:00
The Prime Minister directs local residents living within a 20 km to 30 km
radius from the Fukushima Daiichi Nuclear Power Station to stayindoors.
11:17
Confirmed a large valve of the S/C ventilation valve (AO valve) is closed(due to a decreased working air cylinder pressure).
12:20
Completed freshwater injection.
12:30
A large valve of the S/C ventilation valve (AO valve) opens. (Replaced
the working air cylinders.)
13:12
Seawater injection to the nuclear reactor starts from the FP line using fireengines.
14:15
A radiation dose of over 500 Sv/h at the MP i.e., 905 Sv/h observed
2-61and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental
agencies and other relevant organizations at 14:23.
March 14, 2011 (Mon.)
1:10
Seawater in the pit to feed to the nuclear reactor is running out. Stopped
the fire engines for replenishing seawater into the backwash valve pit.
2:20
A radiation dose of over 500 Sv/h at around the main gate i.e., 751 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified togovernmental agencies and other relevant organizations at 4:24.
2:40
A radiation dose of over 500 Sv/h at the MP i.e., 650 Sv/h observed
and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental
agencies and other relevant organizations at 5:37.
3:20
Seawater injection using the fire engines resumes.
4:00
A radiation dose of over 500 Sv/h at the MP i.e., 820 Sv/h observed
and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental
agencies and other relevant organizations at 8:00.
5:20
Starts opening operation of a small valve of the S/C ventilation valve (AOvalve).
6:10
Confirmed that the S/C ventilation valve (AO valve) is open.
9:12
A radiation dose of over 500 Sv/h at the MP i.e., 518.7 Sv/h observed
and the Station determines that a special event applicable to Paragraph
1, Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental
agencies and other relevant organizations at 9:34.
9:20
Started replenishing seawater from the cargo unloading station to thebackwash valve pit.
11:01
An explosion occurs in R/B, damaging the fire engines and hoses.
Stopped seawater injection.
Around 16:30 Constructed a new line that feeds seawater from the cargo unloading
station to the nuclear reactor. Replaced the damaged fire engines andhoses with new ones.
21:35
A monitoring car recorded a radiation dose of over 500 Sv/h at the MPi.e., 760 Sv/h and the Station determines that a special event
applicable to Paragraph 1, Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster has occurred (abnormal
surge of radiation dose at the boundary of the premises). The same isnotified to governmental agencies and other relevant organizations at
22:35.
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March 15, 2011 (Tue.)
6:50
A radiation dose of over 500 Sv/h around the main gate i.e., 583.7
Sv/h observed and the Station determines that a special event
applicable to Paragraph 1, Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster has occurred (abnormal
surge of radiation dose at the boundary of the premises). The same isnotified to governmental agencies and other relevant organizations at
7:00.
7:00
Notified the governmental agencies and other relevant organizations
that all workers except for monitoring staff and other workers required forrestoration of the Station would evacuate to Fukushima Daini Nuclear
Power Station.
7:55
Confirmed steam floating above the R/B. The same is notified togovernmental agencies and other relevant organizations.
8:11
A radiation dose of over 500 Sv/h around the main gate i.e., 807 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal release ofradioactive substances due to a fire and an explosion). The same isnotified to governmental agencies and other relevant organizations at
8:36.
16:00
A radiation dose of over 500 Sv/h at the main gate i.e., 531.6 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified togovernmental agencies and other relevant organizations at 16:22.
23:05
A radiation dose of over 500 Sv/h around the main gate i.e., 4,548
Sv/h observed and the Station determines that a special event
applicable to Paragraph 1, Article 15 of the Act on Special Law ofEmergency Preparedness for Nuclear Disaster has occurred (abnormal
release of radioactive substances due to a fire and an explosion). The
same is notified to governmental agencies and other relevantorganizations at 23:20.

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Fig. 2.4-1: Fukushima Daiichi Nuclear Power Station Unit 3 accident progress flow
after the earthquake
2-64Water Level in the Nuclear
Reactor (Active Fuel) (A)Water Level (mm)Water Level in the Nuclear
Reactor (Active Fuel) (B)Time

Fig. 2.4-2a: Shift of the Water Level in the Nuclear Reactor (Unit 3)

Pressure (MPa[abs])

Pressure in the
Nuclear Reactor (A)Pressure in the
Nuclear Reactor (B)Time

Fig. 2.4-2b: Shift of Pressure in the Nuclear Reactor (Unit 3)

2-65

D/W Pressure

Pressure (MPa [abs])

S/C Pressure

Time

Fig. 2.4-2c: Shift of the Pressure in the Primary Containment Vessel (Unit 3)

2-66

Water Level in the Nuclear
Reactor (Active Fuel) (A)Water Level (mm)Water Level in the Nuclear
Reactor (Active Fuel) (B)Time

Fig. 2.4-3a: Shift of the Water Level in the Nuclear Reactor (Unit 3: Long Term)

Pressure in the Nuclear
Reactor (A system)

Pressure (MPa[abs])

Pressure in the Nuclear
Reactor (B system)

Time

Fig. 2.4-3b: Shift of Pressure in the Nuclear Reactor (Unit 3: Long Term)

2-67

D/W Pressure

Pressure (MPa [abs])

S/C Pressure

Time

Fig. 2.4-3c: Shift of the Pressure in the Primary Containment Vessel(Unit 3: Long Term)
Temperature of the Water Feed
Nozzle (N4B)
Upper Part of the Head at the
Bottom of RPV
RPV Body Flange
Temperature of Upper Part of thePressure Vessel

Temperature ( C)

Temperature of RPV Stud Bolt
Temperature of Lower Part of the
RPV Body Flange
Leakage at Safety Relief Valve(2-71D)
Leakage at Safety Relief Valve(2-71F)
Leak-Off Temperature of the Main
Steam Isolation Valve (2-86A)
Return Temperature of D/W HVH
RPV Bellow Seal
Water Temperature of S/C Pool A
Water Temperature of S/C Pool B

Time

Fig. 2.4-4c: Shift of Temperature (Unit 3: Long Term)

2-68

2.5 Course of accident at Unit 4
Unit 4 had been undergoing a regular inspection since November 30, 2010 and all
fuel rods had been taken out of the nuclear reactor to its SFP to replace its shroud.
A total of 1,535 fuel rods (97% of the total storage capacity) were in the SFP andtheir decay heat was relatively high (decay heat: approximately 2.26 MWt as ofMarch 11).
The water level in the SFP was around the overflow level and the temperature wasapproximately 27 C. On the nuclear reactor side, the pool gate was closed andfilled with water.
The earthquake that occurred at around 14:47 on March 11 damaged various
systems including equipment in the New Fukushima Substation, causing a total
loss of external power supply. However, a stand-by emergency DG automaticallystarted (another DG was undergoing a regular inspection) and the necessary
power supply to the system was secured.
The massive tsunami that hit the plant at 15:38 on March 11 made the emergency
DG stop and a Station Black Out occurred. This made it impossible to cool down
the SFP. Temporary lighting equipment only was available in the central operatingroom.
After restoration work by the restoration team of the Station’s disastercountermeasures office, a small-sized power generator was installed in the central
operating room and a temporary lighting system became available at 21:58 on thesame day.
Then, the water temperature in the SFP reached to 84 C at 4:08 on March 14.
A very loud sound was heard at around 6:00 on March 15. Damage was found to
the roof of the 5th Floor of the R/B of Unit 4. In addition, at 9:38 on March 15, a fire
was observed around the northwestern corner on the 3rd Floor of the R/B. An
on-site investigation started at around 11:00 on the same day, but the fire hadalready been automatically extinguished.
Workers onboard a helicopter of the Self-Defense Forces made an approach to its
operating floor on March 16. They visually noted the water surface of the spent fuelpool and confirmed that fuel rods in it were not exposed.
Water injection by the Self-Defense Forces and by a high-pressure water canon
owned by the U.S. forces was carried out on March 20 and March 21 respectively.
Another water injection started on March 22 using a concrete pumper.
The workers investigated the R/B, and found that its 5th Floor and western andeastern sides on the 4th Floor were seriously damaged. The Station estimated thathydrogen was present around the 4th and 5th Floors. Water was sampled from thepool. From the fact that only a small amount of radioactive substance wascontained in it as well as from the result of observations via undergroundphotography, the Station considered that most fuel rods were intact. Therefore,
there was only a small possibility that the fuels in the SFP had heated up andgenerated hydrogen.
In addition, the Station assumed that the water level in the SFP had been
maintained in a way that any fall in the water level due to evaporation was being
supplemented by water on the well side poured into the SFP by way of the gate,
and this prevented the fuel rods from being exposed.
2-69

A pipe for the Standby Gas Treatment System (hereafter called “SGTS”) of Unit 4
joins together with that of Unit 3 just before the exhaust pipe. The Station therefore
points out that the flow of gas ventilated from Unit 3 might have flowed into Unit 4
through the SGTS pipes.

2-70

Table 2.5-1 Time-Course Progress (Unit 4)
March 11, 2011 (Fri.)
14:46
Great East Japan Earthquake occurs. Alert Level 3 automatically issued.
15:06
A disaster countermeasures office established in the TEPCO head office (tounderstand damage conditions due to the earthquake and restoration fromblackout).
15:27
First tsunami arrives.
15:35
Second tsunami arrives.
15:38
Station Black Out occurs to Unit 4.
15:42
The Station determines that a special event applicable to Paragraph 1,
Article 10 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (Station Black Out) to Unit 1*, Unit 2*, Unit 3*,
Unit 4* and Unit 5*. The same is notified to governmental agencies andother relevant organizations.
* Corrected on April 24, 2011. The Station Black Out occurred at Unit 1, 2
and 3 only.
15:42
Alert Level 1 issued. An emergency task force established (as joint
headquarters with the disaster countermeasures office).
16:36
Alert Level 2 issued.
20:50
Fukushima Prefecture directs local residents living within a 2 km radiusfrom the Fukushima Daiichi Nuclear Power Station to evacuate.
21:23
The Prime Minister directs local residents living within a 3 km radius fromthe power plant to evacuate and those living within a 3 km to 10 km radius
to stay indoors.
March 12, 2011 (Sat.)
0:30
Completion of evacuation of the local residents confirmed by theGovernment of Japan (confirmed evacuation of people living within 3 km
from the plant in Futaba City and Okuma City, and reconfirmed it at 1:45).
4:55
Development of radiation dose within the plant premises is confirmed. The
same is reported to the governmental agencies and other relevantorganizations.
5:44
The Prime Minister directs local residents living within a 10 km radius from
Fukushima Daiichi Nuclear Power Station to evacuate.
7:11
The Prime Minister arrives at the Fukushima Daiichi Nuclear Power Station.
8:04
The Prime Minister leaves the Fukushima Daiichi Nuclear Power Station.
16:27
A radiation dose of over 500 Sv/h at the MP i.e., 1,015 Sv/h observed
and the Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations.
18:25
The Prime Minister directs local residents living within a 20 km radius fromthe Fukushima Daiichi Nuclear Power Station to evacuate.
March 13, 2011 (Sun.)
8:56
A radiation dose of over 500 Sv/h at the MP i.e., 882 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at the2-71

14:15boundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 9:01.
A radiation dose of over 500 Sv/h at the MP i.e., 905 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 14:23.

March 14, 2011 (Mon.)
2:20
A radiation dose of over 500 Sv/h at around the main gate i.e., 751 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified togovernmental agencies and other relevant organizations at 4:24.
2:40
A radiation dose of over 500 Sv/h at the MP i.e., 650 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 5:37.
4:00
A radiation dose of over 500 Sv/h at the MP i.e., 820 Sv/h observed andthe Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 8:00.
4:08
Temperature in the SFP of Unit 4 is confirmed as 84 C.
9:12
A radiation dose of over 500 Sv/h at the MP i.e., 518.7 Sv/h observed
and the Station determines that a special event applicable to Paragraph 1,
Article 15 of the Act on Special Law of Emergency Preparedness for
Nuclear Disaster has occurred (abnormal surge of radiation dose at theboundary of the premises). The same is notified to governmental agencies
and other relevant organizations at 9:34.
21:35
A monitoring car recorded radiation dose of over 500 Sv/h at the MP i.e.,
760 Sv/h and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified togovernmental agencies and other relevant organizations at 22:35.
March 15, 2011 (Tue.)
Around 6:00 to 6:10
A large sound is heard. Damage to around the roof of the 5th Floor of Unit 4
R/B is confirmed.
6:50
A radiation dose of over 500 Sv/h around the main gate i.e., 583.7 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified to the2-72governmental agencies and other relevant organizations at 7:00.
The Station notifies the governmental agencies and other relevantorganizations that it has confirmed damage around the roof of the 5th Floorof Unit 4 R/B.
8:11
Damage to the Unit 4 R/B is confirmed. A radiation dose of over 500 Sv/haround the main gate i.e., 807 Sv/h observed and the Station determinesthat a special event applicable to Paragraph 1, Article 15 of the Act on
Special Law of Emergency Preparedness for Nuclear Disaster has occurred(abnormal release of radioactive substances due to a fire and an explosion).
The same is notified to governmental agencies and other relevantorganizations at 8:36.
9:38
A fire is confirmed around the northwestern corner on the 3rd Floor of theUnit 4 R/B. The same is notified to the governmental agencies and otherrelevant organizations at 9:56.
Around 11:00 Workers of TEPCO makes an on-site investigation of the fire that occurredat Unit 4 R/B, but they confirm that the fire has been automaticallyextinguished. The same is notified to the governmental agencies and otherrelevant organizations at 11:45.
16:00
A radiation dose of over 500 Sv/h at the main gate i.e., 531.6 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal surge ofradiation dose at the boundary of the premises). The same is notified togovernmental agencies and other relevant organizations at 16:22.
23:05
A radiation dose of over 500 Sv/h around the main gate i.e., 4,548 Sv/hobserved and the Station determines that a special event applicable toParagraph 1, Article 15 of the Act on Special Law of Emergency
Preparedness for Nuclear Disaster has occurred (abnormal release ofradioactive substances due to a fire and an explosion). The same is notified
to governmental agencies and other relevant organizations at 23:20.
7:55

2-73

2.6 Course of accident at Unit 5
Unit 5 had been suspended since January 3, 2011 due to a periodic inspection.
When the earthquake occurred, the reactor was loaded with fuel, and pressure
leak tests of the RPV were being conducted with the control rod fully inserted.
(RPV was full of water, and pressure of the nuclear reactor was approximately 7MPa [gage], and temperature of the nuclear reactor was approximately 90 C). The
water level of the SFP was full (around the overflow level), and water temperatureof the pool was 24 C.
At 14:47 on March 11, the collapse of the steel towers of Yorunomori transmission
line due to the earthquake caused a loss of external power supply, which
automatically activated two emergency DGs.
After the earthquake occurred, the CRD pump that had been applying pressure on
the reactor for the pressure leak test stopped due to a loss of power supply,
leading to a temporary pressure drop in the reactor. In addition, although operationof the spent fuel cooling and filtering system pump also stopped as a result of theloss of power of external power supply, the RHR was put into standby state as thewater temperature of the pool was sufficiently low and time was not pressing.
Then, the two emergency DGs stopped at 15:40 on March 11 due to the hugetsunami, resulting in the station blackout. As a result of this, the RHR and CS
pumps became inoperable.
In the central operating room on the side of Unit 5 only temporary emergency
lighting was available, which then went out. However, some of the monitoring
instruments could be checked thanks to the DC power supply even after theStation Blackout.
As the pressure of the reactor increased due to decay heat, the workers attempted
to reduce the pressure using the RCIC steam line and such like, but no change
occurred. Around 1:40 on March 12, the SRV repeated automatic opening andclosing operations by the safety valve function, and the pressure of the reactor wasmaintained at around 8 MPa [gage]. (The maximum operating pressure was 8.27
MPa [gage]).
At 6:06 on March 12, the workers opened the vent valve on top of the RPV to
reduce the pressure of the RPV.
One emergency DG of Unit 6 was operating. Thus, at 8:13 on March 12, theworkers utilized the power interchange cable to Unit 5, which had been laid as acountermeasure against severe accidents, and supplied electricity to part of the
low voltage power panel of the R/B of Unit 5 from the emergency DG of Unit 6. In
this way, they secured a power supply for instruments and such like. At 18:29 on
March 13, they laid a temporary cable from the low voltage distribution board of
Unit 6 and the MUWC pump was manually started at 20:54 on March 13.
In the early hours of March 14, the SRV was restored so that it could be operated
from the central operating room. After 5:00 on March 14, the workers reduced thepressure with the SRV as appropriate, and the pressure of the reactor wasmaintained at 2 MPa [gage] or lower. In addition, by using the alternative water
injection line installed as a countermeasure against severe accidents, the workers
started injecting water into the reactor from the CST by the MUWC pump at 5:30
2-74

on March 14. From then, they injected water intermittently to adjust the water levelof the reactor. At 9:27 on March 14, they also supplied water to the SFP.
From March 16 to 17, part of the SFP water was drained to the S/C and suppliedby the MUWC pump, and an increase in the water temperature of the SFP wascontrolled.
On March 17, in order to install a temporary submersible pump the workers
removed debris scattered by the tsunami and such like. Then, they prepared a
power supply on March 18, and installed a temporary seawater pump for the RHR
in the water intake trench, and activated it on March 19. As the high voltage power
panel in the basement of T/B could not be used due to flooding, power was
supplied to the RHR pump by directly laying a temporary cable from the power
panel of Unit 6. Then, the RHR pump was started around 5:00 on March 19, andthe SFP began to be cooled.
Then, the Station decided to alternately cool the SFP and reactor by switching thesystem configuration. The workers started cooling the reactor at 12:25 on March
20. At 14:30 on March 20, the reactor achieved a cold shutdown.

2-75Table 2.6-1 Main Chronology (Unit 5)
March 11, 2011 (Friday)
14:46
Great East Japan Earthquake occurred.
14:47
The emergency DGs of Unit 5 were automatically activated.
15:27
The first tsunami arrived.
15:35
The second tsunami arrived.
15:40
Total Blackout
15:42
The Station determines that for Unit 1*, Unit 2*, Unit 3*, Unit 4*, and Unit 5*,a specific event (Total Blackout) based on Article 10 of the Special Law on
Emergency Preparedness for Nuclear Disaster occurred, and theauthorities were notified accordingly.
* On April 24, 2011, a correction was made to the effect that the specific
event had occurred only at Unit 1, Unit 2, and Unit 3.
March 12, 2011 (Saturday)
0:09
TEPCO departed for the site at Unit 5 and Unit 6 to inspect the power
supply system in the station.
About 1:40 The SRV was automatically opened. (From then, pressure of the reactor
was maintained at approximately 8 MPa through repeated opening andclosing.)
6:06
The pressure of the RPV was reduced by opening the valve on the top of
the RPV.
8:13
Electric power interchange to Unit 5 (part of the DC power supply) was
enabled by the provided cable from the DG of Unit 6.
14:42
With the power supply from the DG, the air conditioning system on the sideof Unit 6 of the MCR emergency ventilation and air conditioning system of
Unit 5 and 6 was manually started and cleaning of air in the MCR of Unit 5
and 6 was started.
March 13, 2011 (Sunday)
18:29
Supply of power by the temporary cable from the DG of Unit 6 to the MUWC
started.
20:54
The MUWC pump was manually started.
20:54
The SGTS was manually started.
March 14, 2011 (Monday)
5:00
The SRV was opened and pressure of the RPV was reduced. (From then,the opening operation was conducted intermittently.)
5:30
Water injection into the reactor by the MUWC was started. (From then,water was injected intermittently.)
9:27
Supply of water to the Spent Fuel Pool was started. (From then, the supply
was conducted intermittently.)
March 16, 2011 (Wednesday)
22:16
Change of water in the Spent Fuel Pool was started.
March 17, 2011 (Thursday)
5:43
Change of water in the Spent Fuel Pool completed.

2-76March 18, 2011 (Friday)
13:30
Workers completed making holes (at three locations) in the roof of thereactor building.
March 19, 2011 (Saturday)
1:55
The RHR temporary seawater pump was activated by the temporary power
supply from the power supply vehicle.
4:22
The second DG of Unit 6 was started
About 5:00 The RHR was manually started. (Cooling of the Spent Fuel Pool wasstarted in emergency load mode.)
March 20, 2011 (Sunday)
10:49
The RHR was manually stopped. (Emergency thermal load mode.)
12:25
The RHR was manually started. (Cooling of the reactor was started in stopcooling mode.)
14:30
The water temperature of the reactor fell below 100 C and the reactor
achieved a cold shutdown.

2-77

2.7 Course of accident at Unit 6
Unit 6 had been suspended since August 14, 2010 due to a periodic inspection.
When the earthquake occurred, the reactor was loaded with fuel, and the top lid of
the RPV was tightened by bolts. The reactor was in a cold shutdown condition andthe control rods were fully inserted. The duty person before the earthquake
occurred confirmed that the water level of the SFP was full (around the overflow
level), and that water temperature of the pool was 25 C.
At 14:47 on March 11, the collapse of the steel towers of Yorunomori transmission
line due to the earthquake caused a loss of external power supply, which
automatically activated three emergency DGs. As a result of the loss of external
power supply, operation of the RHR and FPC halted. As the reactor had been in acold shutdown condition before the earthquake, the water temperature of the SFP
was sufficiently low and time was not pressing. Thus, the RHR and FPC were put
into a standby state.
Then, at 15:36 on March 11, affected by the massive tsunami, two emergency DGs(6A, HPCSDG) lost functionality due to the flooding of the seawater pumps or
power panel and such like. However, one emergency DG (6B) continued tooperate as it was of an air-cooled type and the power panel was still usable. This
enabled a supply of power necessary to allow the Station to supply water into thereactor. The RHR, Low Pressure Core Spray System, and High Pressure Core
Spray System (hereafter called “HPCS”) became inoperable due to a loss of poweror submerged seawater pumps.
The pressure in the reactor moderately increased due to the decay heat after theearthquake occurred; however, the rate of increase was more modest than that ofUnit 5 because a longer period of time had elapsed after the halt.
At 13:01 on March 13, the MUWC pump was started, and at 13:20, injection ofwater into the reactor from the CST was started. After March 14, the SRV wasused to reduce pressure as appropriate, and the reactor pressure and water level
were controlled.
The workers tried to cool the SFP, but as the seawater pumps were unusable dueto the massive tsunami, they stirred the water in the pool several times after March
16 with the FPC pump that was still operable with the one remaining emergency
DG.
From March 17, in order to install a temporary submersible pump, the workers
conducted debris removal work, prepared a power supply, and did other tasks.
They installed a temporary seawater pump for the RHR in the water intake trenchand started it on March 19. Then, at 22:14 on March 19, the RHR pump wasstarted and cooling of the SFP was started.
As the soundness of the DG seawater pump (6A) flooded by the tsunami was
confirmed on March 18, the same seawater pump was started at 19:07 on March
18, and the emergency DG (6A) was started at 4:22 on March 19.
Then, the Station decided to alternately cool the reactor and SFP by switching thesystem configuration, and the reactor started to be cooled at 18:46 on March 20.
The reactor achieved a cold shutdown at 19:27 on March 20.

2-78

Table 2.7-1 Main Chronology (Unit 6)
March 11, 2011 (Friday)
14:46
Great East Japan Earthquake occurred.
14:47
The three emergency DGs of Unit 6 were automatically activated.
15:27
The first tsunami arrived.
15:35
The second tsunami arrived.
15:36
The two DGs of Unit 6 tripped.
March 12, 2011 (Saturday)
8:13
Electric power interchange from the emergency DGs of Unit 5 and Unit 6
was enabled.
March 13, 2011 (Sunday)
13:20
With the power supply from the emergency DG of Unit 6, water injection by
the condensate transfer pump was started. (Then, water was injectedintermittently.)
March 14, 2011 (Monday)
14:13
Supply of water into the Spent Fuel Pool was started. (Then, water was
supplied intermittently.)
March 16, 2011 (Wednesday)
13:10
The FPC was manually started. (Cycle operation without heat removal
capability.)
March 18, 2011 (Friday)
17:00
Workers completed making holes (at three locations) on the roof of thereactor building.
19:07
The DG seawater pump was started.
March 19, 2011 (Saturday)
4:22
The second DG was started.
21:26
With the temporary power supply of the power supply vehicle, the RHR
temporary seawater pump was started.
22:14
The RHR was manually started. (Cooling of the Spent Fuel Pool wasstarted in emergency load mode.)
March 20, 2011 (Sunday)
16:26
The RHR was manually stopped. (Emergency thermal load mode)
18:48
The RHR was manually started. (Cooling of the reactor was started in
stopped cooling mode.)
19:27
The water temperature of the reactor fell below 100 C and the reactor
achieved a cold shutdown.

2-79

2.8 Status of Dose dose Rate rate Around around Power power
Stationstation
As the MP halted, the dose rate around the power station could not be measured
with MP. However, measurements were carried out with the monitoring car thatwas deployed as a replacement of MP in the vicinity of the main gate of the station.
Fig. ure 2.8-1 shows the measurement results.
In Unit 1, around 14:00 on March 12, the temporary air compressor for operating
the major valve of the S/C vent valve (AO valve) was installed, and a drop in theD/W pressure was confirmed at 14:30. Since From then, the workers performed
vent operations, including those for Unit 2 and Unit 3, were performed several
times.
For the dose rate, no big change in the background level before and after thesevent operations was observed. However, from 4:00 to 7:00 on March 12, the dose
rate increased. In addition, around 6:14 on March 15, the sound of an explosion
was confirmed, and the background level of the dose rate increased before andafter it.
Around 14:30 to
W/W ventingof Unit 1

Dose Rate (µSv/h)

10:17 toNo pressurechange in D/W of
Unit 1

Around 9:20 toW/W ventingof Unit 3
13:30 toW/W
venting ofUnit 3

15:36
Explosion of
Unit 1 building

11:01 toAround 6:14
Explosion ofSound of
Unit 3 building
explosion
5:20 to16:00 to
W/W
Pressureventing ofchanged in
Unit 3D/W of Unit 3.

16:50 toW/W
venting ofUnit 3The backgroundlevel increased
before venting
started.

Date

Fig. 2.8-1 Fukushima Daiichi Nuclear Power StationFukushima Daiichi Nuclear
Power StationNuclear Power Plant
Dose Rate in the Vicinity of Main Gate

2-80

2.9 Evaluation of Core core Conditions conditions of Unit 1 to Unit 3
at the Time time of Accident accident
TEPCO implemented the event progress analysis on the core conditions of Unit 1
to Unit 3 when the accident occurred, by using the severe accident analysis code.
The analysis result is outlined in the following.
The analysis result indicates that in the shortest case, the reactor core damage
started in Unit 1, 4 hours after the earthquake occurred. In comparison of with theshortest cases, the result also shows that the core damage started in Unit 2 and in
Unit 3, respectively, 77 hours and 42 hours after occurrence of the earthquake.
The time to the core damage in Unit 2 and Unit 3 was extended longer by theperiod of time during which the respective RCIC or HPCI worked.

2.9.1

Evaluation of Core core Conditions conditions of Unit 1 at tthe Timeof Accidentaccident•

In the analysis, tThe analysis was implemented, assuming the condition of
gaseous leak from the containment vessel (about 18 hours after occurrenceof the earthquake) and operating conditions of the IC (it became inoperable
after the station blackout). As a result, the main development is as shown
below:
Time when exposure of the core started:
About three hours after occurrence of the earthquake
Time when the core damage started:
About four hours after occurrence of the earthquake
Time when the RVP was broken:
About 15 hours after occurrence of the earthquake
After the assumed timing of the IC stop, the water level of the reactor
reached the Top of Active Fuel in about two hours, then and this leading to
the rector damage.
Although the reactor pressure increases after the assumed timing of the IC
stop, it remains around 8 MPa (abs) because of the Safety Relief Valve. Afterthe reactor core is damaged, the melted pellets move to the lower plenum.
Then, about 15 hours after the earthquake, the RPV breaks and the reactor
pressure decreases rapidly.

•

•

•
•

•

•

The pressure of the containment vessel temporarily increases because of
the steam released from the RPV and hydrogen gas formed by the reactionof water and metal in the reactor. However, after that, the pressure shows adecreasing trend because of the leakage from the containment vessel
assumed in the analysis, and then it decreases rapidly by the vent operation
on March 12.
The PCV temperature is over 300°C about 18 hours after occurrence of theearthquake, and well exceeds the designed temperature of the containment
vessel (138°C).
Hydrogen is generated almost simultaneously with the core damage, andthere is the possibility that the explosion of March 12 was caused by thehydrogen generated then.
2-81

•

Regarding radioactive substances to be released as a result of core damage,almost all noble gases are released to the atmosphere by the vent operation.
According to the analysis result, approximately 1% of cesium iodide and lessthan about 1% of other nuclides are released.

2.9.2

Evaluation of Core core Condition condition of Unit 2 at the Time of
Accidentaccident•

TIn the analysis, the analysis was implemented, assuming the condition ofgas phase leakage from the containment vessel (about 21 hours after theearthquake) and the following two cases of the amount of water injected by
the fire pumps.
As a result, the main development is as shown below:(Case1) It is assumed that the workers injected an amount of water thatallowed the reactor water level to be almost comparable with themeasured value was injected.

•

•

•

•

•

(Case 2) It is assumed that the workers injected an amount of water that
did not allow the reactor water level to be maintained in the fuelrange was injected.
Time when exposure of the core started:
Approx. 75 hours after occurrence of the earthquake
Time when the core damage started:
Approx. 77 hours after occurrence of the earthquake
Time when the RVP was broken:
About 109 hours after occurrence of the earthquake (iInthe case of Case 2)
The reactor water level gradually goes down after the RCIC stops, and thereactor core starts to be exposed. After the SRV is opened, the reactor coredamage starts.
The reactor pressure is kept high around the SRV operating pressure until
the RCIC stops. As a result of opening of the SRV following the RCIC stop,
the reactor pressure decreases rapidly, then going goes down to nearatmospheric pressure.
The pressure in the containment vessel gradually increases as the watertemperature of the suppression pool rises. The opening of the SRV following
the RCIC stop results in a temporary pressure buildup, and then thepressure shows a decreasing trend because of the leakage from the
suppression chamber assumed in the analysis. (Note that it is currently
unknown whether the assumed leak from the containment vessel actuallyoccurred or whether that was only a problem of instrumentation.)
A large amount of hydrogen results from the reaction of metal and water in aperiod when the reactor core is exposed and the temperature of fuelcladding tube starts to rise.
Of the radioactive substances to be released due to the core damage, noble
gases are released from the pressure vessel to the Suppression Chamber.
With the assumed leak from the Suppression Chamber, almost totall
volume of the noble gases is are to be released into the environment. For
cesium iodide, the analysis result shows that the release rate is 1% or lower,
and most of it exists in the Suppression Chamber. (It should be noted,
2-82however, that behavior of radioactive substances greatly depends on theeffect of the analysis conditions or uncertainty in models.)
2.9.3

Evaluation of Core core Conditions conditions of Unit 3 at the Timeof Accidentaccident•

In the analysis, tThe analysis was implemented, assuming the following twocases of the amount of water injected by the fire pumps.
As a result, the main development is as shown below:(Case1) It is assumed that the workers injected an amount of water thatallowed the reactor water level to be almost comparable with themeasured value was injected.

•

•

•

•

•

(Case 2) It is assumed that the workers injected an amount of water that
did not allow the reactor water level to be maintained in the fuelrange was injected.
Time when exposure of the core started:
Approx. 40 hours after occurrence of the earthquake
Time when the core damage started:
Approx. 42 hours after occurrence of the earthquake
Time when the RVP was broken:
About 66 hours after occurrence of the earthquake (inthe case of Case 2)
The reactor water level gradually goes down after the HPCI stops, and thereactor core starts to be exposed. After the SRV is opened, the reactor coredamage starts.
The reactor pressure is kept high around the SRV operating pressure until
the HPCI stops. As a result of opening of the SRV following the HPCI stop,
the reactor pressure decreases rapidly, then going goes down to nearatmospheric pressure.
The pressure in the containment vessel gradually increases as the watertemperature of the suppression pool rises. Although the pressure temporarily
goes high due to the opening of the SRV following the HPCI stop, it
decreases by venting from the Suppression Chamber. Then, the pressure
repeats repeatedly increasing/es and decreasingdecreases, depending on
the venting operations.
A large amount of hydrogen results from the reaction of metal and water in aperiod when the reactor core is exposed and the temperature of fuelcladding tube starts to rise. The possibility is pointed out that the explosion ofMarch 14 was caused by the hydrogen generated in this period.
Of the radioactive substances to be released due to the core damage, noble
gases are released from the pressure vessel to the Suppression Chamber,and almost total volume ofall the noble gases is are to be released into theenvironment by venting. For cesium iodide, the analysis result shows thatthe release rate is about 0.5%, and most of it exists in the Suppression
Chamber. (It should be noted, however, that behavior of radioactive
substances greatly depends on the effect of the analysis conditions or
uncertainty in models.)

2-83Source Material(1) Report of Japanese Government to the IAEA Ministerial Conference on Nuclear
Safety, 2011 June
• http://www.kantei.go.jp/jp/topics/2011/pdf/houkokusyo_full.pdf• http://www.kantei.go.jp/jp/topics/2011/pdf/app_full.pdf

2-84Chapter 3

Analysis of Causes for Accident Events andExtraction of Problems

Problems of the accident are that cores were damaged and that radioactivity wasdischarged to the environment. We analyzed the accident to identify the causes forthe accident with an event tree and extracted problems to clarify what made theaccident expand and diminish.
In addition, in order to pick up problems exhaustively, we made analyses with thefocus on functional aspects of “to shut off”, “to cool down” and “to confine”.

3.1

Flow of Progress of Accident Events

The event tree of progress of events in each reactor is shown in Fig. 3.1-1.
The event tree was prepared in a configuration that allows to tell how powerfunctions (DC power, external power and emergency DG) were lost by the initial
event (earthquake) and the following tsunami, short-term core cooling operations(systems that don’t depend on AC power such as IC, RCIC, and others) andtransferring operations to cool shutdown (reactor depressurization, water injection
to reactor, container ventilation, and others) or success or failure of mitigation
measures (cooling damaged core and container and reactor building control). Also,it was prepared so that it clearly shows actions in the case where the power isrestored and functions of facilities can be restored (existing and establishedsevere accident management) and actions in the case like this time where power
restoration was not possible and functions of facilities are difficult to restore.
For Reactor No. 1, in addition to loss of external power due to the earthquake, lossof emergency DG due to tsunami made all AC power unavailable. Besides, DC
power was lost too. It was verified that the IC worked until the tsunami attacked thepower plant. It is assumed that the core was damaged after arrival of the tsunami.
Then, it is assumed that hydrogen generated in the core leaked from the container
to the reactor building causing hydrogen explosion and resulted in discharge ofradioactivity to the environment.
For Reactor No. 2, because of loss of external power due to the earthquake andloss of emergency DG due to tsunami, all AC power was lost. Besides, DC power
was lost too. Cooling the core was secured by the RCIC which was started to work
prior to loss of DC power and worked for less than three days until around 13:30 of
14th. But, it is guessed, since it took time to implement alternate water injection
after stop working of the RCIC and injected water amount was too little, the core
was damaged.
For Reactor No. 3, because of loss of external power due to the earthquake andloss of emergency DG due to tsunami, all AC power was lost. Besides, DC power
was lost too. But the RCIC and the HPCI worked and secured core cooling forone-and-a-half-day until DC power was ran out at around 2:40 of 13th. But, it isassumed, since it took time to implement alternate water injection after stop of the
RCIC and the HPCI and injected water amount was too little, the core wasdamaged. After that, it is guessed, hydrogen generated in the core leaked to the
PCV and the R/B causing hydrogen explosion and resulted in discharge ofradioactivity to the environment.

3-1

For Reactor No. 4 (under suspension due to periodical inspection), because ofloss of external power due to the earthquake and loss of emergency DG due totsunami, all AC power was lost. Besides, DC power was lost too. There were fuelsof relatively high decay heat taken out from the core shortly before that time in the
SFP but water in the pool could not be cooled with the FPC due to loss of power.
And, 5th floor of the Reactor No. 4 building was damaged by hydrogen explosion.
Afterward, water injection to the SFP was executed with a concrete pump vehicle.
In the Reactor No. 5 (under suspension due to periodical inspection), fuels were
loaded in the reactor and a test for pressure resistance and leakage of the RPVwith the control rods being fully inserted was performed at the time of theearthquake. Although all AC power was lost due to the earthquake and thetsunami, AC power was secured by the power provided from the Reactor No. 6.
The seawater pumps in the RHR line were damaged by the tsunami, but the
makeshift seawater pumps were used for the RHR and the spent fuel pool and thereactor well were cooled by the RHR line. With this, the reactor got in a sate ofcold shutdown.
The Reactor No. 6 (under suspension for periodical inspection) was in a state ofcold shutdown with all fuels being loaded and the upper lid of the RPV being shut.
Since external power was lost due to the earthquake three emergency DGs
started to work automatically but two units of them became inoperable because
functions of the seawater pump and the power panel were lost due to attack of thetsunami. As AC power was secured with remaining one unit (air-cooling type),
power supply for the reactor, cooling the SFP and for the Reactor No. 5 was
performed. The seawater pumps in the RHR line were damaged by the tsunami,
but the makeshift seawater pumps were used for the RHR and the reactor well
was cooled by the RHR line. With this, the reactor returned to the sate of cold
shutdown.

3-2

Scram signal Reactor shut Alternative Current (AC) DC power sou Core cooling Restoration of Ensuring of long term cold shut-down
Big
ReactorExternal
Emergency DC Power
IC, Turbine
External
RPV
Reactor
RHR
RHR
earthquake scram
power
DG
supplydriven coolant Powerdepressurizatio water
restoratio
sourceinjection
source,ninjectionnsystemEmergency(Including(Including(RCIC,HPCI) DG, Power
alternative
alternativethat do notsupplysources)sources)requireinterchange
alternative

Status of Core
PCV Vent Cold shutdown, Core damage,PCV damage etc.
(Beforecore
damage)Damaged core coolin Containment vessel control
Reactor building Final status
Reduction Reactor PCV
PCV Vent Hydrogen SGTS, Other
Cold shutdown, Core damage, PCV damage etc.
of reactor coolant
water
control
ventilationpressure injection injection(nitrogensubstituti
on etc.)Cold shutdown
Cold shutdown(A)

Cold shutdown(Long term cooling is required)
PCV damage (Overpressure)

Similar to (A)Similar to (A)RCIC/HPCISoundness of PCV or PCV leakage (Overtemperature
Cold shutdown (Long term cooling is required)
Battery iscollected
temporarily(Successful)Radioactive discharge (PCV leakage (Overtempera
PCV damage (Overpressure)(Continuation)

１F-3

PCV damage (Hydrogen explosion)

Earthquake

Restoration is not possible(Not executed)(Continuation)[Tsunami]

Accident sequence
assumed by 1F-3

PCV damage (Overpressure)Core damage(Quantity of water actually injected to the reactor is less)

PCV damage(MCCI etc.)

Core damage
PCV damage (DCH etc.)

Accident sequence
assumed by 1F-2

Soundness of PCV or PCV leakage (Overtemperatu
Similar to (A)Cold shutdown (Long term cooling is required)
Battery iscollected
temporarily

1F-2

１F-1,2RCIC (Starts
before loss of Restoration is not possible
DC power)[Tsunami]

PCV leakage (Overtemperature) and Hydrogen combu(Continuation)
PCV damage (Overpressure)

PCV damage(Hydrogen explosion)(Not executed)(Continuation)

PCV damage (Overpressure)Core damage(Actual quantity of water injected to the reactor is less)

PCV damage(MCCI etc.)

Core damage
PCV damage(DCH etc.)

Similar to (A)Cold shutdown (Long term cooling is required)Soundness of PCV or PCV leakage (Overtemperature

1F-1
ing till arrival of the tsunami, operation period after arrival of tsunami is not clear)

PCV damage (Overpressure)(RPV low pressure state)
Restoration is not possible

Radioactive discharge (PCV leakage (Overtempera
Accident sequence
PCV damage (Hydrogen explosion)
assumed by 1F-1Core damage(Delay in water injection or actual quantity of water injected to the reactor is less(RPV)
low pressure state)Core damage(Not executed)
PCV damage (Overpressure)
PCV damage (MCCI etc.)

Diagram 3.1-1

Event tree of event progress in Fukushima No.1 nuclear power plant, Unit no.1- Unit no. 3

3-3

3.2

Extraction of subject from accident event progress

3.2.1 Cause analysis from accident event progress (event tree)
3.1 The following three items have been extracted as the main factors of coredamage and emission of radioactivity to the environment from the progress flowof accident event (event tree).
1) Inability of all AC power supply
2) Inability of heat removing from nuclear reactor
3) Hydrogen leakage to building and hydrogen explosion
Cause analysis of the above-mentioned three factors was executed to extract
the subject.
Moreover a common factor which obstructed work accomplishment was dug up
through the cause analysis. These factors were analyzed to extract the subject.
(1) Inability of all AC power supply
Figure 3.2.1-1 shows the result of analysis of the cause that AC power
could not to be supplied.
Before strike of tidal waves, the AC power was supplied by the externalsource before the earthquake occurred and it was supplied by the
emergency DG after the earthquake occurred.
The cause where AC power could not be supplied from the view of theequipment and operation includes inability of power supply from theexternal source, inability of power supply by emergency DG, inability of joint
use of power supply and inability of power supply restoration at early stage.

3-4

Analysis of factors
Execution phase

Issues that should be examined[Unit no. 1 - 4]
Operation of protective
device by the contact oftransmission and tower[Unit no. 1, 2]
Unable tosupply powerby External

Reliability of External powersource (substation,transmission line, Switchyard)
for earthquake , Tsunami

Damage to receiving circuit
breaker, Disconnecting
Switch in the switchyard[Unit no. 3, 4]
Assumption of Tsunami inthe switchyard and flood

switchyard immersed in the
water

Interruption of
Supply ofalternate current
source(Station
blackout）[Unit no.1-4 (Air cooling type diesel generator in Unit no.２，４ is excluded)]
It was notpossible tosupply powerby emergency

Circuit of External powersource is concentrated in
1 location at theAssumption for tsunami to
the building where the diesel
generator is installed andflood

Emergency Diesel
generator was immersed in
the water due to tsunami①

Flood to the installation
area of Emergency Diesel
generator equipment

For the factor analysis
of ①：[ Station
Blackout], refer to these
items

Water flooded into thebuilding from the openingof the building,

Loss of cooling function of
auxiliary system(Sea water pump)

For the factor
analysis, refer to ⑥
in the diagram 3.2.12⑥[Air-cooling type Diesel generator of Unit no. 2, 4]
Power panel was
submerged due totsunamiAssumption of Tsunami to thebuilding where emergency
high voltage power panel islocated and the flood②
Flood to the installation
area of emergency highvoltage power panel

Water flooded into the building
from the opening, penetrationof the building

Power could not beinterchanged(Countermeasure for

Early electric
power
restoration was
not possible

Power panel of adjacent
plant was immersed in thewater

Alternate current source ofadjacent plant could not be
maintained

Difficulty in deploymentof power unit
(Emergency
arrangement)

Difficulty in arrival due todamage of roads , traffic

Power panel was
submerged

Power panel is notreusable②

For factor analysis, refer toabove ②

Access failure due todamage of roads inthe premises, floating

Securing heavyequipment for ensuring
the accessibility of thedCable laying operation
became difficult (Weight,
Distance, Workenvironment)

Multiple routes within thepremises

Securing material andequipment to maintain thenighttime, indoor work
environment , Provision ofprocedure/training

Difficulties in the operation caused byenvironment
・nighttime operation
・Outdoor work under the warning ofmassive tidal waves

Loss of meansofcommunication

Loss of power ofcommunication
facility
Loss of power ofalternative
communication facility①

Communication
facility was immersedin the water

For factor analysis, refer
to abovementioned ①

Securing the communication
facility (Means ofcommunication, quantity)and power supply for

Quantity ofalternative
communication④

Work difficulty due toradioactivity, highdose③
Maintaining the number of
persons required tocontrol the radiation

Ensuring radiation
control staff at the timeof emergency

Difficult y in securing required
number of staff for Personal
dosimeter, protective equipment,
Management system equipment

Radiation control
system equipments
were immersed in the

Difficulty in ensuring
necessary personnel due
to radiation exposure

For the analysis of factors
for ③ :[Inability to promptly
restore power supply],refer to following items

Exchange of materials andequipment between
electrical power

Access within thebuilding became difficult
due to high dose rate
Outdoor debris removal
operation became difficult
due to radioactive

Diagram 3.2.1-1 Analysis of causes for interruption of supply of alternate current power
3-5

For the analysis offactors for ④: [loss ofcommunication means],refer to following items

Communication facility
was flooded

Damage of temporary
cable due to explosion of
adjacent plant（After the coredamage)For the analysis offactors for ②:[Submersion of power
panel due to the
Tsunami], refer tofollowing items⑤

For the factor analysis of⑤：[Difficulties in thework] after reactor
damage,refer to these items

a. Inability of power supply from the external source

Inability of power supply from the external sourc
The power supply from the outside was able to be received from the new
Fukushima substation. After the earthquake, however, the power supply
lost the function with the damage of the substation equipment due to theearthquake, with the damage of receiving breaker and disconnecting switch
in the switchyard in the power plant due to the earthquake, and with the
movement of protection instrument caused by the contact of power line andsteel tower etc.
Note that traces where tidal waves reached were confirmed at theswitchyard of Units 3 and 4.
From these points the following is listed as the examining subject tomaintain the power supply by the external source.
 Reliability of external source (substation, power line and switchyard) toearthquake and tidal waves
 Flooding of switchyard by tidal waves
 Provision to concentration of external source lines to one place insubstation (multiplexing of line)
b. Inability of power supply by emergency diesel generator

The power had been supplied by emergency DG after the earthquake. Afterthe strike of tidal waves, functions of seawater pumps were lost except Unit
6 and functions of DG were lost because of flooding into the building.
Although air-cooled emergency DG of Units 2 and 4 did not encounterflooding because of tidal waves, they lost power supply functions because
the high-voltage power supply panel was flooded.
From these points the following is listed as the examining subject tomaintain the power supply by emergency DG.
 Flooding to installation area of emergency power generating machine(DG and power supply panel) Flooding into building from opening and through part of building Flooding on cooling seawater system pump for emergency DG
Note that AC power could be supplied for Units 5 and 6 by air-cooled
emergency DG of Unit 6.

c. Inability of joint use of power supply

Joint use of power supply from adjacent unit prepared for severe accident
was not performed for Units 1 to 4 since the power was lost on all theseunits.
Joint use of power supply could be performed since the AC power had been
supplied for Units 5 and 6 from emergency DG of Unit 6.
d. Inability of power supply restoration at early stage (excluding Units 5

and 6 for which AC power was secured)
Since the AC power was lost, an attempt was made to restore the power
supply by arranging power source cars for emergency for prompt procuringoutside the power plant. As a result, however, the power supply could not
be restored before the core became damaged.
3-6

Work of early power supply restoration was frequently interrupted by the
influence of aftershock and official announcement of tidal wave warning.
Moreover night work or work under official announcement of tidal waves
warning caused to lower its efficiency.
In work in the building (cable laying work), interior lighting was lost alongwith the loss of the power supply. Work was done by using the limited
amount of flashlight etc. In addition the communication equipment (paging
and security telephone) excluding some apparatuses becomes impossible
to use because of flooding or losing the power supply. It became impossible
to use the cellular phone and PHS gradually by the consumption of thebattery. Communication means between work sites, or with MCR or
emergency station were broken off and work became difficult.
From these points the following is listed as the examining subject to restore
the power supply at early stage
Flooding on power supply panel
Moreover the following is listed as the subject to improve the effectivenessof recovery efforts.
 Securing the machine and material to secure working environment innighttime or room Securing the heavy equipment to secure accessibility of road
 Multiple route in the plant Communication equipment (communication means and amount) andsecuring the power supply for communication including batteries Flooding on communication equipment
Furthermore, "Maintenance of procedure and training" is also extracted asa subject necessary to improve the effectiveness of work though it is not
limited to the power supply restoration.
After the hydrogen explosion after reactor core damage had occurred,difficult situation to work was caused.
Workers got injured by the hydrogen explosion of the reactor building. In
Unit 1, a loss such as the damage of cable installed for emergency to
execute alternative water filling using the boric acid water filling pump was
encountered. Moreover labor and time were spent on work to remove
wreckage polluted with radioactivity, and time was required for outdoor
work.
In addition a lot of personal dosimeters and the radiation control system areflooded by tidal waves. Hence personal dosimeters became insufficient andthe number that all workers wore could not be secured.
From these points the following is listed as the examining subject to be
extracted.
 Joint use of the machine and material between electric powers
 Securing of radiation control workers for emergency
 Flooding on radiation control system

3-7

(2) Inability of heat removing from nuclear reactor (excluding Units 4, 5
and 6 being stopped)
Figure 3.2.1-2 shows the result of analysis of the cause that heat could not
be removed from the nuclear reactor.
IC (Unit 1), HPCI (Units 1 - 3) and RCIC (Units 2 and 3) were built as theheat removing equipment and as the high pressure system equipment todecompress the nuclear reactor.
In addition in Unit 1, SRV (relief valve function/automatic decompressionfunction) to decompress the nuclear reactor and CS that conducts injection
to the reactor core at low pressure were installed. In Units 2 and 3, RHR(low-pressure injection mode) was installed in addition to SRV and CS.
Moreover in Units 1 - 3, the containment vessel vent unit to prevent
overpressure of containment vessel and the severe accident measureequipment were provided.
The cause of heat removing impossible from nuclear reactor from the viewof the equipment and operation includes inability of decompression by SRV,
inability of heat removing by CS and RHR, inability of heat removing bysevere accident measure equipment (alternative water filling), and difficultyin containment vessel vent.

3-8

Analysis of factors
Execution phase

Injection by HPCI,RCIC is impossible

Issues that should be examined

Loss of control power
supply①

Station Blackout

Injection by IC is notpossible

Loss of power supply
necessary for valve driving[Unit no. 1, 2]
Storage battery andpower panel were
submerged

Loss of direct current
power supply

Difficult y indepressurization bymain steam relief safetyvalve

Inability to remove
the heat by Core
spray system andresidual heatremoval system

D.C. power supply
system (Storage Battery,
Power panel) was[Unit no. 3]

Loss of power supply
necessary for valve
driving

Depletion ofStorage battery

Difficulty indepressurization
operation by main steam
relief safety valve

Inability toremove the heatfrom the reactor

For analysis of factors,
refer to ① in the diagram3.2.1-1

Getting alternative powersource by the battery installedon the vehicle

Ensuring storage battery
capacity

Ensuring material and equipment(Storage
battery etc.),
Providing procedures and training[Unit no. 2, 3]
Source of power of low
pressure flooder pumpetc. is not dependable

Station Blackout

Loss of cooling function of
auxiliary system (Sea waterpump)

Submersion of sea
water pump①

For analysis offactors, refer to ①
in digram3.2.1-1

Assumption of Tsunami
to sea water system andflood

Ensuring means of coolingby alternative methods

Power supply of sea
water pump could not
be maintained

Difficulty in early
restoration of Sea
water pump

Power panel issubmerged

Access failure due to damage ofinternal roads , floating objects②

For factor analysis,refer to ②in diagram
3.2.1-1

Securing heavy equipmentfor
ensuring the accessibility of the
roads

⑥

Multiple routes within thepremises
Operational difficultiescaused by theenvironment
・Work in the nighttime
・Outdoor work under warning of massive
tidal waves

Huge workload (Laying operation of
power source cable) and task experience

Loss of means ofcommunication

Inability to remove theheat by severe
accidents
countermeasure system

Unusable condensate
water makeup system(Severe accidentscountermeasures)Containment & Vessel
Cooling Water System could
not be used
(Severe accidentscountermeasures)

Securing material andequipment to maintainthenighttime, indoor work
environment , Provision ofprocedures and trainings

For the analysis offactors for ⑥：[loss ofcooling function (Seawater pump) of auxiliary
system). refer tofollowing items

Providing procedures andtrainings④

For factor analysis, refer to④ in the diagram3.2.1-1

Power panel was
submerged due to
Tsunami②

Early restoration ofalternative power source
was not possible③

Power panel was
submerged due to Tsunami②

Early restoration ofalternative power source
was not possible③

For factor analysis, referto ③ in the diagram 3.2.1-1⑥

For analysis of factors, refer
to abovementioned②②

For analysis of factors, refer to② in the diagram 3.2.1-1

For factor analysis, refer to② in the diagram 3.2.1-1

For factor analysis, refer to③ in the diagram 3.2.1-1

For factor analysis, refer to② in the diagram 3.2.1-1

Submersion of sea
water pump

Early restoration of Sea
water was impossible

Fire Protection System
could not be used(severe accidentscountermeasures)

Power panel was
submerged due to
TsunamiDifficulties in making
fire-fighting vehicle
ready to use

Difficulties in
depressurization ofreactor

Difficulties in
the work

For analysis of factors, refer to② in the diagram 3.2.1-1(Unit no. 1, 2 )Power panel was
submerged due to Tsunam②(Unit No. 3)
Drain of Direct current
power supply

Ensuring capacity ofstorage battery

Access failure due to damageof internal roads, floating
objects

Securing heavy equipment for
maintaining the accessibility of theroads
Multiple routes within thepremises

Operational difficultiescaused by theenvironment
・Work in the nighttime
・Outdoor work under the warning omassive tidal waves

Loss of means ofcommunication

Depletion of fire prevention
water tanks

Securing material and equipment tomaintain the nighttime and indoor work
environment , Provision of procedures
and trainings④

For analysis of factors, referto④ in the diagram 3.2.1-1

Securing source of water necessary
for alternative water injection

Evacuation of field,high dose debris,
damage of hose for injection, re-laying due
to explosion of neighboring plant

Reactor container
vent difficulties（After coredamage)Loss of power supply ofMotor Operated Valve

Securing driving source ofvalve that constitutes
pressure resistant vent line

Air Operated Valve driving
source failAcquiring material and equipment in case of lossof valve driving source, Provision of procedures and trainings

Difficulties in work due toradioactivity, high dose⑤

Diagram 3.2.1-2 Analysis of causes of inability to remove heat from the reactor

3-9

For analysis of factors, refer
to ⑤ in the diagram 3.2.1-1

a. Inability of cooling by emergency condenser, high pressure injection

system and reactor core isolation cooling system
IC in the Unit 1 was manually shut down before the tsunami reached thepower station to protect the cooling rate. However the DC power was lostunder the impact of tsunami with the valve closed, operation became
impossible. Thereafter, an operation to open the valve was conducted andgeneration of steam was confirmed. Nevertheless, the extent of theoperation performance is unknown. Actually, we suppose that only limited
level of cooling could be achieved.
The HPCIs in the Unit 1 and Unit 2 were in the stand-by mode, because
they did not reach the start-up condition before arrival of the tsunami.
However, because their DC power was lost under the impact of the tsunami,
their functions were lost. The HPCI in the Unit 3 started up automatically
due to the drop of reactor coolant level. Then, the system must have shut
down due to the drop of reactor pressure, which led to an operation inability
due to deficit of DC power.
The RCIC in the Unit 3 was started up manually after the reactor scram.
The system was started up and shut down for adjustment of reactor waterlevel adjustment. While the operation was continued even after the arrivalof tsunami, the operation must have shut down due to the exhaustion ofbattery power.
Based on above, following items are listed as agenda for review to ensure
maintenance of functions of IC, HPCI and RCIC.
 Submergence of DC power systems (Battery cell, power unit) Security of battery capacityb. Inability of depressurization by main steam safety relief valve

In the Unit 1 and Unit 2, the DC power necessary to drive SRVs was lost
due to submergence under the impact of the tsunami. With regard do theUnit 3, its function was lost with other components opeated by DC power on
March 13 due to exhaustion of DC power, while the DC power was notaffected by submergence caused by the tsunami.
To achieve alternative coolant injection, it was necessary to depressuize
the reactor by operating the SRVs. Therefore, it was necessary to open an
electromagnetic valve for supplying nitrogen as a source to drive the SRVs.
In the Unit 3, an operation to open electromagnetic valves was tried byusing on-board battery of fire engines when they came to the site foralternative water injection. Because the battery cells store in the station
were used for the recovery of instrumentation in the Unit 1 and Unit 2,
batteries were collected from the vehicles of employees who were working
in the on-site disaster countermeasure headquarters and connected to the
instrumentation panels. Finally, after 6 hours from the loss of high pressure
injection system function, the SRV were opened and rapid depressurizationof the reactor could be achieved.
Based on above, following items are listed as agenda for review to maintaindepressurization function by SRVs.
 Inability of supplying DC power（See 3.2.1(1)）
 Submergence of AC power systems（Battery cell, power unit）
 Security of battery capacity
3-10 Security of materials and equipment to ensure SRV operation in theevent of loss of DC power and implementation of the procedure and thetraining thereofc. Inability to remove residual heat by core spray system (Units 1~3) and

residual heat removal system (Units 2/3)
The cause of inability to remove residual heat by using CS and RHR (lowpressure injection mode) is attributable to an inability to supply AC power
and the function loss of sea water pumps for cooling related auxiliarycomponents. The cause of inability to supply AC power was alreadyexplained in 3.2.1(1). The cause of cooling function loss in sea water
pumps was the submergence caused by tsunami that exceeded theestimated highest level (O.P.5.7m). All of the sea water pumps lost theirfunctions simultaneously.
With regard to the recovery of sea water pumps, considerable time wasspent due to multiple factors as below: -Access to sea pump installation
area was extremely restricted due to the damage caused by the
Earthquake and the tsunami, while spare sea water pumps weremaintained; -Works were interrupted repeatedly due to aftershock; -Work
efficiency was poor because of the issue of great tsunami alarm at night etc.
Besides the sea water pumps, the electric units necessary for the functionof component cooling system were submerged. Therefore, early restorationof this system was extremely difficult.
Based on above, following items are listed as agenda for review to maintainresidual heat removal function by CS and RHR.
Estimation of tsunami and submergence of sea water systems
Security of cooling method by using alternative approaches
Inability to remove residual heat by systems for severe accident
countermeasure
As systems for severe accident countermeasure for the loss of injection
function, MUWC, CCS and FP had been prepared.
However, almost none of these could be used for most of the systems due
to loss of entire AC power and the function loss of auxiliary component
cooling system.
While it was stipulated to use FPs as an alternative injection system basedon severe accident countermeasures, function of the motor operated fire
extinguishing pumps was lost due to power loss. While the pumps driven bydiesel motor could be operated temporally, they were shut down later due
to malfunction. It seems that these pumps failed to achieve start-up
operation in the Unit 2, the cause of this failure is unknown, making it
difficult to judge whether or not this event is attributable to the component
failure. In the Unit 3, these pumps operated temporarily and shut down later.
Details of this event are unknown.
While water injection to the reactor was tried by using fire engines that weremaintained as a part of earthquake preparedness after the earthquake that
attacked Kashiwazaki Kariwa Nuclear Power Station, considerable time
was spent before actually injecting water to the reactor due to multiple
factors as below: -Access to the outdoor area was extremely difficult;-Works were interrupted repeatedly due to aftershock; -Work efficiency waspoor because of the issue of great tsunami alarm at night etc. As a
3-11

consequence, it was impossible to inject water before core damage.
Although the fresh water injection to the reactor was started by using thefire water tank as a water source, the water source was switched to the sea
water reserved in back wash valve pit in the afternoon on March 12 due toexhaustion of the fresh water.
Further, injury of operators occurred due to hydrogen explosion in thereactor building. Hoses and fire engines prepared for alternative injection
were damaged by hydrogen explosion and made unavailable. Materials
and equipment left behind on site caused disturbance in outdoor works later,which became a contributing factor of prolonged work period.
Based on above, following items are listed as agenda for review to secureresidual heat removal function by using systems for severe accident
countermeasure: Loss of AC power Loss of sea water pump function
 Security of water source necessary for alternative injectiond. Containment vent work

Purpose of the containment venting is to secure integrity of the reactor
containment unit by relieving elevated containment pressure. Further, theventing may be used to discharge heat when reactor heat is accumulated
inside the containment due to the loss of safety-related systems.
In the Units 1~3 of Fukushima Daiichi Nuclear Power Station, the purposeof the vent work at the initial phase was to relieve pressure.
In the Units 1~3, motor operated valves and AO valves were closed on thelines that constituted the concerned systems before conducting thecontainment vent.
With regard to the motor operated valves, the entire AC power was lost. On
top of it, pneumatic air for actuating the AO valves was lost. These events
resulted in the inability to start early venting by remote control.
In the case of the Unit 1, motor operated valves were opened manually. The
AO valve that constitutes the main line (=large valve) could not be easily
opened by manual operation based on its structure. Therefore, the small
valve was opened, instead. However, the core damage had occurredalready at that time and atmospheric dose in this area was high. Besides,
the work was continued in the darkness in narrow space. These factors
prevented access to the valve. The operators gave up their plan to open thevalve manually and determined to use the temporary compressor for thework. As a consequence, it took a long time until the start of venting.
While venting was finally achieved on the Unit 1 and Unit 3, it took a longtime to reconstruct a vent line due to the loss of power and pneumatic air. It
was not clear whether or not the venting was achieved in the Unit 2,
because the containment pressure failed to reach the actuation pressure of
rupture disc continuously for long time after the reconstruction of the ventline.
Based on above, following items are listed as agenda for review to secure
implementation of containment vent: Security of power to drive valves that constitute the containment vent line Security of materials and equipment to achieve prompt line configurationin the loss of electric power / pneumatic source to drive the valves
3-12 Improvement of procedure and training(3) Hydrogen leakage to building and hydrogen explosion (excluding
Units 5 / 6 where hydrogen explosion did not occur）
Analysis results of the cause of hydrogen leakage to the building andhydrogen explosion are indicated in Figure 3.2.1-3. Hydrogen leakage tothe building is estimated to be attributable to the hydrogen generated fromfuel cladding due to core damage. This hydrogen seems to have leaked out
to the reactor building via containment vessel. With regard to the leakage
from the containment to the reactor building, leakage through the flange
gasket or penetration sealing with the increase of containment pressure issuspected. With regard to the Unit 2, pressure is lost from the S/C and thecontainment at around 6 o’clock on March 15. Further investigation isneeded to identify the leakage route from RPV and the containment upon
convergence of the accident.
With regard to the Unit 4, generation of large amount of hydrogen in the fuelpool is unlikely based on the analysis of nuclide by sampling the fuel poolwater in addition to the result of visual observation of the fuels in the poolusing monitor cameras.
Meanwhile, the SGTS exhaust pipe in the Unit 4 merged with the exhaust
pipe of the Unit 3 before the stack. Therefore, the hydrogen generated in
the Unit 3 may have migrated in the Unit 4 building via the SGTS exhaustpipe, resulting in a hydrogen explosion in the Unit 4 reactor building after
being accumulated there for a while. Based on the radiation measurement
result, radiation dose was the highest at the exit of SGTS filter train in the
Unit 4. The dose gradually deceased when the measurement point is closer
to the inlet. These results suggest the appropriateness of theabove-mentioned scenario. While AO valve was installed on the SGTS
exhaust pipe, the valve was stuck open due to the loss of pressurized
instrumentation air for normal operation as well as inability to get air supply
for valve actuation from back-up system (dedicated air cylinder) in the eventof containment venting due to power loss.
In every plant, prior detection of hydrogen leakage into the reactor building
could not be achieved. Besides, there was no mean available for removing
the hydrogen under the loss of AC power.
Based on above, following items are listed as agenda for review to preventhydrogen leakage to the buildings and the hydrogen explosion in the eventof hydrogen negation due to core damage etc.:
 Hydrogen migration among the units connected via common stack Migration of hydrogen from containment vent line to the reactor building Accumulation of hydrogen in the reactor building Security of materials and equipment to remove hydrogen gas from thebuilding
 Improvement of procedures and trainings

3-13

Factor analysis
Implementation stage

Hydrogen leakage tobuildings and hydrogen
explosion

Prevention ofhydrogengeneration

Hydrogen generated
in reactor pressure
vesselAgenda for review

Maintenance of cooling
functionCore damage

Maintenance of confining
function

Prevention ofhydrogenaccumulation

Hydrogen leakage toreactor building

Further investigation isnecessary to identify
the leakage route frompressure vessel

Damage ofpressure vessel

Units 1/3
Further investigation isnecessary to identify
the leakage route fromcontainment

Damage of containment or leakage from flange
gasket / penetration sealing

Unit 4
Possibility of hydrogenmigration from merger point of
exhaust pipe at Unit 3 venting

Common SGTS exhaust
pipe
Migration of hydrogen
between units via common
exhaust pipe

No equipment to prevent
migration between thebuildings

Hydrogen migration toreactor building from theexhaust pipe

Accumulation ofhydrogen in reactor
building

Air-operated valve wasstuck open due to theloss of actuation air

No equipment to remove hydrogenfrom reactor building

Difficulty of conducting the work to
remove hydrogen from reactor
building (Removal of blow-out
panel)

Migration of hydrogen to thebuilding from containment
vent line

Hydrogen accumulation in thereactor building

Impossible to detectaccumulation of hydrogen inreactor building

Procedure and material/equipment to remove hydrogenwas not accessible

Security of material andequipment to remove hydrogengas from the building

Implementation of procedureand training

Figure 3.2.1-3: Hydrogen leakage to buildings and hydrogen explosion: Cause analysis

3-14

3.3

Arrangement of problems confirmed from a functional
viewpoint

Fukushima Daiichi Nuclear Power Station Reactor Unit 1 to 4, located adjacent
to each other, were different in condition when the earthquake and tsunami
occurred, i.e., unit 1 to 3 were producing their rated power, while unit 4 was in stop
for the periodic-inspection (with all fuel discharged). In addition, unit 5 and 6 of thepower station are located away from unit 1 to 4. These requirements cause
differences in the progress of phenomena.
3.3.1 Causal analysis of the accident in unit 1 to 3 and management ofissuesWhen a nuclear power station is producing its full rated power, the functions
necessary for placing it in a cold shutdown state are "stopping," "cooling," and"confining," and causes of failures in the functions have been analyzed. The
basic requirements (facility environment) necessary for the implementation of
the operations, "stopping," "cooling," and "confining," have also been analyzed.
Common factors (permanently installed power source unavailable, alternative
power supply (power supply car) unavailable, and unavailable coolingcomponents) derived from them have furthermore been analyzed.
Tables 3.3-1 to 3.3-3 show the results of analyses on the causes of theaccident, classified by functions of the units (unit 1 to 3) producing rated
power, and the results of arrangement.

3-15

Table 3.3-1 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 1 and Sorting of the problems (1/2)
Safety function
Emergency reactor shut-downfunction

Stoppage

Alternative reactivity control

Sub-criticality maintenancefunction

Related facility, etc.
Safety protection system,
control rod and control rod
drive system

Function loss or function degradation

ProblemCause analysis○(Normal operation when earthquake occurred)--

Recirculation pump trip (AM)-(Not required as scram was successful)--

Alternative control rod
insertion (AM)-(Not required as scram was successful)--

Borated water injection system-(Not required as scram was successful)-(1) Reliability of switchyard circuit breaker

External power source

X

Loss of external power supply due to the earthquake

Receiving circuit breaker of Ookuma Line 2L in the switchyard of
Unit 1/2 damaged due to the earthquake

Emergency diesel generator

X

Function loss due to submergence

Submergence of emergency diesel generator due to the tsunami(2) Submergence of the emergency diesel generator

Function loss due to submergence

Submergence of 6.9 kV high pressure power supply panels due
to tsunami(3)

Function loss due loss of 6.9 kV high pressure power
supplySubmergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to submergence

Submergence of 480V low pressure power supply panels due totsunami6.9 kV high pressure power
supplyXPower supply function
480V low pressure power
supplyX

125V DC power supply

X

Function loss due to submergence

Power supply interchange(AM)

X

Power supply interchange unavailable from adjacent
plantsSame as (3)(4)

Submergence of 480V low pressure power supply
panels(5)

Submergence of 125V low pressure power supply
panels(6)

Submergence of power supply panels of theadjacent plant(7)

Procurement of alternative power supply fromoutsideSubmergence of 125V DC power supply panels due to tsunami
Loss of station power of adjacent plant (Unit 2) due to tsunami
Difficulty in reaching due to damaged roads and traffic jams

Difficulty in reaching

Unable to connect due to submergence of DC power supply
panels of Unit 1
Supportsystems

Alternative power supply
function

Difficulty in laying cables (weight, isolation, work environment)
Power source car (emergency
arrangement)

X

Power supply interruption

Cables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building

Function loss due to submergence

Submergence of containment vessel cooling sea water pump
due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

X

Submergence of cooling sea water pump due to the tsunami

Function loss due to submergence

Auxiliary cooling function

Sea water cooling system

X

Function loss due loss of component cooling sea
water system

Function to prevent overpressurization of reactor
coolant pressure boundaryX

Main steam relief safety valve(safety valve function)-

High pressure core injection
systemX

Emergency condenser (mainbody)
High pressure core cooling
function
Emergency
condenser(support
system)△(Temporary
operation)

Demineralized
water supply
systemX

Fire protection
system△(Temporary
operation)

Borated water injection system(AM)

Control rod drive water system(AM)

Reactor depressurization
function

Cool
down

Low pressure core cooling
function

Main steam relief safety valve(relief valve function /automatic depressurization
function)Core spray system

XX(9) Communication means during blackouts(10)

Same as (3)(11)

Same as (5)Submergence of cooling sea water pump due to the tsunami
Same as (11)Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)(Controlled with emergency condenser)

Function loss due to loss of DC power supply--

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Deterioration in function due to loss of DC power
supply (remote operation not possible)

Submergence of 125V DC power supply panels due to tsunami

Isolated with isolation signal transmission (Open
halfway due to blackouts)

Pipe break detection circuit operational due to loss of DC power
supplyFunction loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Unable to use electric pump due to loss of external
power supply

Receiving circuit breaker of Ookuma Line 1L damaged due tothe earthquake

Same as (1)Diesel driven extinguishing pump shutdown

Mechanical malfunction (presumed)

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiDifficulty in power supply recovery operation[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Building

Same as (5)(12)

Same as (4)-Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Injection disabled as reactor depressurization function Submergence of 125V DC power supply panels due to tsunami
cannot be used

Same as (5)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Function loss due loss of containment vessel coolingsea water system

Submergence of containment vessel cooling sea water pump
due to the tsunami

Same as (10)X

X

Same as (11)Same as (5)Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Injection disabled as reactor depressurization function Submergence of 125V DC power supply panels due to tsunami
cannot be usedX

Function loss due to blackouts

Function loss due loss of containment vessel coolingsea water system

Fire protection system (AM)X

Injection disabled as reactor depressurization function
cannot be used
Diesel driven extinguishing pump shutdown
Injection disabled as reactor depressurization function
cannot be used
Difficulty in ensuring the required coolant injection
flow rate

Fire engine (Chuetsu coast
earthquake response)

Same as (5)Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to loss of DC power supply

Alternative injection function(low pressure)

Verification of the design of isolation signal for
loss of DC power supply

Function loss due to blackouts

Function loss due to blackouts

Containment vessel coolingsystem (AM)Submergence of component cooling sea waterpump
Same as (3)Submergence of 125V DC power supply panels due to tsunami

Injection disabled as reactor depressurization function Submergence of 125V DC power supply panels due to tsunami
cannot be used
Makeup water system (AM)Submergence of containment vessel cooling sea
water pump

Function loss due to blackouts

X

Alternative injection function(high pressure)(8) Alternative power supply cable laying

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to blackouts
Function loss due to loss of DC power supply

Reactor component coolingwater system

Same as (3), (4)Intermittent aftershocks and continuous issue of large tsunami
warnings

Difficulty in connection work

Loss of communication means due to loss of alternative power
supplyContainment vessel coolingsea water system

Submergence of 6.9 kV high pressure power
supply panels△(insufficient flow
rate, difficulty inworking)

Submergence of 125V DC power supply panels due to tsunami

3-16

Same as (5)Submergence of containment vessel cooling sea water pump
due to the tsunami

Same as (10)Submergence of 125V DC power supply panels due to tsunami
Same as (5)Mechanical malfunction (presumed)-

Submergence of 125V DC power supply panels due to tsunami
Same as (5)Difficulty in ensuring the capacity of the fire protection watertank used as the water source(13) Depletion of fresh water source

Obstacles due to the impact of the earthquake and tsunami(14) Obstacles due to the earthquake and tsunami

Site evacuation due to explosion in the Unit 1 Reactor Building
and high radiation dose(15)

Sea water injection hose damaged due to explosion in the Unit 1
Reactor Building → relaying

Difficulty in working

Same as (3)Deterioration in work environment due to hydrogen
explosion and rise in radiation dose
Same as (15)Table 3.3-1 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 1 and Sorting of the problems (2/2)
Safety function

Related facility, etc.

Function loss or function degradation

ProblemCause analysis
Intermittent aftershocks and continuous issue of large tsunami
warnings-

Loss of communication means due to loss of alternative power
supplyContainment isolation

Main steam isolation valve

Containment vessel coolingsystem○Alternative containment vessel
spray with makeup water
system (AM)

Alternative containment vessel
spray with fire protection
system (AM)

-

X

X

X

-

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due loss of containment vessel coolingsea water system

Submergence of containment vessel cooling sea water pump
due to the tsunami

Same as (10)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

X

Function for removing heatfrom containment vessel
Shutdown cooling system(Normal operation)

Same as (9)Same as (11)Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Unable to use electric pump due to loss of external
power supply

Receiving circuit breaker of Ookuma Line 2L in the switchyard of
Unit 1/2 damaged due to the earthquake

Same as (1)Diesel driven extinguishing pump shutdown
Operation not implemented for alternative
containment vessel spray

Mechanical malfunction (presumed)-

Priority for feed water and alternative coolant injection to
emergency condenser (presumed)-

Submergence of 125V DC power supply panels due to tsunami(Remote operation of air-operated containment vessel ventvalve not possible)
Drop in pressure of Instrument Air due to loss of external power
supplyBlock up
Alternative heat removal
function

Pressure withstanding
reinforced vent (AM)△(Time required
due to blackout)

Difficulty in vent operation

Same as (5)(16) Difficulty in ensuring compressed air

Unable to open the electric valve of cylinder line due to blackout
and rise in radiation

Same as (4) and (15)Difficulty in accessing due to rise in radiation dose

Same as (15)Loss of communication means due to loss of alternative power
supplySame as (9)Difficulty in maintaining the air pressure and power supply of AO
valve

Drywell cooler (AM)

Reactor coolant cleanup
system (AM)Function loss due to blackouts

Same as (4)Function loss due loss of component cooling sea
water system

Submergence of component cooling sea water pump due to thetsunamiSame as (11)Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Submergence of component cooling sea water pump due to thetsunamiSame as (11)Same as (4)X

X
Function loss due loss of component cooling sea
water system
X

Unable to operate due to loss of DC power supply

Submergence of 480V low pressure power supply panels due totsunamiContainment vessel

X

Leakage generation

Deterioration in the seal of flange gasket and penetration due toloss in the function for removing heat from containment vessel

Reactor building

X

Damage due to hydrogen explosion

Hydrogen leakage from containment vessel penetration part

Radioactive material release
Stand-by gas treatment system
reduction function-

Submergence of 480V low pressure power supply panels due totsunami(17)

Heat resistance and pressure resistance of gasket
and penetration seal

Hydrogen generation due to the reaction of zircaloy and water

Other

Hydrogen retention in reactor building(18) Hydrogen retention in reactor building

Air heating and ventilating
Important functions related tosystem in central control room
safetyX

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Function to determine thecondition of the plant during
an accident

Monitoring instruments at thetime of an accident

X

Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Communication equipment

X

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Function loss due to blackouts

Submergence of 480V low pressure power supply panels due totsunamiSame as (4)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Other

Function to determine
abnormal conditions
Emergency lighting

X○: Normal operation; △: Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)

3-17

Table 3.3-2 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 2 and Sorting of the problems (1/2)
Safety function
Emergency reactor shutdown function

Related facility, etc.
Safety protection system,
control rod and control rod
drive system
Recirculation pump trip (AM)

Stoppage Alternative reactivity control Alternative control rod insertion(AM)
Sub-criticality maintenance
Borated water injection system
function

Cause analysis

Problem○(Normal operation when earthquake occurred)

-

-

-

(Not required as scram was successful)

-

-

-

(Not required as scram was successful)

-

-

-

(Not required as scram was successful)-

Function loss or function degradation

Receiving circuit breaker of Ookuma Line 2L in the switchyard of
Unit 1/2 damaged due to the earthquake
External power source

X(1) Reliability of switchyard circuit breaker

Loss of external power supply due to the earthquake-

Circuit breaker for Ookuma Line 2L in new Fukushima substation
damagedPower supply function

Emergency diesel generator

X

Function loss due to submergence6.9 kV high pressure power
supplyX

Function loss due to submergence

480V low pressure power
supplyAlternative power supply
function
Supportsystems

X

Emergency diesel generator submerged in water due to thetsunami (2A) and power source panels submerged (2B)

(3) Submergence of 6.9 kV high pressure power
supply panels

Function loss due to partial submergence

Partial submergence of 480V low pressure power supply panels
due to tsunami(4) Submergence of 480V low pressure power
supply panels

Submergence of 125V DC power supply panels due to tsunami

Function loss due loss of 6.9 kV high pressure
power supply

X

Function loss due to submergence

Power supply interchange (AM)X

Power supply interchange unavailable from adjacent
Loss of station power of adjacent plant (Unit 1) due to tsunami
plants(5) Submergence of 125V low pressure power
supply panels(6) Submergence of power supply panels of theadjacent plant

Difficulty in reaching

Difficulty in reaching due to damaged roads and traffic jams(7) No deployment of alternative power supply

Difficulty in laying cables (weight, isolation, work environment)(8) Alternative power supply cable laying

Power source car (emergency
arrangement)

Sea water system for residual
heat removal system

X

Intermittent aftershocks and continuous issue of large tsunami
warnings

Difficulty in connection work

Sea water cooling system

Reactor component coolingwater system

X

Main steam relief safety valve(safety valve function)○

High pressure core injection
systemX

Reactor core isolation coolingsystemBorated water injection system(AM)

Function loss due to submergence

Submergence of seawater pump for cooling the emergency
diesel generator due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to submergence

Submergence of sea water pump for cooling the residual heatremoval system due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to submergence

Submergence of cooling sea water pump due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

Same as (12)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)(Normal operation)

Function loss due to loss of DC power supply

Core spray system
Cool
down

Submergence of 125V DC power supply panels due to tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Power supply interruption[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Building

Function loss due to blackouts

Unable to receive power due to loss of external power supply andunavailability of emergency diesel generator

Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

X

X

X

X

X

Low pressure core cooling
function

Residual heat removal system(low pressure coolant injection
system)Makeup water system (AM)Fire protection system (AM)(11) Submergence of sea water pump of theresidual heat removal system
Same as (3)(12) Submergence of cooling sea water pump-

Submergence of 125V DC power supply panels due to tsunami△(Operational for Unable to control due to loss of DC power supply
approx. 3 days)

Function loss due to loss of DC power supply

Automatic depressurization
function during transient (AM)Same as (3)-Power supply interruption

Main steam relief safety valve(relief valve function / automatic
depressurization function)(10) Submergence of cooling sea water system for
emergency diesel generator

Same as (5)Alternative injection function(high pressure)Control rod drive water system(AM)

Cables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building

XX(9) Communication means during blackouts

Power supply interruption

X

Auxiliary cooling function

Reactor depressurization
function

Same as (3)125V DC power supply

Emergency diesel generator
facility and cooling seawater
systemHigh pressure core cooling
function

Submergence of the emergency diesel
generator facility

Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Loss of communication means due to blackouts

Function to prevent overpressurization of reactor
coolant pressure boundary

(2)

X

X

X

Difficulty in depressurizing

Same as (5)[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Building
Submergence of 125V DC power supply panels due to tsunami
Deterioration of the steam condensing performance due to riseand pressure and temperature in the pressure suppression
chamber
Insufficient voltage in temporarily installed batteries

Same as (3)Same as (1) - (3)Same as (12)Same as (5)Same as (5)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Injection disabled as reactor depressurization
function cannot be used

Submergence of 125V DC power supply panels due to tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Function loss due to loss of sea water system ofresidual heat removal system

Submergence of sea water pump for cooling the residual heatremoval system due to the tsunami

Same as (11)Injection disabled as reactor depressurization
function cannot be used

Submergence of 125V DC power supply panels due to tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to blackouts
Function loss due to loss of sea water system ofresidual heat removal system

Submergence of sea water pump for cooling the residual heatremoval system due to the tsunami

Same as (11)Injection disabled as reactor depressurization
function cannot be used

Submergence of 125V DC power supply panels due to tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Injection disabled as reactor depressurization
function cannot be used

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Same as (5)Same as (5)Same as (3)Same as (5)Unable to use the diesel driven extinguishing pump
Cause unknown(presumed)

Alternative injection function(low pressure)Fire engine (Chuetsu coast
earthquake response)

Same as (5)Injection disabled as reactor depressurization
function cannot be used

Submergence of 125V DC power supply panels due to tsunami

Difficulty in ensuring the required coolant injection
flow rate

Difficulty in ensuring the capacity of the fire protection water tank (13) Depletion of fresh water source
used as the water source△(insufficient flow
rate, difficulty inworking)

Obstacles due to the impact of the earthquake and tsunami(14) Obstacles due to the earthquake and tsunami

Site evacuation due to explosion in the Unit 3 Reactor Building
and high radiation dose(15) Deterioration in work environment due tohydrogen explosion and rise in radiation dose

[Recovery not possible] Sea water injection hose damaged due
to explosion in the Unit 3 Reactor Building→ relaying

Difficulty in working

Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of alternative power
supplyInterruption in coolant injection because of no fuelContainment isolation

Main steam isolation valve○(Normal operation)

-

3-18

Same as (15)Same as (8)(16) Fuel supply for fire engine-

Table 3.3-2 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 2 and Sorting of the problems (2/2)
Safety function

Related facility, etc.

Function loss or function degradation

Function for removing heat
Residual heat removal systemfrom containment vessel

X

Alternative containment vessel
spray with makeup water
system (AM)X

Alternative containment vessel
spray with fire protection
system (AM)X

ProblemCause analysis
Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Function loss due to blackouts

Same as (3)Function loss due to loss of sea water pumping
function of residual heat removal system

Submergence of sea water pump for cooling the residual heatremoval system due to the tsunami

Same as (10)Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Function loss due to blackouts

Unable to use electric pump due to loss of external 2L receiving circuit breaker of Ookuma Line in the switchyard of
power supply
Unit 1/2 damaged due to the earthquake

Same as (1)Unable to use the diesel driven extinguishing pump
Cause unknown(presumed)

Same as (5)Submergence of 125V DC power supply panels due to tsunami
Drop in pressure of Instrument Air due to loss of external power
supplyBlock up

Alternative heat removal
function

Pressure withstanding
reinforced vent (AM)X

Vent unavailable(17)

Difficulty in ensuring compressed air

Difficulty in accessing due to rise in radiation dose
Loss of communication means due to loss of alternative power
supplyLarge valves on suppression pool side closed due to explosion in
the Unit 3 Reactor Building

Same as (15)[Recovery not possible] Large valves on suppression pool sideclosed due to insufficient compressed air

Same as (17)Same as (8)-[Recovery not possible] Large valves on suppression pool sideclosed due to malfunction of electromagnetic valve (presumed)-

Reconfiguration of system due to rise in drywell pressure andnon-uniform pressure in pressure suppression chamber
Inoperative rupture disk

Same as (1) - (3)Function loss due to blackouts

Unable to receive power due to loss of external power supply andunavailability of emergency diesel generator

Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

Function loss due to blackouts

Unable to receive power due to loss of external power supply andunavailability of emergency diesel generator

Function loss due loss of component cooling sea
water system

Submergence of cooling sea water pump due to the tsunami

X

Unable to operate due to loss of DC power supply

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Containment vessel

X

Leakage generation

Pressure drop (cause unknown)

Reactor building△

Blow out panel opening (no damage to the building) Hydrogen retention prevention

Air heating and ventilating
Important functions related
system in central control room
to safety

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)X

Function to determine theMonitoring instruments at thecondition of the plant during
time of an accident
an accident

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)X

Communication equipment

X

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)X

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels due
to tsunami

Same as (3)Emergency lighting

Function loss due to loss of DC power supply

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Drywell cooler (AM)

Reactor coolant cleanup
system (AM)Radiation release reduction
Stand-by gas treatment system
function
Other

Other

(18) Inoperative rupture disk

Function to determine
abnormal conditions

X

X○: Normal operation; △: Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)

3-19

Same as (11)Same as (1) - (3)Same as (12)Same as (3)-Table 3.3-3 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 3 and Determining the problems (1/2)
Safety function
Emergency
reactor shutdown function
Stoppage

Related facility, etc.
Safety protection system,
control rod and control rod
drive system

Recirculation pump trip (AM)
Alternative
Alternative
control rod insertion
reactivity control(AM)
Sub-criticality
maintenance Borated water injection system
function

Power supply
function

Function loss or function degradation

Cause analysis

Problem○(Normal operation when earthquake
occurred)---(Not required as scram was successful)

-

-

-

(Not required as scram was successful)

-

-

-

(Not required as scram was successful)--

External power source

X

Loss of external power supply due to theearthquake

Damage to external substation facilities due to theearthquake

Emergency diesel generator

X

Function loss due to submergence6.9 kV high pressure power
supplyX

480V low pressure power
supplyX(1)

Reliability of switchyard equipment

Submergence of emergency diesel generator due to thetsunami(2)

Submergence of the emergency diesel
generator

Function loss due to submergence

Submergence of 6.9 kV high pressure power supply panels
due to tsunami(3)

Submergence of 6.9 kV high pressure
power supply panels

Function loss due loss of 6.9 kV highpressure power supply

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (3)Function loss due to submergence

Submergence of 480V low pressure power supply panels due(4)
to tsunami

Submergence of 480V low pressure
power supply panels

Use of DC power supply exceeding the available supply time (5)Depletion of DC power source

Loss of station power of adjacent plant (Unit 4) due totsunami(6)

Submergence of power supply panels of
the adjacent plant(7)

Procurement of alternative power supply
from outside

125V DC power supply△(Will deplete later)(Normal operation)

Power supply interchange (AM)X

Power supply interchange unavailable fromadjacent plants

Difficulty in reaching due to damaged roads and traffic jams
Difficulty in reaching
Alternative
power supply
function

Power source car (emergency
arrangement)

X

Difficulty in connection work

Support systems

Emergency diesel generator
facility and cooling seawater
systemSea water system for residual
heat removal system

Submergence of seawater pump for cooling the emergency
diesel generator due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Function loss due to submergence

Submergence of sea water pump for cooling the residual
heat removal system due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

X

XHigh pressure
core cooling
function

Alternative
injectionfunction (highpressure)

Communication means during blackouts(10)

Submergence of cooling sea water pumpfor emergency diesel generator
Same as (3)(11)

Submergence of sea water pump of theresidual heat removal system
Same as (3)(12)

Submergence of component cooling sea
water pumpSame as (3)Function loss due loss of component cooling Submergence of cooling sea water pump due to the tsunami
sea water system

Same as (12)Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (3)X○High pressure core injection
system△Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)Reactor core isolation coolingsystem△Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Borated water injection system(AM)

Control rod drive water system(AM)

Core spray system

Cool down

Alternative power supply cable laying

Main steam relief safety valve(safety valve function)

Main steam relief safety valve(relief valve function /Reactorautomatic depressurization
depressurizatio
function)n function
Automatic depressurization
function during transient

Low pressure
core cooling
function

Submergence of cooling sea water pump due to the tsunami

Same as (3), (4)Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Function loss due to blackouts
Function toprevent overpressurizationof reactor
coolant
pressureboundary(9)

Function loss due to submergence

Function loss due to blackouts

Reactor component coolingwater system

Loss of communication means due to loss of alternative
power supply
Cables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building

Function loss due to submergence
Sea water cooling system(8)Power supply interruption

X

Auxiliary cooling
function

Unable to connect due to submergence of AC power supply
panelsDifficulty in laying cables (weight, isolation, work
Intermittent aftershocks and continuous issue of large
tsunami warnings

Residual heat removal system(low pressure coolant injection
system)Makeup water system (AM)Fire protection system (AM)

(Normal operation)-

Function loss due to blackouts
X-[Recovery not possible] Power supply recovery difficult due toDifficulty in power supply recovery operation aftershocks and poor working environment-

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Function loss due loss of component cooling Submergence of cooling sea water pump due to the tsunami
sea water system

Same as (12)△Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)△Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)Injection disabled as reactor
depressurization function cannot be used

Use of DC power supply exceeding the available supply time

Same as (5)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (3)Function loss due to loss of sea water
system of residual heat removal system

Submergence of sea water pump for cooling the residual
heat removal system due to the tsunami

Same as (5)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (11)Injection disabled as reactor
depressurization function cannot be used

Use of DC power supply exceeding the available supply time

Same as (5)Function loss due to blackouts

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Injection disabled as reactor
depressurization function cannot be used

Use of DC power supply exceeding the available supply time

Same as (5)Injection disabled as reactor
depressurization function cannot be used

Use of DC power supply exceeding the available supply time

Same as (5)Insufficient flow rate of the required coolant
injection

Difficulty in ensuring the capacity of the fire protection watertank used as the water source

Function loss due to blackouts
X

X

X

X

X

Alternative
injectionfunction (lowpressure)(14) Depletion of fresh water source

Obstacles due to the impact of the earthquake and tsunami(15)
Fire engine (Chuetsu coast
earthquake response)△(insufficient flow
rate, delay)

Site evacuation due to explosion in the Unit 3 Reactor
Building and high radiation dose
Difficulty in coolant injection

Sea water injection hose damaged due to explosion in the
Unit 3 Reactor Building → relaying
Intermittent aftershocks and continuous issue of large
tsunami warnings

3-20

Obstacles due to the earthquake andtsunamiDeterioration in work environment due to(16) hydrogen explosion and rise in radiation
dose
Same as (16)-

Table 3.3-3 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 3 and Determining the problems (2/2)
Safety function

Containment
isolation
Function forremoving heatfromcontainment
vesselBlock up

Related facility, etc.

Main steam isolation valve

Containment vessel coolingsystemFunction loss or function degradation△

X

Cause analysis
Loss of communication means due to loss of alternative
power supply

Operation (generation of isolation signal
related to break before and after the main
steam isolation valve closure)-

Submergence of sea water pump for cooling the residual
heat removal system due to the tsunami

Same as (11)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (3)Function loss due to loss of sea water
system of residual heat removal system

X

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Alternative containment vessel
spray with fire protection
system (AM)△

Unable to use the electric pump due to lossof power supply. Diesel pump temporarily
operational

Submergence of switchyards of Units 3/4 due to tsunami

Same as (1)Use of DC power supply exceeding the available supply time

Same as (5)Drop in pressure of Instrument Air due to loss of external
power supply

Same as (1)Difficulty in accessing due to rise in radiation dose

Same as (16)

Drywell cooler (AM)△Difficulty in vent operation

Loss of communication means due to loss of alternative
power supply
High temperature, pitch dark working environment

X

Submergence of 480V low pressure power supply panels due
Function loss due to blackouts
to tsunami
Function loss due loss of component cooling Submergence of cooling sea water pump due to the tsunami
sea water system

Same as (9)Same as (4) and (5)Same as (4)Same as (11)Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Function loss due loss of component cooling Submergence of cooling sea water pump due to the tsunami
sea water system

Same as (11)X

Unable to operate due to loss of DC power
supplySame as (4)Containment vessel

X

Leakage generation

Reactor building

X

Damage due to hydrogen explosion

Function loss due to blackouts
Reactor coolant cleanup
system (AM)Radioactive
material release
Stand-by gas treatment system
reductionfunctionX

Submergence of 480V low pressure power supply panels due
to tsunami

Deterioration in the seal of flange gasket and penetration due
Heat resistance and pressure resistance(17)
to loss in the function for removing heat from containmentof gasket and penetration seallHydrogengeneration due to the reaction of zircaloy andwater
Hydrogen leakage from containment vessel penetration part
Hydrogen retention in reactor building

Other

Same as (9)Alternative containment vessel
spray with makeup water
system (AM)

Alternative heat Containment vessel vent (AM)removalfunction

Multiple barrier
function

Problem(18) Hydrogen retention in reactor building

Important
Air heating and ventilating
functionssystem in central control room
related to safety

X

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Function todetermine thecondition of theplant during an
accident

Monitoring instruments at thetime of an accident

X

Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)Communication equipment

X

Function loss due to blackouts

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Function loss due to blackouts
X

Submergence of 480V low pressure power supply panels due
to tsunami

Same as (4)Emergency lighting

Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

Same as (5)Function todetermineabnormalconditions○: Normal operation; △: Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)

3-21

3.3.1.1 Problems by function(1) “Stopping” function
a. Emergency reactor shut-down function of the reactors
The reactors scrammed normally due to occurrence of the earthquake. The
emergency reactor shut-down function of the reactors has not been lost in thesafety protection system and control rod drive system.
b. Sub-criticality maintenance function
Normal scram of the reactors prevented a request of the function for control
of alternative reactivity rate (control of reactivity rate by recirculation pump trip
and alternative control rod insertion).
However, the recirculation pump tripped due to loss of external powersource.
c.

Sub-criticality maintenance function
Normal scram of the reactors prevented a request of the function for
sub-criticality maintenance by SLC.
As mentioned above, since the “stopping” function operated normally or thefunction was not requested, no cause and no problem exist.

(2) “Cooling” function
a. Function to prevent over-pressurization in the reactor coolant
pressure boundary
Signs of loss of reactor coolant by the earthquake were not found from themaintenance of main steam flow rate or temperature change in thecontainment air handling system after the earthquake.
In unit 1, no request was made to perform the safety valve function of SRV
because IC controlled the reactor pressure after MSIV was closed.
In unit 2 and 3, SRV stably controlled the reactor pressure with the relief
valve function or safety valve function after MSIV was closed, and performed
its function normally.
b. High pressure core cooling function(a) High pressure core injection system
For water levels of reactors after the scram, unit 1 to 3 were not givenfunctional requests for HPCI operation request, but HPCI was put in thestand-by state. After that, arrival of the tsunami caused unit 1 and 2 to lose
their functions due to loss of 125 VDC power. In unit 3, since the DC power
supply system avoided flood, RCIC and HPCI could be used. With the
reduction of water level in the reactors, the HPCI automatically started andthe water level of the reactors was retained for a while. However, it thenstopped and the function for coolant injection to the reactors was lost. Thiswas caused by the exhaustion of DC power supply.
(b) Emergency condenser (unit 1 only)
High pressure in the reactor automatically started the IC after MSIV wasclosed. After that, the power was lost due to the tsunami during the reactor
pressure control operation by the IC. Since loss of the control power supply(DC power supply) of IC due to the tsunami prevented confirming the
open/close state of IC valve, and the indicator lamps of HPCI were OFF, the3-22

startup was judged impossible. According to an investigation after that,
there is a possibility that deenergizing the detection circuit of break in pipe,
normally made up of an excitation circuit, could originate an isolation signal,
automatically closing valves of the IC. Almost simultaneous loss of all AC
power supplies may have caused the isolation valve inside the reactor
containment, driven by AC power supply, to stop closing with the valve half
opened (the clear opening is unknown). When DC power supply was
temporarily recovered around 18:00 on March 11 (reason for the recovery is
unknown), the isolation valve outside the reactor containment was opened.
However, the degree of IC's core cooling function is unknown according tothe opening of the isolation valve outside the reactor containment. Thereseems to be a high possibility that the IC's function may have been lost until
the temporary recovery of DC power supply after the arrival of the tsunami.
As a facility for supplying coolant to the IC, FP called a makeup water
system purified (hereinafter referred to as "MUWP") is provided. However,loss of the low voltage power supply of 480 V caused it to lose its function.
The FP is divided into a motor pump and a diesel pump. The motor pumplost its function due to the loss of AC power supply. It is confirmed that thediesel driven pump ran temporarily but stopped due to malfunction. The
contents of the malfunction are unknown.
(c) Reactor core isolation cooling system (unit 2/3)
After the scram, manual startup of the RCIC, automatic stop due toincrease of reactor water level, and manual startup were repeated to control
the reactor water level.
In unit 2, the operation state of the RCIC was not confirmed. At around3:00 on March 12, however, discharge pressure of the RCIC pump was
confirmed, and the pump is judged to have been running. After that, thewater source was switched from CST to S/C to perform the operation. After
13:00 on March 14, the water level of the reactor tended toward decreasing,and the function of the RCIC was judged to have lost the function at 13:25.
As a result, the RCIC of unit 2 is considered to have worked forapproximately 3 days. It seems that loss of the power supply set the valveof the RCIC to work "as is" and the valve happened to open with the timingof loss of the power supply.
In unit 3, since the DC power supply was not affected by the tsunami, theRCIC and HPCI, operable with DC power supply, were usable.
Unnecessary load was isolated to expand the operating time of batteries. At
11:36 on March 12, the RCIC automatically stopped and the HPCI
automatically started up. At 2:42 on March 13, the HPCI automatically
stopped and lost its function of injecting coolant to the reactor. As a result,coolant was injected to unit 3 for approximately 1.5 days until theexhaustion of the battery capacity.
c.

High pressure alternate injection function (countermeasure forsevere accident)(a) Stand-by liquid control system
The function was lost due to loss of the power supplied from the low
voltage power supply of 480 V necessary for the operation of the stand-by
liquid control pump in unit 1 to 3. Since the power panel of the stand-by
liquid control system and the main body of the stand-by liquid control were
3-23

not affected by the tsunami, power supply operation from the power supply
car, arranged urgently after the loss of all AC power supplies, were carriedout. However, because hydrogen explosion in the reactor building of unit 1
damaged the cable that had been laid. the power supply car arranged
urgently stopped automatically, and then the SLC function was notrecovered.
(b) Control rod drive water system
The function was lost due to loss of the power supplied from the low
voltage power supply of 480 V necessary for the operation of the drive
water pump in unit 1 to 3.
d. Reactor depressurization function(a) Main steam relief safety valve (manual relief function and automatic
depressurization function)
The manual relief operation and automatic depressurization function
were lost because unit 1 to 3 lost its power supplied from the 125 V DC
power supply necessary for the operation. As the result, the reactor
pressure remained high, systems/facilities having the low pressure corecooling function could not inject coolant into the reactors. However, since
the systems having the low pressure core cooling function lost its power
supplies when all AC power supplies were lost, they lost the reactor
injection function.
Since unit 2 required the depressurization of the reactor for fire engines
to inject coolant to the reactor, temporary batteries were used to assure thepower supply necessary for SRV to operate, so that the reactor wasdepressurized.
e. Low pressure core cooling function(a) Core spray system
In unit 1 to 3, the core spray pump lost its function due to loss of thepower supplied from the high voltage power supply of 6.9kV necessary forthe operation of the core spray pump.
(b) Low pressure coolant injection system (residual heat removal
system) (unit 2/3)
The residual heat removal pump (low pressure coolant injection mode)
lost its function due to loss of the power supplied from the high voltage
power supply of 6.9 kV necessary for the operation of the residual heatremoval pump.
f.

Low pressure alternate injection function (countermeasure for severe
accident)(a) Makeup water system
In unit 1 to 3, the makeup water pump lost its function due to loss of thepower supplied from the low voltage power supply of 480 V necessary forthe operation of the makeup water pump.
(b) Fire protection system
The fire protection system pump lost its function due to loss of the power
supplied from the low voltage power supply of 480 V necessary for the
3-24

operation of the fire protection system pump. In addition to motor driven
system pumps, diesel driven pumps were used as fire protection system
pumps. In unit 1, it is confirmed that they ran temporarily but stopped due tomalfunction. In unit 2, startup operation seems not to have been performed.
However, details are unknown, e.g., whether the operation was not done
due to failures. In unit 3, pumps ran temporarily but then stopped. Details
are unknown.
Alternate coolant injection was carried out using fire engines prepared asa countermeasure taken after the Chuetsu Offshore Earthquake. However,since the volume of the fire protection tank used as a water source waslimited, the required volume of coolant was insufficient and, therefore,
preparations were made for the injection of sea water. Operations for the
injection of coolant into the reactors had difficulties caused by obstacles
produced by impact of the earthquake and tsunami, evacuation from thefield site due to explosion of the reactor building of unit 1, high-dose debris,
relaying of sea water injection hoses replaced with damaged ones,consecutive aftershocks, continuation of large tsunami warning, loss ofcommunication methods due to loss of AC power supply, etc. In unit 2, fireengines started sea water injection after the reactor depressurization.
However, since they ran out of fuel and stopped, injection of coolant to thereactor was suspended.
(c) Containment & vessel cooling water system
The containment & vessel cooling water pump (unit 2/3 was put in thecontainment & vessel cooling mode for RHR pump) lost its function,
because the power supplied from the high voltage power supply of 6.9 kVand the 125 VDC power supply necessary for the operation on thecontainment & vessel cooling water pump were unavailable or because thepump could not be started up due to flooding of the containment & vessel
cooling sea water system (residual heat removal cooling sea water system
in unit 2/3) and loss of the power supply.
For the reasons above, problems on the "cooling" function by facility are
as follows.
・ Submergence of power supply systems
・ Exhaustion of sea water pump・ Exhaustion of fresh-water source
・ Deteriorationofworkingenvironmentsduetohydrogen
explosion/increase of radiation dosage
・ Obstacles caused by the earthquake and tsunami
・ Loss of communication methods during loss of AC power supply
・ Refueling of fire engines
・ Exhaustion of DC power supply
・ Origination of IC isolation signal with DC power supply lost(3) "Confining" function
a. Reactor containment isolation function
For unit 1 to 2, the MSIV is normally closed by the isolation signal
generated due to loss of external power source. No leakage occurs in themain steam piping because the flow rate of main steam is zero after the MSIV
is closed and because the reactor pressure has been increased.
3-25

The reactor containment isolation function works normally immediately after
the occurrence of scram, and there are no causes of accidents or problems.
b. Heat removal function from the containment
CCS (unit 1: CCS, unit 2/3: RHR containment cooling mode) was started up
manually after the scram, and unit 2 was started up manually to cool water in
the S/C pool. After that, the CCS pump lost its function due to loss of thepower supplied from the high voltage power supply of 6.9 kV necessary forthe operation of the CCS pump. Unit 3 was not started up due to loss of power
supply.
c.

Alternative cooling function (facilities for countermeasures forsevere accident)(a) Alternative containment spray by the makeup water system
The makeup water system pump lost its function due to loss of the power
supplied from the low voltage power supply of 480 V necessary for theoperation of the makeup water system pump.
(b) Alternative containment spray by the fire protection system
The fire protection pump lost its function due to loss of the power
supplied from the low voltage power supply of 480 V necessary for theoperation of the fire protection pump. In addition to motor driven system
pumps, diesel driven pumps were used as fire protection system pumps.
For the situation, refer to the description in f. "Low pressure alternate
injection function (countermeasure for severe accident)."(c) Containment vessel pressure reinforcement ventThe vent to be performed based on the primary operation procedure
could not be done because no power was supplied from 125 VDC power
supplies necessary for the operation and the air pressure for
instrumentation was decreased.
For unit 1 and 3, an attempt was made to perform manual release
operation. However, the operations had difficulties because access wasdifficult caused by increase of radiation dosage or by loss ofcommunication methods due to loss of AC power supply. As a result, ventis considered to have been done because the pressure of the containment
was decreased.
In unit 1, when operations for opening vent valves of S/C (small valve)were performed several times in expectation of residual pressure ofcompressed air for instrumentation, the indication value of MP increased.
Immediately after that, the indication value of MP radiation dosage
decreased. It was therefore confirmed that there was a possibility that thevent might not be effective. The cause of this is assumed to be that the ventvalves of the containment were closed immediately because the residual
air pressure for instrumentation was little and the valves could not be kept
open. Considering the vent of the containment of unit 1 to take precedence
over that of the other reactors, the Minister of Economy, Trade and Industry
issued an order to vent it at 6:50 on March 12. After the Prime Minister's
visit to Okuma Town (Kuma area), confirmation of evacuation, etc., thereactor was vented at 9:15.
3-26

Although unit 3 also had difficulties in operation, the vent was judged tohave been done. After that, since the air pressure and power supplies were
not retained, valve opening operations were conducted several times.
In unit 2, the pressure of S/C remained lower than the working pressureof the rupture disk. On the other hand, the release operation was carriedout for line configuration because the pressure of D/W was increasing.
However, the pressure of D/W decreased around 6:00. For the reasons
above, the implementation state of venting the containment of unit 2 is notclear at the current stage.
The pressure of D/W increased, while the pressure of /C decreased.
However, the factor for indicating such behaviors is unknown at the current
stage.
(d) Alternative heat removal (countermeasure for severe accident) by
dry well cooler and reactor coolant cleanup system
In each case, the function was lost because the power was not supplied
from the low voltage power supply of 480 V and the reactor component
cooling water system stopped.
d. Function for reducing the discharge of radio-active materials
The SGTS lost its function because the power was not supplied from thelow voltage power supply of 480 V.
e.

OthersSince the heat removal function from the primary containment vessel(hereinafter referred to as "PCV") was lost, and temperature/pressure of thecontainment atmosphere increased excessively, the gasket of flange area andseal of the penetration are assumed to have deteriorated, causing theleakage. It is conjectured that, since leaked hydrogen from this penetration of
the containment was retained in the reactor building, and increase ofhydrogen concentration exceeded the flammability limit, hydrogen explosion
occurred.
In the reactor building of unit 2, the blowout panel opened is conjectured tohave prevented retaining hydrogen in it.
For the reasons above, problems on the "confining" function by facility are
as follows.
・ Submergence of power supply systems
・ Submergence of sea water pump・ Deterioration of working environments due to hydrogen explosion/increaseof radiation dosage
・ Sealing property in a state exceeding the design conditions of gasket andseal of the penetration
・ Retention of hydrogen in the reactor building・ Assurance of compressed air to keep the vent valves of the containment
open
・ Non-operation of rupture disk for venting the containment at low pressures

3-27

(4) Basic requirements for implementing the operation (facility
environments)
a. Habitability of the central control unit
When the accident occurred, the air heating and ventilating system toassure the habitability of the MCR lost its function because the power was not
supplied from the low voltage power supply of 480 V.
b. Function of seizing the status of the station at the time of an accident
The monitoring instruments at the time of an accident, which was a functionof seizing the status of the station at time of an accident, lost the function,
because DC power for instrumentation was not supplied to unit 1 and thepower from the low voltage power supply of 480 V was not supplied to 2 and3.
c.

Functions of seizing an abnormal stateCommunication equipment and emergency lighting, which were functions
for seizing abnormal states, lost their functions, because power was suppliedneither from the low voltage power supply of 480 V nor 125 VDC power
supply.

For the reasons above, the following items are listed as the basic
requirements for implementing the operation (facility environments).
・ Submergence of power supply systems
・ Exhaustion of DC power supply
3.3.1.2 Problems related to common factors/failures
Extracted problems by function in 3.3.1.1 include "without power supplied
from power supply system" and "stoppage of cooling water system." Therefore,
analysis of the causes and extraction of problems were furthermore carried out
in terms of "power supply" and "auxiliary component cooling function."(1) Power supply function
a. External power source
All unit 1 to 4 receive power supplied from external power sources via the
Shin-Fukushima Transforming Station. The power supplies were lost caused
by, e.g., damages of facilities in the transforming station due to theearthquake, damages of power receiving breakers and disconnecting
switches within the switchyard of the station due to the earthquake, andoperations of protective devices due to contact between power transmission
lines and steel towers due to the earthquake. In addition to the damages dueto the earthquake, the switchyards for unit 3 and 4 became difficult to recoverexternal power sources at an early date because they were flooded by thetsunami.
b. Emergency diesel generator
After the loss of the external power sources, unit 1 to 3 automaticallystarted up emergency DGs to supply power to the emergency bus. After that,
however, the tsunami flooded them (although the main unit of 2BDG escaped
the tsunami-caused damages, the high voltage power panel of 6.9 kV wasflooded), they lost their functions, and the reactors then lost all AC power
supplies.
3-28

c.

On-site power
The high voltage power supply of 6.9 kV lost its function because the power
panel was flooded. Therefore, the low voltage power supply of 480 V, used as
the load, also lost its function. The 480 V power panels for unit 1 and 3 werealso flooded.
The 125 VDC power for unit 1 and 2 could not be supplied from batteries,
because the 125 VDC power panel was flooded by the tsunami. The 125 VDC
power panel for unit 3 escaped the flood. After the batteries worked forseveral days, they were exhausted and lost the function.

d. Alternative power supply function
Loss of all AC power supplies prevented the power interchange fromadjacent reactors, which had been prepared as a countermeasure for severe
accidents.
Operations for laying and connecting cables were extremely difficult forvarious reasons, e.g., delay in arrival of power supply cars arranged urgently
after the loss of all AC power supplies due to damages of roads caused by theearthquake and traffic congestions, submergence of all AC power panels,
weight of cables, laying distance, poor working conditions, intermittent
aftershocks, continuation of large tsunami warning, and loss ofcommunication methods. In addition, hydrogen explosion in the reactor
building damaged laid cables, and the power supply cars stopped
automatically.
For the reasons above, problems on power supply functions by facility arelisted as follows.
・ Reliability of external power sources against earthquakes and tsunami
・ Reliability of breakers in switchyards when earthquakes occur
・ Submergence of power supply systems
・ Procurement of alternative power supplies from outside
・ Laying of cables for alternative power supplies
・ Communication methods during the loss of AC power supply(2) Auxiliary component cooling function
Immediately after the external power sources were lost due to theearthquake, the sea water systems requiring the auxiliary component cooling
function received power from the emergency DG and operated normally.
However, since the sea water pump was flooded due to the tsunami, and thehigh voltage power panel of 6.9 kV for supply power was flooded, the function
was lost.
For the reasons above, problems on the auxiliary component cooling
function by facility are listed as follows.
・ Submergence of sea water pump・ Submergence of power supply systems
Electric power systems and auxiliary component cooling systems are
facilities having highly important safety functions. Since all the systems lostthe functions at a time due to the tsunami, the accident was enlarged. The
facilities of countermeasures against failures could not put the expansion of
3-29

the accident under control, because they lost their functions due to the loss ofelectric power systems and auxiliary component cooling systems.
For the reasons above, the following problems can be listed ascountermeasures against severe accidents for power supply systems andauxiliary component cooling systems.
・ Prevention of tsunami from damaging all facilities a time.
・ Means of preventing tsunami from affecting (breakwater, or deployment inrising ground, etc.) has been prepared so that damages by tsunami can be
avoided.

3-30

3.4

Causal analysis of the accident in unit 4 and management ofissues

Causes of the loss of cooling function of the spent fuel pool and the hydrogen
explosion in Fukushima Daiichi Unit 4 are summarized. The result of the causal
analysis of the accident in Unit 4 and the management of the issues is shown in
Table 3.4-1.
After the earthquake occurred, loss of an external power source, that is a station
blackout, occurred that was caused by loss of emergency DG due to the tsunami.
The DC power source and a sea water pump were also lost.

3-31

Table 3.4-1 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Plant Unit 4 and Determining the problems (1/1)
Safety function

ower supply functi

upport syste

Related facility, etc.

X

Emergency diesel generatorX

6.9 kV high pressure power
supplyX

480V low pressure power
supplyX

Function loss due to submergence (only power
Submergence of 480V low pressure
Submergence of 480V low pressure power supply panels due(3)
supply panels installed in auxiliary public facilities
power supply panels
to tsunami
in operation)

125V DC power supply

X

Function loss due to submergence

Submergence of 125V DC power supply panels due totsunamiFunction loss due to submergence

Submergence of sea water pump of theSubmergence of sea water pump for cooling the residual heat(5)
residual heat removal systemremoval system due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Function loss due to submergence

Submergence of cooling sea water pump due to the tsunami

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (2)Function loss due loss of component cooling sea
Submergence of cooling sea water pump due to the tsunami
water system

Same as (6)Sea water cooling system

X

Reactor component coolingwater system

X

Fuel pool cooling system

XX

olant injection func Makeup water supply system
Secondarycontainment
vesselReactor building(4)

Submergence of 125V low pressure
power supply panels

X

Cooling of Cooling functionused fuelResidual heat removal system
pools

Block up

ProblemCause analysis

Loss of external power supply due to theDamage to external substation facilities due to the earthquake (1) Reliability of switchyard equipment
earthquake
1 out of the 2 installed generators is underroutine inspection.
Submergence of 6.9 kV high pressure
The remaining generator cannot be used as the Submergence of 6.9 kV high pressure power supply panels(2)
power supply panels
due to tsunami
power supply panels are submerged (theemergency diesel generator is not submerged inwater).
Submergence of 6.9 kV high pressure power supply panels
Same as (2)Function loss due to submergence
due to tsunami

External power source

Sea water system for residual
heat removal system

xiliary cooling func

Function loss or function degradation

Same as (2)(6)

Submergence of component coolingsea water pump

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panelsand 480V low pressure power supply panels due to tsunami

Same as (3), (4)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panelsand 480V low pressure power supply panels due to tsunami

Same as (3), (4)Function loss due to loss of sea water system of Submergence of sea water pump for cooling the residual heat
residual heat removal systemremoval system due to the tsunami

Same as (5)Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panels
due to tsunami

Same as (2)X

Function loss due to blackouts

Submergence of 6.9 kV high pressure power supply panelsand 480V low pressure power supply panels due to tsunami

Same as (3), (4)X

Damage due to hydrogen explosion○: Normal operation; △: Insufficient operation; X: Unable to operate; -: Operation not

3-32

Hydrogen retention in reactor building(7) Hydrogen retention in reactor building

Inflow of vent gas in Unit 3(8)Wraparound of hydrogen gas in units
that share the vent stack(1) Loss of cooling function of the spent fuel poola. Spent fuel pool cooling function
The FPC that had been operating right before the earthquake occurred
stopped because the external power source was lost due to the earthquake.
After that, the SFP cooling function was lost because the function of thecomponent cooling water system was lost due to the Tsunami and the 480V low
voltage power source that is necessary for the operation of the FPC was lost.
The RHR that had been operating right before the earthquake occurred
stopped because the external power source was lost due to the earthquake.
After that, the SFP cooling function was lost because the function of the RHR
sea water system was lost due to the tsunami and the 6.9kV high voltage powersource that is necessary for the operation of the RHR was lost.
The MUWC has a function of supplying water to the SFP. However, its
function was lost because the 480V low voltage power source that is necessary
for the operation of the MUWC was lost.
As a result, the issues of the respective facility regarding the SFP cooling
function were as follows.
・ Soaking of the sea water pump・ Soaking of the power source facility(2) Hydrogen explosion
a. Main cause of hydrogen generation
An explosion that is considered to be due to hydrogen occurred in the reactor
building of Unit 4 at about 6 am on March 15.
When the earthquake occurred, Unit 4 was stopped because it wasundergoing a periodic inspection, and all fuel was taken out from the reactor to
the spent fuel pool. Because of this, there was no possibility of hydrogengeneration from the reactor. As a result of observing the fuel in the pool bycamera, it can be assumed that damage of the fuel was not found, and that
there was no large-scale damage of the fuel from the result of analyzing
nuclides of the fuel pool water taken from the fuel pool of Unit 4. Consequently,
the possibility of generating a large amount of hydrogen in the fuel pool of Unit 4
was low.
On the other hand, the SGTS exhaust pipe of Unit 4 joined with the SGTS
exhaust pipe of Unit 3 before the vent stack. There is a possibility that thehydrogen explosion occurred as a result of the hydrogen in the PCV of Unit 3
flowing into the R/B of Unit 4 through the SGTS exhaust pipe via a container
vent that was carried out in Unit 3, and the hydrogen was retained. The radiationdose of the SGTS filter train of Unit 4 was measured, and it was found that theradiation dose was high on the exit side and got lower going toward theentrance side.
As a result, the issues of the respective facility regarding the hydrogen
explosion in Unit 4 are as follows.
・ Hydrogen retention in the reactor building
・ Wrapping around of hydrogen gas between the units that share the ventstack
・ Wrapping around from the container vent line to the building

3-33

3.5

Management of the events in units 5 and 6

Because the reactors of Units 5 and 6 were not working and there was enoughtime for correspondence, the power source was successfully accommodated fromthe DG (6B) whose function was not lost due to the tsunami and alternative
coolant injection was successful. Conclusively, the residual heat removal function
by the temporary sea water pump was able to recover.
The progress of the events of Fukushima Daiichi Units 5 and 6 is shown in
Figure 3.5-1 using an event tree.
Accidents were successfully prevented from extending in Units 5 and 6 bydirectly connecting a power source of one air-cooled emergency DG to a safety
component, and it is considered that safety can be ensured when the powersource is secure.
Hence, having variety of cooling types of the emergency DG and having avariety of installation sites from the viewpoint of soaking can become effective
countermeasures.
However, it should be kept in mind that there was a necessity of supplying
power by direcly drawing a cable from a bus of the emergency DG to the safety
component because the power panel was affected by the tsunami. Because the
distribution of electricity to a plurality of the safety components will besimultaneously lost when the power panel is submerged, it is necessary toinvestigate a plan for supplying power to the necessary components from theavailable power sources even when soaking occurs while ensuring soaking
countermeasures in order to increase the security of the dependable powersource.

3-34

Events
occurred
Earthquake andTsunamiReactorshutdown
Reactorscram

DC power
Electric power
AC source
Ensuring long-term cold shutdown
Core cooling
Status of reactor coresourcerecoveryDC power External
Emergency IC, turbineExternal
RPV
Reactor
RHR
RHR
PCV vent Cold shutdown, core damage,sourcepower
DGdriven coolant power source, depressuri coolant
PCV damage etc.
recovery (before
sourceinjection
emergency
zationinjectioncore
system
DG, power(Including (including
damage)
RCIC,HPCI) interchange alternative alternative
for which AC
sources) sources)
source is not

Degraded core coolin
Depressu Reactor
rization of coolant
nuclearinjection
reactor

Reactor
Final statusbuilding control
PCV vent Hydrogen SGTS,
Cold shutdown, Core damage, PCV damage etc.
control
ventilation(Nitrogenreplacement etc.)

Reactor container control
PCV
coolant
injection

Cold shutdown

Cold shutdown

1F-6 sequence

Cold shutdown (Long term cooling is required)
PCV damage (Overpressure)
Cold shutdown
1F-6

Cold shutdown

Not required

1F-5 sequence

Temporarily installed seawater pump
Cold shutdown (Long term cooling is required)
PCV damage (Overpressure)Soundness of PCV or PCV leakage(Overtemperature)

Cold shutdown (Long term cooling is required)

PCV leakage (Overtemperature) and hydrogen
combustion in the building

Unit 6 power supply interchange[Earthquake]
1F-5

PCV damage (Hydrogen explosion)
Not required

PCV damage (Overpressure)[Tsunami]

PCV damage (Overpressure)Core damage

Shutdown

PCV damage (MCCI etc.)
Core damage
PCV damage (DCH etc.)

Soundness of PCV or PCV leakage(Overtemperature)
Cold shutdown (Long term cooling is required)

PCV leakage (Overtemperature) and Hydrogen
combustion in the building
PCV damage(Hydrogen explosion)

PCV damage (Overpressure)
PCV damage (Overpressure)Core damage
PCV damage(MCCI etc.)
Core damage
PCV damage(DCH etc.)

Cold shutdown (Long term cooling is required)Soundness of PCV or PCV leakage(Overtemperature)

PCV damage (Overpressure)

PCV leakage (Overtemperature) and Hydrogen
combustion in the building

Core damage

PCV damage(Hydrogen explosion)

Core damage

PCV damage (Overpressure)
PCV damage(MCCI etc.)

Figure 3.5-1 Event tree of event progress in Fukushima Daiichi Nuclear Reactor Unit 5, 6

3-35

(1) Unit 5
Refer to “2.6 Status of Progress of the Accident in Unit 5” in this article for the
behavior of the plate when the earthquake occurred.
a.

Status of “shutdown” function
When the earthquake occurred, Unit 5 was stopped because it wasundergoing a periodic inspection, the pressure of the reactor was increased
and maintained to 7.2 MPa for a pressure resistance and leakage
inspection, and all control rods were inserted.

b. Status of “cooling” function
All AC power source was lost due to the influence of the tsunami, and theoperations of the RHR, the low pressure core spray, and the HPCS became
impossible. The pressure of the RPV was reduced a day after theearthquake by opening a valve at the top of the RPV.
On March 13, the pressure and the water level of the reactor were
controlled by starting a condensate water transfer pump using the powersource that was exchanged from Unit 6, by properly reducing the pressure
with the SRV, and at the same time by repeating an operation of supplying
water from the CST to the reactor with the condensate water transfer pump.
Then, both the spent fuel pool and the reactor were cooled by building atemporary sea water pump to take sea water and by alternating cooling of
the spent fuel pool and the reactor by switching system configurations of
the RHR.
c.

Status of “trapping” function
A stable value was shown for the stack radiation monitor, and noabnormality was found.

d. Status of the spent fural pool cooling system
The water level of the spent fuel pool was full and the water temperatureof the pool was about 24°C before the earthquake, and it was not a
condition of which cooling of the fuel was impeded during an early stage.
Therefore, the cooling of the pool was not yet carried out before the tsunami
arrived. Because all AC power was lost due to the tsunami, operation of the
RHR became impossible. Then, both the spent fuel pool and the reactor
were cooled by building a temporary sea water pump to take sea water andby alternating cooling of the spent fuel pool and the reactor by switchingsystem configurations of the RHR.
e.

Status of “power supply” function
Because the external power source was lost due to the earthquake, twoemergency DGs were automatically started. Although all AC power was lost
due to the tsunami, one diesel generator continued to operate in Unit 6 andpower that is necessary to accommodate the power source to Unit 5 was
supplied.

(2) Unit 6
Refer to “2.7 Status of Progress of the Accident in Unit 6” in this article for the
behavior of the plate when the earthquake occurred.

3-36

a.

Status of “shutdown” function
When the earthquake occurred, Unit 6 was stopped because it wasundergoing a periodic inspection, and the RPV top head was tightened witha bolt. The reactor was in a status of cold shutdown, and all control rods
were inserted.

b. Status of “cooling” function
The reactor was in a status of cold shutdown before the earthquake, andit was not a condition of which cooling of the fuel is impeded during theearly stage. Therefore, the cooling of the pool was not yet carried out before
the tsunami arrived.
The function of two diesel generators was lost due to the influence of the
Tsunami, and the operations of the RHR, the low pressure core spraysystem, and the HPCS became impossible. On the other hand, one diesel
generator continued to operate and power that is necessary to maintain thefunction of supplying water to the reactor was supplied. The pressure andthe water level of the reactor were controlled by starting a condensate
water transfer pump on March 13, by properly reducing the pressure with
the SRV, and at the same time by repeating an operation of supplying waterfrom the CST to the reactor on and after March 14.
Then, both the spent fuel pool and the reactor were cooled by building atemporary sea water pump to take sea water and by alternating cooling of
the spent fuel pool and the reactor by switching system configurations of
the RHR.
c.

Status of “trapping” function
A stable value was shown for the stack radiation monitor, and noabnormality was found.

d. Status of the spent fural pool cooling system
The water level of the spent fuel pool was full and the water temperatureof the pool was about 25°C before the earthquake, and it was not a
condition of which cooling of the fuel was impeded during the early stage.
Therefore, the cooling of the pool was not yet carried out before the tsunami
arrived. Because two diesel generators were lost due to the tsunami, theoperation of the RHR became impossible. Then, both of the spent fuel pooland the reactor were cooled by building a temporary sea water pump totake sea water and by alternating cooling of the spent fuel pool and thereactor by switching system configurations of the RHR.
e.

Status of “power supply” function
Because the external power source was lost due to the earthquake, three
emergency DGs were automatically started. Although the function of twodiesel generators was lost due to tsunami, one diesel generator continued
to operate and power that is necessary to accommodate the power source
to the reactor for cooling and to Unit 5 was supplied.

3-37

3.6

Comparison of progress of accidents of Fukushima Daiichi
Nuclear Power Station with that of other stations

In the accident at this time, a large amount of radioactive substance wasdischarged from Fukushima Daiichi Nuclear Power Station (Fukushima Daiichi)
into the environment. However, we decided to learn a lesson also by comparing astation that ended up discharging a radioactive substance with a station that didn’tdischarge even though the tsunami hit. Fukushima Daini Nuclear Power Station,
Onagawa Nuclear Power Station (referred to as “Onagawa” below), and Tokai
Daini Nuclear Power Station (referred to as “Tokai Daini” below) that were affectedby the tsunami were selected as stations that didn’t discharge a radioactive
substance, and an investigation and comparison of Fukushima Daiichi Units 1 to 3
with these stations were performed.
As shown in Fig. 3.6-1, the transition of the event in Fukushima Daiichi Units 1
to 3 and other stations is branched from the heading of the AC power source (theexternal power source or the emergency DG), and the power source was secured
in all the other stations as a result, which led to prevent the accident fromextending.
Hence, it is found that the main cause of the extention of the accident inFukushima Daiichi is that the external power source and the emergency DG werelost due to the effect of the earthquake and the tsunami and all of the safety
components and the facilities to counter severe accidents couldn’t function
excluding some of the components that don’t need a power source.
As shown in Table 3.6-1, the main reason that the power source was secured inother stations is considered to be that their sites were relieved from the infiltrationof the tsunami. The tsunami did infiltrate the site of Fukushima Daini, and theexternal power source, the power source panel, the emergency DG, etc. weredamaged. However, the accident was settled by using a power source and facility
that were not damaged because the infiltration of the tsunami was limited.
The core cooling and the ultimate heat sink can become a common cause of
making decay heat removal impossible and can lead to the extension of theaccident. The core cooling by the RCIC and the HPCS was possible in Fukushima
Daini, Onagawa, and Tokai Daini, and cooling of the reactor by the core coolingsystem was sufficiently effective as long as the power source could be secured.
Ultimately, cooling by the RHR was secured and the event was settled in all of
the above described stations. In the case of Tokai Daichi, tsunami
countermeasures were taken in the sea water pump area for the sea water systempertaining to the ultimate heat sink. When the components in which thecountermeasures were already taken operated, a smooth settlement of the event
became possible. Hence, the importance of tsunami countermeasures for the sea
water system (soaking prevention countermeasures) can be seen.
Details of the investigation of each station are attached as Appendix-1.

3-38

Events
occurred
Earthquake andTsunamiReactorshutdown
Reactorscram

DC power
AC source
sourceDC power External
Emergenc
sourcepower
y DG
sourceElectric power
Ensuring long-term cold shutdown
Status of reactor core
recoveryIC, turbineExternal
RPV
Reactor
RHR
RHR
PCV vent Cold shutdown, core damage,driven coolant power
depressuri coolant
recovery (before
PCV damage etc.
injection
source,zationinjectioncore
systememergency(Including (including
damage)
RCIC,HPCI) DG, power
alternative alternative
for which AC interchange sources) sources)
source is notCore cooling

2F-3 SRV
O-2
O-3 SRV

2F-3 RCIC
operational
O-2 not operational
O 3 RCIC

2F-3
RCIC,MUWC,RHR
O-2
O 3 RCIC MUWC

Cold shutdown

Depressu Reactor
rization of coolant
nuclearinjection
reactor

Reactor
Final statusbuilding control
PCV vent Hydrogen SGTS,
Cold shutdown, Core damage, PCV damage etc.
control
ventilation(Nitrogenreplacement etc.)

Reactor container control

Degraded core coolin

PCV
coolant
injection

2F-3 3/12 12:15
O-2 3/11 14:49 (The reactor water temperature
at the time of earthquake was less than 100 °C)
O 3 3/12 1:17

2F-3,O-2-3
2F-1 3/14 17:00
2F-2 3/14 18:00
2F-4 3/15 07:15
Cold shutdown (Long term cooling is required)
Cold shutdown
2F-1,2,4[Tsunami]

2F-1-4,O-2-3

PCV damage (Overpressure)O-1 RCIC,CRD
Tokai RCIC,HPCS
O-1 SRV
Tokai

Cold shutdown

O-1 3/12 0:58
Tokai 3/15 0:40

Cold shutdown

O-1 D/G（A,B)
O-1 RCIC
Tokai

Cold shutdown (No RHR)
PCV damage

O-1
Tokai
Cold shutdown
Cold shutdown
（A)

1F-3

(1) Working with SBO with which facility
can be receoved

Cold shutdown (Long term cooling is required)

2F1-4
O-1-3
1F-3

PCV damage (Overpressure)Soundness of PCV or PCV leakage(Overtemperature)

Cold shutdown (Long term cooling is req(Continuous)

PCV leakage (Overtemperature) and hydrogen
combustion in the building

RCIC/HPCIPCV damage (Hydrogen explosion)

1F-3(Successful)

Unable to use S/Y due
to 1F3 work andtsunami
O-1 startup
transformer shutdown(M/C fire)
Tokai earthquake

PCV damage (Overpressure)
SRV
Recovery failed(Not implemented)(Continuous)

PCV damage (Overpressure)

1

Reactor core damage(Quantity of water actually injected to the reactor is less)Submerged due to
1F3 tsunami

Accident
sequence

PCV damage (MCCI, etc.)

Reactor core damage
PCV damage (DCH, etc.)
Accident
sequenceestimated with
1F-2
Soundness of PCV or PCV leakage(Overtemperature)

Same as (A)1F-1/2[Tsunami]

Cold shutdown (Long term cooling is require(Continuous)
1F-2

PCV leakage (Overtemperature) and hydrogen
combustion in the building
PCV damage (Hydrogen explosion)

RCIC

PCV damage (Overpressure)[Earthquake](Not implemented)(Continuous)

Recovery failed

PCV damage (Overpressure)

Reactor core damage(Quantity of water actually injected to the reactor is less)

PCV damage (MCCI, etc.)

Reactor core damage
PCV damage (DCH, etc.)[Tsunami]

1F-1

Cold shutdown (Long term cooling is required)Soundness of PCV or PCV leakage(Overtemperature)

PCV damage (Overpressure)

PCV leakage (Overtemperature) and hydrogen
combustion in the building

nami, operation period after tsunami is not known)
Recovery failed(RPV low pressure)

PCV damage (Hydrogen explosion)
Reactor core damage (RPV low pressure)(Water injection delayed or Quantity of water actually injected to the reactor is less)(Not implemented)
PCV damage (Overpressure)

Accident
sequenceestimated with 1F-

PCV damage (MCCI, etc.)

Figure 3.6-1 Event tree of event progress in Fukushima Daiichi Nuclear Plant Units 1 - 3, Fukushima Daini, Onagawa, Tokai Daini

3-39

Table 3.6-1 Comparison of conditions and specifications of plants that were damaged in the tsunami following The Great East Japan Earthquake

Condition of plant when earthquake
occurred2

Operational

Operational

Condition of the plant after tsunami

Emergency

1

Suspended

Suspended

Operational

Emergency cooling system(Including Emergency condenser(IC))

IC (operationaltime unknown)

RCIC(Operational for
approximately
3 days)

○

?

○

-

-

-

×

×

×

×

×→○

○

NormalEmergency

Principal
buildingsurrounding
immersion depth [Inundation height]

Operational

Operational

No damages

Damaged(part M/C, P/C
usable)No damages(part P/C unusable)(Suspended)

RCIC

No damages
RHRS
No damages to
No damages
RHRS(B) only
[loss of power[Loss of power]
to A]

RCIC

RCIC

No damages to
RHRS(D) only
[Loss of power]

RCIC, HPCS

(Vent line configuration complete)○○○

O.P.+5.7m

O.P.+5.2m

Approximately +13m2○

O.P.+10m

O.P.+13m

Approximately +9m 2
O.P.+12m

Approximately 1.5 - 5.5m
[O.P. approximately +11.5 - +15.5m3]

Approximately 1.5m or less
[O.P. approximately +13 -+14.5m]

Approximately 2.5m or less (almost zero except surroundings of Unit 1)[O.P. approximately +12 -+14.5m4]

DG

T/B B1F

T/B B1F
Commonpool1F

Power
panel

T/B 1FC/B B1F

T/B B1F
T/B 1F
Common
B1F

Power
panel

T/B 1FT/B B1F
T/B 1F
2SA building 1F
N

Operational

4

DamagedRHRS
No damages[Loss of power]

RCIC(Operational for
approximately 1
day),
HPCI
(Operational for
approximately
half a day)Height of tsunami at arrival
Site height

Diversity in external power supply(diversity in no. of lines, switchyard)

No damages

DamagedSevereaccident Containment vessel ventcounterm
easureMUWC alternativeequipmen
injectionts
Expected height of tsunami (Societyof Construction Engineer)

Availability of Air-cooling DG (Y/N)

No damages

Damaged (except part P/C)

Emergency cooling system sea water
pump (main body)

Fukushima Dai-ni
2
3

Among 4 lines (1 line under construction), 1 line usable1

Damaged (except part P/C)

NormalDeploymentofpower panel (M/C,
P/C),
emergency
DG(building,hierarchy)

Suspended

6

Existing DG (1A, 1B, 2A, 3A, 3B, 4A): Damaged
Extension DG (2B, 4B): No damages

Emergency DG (main body)Specifications of each plantOperational

5

Among 6 lines (1 line under construction), all unusable1

External power supply

Power panel (M/C,
P/C, DG)

Fukushima Dai-ichi
3
4

1

T/B B1F

pool

Y(DG2B)

T/B B1F
Common1F

T/B B1F

T/B B1F
T/B 1F
CommonB1F

T/B B1F
C/B B1F

T/B B1F
T/B 1Fpool

pool

T/B B1F

C/S B1F
D/G building 1F

C/S B2FC/S B2FC/S B2FC/S B2F

T/B B1F

C/S B2FC/S B1F
C/S 1FDG building B1F

C/S B1F
Hx/B 1F

C/S B1F
Hx/B 1F

C/S B1F
Hx/B 1F

C/S B1F
Hx/B 1F

T/B B1F
T/B 2FC/B B1F

T/B B1F

C/B B1F
C/B 1FC/B B1F
C/B 1FC/B B2F
C/B 1FC/B B2F
C/B 1F

N

N

N

N

Y
(DG4B)

N
6 linesN

Y(DG6B)

4 lines

Although the sea water system was installed in the building, electrical
power systems were damaged(except B system of Unit 3)
T/B: Turbine building, Common pool: Common pool building, C/B: Control building, C/S: Combination structure (Waste disposal building, Gaseous waste disposal building are combined and stored in 1 building around theconventional reactor building), Hx/B: Heat exchanger building1: Damages to transmission tower, switchyard etc are the damages due to earthquake, 2: Height of tsunami in tide station installation position of both the power plants. For instrument damage, the actual height of tsunami intide station is unknown, 3: O.P. approximately +16 - +17m [immersion depth approximately 6 -7m] locally in the southwest of the said area, 4: O.P. approximately +15 - +16m [immersion depth approximately 3 – 4m] locallyfrom the south of Unit 1 till the seismic isolation significant buildings
3-40
Water-tightening
building)

(pump

area,

Sea water system installed outside the buildingTable 3.6-1 Comparison of conditions and specifications of plants that were damaged in the tsunami following The Great East Japan Earthquake
Onagawa
Condition of plant when earthquake
occurred1

2

3

Operational

Start-up

Operational

External power supplyPowerpanel(M/C,P/C,DG)

No damages

NormalM/C 6-1A Fire

Emergency cooling system sea
water pump (main body)

RHRS, ECWS
No damages

Emergency cooling system(Including Emergency condenser(IC))
Severeaccident Containment vessel ventcounterm
easureMUWC alternativeequipmeinjectionnts
Expected height of tsunami (Societyof Construction Engineer)

No damages
No damages

RSW (B) system: Damaged
RSW (A) system and HPSW
system no damages

RCIC, HPCINo damages

No damages
RSW, HPSWNo damages

RCIC, HPCS

RHRS No damages [A system Loss of power]
DGSW(2C) damaged
RCIC, HPCS

-

-

-

-

-

-

○

Altitude + 4.9m

Height of tsunami at arrival

O.P.+13.0m

Altitude + 4.8-5.3m

Site heightOP+14.8m
 Crustal movements around the power station due to the earthquake has been estimated to beapproximately 1m, currently it is 13.8m

T.P.+8.0m
 Value before earthquake

Main building surrounding immersion
depth[Inundation height]

No immersion

No immersion

Deployment of
power
panel(M/C,P/C),
emergency DG(building,hierarchy)

Emergency

O.P.+13.6m

DG

C/B B3F

R/B 1FR/B 1FC/S B1F

Power
panel

T/B B1F, C/B B2F

R/B B1F

R/B B1F

C/S B2FC/S B1F
C/S 1F (direct current power supply panel)

Norm
al

Condition of the plant after tsunami
Specifications of each plant

No damages In Unit 2, as the RCW (B) system and HPCW system were flooded, DG (B) and DG (H) could not be
used.

Emergency

Operational
Among 3 lines, all unusable1

Among 5 lines, 1 line usable

Emergency DG (main body)

Tokai Dai-ni

Power
panel

T/B B1F

C/B B1F

S/B B2FC/S B2FC/S B1F

Availability of Air-cooling DG (Y/N)

N

N

Diversity in external power supply(diversity in no. of lines, switchyard)

5 lines

3 lines

Installed tsunami countermeasure side wall of pump area T.P.+6.1m
Affected the under construction sealing work of the wall penetration of
one side pump area
T/B: Turbine building, Common pool: Common pool building, C/B: Control building, C/S: Combination structure (Waste disposal building, Gaseous waste disposal building are combined and stored in 1 building around theconventional reactor building), Hx/B: Heat exchanger building1: Damages to transmission tower, switchyard etc are the damages due to earthquake
Water-tightening (pump area,building)

Sea water system installed outside the building

3-41

3.7

Causal analysis

An event tree analysis of the Fukushima Daiichi accident has shown that the
major cause for aggravation of the situation was the loss of all safety systems dueto the failure of power-supply sources.
According to the same analysis, other nuclear power stations averted a similar
deteriorating situation largely due to the securing of power sources.
Nuclear powers stations are equipped with emergency DGs and designed topromptly replace external power supplies. At Fukushima, the emergency DGs
were submerged. Furthermore, even if the external power sources had been
restored, emergency power-supply devices (such as emergency DGs andpower-supply vehicles) could not have been connected because the power panels
were also submerged as a result of the massive tsunami. This was a major reason
for exacerbation of the situation.
In short, the real issue was the lack of preparation for such a state of emergency.
Stricter safety measures could have ensured different results. A worst-casescenario and countermeasures for it should have been developed. For example, “ifa tsunami reaches station premises, water may come into buildings and importantsafety equipment including power panels may be submerged/compromised. With
this in mind, seal such important systems to prevent water from seeping in or
encase them in a watertight compartment. Reinforce facilities and equipment in
the event such protection is lost, and furthermore, be prepared to deal with such asituation in case such enhancement is not sufficient.”
The major cause for aggravation of Fukushima Daiichi situation was the loss of
power-related facilities/equipment such as external power sources, emergency
DGs and power panels, as well as the lack of preparedness for dealing with suchan emergency.
While securing power supply could have mitigated the emergency to a certain
extent, securing cooling functions would also have been needed. During thedisaster, Fukushima also lost component cooling systems as a result of the lack of
sea water pump functions. Therefore, stringent emergency measures are needed
for power supplies as well as for cooling systems.
We have classified identified issues into the following five categories. Chapter 4
will describe these issues and required preparation/measures:・Countermeasures against earthquakes/tsunamis (natural hazards)
・
Securing of power supply
・Measures against loss of heat sink systems
・Measures against hydrogen leakage
・Preparation for emergencies
Category
Countermeasures against
earthquakes/tsunamis

Issues/requirements
・ Seismic adequacy of external power sources
・ Appropriate arrangement for switchyards
・ Appropriate arrangement for sea water systems
・ Protection of sea water pumps from submergence・ Appropriate arrangement for emergency DGs
・ Air/water tightness of buildings

3-42Category
Securing of power supply

Measures against loss ofheat sink systems

Measures against
hydrogen leakage

Preparation for
emergencies

Issues/requirements
・ Only one incoming panel to receive external power
・ Reliability of power sources in case of tsunami
・ Submergence of power supply systems
・ Runout of DC power supply
・ Securing of battery capacity
・ Obtaining power from external sources (e.g.,
power-supply vehicles)
・ Securing of AC power supply
・ Cable installation for alternate power sources
・ Submergence of sea water pumps
・ Ensuring entire facilities/equipment so as it cannot
be put out of commission by a single event
・ Setting up heat sink systems that will not be nullified
by tsunamis
・ IC isolation signal in the event of loss of DC power
supply・ Securing of equipment to cope with valve drive
failure・ Establishing emergency procedures and training
・ Loss of freshwater sources
・ Earthquake protection and tsunami
countermeasures for FP piping
・ Procurement of fuel for fire-extinguishing vehicles
・ Enhancement of drive source for pressure-retaining
vent line valves
・ Inoperability of rapture disks at low pressure
・ Securing of compressed air to maintain opening
container ventilation valve
・ Reinforcement of container gasket and penetration
area sealing・ Leakage from pressure-resistant ventilation line tobuildings
・ Circulation between buildings that share the same
exhaust stack・ Hydrogen retention in R/Bs
・ Detection of hydrogen
・ Securing sufficient personnel for radiation
management/monitoring
・ Protection of radiation control facilities from tsunami
damage・ Obstructions created by earthquakes/tsunamis
・ Communication methods in the event of loss of ACpower
・ Tools/equipment to support nighttime/indoor
operations
・ Machines/equipment to secure road access・ Securing of more than one access route within thepremises
3-43

CategoryIssues/requirements
・ Support systems between power companies
・ Deterioration of environment due to hydrogen
explosion and rising radiation

Source:(1) Report of Japanese Government to the IAEA Ministerial Conference on Nuclear
Safety (June 2011)・ http://www.kantei.go.jp/jp/topics/2011/pdf/houkokusyo_full.pdf
・ http://www.kantei.go.jp/jp/topics/2011/pdf/app_full.pdf

3-44Chapter 4

Lessons Learned and Measures To TakeAs described in Chapter 3, the situation at the Fukushima Daiichi Nuclear Power
Station became serious due to the loss of power sources as a result of destruction ofmost safety equipment by the tsunami, as well as the failure of backup measures(severe accident measures) that were expected to control the situation.
From
this
accident,
wehavelearnedthat
thefollowing
preparations/countermeasures are critical:
・ Countermeasures against natural hazards
・ Securing of power supply
・ Measures against loss of heat sink systems
・ Measures against hydrogen leakage
・ Preparation for emergencies (training/drills)

4.1

Countermeasures against natural hazards

This document describes the damage caused by the earthquake andsubsequent tsunamis and our recommendations on countermeasures.
The size of the earthquake that occurred on March 11, 2011 was mostly within
the predicted values for the facility design. The post evaluation conducted by
TEPCO utilizing acquired seismic waves reflected that the safety
devices/equipment would not have been inoperable. However, the combination ofseveral earthquakes within a close proximity created a massive tsunami. The
possibility of this phenomenon had never been considered. The height of possible
tsunamis needs to be re-evaluated based on this newly recognized phenomenon.
Power sources for the station were damaged by the tsunami. Calculation basedon outdated knowledge did not anticipate that the height of a tsunami would
exceed ground level at the station. Therefore, the possible consequences of sea
water infiltration were not carefully considered and sufficient countermeasurestaken.
While the precise size of natural hazards such as tsunami and typhoons is notforeseeable, we must prepare for hazards that may exceed the calculated designbasis. If there is a possibility of a severe power station accident, even through theprobability of that possibility is minimal, we must take steps to protect existing
properties and bring more facilities/equipment to bear in order to improveendurance and resolve vulnerability issues.

4.2

Securing of power supply

In the event the power supply for a station is cut off, there are external powersources that may be tapped to supply electricity. If for some reason these external
power sources are not available, emergency DGs are programmed to start up andprovide power to safety equipment. There usually are two groups (or three groups,
depending on reactor type) of 100% capacity emergency DGs. Power sources aredesigned in view of multiple performance and diversity.
At Fukushima Daiichi, the external power supply was cut off due to theearthquake, and subsequently all AC power sources became inoperable because
the power panels and emergency DGs were submerged as a result of the tsunami.
Although some DC sources managed to escape water-damage, battery life wasexceeded before the power supply was reestablished. As a result, most facilities
ceased to function and the situation became critical. On the other hand, an
air-cooling emergency DG for Fukushima Daiichi, Unit 6, was not affected by the
4-1

tsunami and maintained continued to operate. Decay heat at this unit was
relatively low because it was under suspension and there was adequate time tocool down. The fact that this unit was able to successfully reach cold shutdown
can be used as an example when planning countermeasures in the future.
As for power supply, we need to enhance existing facilities/equipment in
preparation for natural hazards and establish reliable backup measures in theevent they should fail to function.
Possible appropriate measures include ensuring that the facilities are watertightin order to protect them from possible tsunamis, and using power-supply vehicles
to provide electricity in case existing equipment fails to function.

4.3

Measures against loss of heat sink systems

As mentioned earlier, one of the major causes behind the aggravation of the
Fukushima situation was the functional failure of emergency DGs due to loss ofthe power supply. While some of the emergency DGs failed to function as a resultof submergence/the submergence of the power panels, lack of cooling water due
to damage to sea water pumps caused by the tsunami also contributed to thefailure of some DGs.
Since the power failure caused severe equipment damage, the sea water pump
failure was not very conspicuous. However, loss of sea water pumps may lead tothe simultaneous functional failure of other various equipment (such as coolingwater destination pumps and heat exchangers), and result in further aggravationof the situation.
Therefore, protection of sea water pumps from tsunami damage is required andmeasures to immediately restore their functionality are also important. Reactor
cooling system must also be secured in case the sea water pumps become
inoperable. In view of the need to cool reactors during power outages, alternate
coolant injection by utilizing existing equipment or provisions for the establishmentof temporary systems should be planned, and water sources should be secured toprovide continuous cooling capability.

4.4

Measures against hydrogen leakage

Explosions allegedly by hydrogen at Fukushima Daiichi, Unit 1, 3 and 4
destroyed the roofs and walls of these R/Bs and brought about obstacles to deal
with the situation.
Under such circumstances, the cooling down of the reactors and maintenanceof the cooling effort must be the highest priority regardless of whether the core isdamaged or not. Accurate data on the reactor core may not be available during astate of emergency at a nuclear power station and the possibility of core damage
and hydrogen leakage must always be considered. Hydrogen is a substance thatcan easily escape. It is necessary to keep it in mind that the hydrogen leak may
spread beyond the normal containment area.
A PCV is usually used to contain hydrogen. PCVs also act as recombiners inwhich nitrogen is added in order to inactivate the hydrogen.
However, in situations where the high temperatures/pressures exceed thedesign parameters, hydrogen may leak from penetration or the flange gasket
sealing of PCV and then accumulate inside of R/Bs. As the containment vessel
ventilates, hydrogen then may go enter the pipes connected with exhaust stacks.
Under the presumption that hydrogen is susceptible to leakage,
countermeasures should be established to stop the accumulation/leakage, and4-2

provisions for controlled burns should be developed. Power outages also need to
be assumed whenever these measures are taken.

4.5

Preparation for emergencies (training/drills)

The employees and contractors at Fukushima Daiichi had to work under
extreme circumstances: a power blackout, loss of meters and gauges, loss ofcommunication ability, high radiation, a highly contaminated environment,
scattered rubble and debris, ongoing tsunami warnings, multiple reactor units
accidents, as well as having to deal with other emergency operations that included
the need to arrange for temporary hoses/cables and radiation monitoring
equipment. Few of those workers had ever had such experience and that was oneof the reasons that it took an inordinate amount of time to carry out theseemergency tasks. On-site personnel were not the only ones who faced difficulties.
TEPCO Headquarters’ command structure was also plunged into turmoil due tothe number of accidents and the rapidly spreading damage.
We must establish provisions for dealing with such situations with restrictedconditions.
The seismic isolated building effectively functioned as an emergency command
post. This fact is an excellent case example for other power stations. Based on theexperience gained from this accident, various facilities/equipment have been
added and new emergency manuals have been established. Training and drills forpossible stringent conditions must be conducted by using currently available
facilities/equipment and manuals in order to be proficient in responding to an
emergency. Crisis handling systems, line of command and efficacy of eachmeasure must be evaluated and improved upon, if necessary, during suchtraining.

4-3

Examples of emergency measures are as follows:

4.6

Countermeasures against earthquakes/tsunamis (example
measures)4.6.1 Assumption of earthquake/tsunami strength
Although the strength of the earthquake on March 11, 2011 exceeded
maximum response acceleration value against reference seismic movement, Ss,
all the emergency functions, “stop”, “cool” and “contain” at Fukushima Daiichi
were functioning during and right after the quake until the tsunami descended
upon the site.
Sea water pumps and emergency DGs became inoperable once the massive
tsunami reached the grounds of the station site and after the quake had
disrupted the external power supply. Furthermore, all power sources andultimate heat sinks were lost simultaneously as a result of submergence of themetal clad switchgears, power center and DC power facilities.
Road damage and debris caused by the quake/tsunami interfered with
operations to connect external power sources. As a result, the SBO state lasted
for hours and coolant injection into reactors was delayed.
Findings/lessons learned
・ The combination of multiple hypocenters created a massive earthquake (inmagnitude, size of affected area and length of shift). Such interrelated
earthquakes had not been anticipated.
・ Despite the strength of the quake, the earthquake response spectrum locatedon the foundation of the R/Bs showed about the same value as the response
spectrum for the design basis (earthquake ground motion), Ss. Essential
safety facilities/equipment continued functioning.
・ Large shift of a fault triggered huge tidal waves. The combination of suchwaves created a massive tsunami that exceeded predicted the tsunami waterlevel and descended upon the station, causing extensive damage.
Measures to take
Given the above findings, the following measures should be taken:
・ The current design basis for earthquake ground motion, Ss probably
correspond to future earthquakes. However, possible combination of multiple
hypocenters in the neighboring waters should also be considered in order toensure safety.
・ To calculate height of future tsunami waves triggered by earthquake,possible combination of multiple hypocenters should be considered as awave source at the boundary of ocean-trench plates. The value of distanceof possible shift at a hypocenter should also be set to larger value as
appropriate.
Measures to deal with obstacles that interfere with emergency operations
such as the handling debris are described in “4.5.4 Preparation for emergencies(heavy equipment and rescue efforts), and emergency cooperative system.”
4.6.2 Protection of the site from tsunami
The massive tsunami easily exceeded the elevation of the Fukushima Daiichi
site. The reactor cooling system ceased functioning as a result of damage to thesea water systems and the loss of the power supply including emergency power.
4-4

Tanks that were flushed away by the wave blocked roads causing further delayin the preparing of injecting coolant by fire trucks.
Findings/lessons learned
・ Station equipment/facilities may submerge depending on their location andsea water may prevent safety equipment from properly functioning.
・ Debris/ wreckage brought by tsunami may block emergency traffic fromreaching the scene.
Measures to take
Given the above findings, the following measures should be taken:
・ Safety requires that all power-supply facilities be located in an area thatwill not be affecting by tsunamis. Coastal levees (tide barriers) or
breakwaters must be installed. Alternatively, protection barriers or
multilayered water protection measures need to be taken to ensure thesafety of the facilities.
・ Barriers that prevent structures from becoming floating wreckage andblocking access of emergency vehicles needs to be installed. Heavy
equipment needs to be available in order to remove debris/wreckage.
4.6.3 Protection of buildings
Buildings were flooded by water from the tsunami causing isafety equipment
to become inoperable.
(1) Water tightening of critical facility areas
Almost the entire area around the main buildings of Fukushima Daiichi were
flooded by the tsunami. The water entered the buildings allegedly fromground-level openings such as doorways and equipment hatches, ventopenings (louvered doors) and openings connected to under-ground trench
ducts (cable/pipe pass-through slots).
Most emergency power panels in Unit 1 through 5 and DC main busbar
panels in Unit 1, 2 and 4 were submerged. (There was no inundation in Unit
6.)
Emergency DGs also became inoperable in Unit 1 through 5 due to thesubmergence of the DGs themselves or their power panels. Air-cooled
emergency DGs were installed in Unit 2, 4 and 6, and the ones in Unit 2 and 4
became inoperable due to submergence of power panels. (The water-cooled
emergency DG in Unit 6 also became inoperable due to the inoperability ofthe sea water cooling system stemming from damage as a result of thetsunami.)The framework of main building including outside walls and columns did not
suffer significant damage from tsunami. However, inundation into thebuildings from openings caused the loss of power-supply equipment resulting
in loss of all DC/AC power sources and ultimate heat sinks.
Findings/lessons learned
・ Insufficient preparation to prevent inundation and inadequate endurance towave power allowed sea water to enter the main buildings from broken
doors on the sea side and from other openings such as louvered doors.
・ As a result of the tsunami, sea water overflowed into importantfacilities/equipment (such as electrical facilities) from entrance doors andother openings including pipe penetration and resulted in thosefacilities/equipment becoming inoperable.
・ Recovery operations were delayed due to accumulated sea water inside
4-5

the buildings.
Measures to take
Given the above findings, the following measures should be taken:
・ Steps to better protect against inundation including improving sealing
capability at penetration and openings (such as doors and vents) must be
taken.
・ Impact of inundation must be minimized by the water tightening of doors to
key safety facilities.
・ Portable drain pumps need to be equipped in buildings in order to be able
to drain sea water and ensure speedy restoration.
・ Doors facing the ocean and that may receive direct impact from a tsunami
must be reinforced.
(2) Protection of sea water systems against inundation
At Fukushima Daiichi, sea water systems ceased functioning due to theunanticipated size of the tsunami, but the system at Unit 5 and 6 wasreestablished by using substitute pumps. At Fukushima Daini, all sea water
systems except the one at Unit 3 ceased functioning, but the system waseventually brought back on line by using replacement motors. At Tokai Daini,
all sea water pumps remained operational except one because a floodwall
was under construction around the sea water pump area. No stations
experienced any blockage or damage of intake channels due todebris/wreckage.
Findings/lessons learned
・ Fukushima Daiichi lost all sea water pumps due to the tsunami.
・ Specific measures must be taken so as not to lose the operation of all ofsea water pumps at the same time.
Measures to take
Given the above findings, the following measures should be taken:
・ Protection against inundation including the building of floodwalls aroundsea water pumps must be initiated.
・ Extra motors must be reserved to ensure a speedy recovery.
・ Portable sea water pumps should be held in reserve or waterproof pumps
should be installed.

4.7

Preparation for power supply (example measures)

4.7.1 SBO and loss of DC power sources
Transmission of the external power supply broke down and external power
was lost due as a result of the earthquake. The emergency DGs started up rightaway as intended and the required power supply for the station wasreestablished.
However, due to the inordinate size of the tsunami, power-supply facilities,
including emergency DGs that were not capable of withstanding inundation,
were subsequently submerged and all AC power sources were lost. Power fromDC power-supply facilities that were not submerged ceased later due to theexceeding of battery life. As a result, Fukushima Daiichi lost all power sources.
Traces of the tsunami showed that it reached the area around the switchyard
and incoming electricity transformers. The power supply line from neighboring
station, which is one of the severe accident response measures, failed because
4-6

the closest station also went down.
Fukushima Daini, Unit 1, 2 and 4, also lost their sea water cooling systems.
However, core cooling was maintained by RCIC followed by an alternate cooling
operation. Meanwhile, the sea water system was reestablished and theradiation cores went into a cold shutdown state. Thus, the safety of power
station can be ensured through the use of emergency equipment/facilities
provided the power supply is secured.
In order to recover the power supply, Fukushima Daiichi tried to bringpower-supply cars and connect them to the station facilities. Access to thestation was hindered due to congestion of the surrounding roads and roaddamage as a result of the earthquake. Although batteries and manpower to lay
heavy cables was brought to Fukushima to assist with the recovery effort,
submergence of power panels resulted in an extended power blackout period.
Findings/lessons learned
・ Emergency DGs were installed to cover the loss of external power sources.
However, emergency DGs also became inoperable due to the tsunami.
Further backup systems are needed.
・ In order to ensure the reliability of incoming electricity transformers andswitchyards, the external power supply for them needs to be protected fromtsunamis.
・ Emergency DGs and related facilities were submerged and became
inoperable as a result of the tsunami. These DGs need to be protectedagainst such damage.
・ Some of DC power-supply facilities were submerged and lost functionality.
Other DC power-supply facilities that were not affected by the tsunami ran out
power and became inoperable because they could not recharge. Suchlong-term needs have to be considered.
・ Severe accident response facilities (power panels to bring electricity fromother stations) were submerged by the tsunami. These facilities need to beprotected from inundation.
・ As an emergency response, power-supply cars were sent to Fukushima
Daiichi, but were delayed because of the adverse conditions brought on by
the earthquake/tsunami. The mobility of power-supply cars is very useful but
the way to provide speedy access under adverse circumstance must also be
developed.
Measures to take
Given the above findings, the following measures should be taken:
・ Backup power supply in the event emergency DGs become inoperable－Reserve earthquake/tsunami-proof power-supply cars or large-capacity
batteries (gas turbines or diesel generators) as backup power sources
and the establishment of emergency procedures (e.g. steps to obtain fuelsuch as light oil and how to connect power sources).
・ Protection of external power sources against inundation－Review location/arrangement of incoming electricity transformers andswitchyards, and make them watertight as needed.
・ Protection of emergency DGs and related facilities against tsunamis－Review location/arrangement of emergency DGs, power-supply facilities(high and low voltage) and cooling systems for emergency DGs, andsecure in watertight structures as needed.
・ Protection of DC power-supply facilities against inundation
4-7

－Review location/arrangement of DC power-supply facilities, and protect
against possible water damage as needed.
－Establish DC power-supply (battery) charging routes from backup powersources.
・ Protection of inter-building power-supply facilities (severe accident
measures) against inundation－ Review protection of inter-building power-supply facilities (severe
accident measures) against inundation, and improve reliability as
needed.
In order to improve the seismic capacity of external power sources, all powercompanies have been directed by the government to review the seismic
adequacy of their transmission wires and improve the reliability of the facilities at
locations where landslides may occur. Power companies are also to address thereliability and performance of multiple power systems.
Expansion of battery capacity is a logical strategy, but feasibility isquestionable due to required space for such a system and its limited
effectiveness. Therefore, in order to secure DC power supply, the first step will
be to secure power sources to feed the DC power supply. The second step will
be to establish methods to charge batteries as soon as the power source has
been lost.
Factors that could interfere with power recovery operations are common toother operations. These factors and approaches are described in “4.10Preparation for emergency situation.”

4.8

Examples of measures addressing the heat sink loss

4.8.1 Water Injection to the reactors
In this accident, the electric-driven water injection system of ECCS could not
be activated due to loss of power supply while the steam-driven water injection
system could not continue to operate due to loss of the control DC power supply
batteries after a certain period of time or for some unknown reason.
In this case, the alternative low-pressure cooling water injection system
required some time before starting cooling water injection because it needed to
first depressurize the reactors which lost the driving power supply and thedriving air pressure of SRV (ADS function, relief valve function). In addition,
MUMC, an alternative water injection system, could not operate because thepump was inundated and the FP diesel fire pump failed. Furthermore, thealternative high-pressure injection facilities (CRD, SLC) were in a state that
could not be started due to the complete loss of power supply.
As stated above, the water (fresh water) injection to reactors could not be
implemented using the existing injection facilities, and eventually fire engines
were used for emergency injection of seawater.
In addition, as for water injection facilities, the actual situation of damages
was unknown because they could not function due to loss of power supply,
except some of the equipment was known to be damaged by seawater flooded
into the building.
Findings/lessons learned
・ It was unsuccessful in dealing with the situation that all ECCSs lost theirfunctions due to complete loss of power supply for a long time caused by theearthquake and tsunami, and that the facilities for preventing severe
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accidents could not be used. Therefore, it is necessary to examine themeasures to ensure long-term water injection (including securing water
sources) even if tsunami and earthquake hit.
・ In the event that the high-pressure injection system loses its function, thealternative low-pressure injection should perform to depressurize the reactor.
However, it is difficult to reduce the pressure due to loss of the source which
drives the valve for reactor depressurization. It would take some time before
the alternative low-pressure injection system can be used. Therefore, it isnecessary to examine the measures to cope with loss of driving source offacilities required for reactor depressurization.
・ For the body of the water injection facility, it is also necessary to havemeasures to deal with seawater flood into the building in a tsunami.
Measures to takeIn light of those described above, the following measures should beconsidered to cope with, for example, the events of BWR.
・ To ensure the water injection function to achieve secure water injection toreactors in the event of loss of existing driving sources for a long period oftime.
－To improve the power supply reliability for the existing injection system bydeploying a backup power unit or large-capacity power supply－To secure alternative means of injection to reactors using portable powerpumps and hoses that do not depend on the existing power supply－To prepare backup power supply and reserved air cylinders necessary for
driving to ensure reliable depressurization of reactors through SRV－To secure water source (including seawater as the last resort)・ Among the severe accident prevention facilities, there should beanti-inundation measures by establishing the seawall and watertight doors to
the extent expected as the last resort in combination with other alternativemeans.
・ To establish anti-inundation measures by ensuring the watertightness ofcompartments of safety equipment, such as emergency cooling systems forreactor cores.
In PWR, there should be cooling means of the secondary system required
when a station blackout occurs. In these means, water will be supplied to thesteam generators through the turbine-driven auxiliary water feed pumps withsteam as the driving source; and a steam generator is available for discharging
the steam through the main steam relief value. This is equivalent to secure thefunction of water injection to reactors in the BWR; therefore, it is important toensure the means of water supply to steam generator and steam dischargemeans. In order to achieve continuous heat removal for a long time, thefollowing measures are suggested.
・ Improve the power supply reliability for the existing water injection system bydeploying a backup power unit or large-capacity power supply
・ Secure alternative means of water injection to steam generators using
portable power pumps and hoses that do not depend on the existing power
supply・ Secure more reliable measures to cool the reactors with the steam
generators through the main steam relief valve (including reserved air
cylinders)
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・ Secure water source (including seawater as the last resort)
As stated in the report, “the fire hydrant erupted water, but the filtered water
could not be used as the water source”. At this investigation meeting, there wasan argument whether it is necessary to improve the reliability of fire-fighting
water system. However, Tokyo Electric Power Co., Inc. gave the explanation, “itwas difficult to respond to the accident when the FP was damaged by theearthquake”. It is unknown yet whether it is a fact; therefore, the need to
examine FP will be decided after the investigation results of Tokyo ElectricPower Co., Inc. become available.
4.8.2 Loss of seawater cooling
As the tsunami was stronger than expected in design, the seawater pump
motor and ancillary equipment were damaged and could not operate. The heatremoval route “reactor → residual heat removal system → reactor building
closed cooling water system → reactor building closed cooling sea water
system → seawater” for final heat relief (final heat sink) loses its function, so theheat generated by reactor could not be removed.
In addition, due to the effects other than loss of function of the seawater pump,
the ancillary equipment cooling water pump and heat exchanger required forcooling by seawater pumps could not be used; therefore, ECCS and ancillary
reactor equipment could not operate even without the power loss.
Findings/lessons learned
・ It is necessary to examine the measures for inundation caused by tsunami
because the seawater pumps will no longer be used in a tsunami stronger
than expected. (Refer to the section of Tsunami for measures)・ It is necessary to examine the means to recove inundated seawater pumps.
・ It is necessary to examine a way to secure the heat removal route in the eventthat the seawater pumps could not be recovered early.
Measures to takeIn light of those described above, the following measures are suggested.
・ Alternatives to sea water pumps－Deploy reduandant sea water pump motors and develop a replacement
procedure after loss of functionality－Ensure the power supply which can back up the minimum functions
required for sea water pumps
・ Have measures for early recovery of sea water pumps－Have the cleaning and drying materials available for sea water pump
motors・ Ensure an alternative heat removal route－Disperse the heat to atmosphere through the containment vent (see thefollowing section)
In addition to the need to take the same measures as above for PWR, there
should be an alternative heat removal route “reactor → steam generator →atmosphere” using the secondary system with steam generator in the event thatthe heat removal route “reactor → residual heat removal system → reactor
building closed cooling water system → reactor building closed cooling sea
water system→ seawater” as final heat sink loses its function.

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4.8.3 Containment ventIn this accident, the route for heat removal to the final heat sink (sea) failed tofunction due to loss of sea water cooling; therefore, the heat from reactor washandled according to the procedure using the route “reactor → containment →atmosphere (vent)”.
To allow a rapid depressurization by opening the SRV, it was necessary tolower the containment pressure through the vent before discharge of steam.
Furthermore, when the containment pressure rose, the venting anddepressurization were performed in order to ensure the integrity of containment.
In this case, it needed some time before starting the vent because of thefollowing reasons.
・ The containment pressure could not be properly understood because ofloss of power supply to the metering instruments.
・ The remote control was disabled due to loss of power supply to the ventvalve and the cylinder pressure. Although alternative manual operation
was planned, some time was needed to examine and establish the ways
for implementation.
・ In addition, due to the increase in building radioactive dose andsurrounding room temperature, it was extremely difficult to access the ventvalve area.
・ The evacuation of the surrounding residents was finally confirmed.
Findings/lessons learned
・ The temporary rechargeable batteries that could be used for metering
instruments, and temporary power supply and air cylinder for driving the ventvalves were not prepared in advance for the event when the station blackout
continued for a long period of time; the radioactive dose was very high aroundthe vent valve area when the reactor core was damaged, and some time wasneeded for the vent operation. Therefore, it is necessary to examine how to
improve the reliability of vent operation in the event a station blackout
continues for a long time.
・ It is necessary to remove the radioactive substances other than rare gas
caused by the scrubbing effects of pool water of S/C while strengthening theprevention measures for over-heating damage in order to prevent damages tocontainment while performing the vent.
Measures to takeIn light of those described above, the following measures are suggested incase of BWR.
・ Improve the reliability of vent operation during a complete loss of power
supply－ Deploy temporary power supply to feed the monitoring instruments
required to determine the vent preparation and vent implementation－Deploy a backup power supply and driving source for vent operation
・ Optimize the operating conditions of the vent
－Conduct future examinations for optimization of the time to start the ventand perform a review as needed.
・ Reinforce the alternative containment spray after damages of the reactor
core
In addition, in the PWR, there should be a secondary cooling system withsteam generator even if the heat removal to the final heat sink fails due to loss
4-11

of sea water cooling. This system shoud be able to continue to remove the heatgenerated by the reactor. Therefore, for PWR, it is not necessary to discharge
the energy in the containment through the vent.
The design of rupture disk has been established, and will include a function toprevent discharge into environment of the containment atmosphere caused bymalfunction of valves, and the operation pressure will be set higher than thedesigned pressure in consideration of the malfunction. If the valve of the ventsystem can be reliably opened during the pressure rise within the containment,
it will achieve its original function of lowering the pressure; therefore, it isbelieved not necessary to make a change in the design of the rupture disk.
However, the measures to lower the set value for operation of rupture disk, or to
remove the rupture disk, will have a wider choice in response to accidents asthese measures might depressurize the containment early; and whether thesemeasures should be adopted will be determined based on judgment of electric
power, considering that there would normally be a small amount of emission ofradioactive materials associated with malfunctions.

4.9

Hydrogen measures

In this accident, the hydrogen leaked and was deposited in the reactor building
caused by the damage to the reactor core. As a result, the hydrogen-induced
explosions occurred in the No. 1 reactor building at 03:36 PM on March 12, and in
the No. 3 reactor building at 11:01 AM on March 14.
Due to these explosions, it was difficult to lay power cables and connect thehoses.
Also, an explosion, possibly caused by hydrogen, occurred in the No. 4 reactor
building at around 6:00 AM on March 15.
Findings/lessons learned
・ As a result of the continued leakage and deposit of hydrogen caused bydamage to the reactor cores, the hydrogen-induced explosions occurred in thereactor buildings. It is necessary to consider measures to prevent such asituation.
・ In the case of containment vent using the reinforced pressure-enduring vent line,
it is necessary to examine the measures to prevent the roundabout of hydrogenfrom SGTS exhaust pipe, or to prevent the roundabout of hydrogen through thejunction of the exhaust pipes shared with other units.
・ If temperature rises in the containment and pressure exceeds the design value,
the hydrogen from containment penetration and gaskets will leak into the reactor
building, which may lead to a hydrogen explosion.
Measures to takeIn light of those described above, the following measures are suggested.
・ In order to prevent the hydrogen-induced explosion, it is necessary to examine
measures to prevent or reduce emission of hydrogen which will be deposted in
the reactor building.
・ In this accident, it is necessary to examine the route of hydrogen leaked into
the reactor building, and to examine the need to set gas detectors toappropriately monitor the deposit of hydrogen in the reactor building if needed.
・ If branch pipes are installed in the containment vent line, it is necessary to
examine the measures to prevent the roundabout of hydrogen from such
exhaust pipes.
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・ If an exhaust pipe is shared, it is necessary to examine the measures toprevent the roundabout of hydrogen to other units, and ensure independence
in terms of engineering.
・ It is necessary to have trainings and procedures designed to cope with leaks
and deposit of hydrogen.
As for the containment penetration and seals, it is unrealistic to deman proper
sealing under a condition exceeding the designed, and measures should be
proposed to avoid such a condition. Considering that hydrogen is easy to leak if
the reactor core is damaged, it is necessary to take corresponding measures tocope with the hydrogen leaks that may occur.

4.10 Emergency preparedness (Especially training)
4.10.1 Training
Because of severe work environment of the incident response site in this
accident where there were hardly any lightings or communication means, andhigh-dose of radiation with scattered wreckage of building etc., it was very
difficult to perform fieldwork such as cable laying from power-supply car and join
up, laying alternative injection hose and so forth. Usually the fieldwork needs totake a lot of time and trouble, but we tackled it within a relatively short timethrough workers' effort. The fieldwork required this time has not been assumed
in the countermeasures training for severe incidents so far.
And this accident is a multi-unit and coincident accident that is not presumed
in the conventional training, therefore prompt decision-making and response
was required with a limited number of staffs on a moment-to-moment basis.
Findings/lessons learned
・ The countermeasures training for severe incidents should include field
operation trainings considering the fieldwork under the severe conditions
based on real actions, and train on a routine basis so as to take systematicalactions in the event of an accident.
・ Personnels should get training for various situations on a routine basis inorder to take actions in line with time in the event, and are encouraged tounderstand how the incident response activities affect the course of events.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ Assemble knowlege/proficiency－Assemble the knowledge and proficiency requested for each personnel
during a severe accident, and provide each personnel with a data-type
education and proficiency training in an appropriate manner.
・ Field trainings based on real actions－Provide personnels with the field trainings for the fieldwork presumably
required as countermeasures for severe incidents so as to take thepersonnels' proficiency in procedures and methods to a higher level.
－ Simulate the fieldwork during nighttime or without communicationmeans, and the fieldwork with full range of protective gears such as aprotection clothing and a full-face mask etc. Also consider an impact of
obstacles on approaching the field.
・ Training based on proceeding with time in the event
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－ Provide a training having effectiveness with a focus on reliable
cooperation and decision under the the same condition as real such as
blind training and real-time training etc. in addition to a training basedon a scenario.
－Confirm time required for such as approaching the field, wearing theprotective gears, and performing fieldwork etc., and then ensure how
his/her activities affect the course of events.
－Make use of simulator etc. to enable each personnel to take actions inline with time in the event.
The trainings mentioned above are preferably provided on a regular basis toremind the lessons learned and to maintain/improve the proficiency
continuously.
4.10.2 Air conditioning and shield at Central control room
In this accident, operators couldn't enter MCR or couldn't stay at MCR for along time because of high radiation dose.
And MCR ventilation system having a charcoal air filter for iodine removal
didn't work due to loss of electric power for a long time, therefore it is necessary
to wear a full-face mask even in MCR.
These conditions made MCR habitability lower, which was an obstacle to theincident response activities.
Findings/lessons learned
・ Loss of electric power for a long time disabled the MCR ventilation system. In
response to this situation, it is required to consider a countermeasure.
・ Due to high radiation dose, operators couldn't enter MCR or couldn't stay at
MCR for a long time. In response to this situation, it is required to consider acountermeasure.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ A countermeasure to ensure MCR habitability for a long time
－ Ensure power supply to facilitate the MCR ventilation system during
emergencies. And therefore put the operation procedure for it in place.
－ Examine the reason why high radiation dose is increased so as tostrengthen countermeasures of the radiation protection.
4.10.3 Measurement during accidents
In this accident, instrument power supply was lost, because DC power supply
unit was loss in function in addition to Station blackout for a long time. Therefore,
important parameters such as Reactor water level, pressure, and temperatureetc. were not measured or records for these parameters were lost, status of areactor like the injection status was not determined. To restore the instruments,
it was required to connect batteries collected from vehicles to the MCR
instrument board as temporary power supply.
And from the viewpoint of instrument data reliability, water level of reactor
varied from instrument to instrument. According to comparative result of a waterlevel instrumentation installed temporarily to confirm the reliability of
instrumentations, actual water level of reactor might be less than themeasurement limit of the water level instrumentation in the fuel zone. Therefore,
4-14

trend monitoring for reference-plane water level of the water level
instrumentation indicated that the reference-plane water level might be lowered
continuously by evaporation due to the effect of high temperature in thecontainment vessel.
With the progress of the accident, unexpected measurement were requested
such as water level measurement due to evaporation of pool water at the spent
fuel pool, hydrogen-concentration measurement to determine hydrogen leakage
and accumulation from the containment vessel at the reactor building and soforth. In fact, there was no means of measurement, so it became difficult to do
incident response. Moreover, the building exposed to high radiation prevented
staffs from approaching to the instrumentations so that restoring themeasurement system became difficult.
Findings/lessons learned・ (Dependable source of power) Losing MCR monitoring function due to loss ofelectric power for a long time was beyond the scope of assumption. In
response to this situation, it is required to be considered.
・ (Enlargement of measurement specification) Because Reactor water level
was less than the measurement limit of the instrumentation during a severe
accident, it is required to consider the outbreak of a severe accident in termsof the measurement system of important parameters that are necessary tounderstand the plant status.
・ (Addition to measurement parameters) There was no means of handling
unexpected measurement requests which was not expected in conventionallyassumed event occurrence, such as hydrogen-concentration in the reactor
building. In response to this situation, it is required to be considered.
・ (Ensuring measurement reliability) There was no means of confirming thereliability of the measurement system (measurement data) regarding theimportant parameters such as Reactor water level during a severe accident.
In response to this situation, it is required to be considered.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ (Considering dependable source of power) Consideration for restoring
instrumentation early in case of loss of electric power for a long time.
－ Prepare temporary storage battery and connection cable near MCR as a
backup of power supply.
－ Reconsider the instrumentation power supply such as utilizing
power-saving technology or uninterruptible power supply if necessary.
・ (Considering enlargement of measurement specification) Consideration of themeans to measure important parameters such as Reactor water level tounderstand the plant status during a severe accident.
－ Consider development of the measurement system having an enlarged
measurement limit of Reactor water level, and means to understand thereactor status by measuring the containment vessel even when reactor
instrumentation is lost in view of the outbreak of a severe accident.
・ (Considering addition to measurement parameters) Consideration of selecting
parameters that are newly requested to measure in this accident, andmeasurement system for them.
－ Establishaparameter
measurement
systemsuch
ashydrogen-concentration in the reactor building.
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・ (Considering securement of measurement reliability) Consideration of a way
to ensure reliability of the measurement system (measurement data)
regarding the important parameters during a severe accident.
－ Develop a measurement system that strengthens environment
resistance and a variety of measurement means in view of the outbreakof a severe accident.
4.10.4 Emergency Operation Facility
Emergency Operation Facility is a facility to give countermeasure instructions
during a accident and placed within Examination Guide for Safety Design for
Nuclear Power Facilities. However, in this accident the available communication
method between Emergency Operation Facility and MCR were only hot-line andland line after Station blackout. Communication environment became worse sothat the communication between Emergency Operation Facility and the field
was hardly established, which impacted on discussion on countermeasures andposed a problem for giving instructions. And personnel went to theadministration building that was restricted area to get valve drawings while
aftershock occurrence so as to study the procedure of vent operation for thecontainment vessel after Station blackout. The storage place of valve drawings
should be reconsidered when discussing stored judgment material in
Emergency Operation Facility.
From the viewpoint of radiation control in Emergency Operation Facility, sinceoriginally any buffering area was not specified to put on/off radiation protection
clothing in the earthquake-proof principal building where Emergency Operation
Facility was located, the concentration of radioactive materials in the air ofbuilding was beyond the limits by law to the day of April 3rd, so that it didn't
protect personnels from the risk of internal dose even in the building and it didn't
provide enough space for relaxation for staffs who have been working over along time.
However, the earthquake-proof principal building could withstand theearthquake and Tsunami with independent emergency electric power supply
unit and air conditioning system. Though it had problems as mentioned above, it
is noticeable that it functioned as a base for incident response activities.
Findings/lessons learned
・ Emergency Operation Facility is a facility to give countermeasure instructions
during accident. It inevitably ensures the communication means to MCR or
the field and enables to get plant parameters required for determining
countermeasures even under the severe conditions such as Station blackout.
Also judgment materials for emergency countermeasures should be preparedat Emergency Operation Facility and personnel should be capable to get themeasily if required.
・ Emergency Operation Facility is a facility where personnels stay and use
when an emergency arises including radioactive release. It needs adequate
radiation protection including preventing the internal retention of radiation andconsidering the external environment when used.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ Emergency Operation Facility is expected to use under the severe natural
phenomenon such as an earthquake, tsunami etc., and under the severe

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conditions such as Station blackout for a long time, releasing large amount ofradioactive material into the environment etc. so that it should be prepared toprovide its functionality considering above mentioned situations. Especiallyconsider the following points.
－ Building designed and located to withstand an earthquake and tsunami－ Plant capable of using in case of loss of external power supply for a longtime and ensuring independent emergency electric power supply－ Collecting the plant parameters required for the countermeasure
instructions and ensuring the communication means to MCR and the field－ Radiation protection countermeasures considering large volume ofradioactive material release－ Ensuring habitability for long-stay personnels－ Ensuring emergency material for severe environment－ Scrutinizing and ensuring judgment material required for emergency
countermeasures
4.10.5 Radiation control/working control
In this accident, APDs which were used for personal exposure control,
submerged in water and part of them became unusable, due to the effects oftsunami. Therefore, number of APDs was not sufficient for all workers. Onlysome of workers could use APD. And the exposure control system didn't work
due to loss of power, so we had to manually record the irradiated dose.
Charcoal filter masks which used as iodine adsorbent were also in short, as aresult, risk of internal exposure increased. Holding-counter type WBC for
measuring internal exposure became unavailable partly because background ofradioactive materials on site increased. Measurement using mobile type WBC
and measurement in other power station was performed, but it was not enough.
It caused confusion because of shortage of all sort of radiation measuring
instruments and radiation protection clothing, and also loss of related function.
In the midst of the confusion, exposed persons beyond dose limit were found.
It confirmed that multiple staffs involved in emergency works exposed beyond
the dose limit 250mSv. And two female staffs who have stayed in theearthquake-proof principal building exposed beyond the dose limit 5mSv per 3
months in total of external exposure and internal exposure. And 3 workers fromthe contracted company exposed beyond 170mSv ignoring alarmed APD.
Especially two of them, wearing low quarter shoes, got their feet wet in radiatedwater. Since possible beta ray burns were suspected on the basis of radioactive
materials attachment, they were taken to a hospital and then to National Instituteof Radiological Sciences (NIRS).
In the earthquake-proof principal building, since any buffering area was not
specified to put on/off radiation protection clothing, the concentration ofradioactive materials in the air of building was beyond the limits by law to the
day of April 3rd.
In this accident, because of the vent operation for the containment vessel,
radioactive material release to environment due to hydrogen explosion, andscattered wreckage with high-dose of radiation, radiation control service
including measurement of environment increased abruptly so that it was very
difficult to handle the radiation control service.

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Findings/lessons learned
・ Due to tsunami and loss of electric power for a long time, it is beyond thescope of assumption that personal dosimeters like APD became heavily
unavailable, that radiation protection clothing like mask run short due to rapid
progress of the accident, and that WBC became unavailable due to highradiation dose.
・ Because radioactive material possibly releases to environment such as thevent operation for the containment vessel during an accident, internal
exposure by iodine might become beyond expectation if there is no radiation
protection clothing like mask. A base for incident response activities also
might have radiation pollution, and as progress of an accident, some of theworkplace might increase dose rate or be polluted locally, radiation control
should manage both external exposure and internal exposure depending on
the situation. And working control should assume it previously.
・ During an accident, radiation control service might increase abruptly so it
should reinforce the system for acting more quickly.
・ Since the doctor who stayed in the earthquake-proof principal building had
knowledge of emergency exposure medical treatment, he examined two
workers who possibly had beta ray burns and swiftly took them to National
Institute of Radiological Sciences. It is an anecdotal experience and weshould continuously promote understanding associated with emergency
exposure medical treatment.
Measures to take
Based on the above results, some examples of the countermeasures are shown
below.
・ Prepare materials and equipment such as radiation measuring instruments
and radiation protection clothing so as to do adequate radiation control.
Following measures are listed for example.
－ Ensure diversification of electric power sources like battery-operated forthe radiation measuring instruments and enough electric power sources
to use for a long time. Place required number of batteries in the place
unaffected by tsunami. And make a plan arranging to interchange
required radiation measuring instruments beforehand.
－ Ensure mobile WBC such as WBC on-vehicle, or arrange to use WBC ofother facilities.
－ Ensure required number of the radiation protection clothing considering
rapid progress of an accident.
・ Make Emergency Operation Facility which becomes a base for incident
response activities during an accident to be available on the assumption thatradioactive material exists in the environment. Let all staffs know the latest
information regarding dose rate and pollution in the workplace with theprogress of an accident, and open their eyes to the importance of exposure
dose reduction.
・ Reinforce the system in terms of staffs for acting more quickly to handle theradiation control service which might increase abruptly. For example,
arrange a support system that includes radiation management staffs fromother sites and staffs who count up the radiation dose without the knowledgeof radiation control so that radiation management staffs focus on more
important operations.
・ Prepare to arrange an emergency exposure medical treatment team in case4-18

of an accident.
4.10.6 Organization/command or order
Originally director of a plant can decide to do the vent operation for thecontainment vessel as written in the internal manual. But in this accident, we
needed to ask government authorization and to negotiate with local government,
when the vent operation became inevitable. External adjustment took time
because, for example, it needed to wait until evacuation completed.
And when doing sea water injection at Unit 1, it was proceeded with based ona confirmation and agreement of company president. After that, as a result of
discussion between head office and the power plant, the sea water injection was
decided to temporarily stop, but it was continued based on a judgment by the
director of the power plant. There was a confusion of chain of command ororder.
Findings/lessons learned
・ Regarding emergency measures, in the confusion during emergencies it
needs an adjustment with associated organizations, therefore, there is apossibility of delay or confusion in a decision of execution.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ Clearly articulate a timing of judgment for the emergency measures that
affect badly convergence of an accident, and proceed with external
adjustment for them at an early date preparing to do a prompt operation.
4.10.7 Communication
In this incident response activities, due to water exposure by tsunami andStation blackout for a long time, internal PHS or paging device etc. which was
ordinary communication means among the fields, countermeasures headquarter,and MCR, was hardly used.
Because field worker put on full range of protective gears such as protection
clothing and a full-face mask etc., they had to work under a severe environment
and without communication one another. Therefore, it was very difficult to do theincident response activities because it took time to confirm the field status and toprovide direction of a fieldwork.
On the other hand, regarding communication between Power station andoutward, in fact the leased line network to internal and external organization wasavailable in this accident, but there is always a risk to be disrupted like internal
network in Power station due to an earthquake or tsunami etc.
Findings/lessons learned
・ The communication means within Power station and to outside should bebuilt/maintained by means which is free of the influence of an earthquake ortsunami, and which is capable of using power source even when Station
blackout for a long time.
・ One communication means between field workers and anothercommunication means among field worker, Countermeasures headquarterand MCR should be prepared considering use with full range of protection
gears and under a severe environment during an accident.
Measures to take4-19

Based on the above results, some examples of the countermeasures areshown below.
・ Countermeasures for communication means within Power station and tooutside which is free of the influence of an earthquake or tsunami, and whichis capable of using power source even when Station blackout for a long time.
－ Prepare backup power such as battery which is free of the influence ofan earthquake or tsunami, and which is independent from the reactor
facility.
－ Design facilities used on an emergency basis such as servers,
communication equipment like switch, and cable communication line or
wireless staging base so that they can't just be disrupted by natural
disaster such as an earthquake and tsunami.
－Deploy multiple communication devices such as leased line, satellite
phone, and wireless line, and clarify the way of their operation.
・ Countermeasures for facilitating communication between field workers, oramong worker, MCS and Emergency Operation Facility.
－ Deploy required number of communication devices including the field.
－ Clarify the way of operation for the deployed communication devices andimprove worker's proficiency through training at the same time.
4.10.8 Environmental monitoring
According to government's "Basic disaster prevention plan", in terms ofenvironmental monitoring at the time of nuclear disaster, business operator
takes charge of border line and inside of the site boundary, and mainly local
government takes charge of outside of the site boundary.
In this nuclear disaster, regarding environmental monitoring for inside of thesite boundary and border line which business operator takes charge of,measurement at MP within the site boundary and on border line became
unavailable due to loss of external power supply caused by tsunami. Therefore,
until the external power supply was restored, the environmental monitoring wascontinued by use of monitoring vehicle, temporary MP and survey meter etc.
And the measurement results were released on the Internet as applicable,
though it was limited due to manual creation.
In case of local government's environmental monitoring, environment
monitoring equipments and facilities required were not available due to theeffect of power outage, communication line disconnected, and local governmentbuilding swept away by tsunami. And also, the environmental monitoring
activities had major obstacle, because persons in local government had to focus
on responses to the earthquake and tsunami occurred in a wide area, andbecause the onsite countermeasures headquarter had to move from the offsite
center to Fukushima prefectural government. Moreover, related government
offices should help local government to execute and support monitoring basedon local government's request, but they were engaged in other earthquake
responses such as a lot of missing persons’ investigation that covered a wide
range, so it was difficult to build the environmental monitoring system just after
disaster of the earthquake.
On the other hand, business operators except Tokyo Electric Power Co., Inc.
dispatched personnels based on Atomic energy cooperation agreement andsupported the environmental monitoring outside the site boundary executed by
local government utilizing a monitoring vehicle.
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As mentioned above, there was a lot of problems such as monitoring facilities
or shortage of personnels etc., but monitoring activities have been done asmuch as possible. However, some of mass media criticized the delay inreleasing monitoring data, and low reliability of offshore monitoring data, anddoubted some of monitoring-vehicle measurement data per 2 minutes by Tokyo
Electric Power Co., Inc. was not released by reason of complementary data.
Findings/lessons learned
Since an entity taking charge for the environmental monitoring is different
depending on inside or outside of the site boundary, we only describe themonitoring executed by business operator.
・ Due to the effect of power outage by tsunami, the permanent monitoring
facility of business operator became unavailable, so alternative means had to
be used.
・ Although accepting personnels based on Atomic energy cooperationagreement and working with environmental monitoring activities conducted by
local government to do monitoring activities continuously, it took a long time tobuild the system to do the environmental monitoring under the condition that
extensive natural disaster and nuclear accidents occurred at the same time.
・ The monitoring data by business operator was released on Internet or by themass media if applicable, but from the viewpoint of promptness of release andreliability of released value, it failed to satisfy general public and the mass
media.
Measures to take・ Public-private partnership such as cooperation/support in the environmental
monitoring conducted by local government, and private-private partnershipsuch as cooperation among businesses should be reconstructed. Specificdetails are shown below.
－ Reconfirm the monitoring equipments and facilities that are required tointerchange on an emergency basis, and deploy required number ofthem.
－ Reconsider the dispatched personnels on an emergency basis, andclarify rules associated with the dispatch.
・ Considering this natural disaster (earthquake, tsunami), the utility used forthe permanent MP should be reinforced. Specific details are shown below.
－ Reinforce electric power supply and transmission line
－ Reinforce resistance characteristics to an earthquake and tsunami
・ Monitoring procedure using alternative means should be clarified in case thepermanent MP becomes unavailable. Specific details are shown below.
－ Reconsider the measurement procedure by use of alternative means
such as temporary monitoring equipment－ Confirm the required equipments and facilities by use of alternativemeans, and deploy them・ Required information and transmitted means for common people, relatedorganization, and personnels involved should be clarified. Specific details
are shown below.
－ Streamline the contents of transmitted information, how to coordinate
them, and required personnels－ Consider transmitted means (Newspaper, HP etc.) and timing of release
4.10.9 Preparing disaster countermeasures (heavy equipment/rescue),
4-21

emergency cooperative framework
In this accident, extensive natural disaster and nuclear accidents occurred at
the same time. Therefore, it was very difficult to do contact/communication,
emergency call up, and procurement of goods etc., and it was not quickly andsufficiently to accept the materials and equipment for use in emergency
measures or rescue team who support the accident management activities at
the initial reaction of the accident. Also, the high radiation lever at the field
became an obstacle to human responses. Other conditions such as looking out
for aftershock and its tsunami, darkness, high contamination status, ensuring
electric power supply in anticipation of worse environmental conditions likecontaminated water, arranging materials and equipment and temporary work,
removing blockages to pass through on and off site, were there. These
conditions greatly influenced following incident response activities.
Findings/lessons learned
・ Due to earthquake and tsunami, route to access such as public highway,
general road, and entranceway were damaged within and outside the power
plant, so it took time to bring in materials and equipment for use in incident
response and to call up supporting staffs. it worked against doing the incident
response activities smoothly. Materials and equipment that facilitates
constrained conditions for various sort of work were expected.
・ Preparing a multi-unit and coincident accident like this accident was notenough, so that protection clothing, dosemeter and mask etc. for use inincident response were not deployed in an appropriate manner. And human
work under high radiation for a long time was not assumed specifically.
Because preparation for work under these conditions didn't exist, incident
response was not able to proceed promptly.
Measures to take
Based on the above results, some examples of the countermeasures areshown below.
・ On the assumption that route to access such as public highway, generalroad, and entranceway are blocked within and outside the power plant due
to natural disaster (earthquake, tsunami), preparing heavy equipments toremove scattered wreckage and staffs and diversifying means of transport
for materials and equipment are required. Considering cooperation withrelated organizations, specific details are shown below.
－ Clarify required heavy equipments except the loader which was deployed
as severe accident countermeasures, and deploy them.
－ Establish means for sea transportation and air transport instead of landtransport, and deploy the system.
・ It is required to ensure required materials and equipment for use in incident
response, and to reconstruct a system to interchange among business
operators on a emergency basis. Considering cooperation with relatedorganizations, specific details are shown below.
－Clarify required materials and equipment based on the incident responseof this accident, confirm required number of them, and deploy
them.(including temporary light, temporary air conditioning, andtemporary drainage equipment etc.)－ Reconfirm a system to interchange among business operators on a
4-22

emergency basis and deploy means of interchange.
－ Focus on a remotely-operable facility/equipment(robot, unmanned
helicopter) for Nuclear Emergency Preparedness that can work underhigh radiation level, participate in review session associated with its
development and operation, and deploy a cooperative system withbusiness operators and related organizations for actual installation.

4.11 Security of interigty of the spent fuelAll external power sources were lost lost due to the earthquake. However, allemergency DGs were activated excluding one emergency DG that was underinspection. However, all of the emergency DGs excluding the 6B emergency DG
of Unit 6 were stopped due to the tsunami that hit the power station afterwards. In
addition, the cooling function of the spent fuel pool was lost in all units because thefunction of the sea water pump was lost due to the tsunami.
Further, it is considered that there is no exposure of fuel in the spent fuel pool.
Findings/lessons learned
・ When the cooling function of the spent fuel pool was lost, the security of thewater level of the spent fuel pool is extremely important, and maintaining andstrengthening the water level of the spent fuel pool and the monitoring
function of the pool water temperature are also important.
・ From the viewpoint of preventing a decrease of the pool water level anddamage on the spent fuel followed by the loss of cooling function of the spent
fuel pool, it is considered with this accident as a turning point that new
countermeasures have to be additionally prepared in order to maintain thecooling function of the spent fuel pool when all of the AC power sources werelost in a plurarity of units including adjacent units and when the function of thesea water pump was lost.
Measures to take
Based on the above results, some examples of the countermeasures are shown
below.
・ It is necessary to secure the alternative coolant injection means to secure
the water level of the spent fuel pool.
－ Supplying water by fire engines, pump trucks, etc. and securing a
coolant injection route and a water source
・ It is necessary to secure and strengthen the water level of the spent fuel pooland the monitoring function of the pool water temperature.
－ Securing the power source supply from the emergency power source tothe water level meter and the thermometer of the spent fuel pool
－ Strengthening the monitoring function of the status of the spent fuel pool(connection of the emergency power source to the ITV, battery
connection, etc.)
・ It is necessary to investigate the maintainance of the cooling function of the
spent fuel pool as a continuous cooling means after the water level was
secured.
－Securing the alternative cooling function of the spent fuel pool water.

4.12 Summary of the countermeasures
As described in Chapter 3 “Summary of the Causal Analysis”, to prevent
reoccurrence of the accident it is important to increase the safety margin by
4-23

assuming a scenario of the accident extending and by preparing for it by taking
appropriate countermeasures.
The profoundness of the system of countermeasures are summarized andoutlined in Table 4.12-1 by combining the countermeasures that have alreadyexisted and the countermeasures that are exemplified here as new
countermeasures.
In the causal analysis, the main cause of the extension of the accident
extending was the loss of the power source, and it was described that the functionof the safety components was lost due to the loss of the power source. Therefore,
we decided to confirm the status of the system of countermeasures regarding thepower source. Further, there was a loss of a heat sink (including the auxiliarycomponent cooling system) as a main factor causing the loss of function of thesafety components other than the power source, and a confirmation is performed
from that viewpoint. Furthermore, a confirmation is performed for hydrogenbecause the hydrogen explosion largely affected the discharge and diffusion ofradioactive substance in addition to becoming a hindrance to taking action forresponding to the accident.
The countermeasures are outlined, and basically the following approach can befollowed.
First, resolution of an assumed major accident at the stage of designing thecountermeasures can be attempted with the safety components (the sea water
pump, the emergency DG, the ECCS, etc.) that have been already installed.
Reviewing what happened this time, protection of these safety components thathave been already installed shall be strengthened.
The safety components that have been already installed have multiplicity andvariety. Severe accident countermeasures (accident management) have been
prepared independently at each nuclear power station in the 1990s by assuming acase where the functions of all of these safety components are lost. In the
Fukushima Daiichi accident, most of the accident management could not beapplied due to various constraints such as the loss of the power source and highcontamination. Because of this, countermeasures for ready application shall beconsidered.
Because this accident management did not sufficiently function, severe accident
countermeasures that will be added this time shall be prepared.
Therefore, as shown in the confirmation table, selectively treating thecountermeasures means adding protection to the safety systems and severe
accident countermeasures that have been already installed.
In such way, a layered protection system can be prepared by adding severe
accident countermeasures to the safety components that have been alreadyinstalled and to the accident management components, and this is considered to
become an effective countermeasure for preventing progress of the accident.
－ Power Source
The power source shall be secured surely against an earthquake and atsunami by layered countermeasures.
・ The power is supplied by an external power source during a blackout in theconventional design. However, the power source is supplied by an emergency
DG when external power source is lost. Protection for this is performed.
・ Assuming a case that these functions are lost, the power source is supplied byan alternative means such as deployment of a power source truck.
・ Valves, etc. are individually addressed by deploying a storage battery, etc.
4-24

－ Heat sink (coolant injection and cooling)
The core shall be certainly cooled when an accident occurs by layeredcountermeasures.
・ The sea water system and the ECCS component that are expected to preventan accident from extending in the conventional design shall be protected.
・ Spares for the sea water system are deployed as countermeasures when theprotection is broken. These are protected by the facility of severe accident
countermeasures (accident management) against the loss of the function of
the ECCS.
・ Considering a case that these do not function effectively, a pump that canseparately inject water is deployed and the water source is secured.
Countermeasures shall be taken for reducing the RPV pressure to secure theinjection of water.
・ Countermeasures shall be taken in the BWR for certainly performing acontainer vent that is a final cooling means when an accident extends.
－ Countermeasures for a hydrogen explosion
An explosion shall be prevented by countermeasures for each stage of theevent such as prevention of hydrogen generation, prevention of leakage,
and reduction of discharge.
・ The generation of hydrogen due to damage of the fuel is prevented in theconventional design by securing the cooling of the core. An explosion isprevented in the BWR by confining the generated hydrogen in a container of anitrogen atmosphere. Therefore, the countermeasures of the water injection
and cooling are effective also as countermeasures for hydrogen.
・ Hydrogen is generated when the core is damaged. However, countermeasures
are taken for discharging or reducing hydrogen so that it does not stay in thebuilding by considering that hydrogen leaks from the container becausehydrogen easily leaks.
・ Countermeasures are taken for the leaked hydrogen not going into other units
from a vent line.
Specifically for investigating (adapting and combining) other layeredcountermeasures, it is effective to consider the issues such that・ an appropriate balance shall be taken among the prevention of core damage,the prevention of container damage, and the degree of prevention,
・ we shall not be biased toward administrative measures,・ the protection against possible common causal troubles shall not be decreased
and new common causal troubles shall not be induced,
・ independency of barriers shall not be decreased,・ preparation for human error shall be maintained, and・ functions shall be secured that are planned in a plant design standard.
In order to determine priority among individual countermeasures, the following
criteria shall be used.
・ The means such as the UHS and the power source which have an effect andinfluence on a larger number of functions are more important.
・ The means and the measures having a larger contribution to the “thickness” oflayers have higher priority.
4-25

・ Qualitatively, an individual step is considered to be more important as thenumber of steps is smaller.
For example, a step that is added to one existing step is more important than astep that is added to two existing steps.
The layers of the countermeasures that are prepared are determined dependingon the extent of internal and external hazards and failures.
By combining various countermeasures at this time with the countermeasuresthat have been considered at the stage of designing and constructing,countermeasures shall be taken that are sufficiently effective for securing safety
and for effectively preventing an accident where a large amount of radioactive
substance is discharged to the environment even when the prepared functions arelost one by one.
As a result of outlining the points as described above, countermeasures that areconsidered to have high priority from the viewpoint of preventing reoccurrence of
the accident among the 80 countermeasures exemplified in the present
investigative commission are identified in a Table. We suggest that these items
shall be adopted in the implementation of countermeasures by each company.
－ Power source
・ Establishment of a tide embankment and a breakwater as a facility that is
important for safety
・ Prevention of flooding of the area where components that are important forsafety are provided
・ Prevention of soaking of an opening and a penetration part of an air supply portsuch as improvement of sealing the opening corresponding to the soaking
height・ Sealing the sites where the transformer for receiving and the switching station
are arranged from water or countermeasures for soaking of the components
・ Sealing the sites where the DC power source facility is arranged from water orcountermeasures for soaking of the components
・ Deployment of a power source truck or a power source having a large capacity(a gas turbine or a diesel generator) and preparation of its emergency
procedure・ Preparation of a DC power source charging route by the backup power source
・ Improvement of reliability by countermeasures for soaking of the facility that
accommodates a power source between the units (severe accident
countermeasures)－ Heat sink (water injection and cooling)・ Measures for preventing soaking of a bulkhead, etc. around the sea waterpump
・ Disposition of spares for the sea water pump・ Prevention of soaking of areas where safety components such as the
emergency core cooling system are installed
・ Prevention of soaking of the facility for severe accident countermeasures
・ Disposition of a movable coolant injection pump・ Preparation of the backup power source for driving the SRV, spare air tanks,4-26

etc. (BWR)・ Security of the core cooling through the SG by a main vapor releasing valve(PWR)
・ Disposition of a backup power source for operating a vent and a driving source(BWR)・ Improvement of reliability of the coolant injection system by deployment of a
backup power source truck and a power source having a large capacity
・ A portable power pump, a hose, etc. that do not depend on the existing facility・ Security of the water source
・ Releasing heat to the air by the container vent (BWR)－ Hydrogen
・ Discharge and decrease of remaining hydrogen
・ Prevention of wraparound of hydrogen from a pressure resistant strengthened
vent line
・ Prevention of wraparound of hydrogen between the units sharing an exhaust
pipe
Further, countermeasures are combined by considering safety of the plant as awhole and selecting for implementation. Various combinations of thecountermeasures can be considered corresponding to the condition of each power
station and the strategy for securing safety, and they are not limited to theabove-described countermeasures. All countermeasures that were identified aresummarized and shown in Fig. 4.12-1.

4-27

Table 4.12-1 Checklist processed in terms of defense in depthFirst layer(normal operation)
Purpose Prevention of abnormal operations and failures

Multiple levels of protection

Mandatory measures High quality in conservative design, construction and operation

SourceSecond layer(Anomaly in the expected operation)
Control abnormal operations and detect failures

Prevent accident occurrence/
Mitigating the impact of accident
Third layer
Forth layer(Design basis accident and event during combined operation)(Severe accident)
Control the accidents in the design base
Progress in preventing accidents

Control, limitation and protection systems

・ Assess the expected tsunami height by basic design earthquake
・ Backup with emergency DG when there is a loss of AC power.
ground motion. Tsunami with the height greater than site height is not Install 100% emergency DG in 2 systems serially.
expected.
・ Assure independence for power supply by installing the steamSupply the power by external power source if the power generation of driven equipment's
Beforethe plant is stopped.
・The water immersion through the openings such as building
earthquake ・ Evaluated that seismic capacity of transmission tower is enough.
discharge valves by tsunami are not considered.
Implementation of measures to improve the seismic reliability are inprocess,
・ Setup the storm surge barrier and breakwater in the importantsafety related equipment.
Proposedmeasures

Beforeearthquake

Core・ Assess the expected tsunami height by the latest knowledge of
construction engineer. The measures for these has already been
carried out.

・ Measure to prevent the water immersion like bulkhead around the
Sea water pump
Heat sink(Injection/cooling)

Proposedmeasures

Existing
state *

ECCS and accident procedures・ Not considered the flooding.
・ Ensure the emergency D/G and power of DC battery.

・ Measures to prevent water immersion into the vital areas of safety ・Anti-flood device or water tightness of arrangement parts for
related major components
switchyard and receiving voltage converter
・ Measures to prevent water immersion through the openings like air ・Anti-flood device or water tightness of vital arrangement parts ofsupply opening depending on the height of water by improving seal
D.C. power supply systemfor penetration area
・Place the drainage pump for movable

・ Install a series of 2 safety systems such as ECCS.
・ Develop measures for severe accidents (accident management).
・ Install systems (IC, HPCI, RCIC) that function only with DC power Use the existing equipment as alternative injection facility if required.
supply.
・ Loss of power and flood is not expected.
・ Multiplicity, diversity and independence has been ensured.
・ No alternative driving sources for various types of valves.
・ Although sea water system has unwanted HPCI and RCIC in thecooling of oil and bearing, if the DC power supply is cutoff, it will fail tooperate.

・ Place the spare part for sea water pump motor
・Wash the sea water pump motor and place the dry material

・ Measures to prevent the water immersion in the vital areas related ・ Place mobile injection pump・Loose heat to atmosphere by containment vent (guarantee theto Emergency Core Cooling System
・Prepare SRV drive backup power supply and spare air cylinders
containment integrity)
・Install waterproof pumpsetc.
・Deploy the backup power supply and drive source for Vent
・Ensure the injection procedures by deploying the backup power unit operation Optimization of operating conditions of ventor high-capacity power supply
・Ensure the injection, injection route and river head due to fire・Measures to prevent water immersion into AM system
・Portable pumps and hoses independent of existing power supply
・Ensure river head (also include sea water as last resort)・Loose heat to atmosphere by containment vent (emission of heatfrom S/C)・Deploy the backup power supply and drive source for Vent
operation
・Optimization of operating conditions of vent・ Prevent hydrogen generation by core cooling due to ECCS etc.
・ Set well below the flammability limit even though there is ahydrogen leakage by encapsulating the nitrogen in the PCV and by
setting inactivated environment.

Proposedmeasures・ Install the equipment with sufficient cooling capability for decay heat ・ Set the height of discharge pipe above the required water level and ・ Loss of cooling procedure is not expected.
of maximum storage capacity.
maintain the required water level even though there is a leakage inpipe due to siphon break.
・ Contains the supply function of river head of seismic S class.
Maintain the water level by supplying the water from water supply
system・ Ensure the injection route and river head due to fire

4-28

・ No development of injection measures other than AM measure.

・ Hydrogen leakage is likely to occur from containment if core isdamaged.
・ Reverse flow of gas between the units is not considered.
・ Reduce emission of retained hydrogen・Measure to prevent wraparound from pressure reinforced vent line
・Prevent hydrogen wraparound in the different units that share thevent stack

Proposedmeasures

SFP・ Place power unit or high-capacity power supply (diesel generators ・ Improve reliability through the tsunami measures of the powerand gas turbines), develop emergency proceduresinterchange facilities between different units
・ Develop root of DC power supply by backup power source

・ Install the series of 2 100% Sea water systems or 4 50% Sea
water systems. Ensure the diversity.
・ Enable handling of transient by operational action without safetysystem equipment's such as ECCS etc.
・ The loss of functionality of all equipment's at the same time due tothe immersion of water into Sea water pump and power panel is notexpected.

Hydrogen (H)BeforeInjection, cooling,earthquake
source, waterlevel

Forth layer(Severe accident)
Mitigation of SA
AM included the protection ofComplementary measures and accident management
complementary measures and PCV・ It is estimated that AC power is restored early (in about 8 hours),
・ Improve the flexibility of power between the Unit as AM measure.
long term power supply is not expected.
However, the loss of power for multiple units at the same time is not
・ The core cooling will be performed by DC-powered equipment after expected.
station blackout and AC power will be restored.

・ Ensure alternative cooling function (or early recovery of auxiliary
cooling system)

・ Reduce the emission of retained hydrogen in the containment

福島第一原子力発電所事故調査検討会、―

PREPARATION AGAINST EARTHQUAKES, TSUNAMI 1, 6, 7, 9, POWER SUPPLY PREPARATION 3, 5, CORRESPONDING HEAT SINK LOSS (BWR) 2, 4, 8, 10, 11 [Injection into Nuclear Reactor][Measures to prevent flooding of building]

2(1) Preparation of backup power and reserve air cylinder for SRV drive, etc.
[Sea-water cooling failure](2)Heat Relief into atmosphere from container vent[Container Vent](3) Consideration of provisional/temporary power supply to the monitoring
instrument necessary for determination of vent preparation and vent execution(4) Consideration of backup power and drive source for vent operation.
(5) Review of optimization of vent opening time.
(6) Strengthening of alternate container spray1

1. Depending on the height of the flooding level, it is theprevention of flooding of the outer door of thebuilding, the opening of the air supply vent, etc,
sealing improvement corresponding to penetration
part.
2. Measures to prevent flooding of equipment
installations important for safety.
3. Deployment of portable drainage pump
4. Consideration of wave power for outer door that is
directly affected by the tsunami

Reactor Building
External Power source (normally (275KV) )[Injection to nuclear reactor]
11(13) Flooding counter-measures for layout areasuch as counter measures for severe
accidents, core spray system.

Central Control Room

Spent Fuel Pool

External power source
Reactor Containment Vessel

used fuel cooling system[Injection to nuclear reactor]

10(1) Consideration of generator car or high capacity
power source (Gas Turbine, Diesel Generator),streamlining of procedures for emergency.
(2) Transformer for receiving power andwater-tightness and flooding countermeasures forswitchyard layout points(3) water-tightness and flooding countermeasuresfor layout area of D.C. power supply system.
(4) Maintenance of DC power supply charging route
through backup power source.
(5) Improvement in the reliability due to flooding
countermeasures of interchange facility between
power supply equipment. (Countermeasuresagainst sever accidents)[Injection into Nuclear Reactor]

condensor

water

Water outlet
Water supply pumpControl rods

Pump for Pressurized Water injection

Cooling water (seawater)

Emergency Diesel Generator

Pressure Control PoolFire engine(Backup power source)

4(7) Improvement in reliability of existing injection
system by deployment of generator car for
backup or high capacity power source.
[Failure of Sea-water cooling](8) Power supply that can provide backup forthe minimum essential functions of sea-waterpump.

Diesel Fire Extinguisher Pump
Water outlet[Earthquake, tsunami anticipation]

9[Station Blackout and D.C. power source failure]5

Sea-water pump for cooling of(6) Water-tightness and flooding counter-measuresfor layout area of emergency D/G, power source
equipment, emergency D/G cooling equipment.

Emergency Diesel Generator(10) Basic earthquake ground motion
establishedasSs
consideringinterlocking of multiple epicentres.
(11) Review of setup conditions at the timeof assessing the tsunami height[Seawater cooling failure]

3

steam

Reactor Pressure Container(11) Injection by portable power pump or hose where there is no
pre-existing power.
(12) Ensure source of water (as a last resort, include seawater).

[Station Blackout and Direct current failure]

Seawater

8(9) Establishment of seawater pump motor
reserves and replacement procedure after
function failure.
(10) Setting up of cleaning and drying material
for seawater pump motor.

[Flooding counter-measures for seawater]

7(7) Flooding prevention measures like protective wallaround the seawater system.
(8) Establishment of motor spare parts.
(9) Deployment of portable seawater pump or
establishment of preventive pump.

Figure 4.12-1 Examples of Countermeasures 1/3
4-29[On-site flooding counter-measures against tsunami]

6(5) Establishment of protective seawalls in locations thatare important from the viewpoint of safety of important
machinery.
(6) Protection against tsunami of tank etc that may become
an obstacle at the time of emergency by drifting. Setting
up of heavy machinery for removal of drifting material.

福島第一原子力発電所事故調査検討会-

Hydrogen Countermeasures 14, Emergency Preparedness 12, 15, 16, 17, 18, 20, 21, Integrity Assurance of Used Fuel 13, 19 -[Radiation Control / Work Management]

12(1) Diversification of power supply sources for the radiation
measurement instrument (battery cell type), assurance oflong-term power supply, preparedness for high altitude places,and power interchange from other companies.
(2) Assurance of WBC from portable and other facilities(3) Assurance of preventive protective mechanism(4) Radiation protection of important buildings with base isolation,
providing information of site dose rate, consolidation of support
system for radiation control personnel(5) Promote understanding of emergency treatment for radiation
exposure, consolidation of system.

[Integrity assurance of used fuel]13(1) Used fuel pool level, Supply of power to watertemperature thermometer from emergency powersource, enhancement of condition supervision.
(2) Assurance of alternate cooling functions.

Outside[Communication]21(30) Independent communication facility backup power source that is
not affected by earthquake or tsunami.
(31) Communication equipment that will not be broken by natural
disasters.
(32) Setting up of multiple communication equipment and clarification of
methods of operation.
(33) Organization of communication equipment with onsite workers.
[Environment monitoring](34) Review of instruments and the number of instruments that need to
be interchanged at the time of emergency.
(35) Review of support staff, adjustment of rules for dispatch.
(36) Power source for permanent monitoring post, strengthening oftransmission line.
(37) Resistance intensification of permanent monitoring post(38) Clarification of alternate means for permanent monitoring post.
(39) Confirmation and organization of instruments required formeasurement in alternate means, number of equipment required.
(40) Clarification of content and means of information transfer togeneral public, related institutions, concerned persons at the site, etc.
【Preparedness of countermeasures for disaster, cooperation systemat the time of emergency】(41) Diversification in the means of sending material, equipment andpersonnel when the access route is cut off.
(42) Assurance of necessary material and equipment.
(43) Re-organization of interchange system between workers[MCR air-conditioning, shielding][Hydrogen Countermeasures](1) Release of accumulated hydrogen,measures for reduction.
(2) Setting up of hydrogen gas detector(3) Prevention of bifurcation to branch pipe fromthe containment vessel vent pipe.
(4) Prevention of hydrogen diversion between
facilities providing stack(5) Drill in anticipation of hydrogen leak,accumulation

Important base-isolated buildingCountermeasure

Reactor building

point

CentralControl
Room

Spent fuel pool

Steam
Used fuel cooling system
fuel

Reactor pressurized container
condensor

Water

Water outlet
water
Cooling water
Control rods
Emergency

Pressure control pool

[Measurement at the time of accident](8) Assurance of temporary storage battery
for gage and connecting cable(9) Review of gage power source.
(10) Selection of parameters with newmeasurement requirements, inspectionof measurement system.

[Communication]

Water supply pumpinjection system pump(6) Assurance of power source responsible
for functioning of emergency MCR
ventilation system, maintenance of
operation procedures.
(7)
MCR
radiation
prevention
counter-measures.

dieselgenerator

16(11) Development of measurement
system after considering severe
accident, investigation of alternatemonitoring means(12) Development of measurement
system with further diversification of
measurement methods and resistanceto environment.

Reactor Containment Vessel

Pressurized

15[Measurement at the time of accident](seawater)

17(13) Familiarization of facilities and operation ofcommunication equipment between workers andcentral control room.

(backup power source

Fire engine[Drill]Water outlet
Diesel extinguisher pump
Seawater pump for cooling of

emergency diesel generator

Seawater pump for cooling of emergency

diesel generator[Counter-measure points at the time of emergency]

14seawater

20(22) Location and construction that can withstand earthquake and tsunami.
(23) Assurance of emergency power source that is independent from the plant.
(24) Assurance of means of coordination with the site and central control room and collection ofplant parameters.
(25) Radiation protection counter-measures(26) Assurance of livability(27) Assurance of emergency material(28) Careful examination and assurance of material necessary for emergency counter-measures.
[Organization / Command, Orders](29) Preparedness of rapid operation by early coordination outside the company and clarificationof judgment timing[Integrity assurance of used fuel]19(3) Assurance of water injection by fire-engine, injection route,water source

Figure 4.12-1 Examples of Counter-measures 2/3
4-30

18

(14) Identify the required knowledge and skills and execute drill.
(15) Onsite activity drill for severe accident counter-measures.
(16) Onsite activity drill for simulated situations like obstacles,
multiplexing, conditions where normal communication means
cannot be used and night-time work.
(17) High efficiency drills such as blind drills, real-time drills, etc,
which place emphasis on the coordination and judgment inconditions close to reality.
(18) Drills for confirmation of time required for access to site,
attaching and detaching equipment, work, etc.
(19) Simulation activity[Preparedness of disaster management, cooperation system atthe time of emergency](20) Deployment and removal of heavy machinery forelimination of rubble(21) Development of facilities that can be operated remotely foratomic power disaster prevention, cooperation towards
introduction, coordination system.

福島第一原子力発電所事故調査検討会-

Heat Sink Failure Response (PWR) 1, 2, 3 Other counter-measures (preparedness
against earthquake, tsunami, preparationofpowersource,hydrogencountermeasures, etc) are same as theexamples given for BWR.
[Water injection to nuclear reactor][Water injection to reactor]

2(2) Assurance of alternate water injection means
to the Steam Generator by portable power
pump, hose, etc.
(3) Nuclear reactor cooling via Steam Generator
by main steam relief valve. (Includes spare air
cylinder, etc)

1(1) Assurance of water injection means byorganization of backup generator or highcapacity power source.

[Water injection to reactor]

3(4) Assurance of source of water for water injection to SteamGenerator (includes sea-water as a last resort)

Figure 4.12-1 Examples of Counter-measures 3/3
4-31

Chapter 5

Timeline of the Accident to the Present

The Tohoku earthquake and tsunami which occurred at 2:46 PM on March 11, 2011
struck the Fukushima Daiichi and Daini Nuclear Power Stations, a long-term nuclear
accident unprecedented in scale.
In units 1 to 3 of the Fukushima Daiichi Nuclear Power Station, water continued to beunable to be injected into the RPV for a period, exposing the fuel in the cores andleading to core meltdown. A portion of the melted fuel pooled below the RPV.
A large quantity of hydrogen was produced from a chemical reaction between
zirconium in the cladding and other fuel rod parts and steam, the fuel rod cladding wasdamaged, radioactive substances in the fuel rods were released into the RPV, andhydrogen and radioactive substances were released into the storage container during
the RPV decompression process.
The internal pressure in the RPV that lost core cooling functionality rose and leaked
into the storage container through the safety valve, raising the pressure inside thestorage container. To prevent damage to the containment buildings of units 1 to 3, they
were vented to the wetwells a number of times.
After the venting of the first and third units, an explosion thought to be caused tohydrogen leaking from the containment building occurred in the upper part of thereactor building and the operation floor collapsed.
An explosion which appears to have been caused by hydrogen occurred in the
nuclear building of the unit 4, destroying the upper portion of the building. Around 6 AM
on March 15, the sound of a large impact was confirmed, but at this time, it is not clear
whether this was due to a hydrogen explosion, or the location where it occurred. Nearly
simultaneously, the S/C pressure indicated 0 Mpa (abs), and the reason for that isunclear.
Cooling of the used fuel pools for units 1 to 4 was shut down due to the loss of power,
and the evaporation of water due to heat from the used fuel caused the water level tocontinue to drop. To address the water level, helicopters and spray vehicles were used
to inject water into the used fuel pools, though ultimately a concrete truck pump was
secured, which pumped seawater, followed by freshwater.
On March 11, the government declared a nuclear emergency situation andestablished a Nuclear Emergency Response Headquarters and a Local Nuclear
Emergency Response Headquarters, with the prime minister serving as the
director-general. Further, on March 15, the Accident Measures Integration
Headquarters was established to integrate efforts of the government and nuclear
operators.
On March 11, the Nuclear Emergency Response Headquarters set the area within athree-kilometer radius from Fukushima Daiichi as an evacuation zone and the area
within a three- to 10-kilometer radius as an indoor evacuation zone. On March 12, withsubsequent development of the accident, the area within a 20-kilometer radius wasestablished as an evacuation zone and that within a 20- to 30-kilometer radius as anindoor evacuation zone. On April 21, the area within a 20-kilometer radius wasestablished as a no-entry zone, with only restricted entry allowed.

5-1

Although the Local Nuclear Emergency Response Headquarters was initiallyestablished off-site, it was subsequently moved to the Fukushima Prefectural Officewhen the location subsequently became a high-radiation environment, causing
communication interruptions and impeding logistics.
The provisional assessment of the accident according to the INES was initially alevel 3 due to the determination of inadequate water in the emergency core coolingsystem on March 11. On March 12, this was raised to level 4 due to the state ofradioactive material emissions, and on March 18, it was raised to level 5 in response to
an increase in the quantity of radioactive material being emitted. On April 12, the total
quantity of emitted radioactive material was determined to be roughly one-tenth of thatof Chernobyl, and the level was raised to level 7.
As a result of the total loss of AC power at Fukushima Daiichi, the internal Personal
Handy-phone System (PHS) and Safety Parameter Display System for ascertaining
station status became non-functional.
The Emergency Response Support System (ERSS) for predicting nuclear corestatus and developments during a nuclear accident was unable to obtain the required
station information and was otherwise unable to perform its primary functions. Further,
the System for Prediction of Environment Emergency Dose Information could notacquire emission source data or otherwise perform its primary function of making
quantitative predictions, including how the concentration of radioactive materials in theatmosphere would change.
Towards restoration of the power facilities, power supply vehicles from all branches
were dispatched to Fukushima Daiichi; however, progress was impeded by roaddamage and traffic congestion. Air transport of the power supply vehicles could not beprovided by the Self-Defense Forces because the power supply vehicles were beyond
their weight capacity. Before daybreak on the 12th, power supply vehicles that had
been secured were used and cables laid with the aim of restoring power. On March 15,
external power was restored to the internal switching stations for units 1 and 2, and on
March 24, MCR lighting was restored for unit 1. Further, on March 18, external power
was restored to the mobile M/C installed on the unit 3 premises, and on March 22,
MCR lighting was restored for unit 3.
Taking into account the records collected from MCR and other locations as well as
the operating status of equipment, a Modular Accident Analysis Program composed ofsevere accident analysis code was used to assess the status of the reactor core, andfound that in the most rapid scenario, fuel exposure began approximately two hours
after the tsunami struck and approximately three hours after the earthquake struck,with core damage occurring about one hour after that. As no water was being injected
into the nuclear core at that time, melting of the fuel progressed, and when water
injection commenced around 6:00 AM on March 12, the melted fuel had alreadymigrated to the lower part of the RPV, and apparently leaked to the containment
vessel.
At unit 2, fuel exposure apparently began approximately four hours after 1:25 PM on
March 14, when it was determined that the RCIC shut down, approximately 75 minutes
after the earthquake struck, followed by damage to the core approximately two hours
later. At unit 3, fuel exposure apparently began approximately four hours after 2:24 A.M.
on March 13 when it was determined that the HPCI shut down, approximately 40
5-2minutes after the earthquake struck, followed by damage to the core approximately twohours later.
With regards to stable cooling of the nuclear reactors, before carrying out theflooding operation (filling the containment vessel with water up to the upper fuel region)
initially planned, a review was made and it was decided to establish a “recycled water
injection cooling” to process contaminated water that had accumulated in the reactor
building for reuse as nuclear reactor injection. Additionally, the reliability of the injection
water was ensured (with measures to address abnormalities and multiple water
injection methods), a hydrogen explosion was avoided through filling the containment
vessel with nitrogen, and “stable cooling” was reached, the second step of the roadmap.
Currently, the temperature of each part of the RPV has stabilized, and thetemperature of the bottom portion of the RPVs at units 1 and 3 is stable at no greaterthan 100°C. The quantity of injection water was experimentally varied at unit 2, and astable temperature of no greater than 100°C was confirmed for the bottom portion to
the RPV.
Concerning cooling of the used fuel pools, a heat exchanger was used for circulating
cooling at all units, 1 to 4, and “more stable cooling” was achieved, step 2 of the roadmap, by August 10.
Seawater and underground water from the tsunami, as well as water injected into the
nuclear reactor and used fuel pools leaked, resulting in an accumulation of highly
radioactive water in the reactor building, which leaked to the cable trenches outside
and then to the ocean from cracks in the concrete frame. Measures were taken toprevent leakage to the ocean, including capping the trenches and stopping the water;
additionally, treatment equipment was installed to treat the accumulated water so thatthe water level would not overflow. To carry out treatment of the accumulated waterand inject it into the nuclear reactor in a stable, efficient manner, anevaporation-concentration apparatus was put into service on August 7 as a second lineof treatment equipment to increase the desalting capacity, and the operation moved
forward smoothly. On October 9, the augmentation of the salt treatment equipmentwith evaporation-concentration apparatuses was completed, making nuclear reactor
water injection more stable.
Currently, progress is at the second step of the road map, and work on measures tocontrol the release of radioactive materials is progressing.

5-3Chapter 6

Conclusion

The accident at the Fukushima Daiichi Nuclear Power Station has engendered
distrust in nuclear power not only in Japan but around the world as well.
Cause analysis and recommendations for action have been provided by a variety ofagencies, including a national accident measure commission. In this report, electricaloperators having the primary responsibility for construction and operation of nuclear
power stations have worked with electrical engineers directly involved with the designand construction of power stations, voluntarily summarizing the results of their impartial
analysis of the causes of the accident, their deductions of the most significiant lessons tobe learned, and countermeasures they have considered.
JANTI is composed of many companies involved in the nuclear power industry as
well as electrical operators, and through this association, power engineers and stationengineers have been able to come together to hold discussions. The positions of power
production and station construction were separated, and the abilities of experienced
specialists were employed to objectively study the issues from an engineering
perspective to further delve into the results that were assembled by the power and station
specialists. These efforts were also reflected in this report.
Furthermore, specialists of the technical analysis subcommittee of the Atomic Energy
Society of Japan have provided a review of this report, providing additional perspectives.
Central to this investigation have been the cause analysis into why it was not possible
to avoid core meltdown at the Fukushima Daiichi Nuclear Power Station after the tsunami
struck and proposals for countermeasures. In short, the factors may be summarized as
being that the cooling, control and monitoring functions of the internal electricalequipment, particularly the emergency AC and DC electrical panels were lost due toincoming water, and that due to the time required to establish a reactor core coolingmeans as an alternative to the regular power supply, flooding of the reactor core could
not be maintained. At power stations around Japan, emergency safety measures havealready been implemented for the present to prevent incidents caused by tsunamis, andthrough this investigation, multilayered countermeasures have been deduced, including
those already implemented in power stations throughout Japan and those planned for
implementation.
In this investigation, however, it was not possible to directly interview the operators
working at the time of the accident. By conducting an investigation with a perspective
focused on finding room for improvement, if only to alleviate the burden on the station
operators, the experience of operating during such an emergency situation could likely be
used to acquire knowledge useful in further improving measures against other incidents.
Recommendations in this investigation on alleviating the effects of post-meltdown
radioactive materials on the environment, estimates and criteria of the quantity ofradioactivity that people in the vicinity were exposed to, measures for resident evacuation
facilities, and disaster-related matters such as information dissemination and disclosure,and a system for instructions and orders were based solely on information obtained fromthe industry. In relation to these matters, however, there are likely to be many issues that
ought to be reconsidered and many lessons to be learned when the perspectives ofpeople in the local communities and others are taken into consideration, and those issues
should be the focus of investigations going forward.
When engineers design facilities, they must ensure that certain conditions, including
regulations, are satisfied. The thinking of the people who set those conditions is

6-1

susceptible to various influences, and phenomena may occur that exceed thoseconditions. Given this premise, it may be that the most important lesson that nuclear
power engineers should learn from this accident is that they should appreciate anew theimportance of continuing the search for rational means to solve engineering issues.
Particularly when your adversary is a natural phenomenon such as a tsunami, it is criticalto take yet another look at whether any of a variety of methods are needed to minimize
the effects of a phenomenon should it exceed expectations. Deep consideration should
also be given to the import of the words, “The prime responsibility for safety must restwith the person or organization” in the IAEA’s Fundamental Safety Principals. Moreover, it
is surely the responsibility of the engineer to continuously, repeatedly question safety
without falling into the illusion that satisfying regulatory requirements is an adequate
discharge of one’s duties, and we should demand that nuclear engineers recall to mind
what their purpose is in being engineers.
Whether such investigations will be conducted with due seriousness and without
getting caught up in the past history, and whether the results of those investigations will
be reflected in stations to improve safety is truly a call to the ability of the nuclear powerindustry and to each and every nuclear engineer. It is hoped that the industry andengineers will return to the roots of their calling as engineers and move forward in making
on-going improvements.
The investigation described in this report was conducted based on information
currently available, as described above, and was carried out with our best efforts. Whenothers view the report from their various perspectives, they will likely notice many
oversights and consider measures superior to those offered here. We hope that thereader will subject this report to the strictest of scrutiny and point out all such
shortcomings.
In closing, as members of the Fukushima Daiichi Nuclear Power Station Accident
Investigation Commission, we would like to express our deep gratitude to all of those who
earnestly participated in the long, frequent discussions despite the heavy workload that
dealing with the accident entailed for all concerned.

6-2

Appendix-1 A Comparison of the Development of theAccident at the Fukushima Daiichi Nuclear Power
Station and at Other Stations (Detailed Version)
Section 3.6 “Comparison to Other Stations” summarizes a comparison of Fukushima
Daiichi units 1 to 3 to Fukushima Daini, Onagawa and Tokai Daini. Details for eachstation are provided below.

A Summary of the events at the Fukushima Daini Nuclear Power Station
Although the functionality of the sea water cooling systems were lost at the
Fukushima Daini units 1, 2 and 4 due to the tsunami, RCIC operation ensured thecapability of reactor core cooling, and subsequently, a make-up water system wasused to provide an alternative means for injecting water into the nuclear reactors andcontainment vessels. As a result, cooling of the reactor cores and SFP fuel wasmaintained for roughly three days without venting the containment vessels.
Meanwhile, the sea water system was restored, and residual heat removal
functionality was finally restored through the resumption of the RHR equipment, anda transition was made to a cold shutdown. Because the power supply was secured, it
was possible to secure extra time by utilizing safety equipment for serious accidents
even without the functionality of the sea water system, and it was possible totransition the station into a safe state.
At unit 3, reactor core cooling was ensured with RCIC operation, followed byinjection of water from an alternate source. The residual heat removal system using
RHR was secured with the B system.
The development of events at Fukushima Daini are shown in an event tree inAppendix 1-1.

App.-1-1

Events
occurred

Reactorshutdown

Earthquak Reactor
e and
scram
TsunamiDC power
sourceAC source

DC power ExternalsourcepowersourceCore cooling

Emergenc IC, turbiney DG
driven coolant
injectionsystem
RCIC,HPCI)and HPCS for
which ACsource is not
requiredElectric power
recovery
External
powersource,emergency
DG, powerinterchange

Ensuring long-term cold shutdown
RPV
Reactordepressuri coolant
zationinjection(Including (including
alternative alternative
sources) sources,continuous
coolant
injection)

RHR

2F-3

RHR
recovery

Status of reactor core
PCV vent(beforecore
damage)Cold
shutdown，
core
damage，
PCV damage
etc.

Cold shutdown
Cold shutdown

2F-1,2,4
SRV[Tsunami]

2F-3 3/12 12:15
2F-1 3/14 17:00
2F-2 3/14 18:00
2F-4 3/15 07:15

Cold shutdown (long-term cooling required)
PCV damage (over-pressurization)(Successful)

Cold shutdown
Cold shutdown
Cold shutdown (long-term cooling required)
PCV damage (over-pressurization)

Cold shutdown
Cold shutdown(Successful)
Cold shutdown (long-term cooling required)
PCV damage (over-pressurization)

Figure Appendix 1-1 Fukushima Daini Nuclear power plant disaster identification events treeApp.-1-2

(1) The effects of the earthquake and tsunami
a. The effects of the earthquake
The maximum acceleration rate obtained in the reactor building base mat (thebottommost underground floor) due to the earthquake was confirmed to be lessthan the maximum response acceleration for basic earthquake ground motion Ss,
formulated to include the revised guidelines for seismic design review.
Also, although the response spectrum of the seismological record exceeded the
response spectrum for basic earthquake ground motion Ss during a portion of theperiodic band, it was confirmed as generally being equivalent or lower.
The seismological record was used to analyze the seismological response of
the critical safety equipment, and it is believed that the safety functions weresecure during and subsequent to the earthquake.
b. The effects of the tsunami
A study on the tsunami causd by the Tohoku earthquake that struck on March 11
showed that the water level and tsunami flooded areas on the ocean side andmajor building and equipment areas were as follows (ground movement due to theearthquake is not included):
1) Water level(a) Ocean side area (site O.P. + 4 meters)
・ Approximately + 7 meters* (tsunami water depth, approximately 3
meters)* There were local levels that were higher such as on the south side of the unit 1 heatexchanger building.

(b) Primary building and equipment area (site O.P. + 12 meters)
・ Approximately + 12 to 14.5 meters* (tsunami water depth, approximately
2.5 meters or lower)* Locally, O.P. + approximately 15 to 16 meters from the south side of the unit 1 building
to the base-isolation structure(Flooding depth: approximately 3 to 4 meters)2) Flooded areas(a) Although the entire ocean side area was affected, flooding was not found in themain building and equipment area beyond the slope of the ocean side area(b) Flooding was concentrated on the roads from the southeast side of the mainbuilding and equipment area to the base-isolation structure, and the building
environments of units 1 and 2 as well as the south side of the building of unit 3
were inundated (there was no flooding near the building of unit 4)(2) Unit 1
a. Station behavior upon occurrence of the earthquake
During the constant operation at the rated thermal output, Unit 1 automaticallyshut down at 14:48 p.m. on March 11, 2011 due to the “earthquake-accelerated
large trip” caused by the earthquake occurred at 14:46 p.m. on the same day (theepicenter was Sanriku-oki). The maximum acceleration observed was 305 gal at
the second floor of the basement of the Unit 1 building of the nuclear power station.
All reactors automatically shut down because their protection system became
activated properly as designed. Immediately after the automatic shutdown of thereactor, the following facts were confirmed: the reactor was subcritical; all control
rods were fully inserted; and all the facilities required for cold shutdown of thereactor and cooling of SFP were in normal and stable conditions.
However, such facilities required for cold shutdown of the reactor and cooling of
SFP became disabled by the tsunami which occurred after the earthquake (theApp.-1-3

first wave of the tsunami was visually confirmed at 15:22 p.m. on the same day,March 11). In addition, due to loss of the reactor’s heat removal function, S/C
could not be cooled down, Therefore, the S/C water temperature gradually
increased to over 100°C.
Subsequently, in order to partially restore the facilities required for cold
shutdown of the reactor and cooling of SFP to their usable states, the flooded
facilities were inspected and repaired, and power was supplied from a temporary
power source. After recovering the heat removal function of the reactor, the S/Cwater was cooled down to a temperature lower than 100°C. Since then until 17:00
p.m. on March 14, the reactor’s water temperature had been maintained at atemperature lower than 100°C (i.e., in the cold shutdown state) through the RHR1
system, while at the same time the SFP system had been continuously cooling
down. Currently, the station is under stable conditions.
b. State of the “shut down” function
Upon occurrence of the “earthquake-accelerated large trip” at 14:48 p.m.
caused by the earthquake (at the second floor of the basement of the reactor
building, set point for operation in a vertical direction: 100 gal), all control rods
were fully inserted immediately. The reactor automatically shut down as designed,
and the reactor became subcritical at 15:00 p.m. on the same day.
At 05:58 a.m. on March 12, the alarm went off, detecting the abnormality in theposition indicating probe (hereinafter referred to as PIP) of the control rod 10-51.
Although the alarm condition was cleared once at 10:30 a.m. on the same day, thissituation was repeated several times. There are two ways to indicate the control
rod position. One is to indicate the “full insertion” state. The other is to indicate theposition of the rod itself. When the alarm went off, while the PIP’s “full insertion”
indicator lamp was not on, the position indicator lamp was indicating that thecontrol rods were fully inserted. When the alarm condition was cleared, the fullinsertion indicator lamp was on.
Furthermore, when the alarm went off, no significant change was observed inindication of the start-up range neutron monitor (hereinafter referred to as SRNM)
and the reactor’s subcritical state remained unchanged. Although the alarm
condition was cleared at 12:02 p.m. on March 13, the control rods were isolated(bulb-out) at 15:18 p.m. on the same day to avoid activation. Since then, no
significant change has been observed in the SRNM indication and the reactor’s
subcritical state has been maintained.
c.

State of the “cooling” function
Immediately after the automatic shutdown of the reactor, the reactor output
rapidly declined, thus decreasing the void in the reactor core. As a result, thereactor water level dropped to the “reactor water level low (L-3).” Subsequently,
the reactor water level returned to the automatic-initiation water levels*2 of the
ECCS pumps*1 and the RCIC system without any fall due to the water supply fromthe reactor feed water system.
*1 ECCS pumps (common among reactors 1 to 4 of this power station)
・ HPCS pump・ Low-pressure core spray (hereinafter referred to as LPCS) pump・ RHR pumps (A, B, C) low-pressure coolant injection (hereinafter referred to as
LPCI) mode
*2 Automatic-initiation water levels (common among reactors 1 to 4 of this power
station)App.-1-4

・ HPCS and RCIC • • • • • • • •L-2
・ LPCS and RHR (LPCI) • • • • • • • •L-1
MSIV was completely closed at 15:36 p.m. on March 11 and the reactor
pressure was controlled through SRV in preparation for the following situations:the circulation water pump (hereinafter referred to as CWP) may shut down due toa tsunami, and may thus cause malfunction of the main steam condenser; and theturbine gland sealing steam may become lost due to the auxiliary boiler shutdown
caused by earthquakes. Furthermore, upon full closure of the MSIV, the RCIC
system was manually activated at 15:36 p.m. on the same day in order to pourwater onto the reactor. After the RCIC system automatically shut down at 15:40
p.m. on the same day due to the “reactor water level high (L-8)” the reactor waterlevel was adjusted via the manual activation and automatic shutdown of the RCIC
system.
It was decided that none of the pumps for the emergency equipment coolingsystem*3 was activatable (it was later confirmed at the scene that some motors
and emergency power supplies (P/C 1C-2, 1D-2) were unusable due tosubmersion) because the sea water heat exchanger building was flooded by thetsunami and the operation/shut down indication lamp indicated that these pumps
shut down. All the ECCS pumps therefore became unactivatable, thus causing
loss of the reactor’s residual heat removal function. Moreover, the emergency
power supplies (M/C 1C and 1HPCS) became unusable due to submersion of theannex of the reactor building caused by the tsunami, thus causing theunactivatable conditions to the LPCS pump, RHR pump (A) and the HPCS pump.
*3 Pumps for the emergency equipment cooling system:
・ Residual heat removal cooling system (hereinafter referred to as RHRC
pumps) pumps (A, B, C, D)・ Residual heat removal cooling system via sea water (hereinafter referred to as
RHRS) pumps (A, B, C, D)・ Emergency exchanger cooling water (hereinafter referred to as EECW) pumps
A, B)
・ High pressure core spray system (hereinafter referred to as HPCSC) pump・ HPCS cooling sea water system (hereinafter referred to as HPCSS) pump
Initially, water was poured onto the reactor using the RCIC system. However,
the alternative method using MUWC was introduced at 00:00 a.m. on March 12 as
the countermeasure for severe accidents and implemented along with the RCIC
system for pouring water. Furthermore, rapid reactor depressurization was
initiated at 03:50 a.m. on the same day because the relationship between thereactor pressure and the S/C water temperature turned the situation into theoperation prohibition status because of the heat capacity limitation. The RCIC
system was manually shut down at 04:58 a.m. on the same day due to thedecrease in the pressure of the steam to drive the RCIC turbine due to the rapid
depressurization of the reactor. Subsequently, the reactor water level was
adjusted through the alternative water pouring method using MUWC.
At 17:35 p.m. on March 11 the alarm went off after detecting the state of “D/W
pressure high” (set point: 13.7kPa [gage]). There was an entry (system A)
describing the state of “MSIV reactor water level low (L-2)” in the alarm typer thatwas recorded at 15:37 p.m. of the same day.
It was later discovered that the state of “MSIV reactor water level low (L-2)”(system A) was caused by loss of the MSIV trip logic circuit power supply due toApp.-1-5

shutdown of the AC120V station vital panel board 1A that was caused by thetsunami. Although the automatic activation signals of all the ECCS pumps properly
worked upon detection of the “D/W pressure high” state, of those ECCS pumps,
the LPCS pump, the RHR pump (A) and the HPCS pump did not automatically
activate because of the unavailable emergency power supplies (M/C1C, 1HPCS).
Also, the RHR pumps (B, C) were manually shut down because the RHRC pumps(B, D), the RHRS pumps (B, D) and the EECW pump (B) were unusable. After thisincident, the preventive measure for automatic activation (the control switch ismaintained at the “pull” position) was undertaken.
Subsequently, the S/C water temperature increased to a temperature over
100°C at 05:22 a.m. on March 12. It further increased up to approximately 130°C(at 11:30 a.m. on March 13). At 06:20 a.m. on March 12 in order to cool down the
S/C, the coolant (MUWC) was poured into the S/C from the cooler of the
flammability (flammable gas) control system (hereinafter referred to as FCS) via
the coolant drainage line. At 07:10 a.m. on the same day the alternative method
using MUWC was undertaken. D/W spraying was initiated at 07:10 a.m. and thecooling method was switched to the S/C spraying at 07:37 a.m. Thus PCV
alternative cooling was conducted by alternating these two methods according to
the situation.
Furthermore, along with the implementation of the alternative water pouringmethod using MUWC, the PCV alternative cooling method and the S/C coolingmethod using the FCS coolant (MUWC), the following pumps were inspected andrepaired: the PHRC pump (D); the RHRS pump (B); and the EECW pump (B).
(Motors were replaced for the RHRC pump (D) and the EECW pump (B).)
Additionally, because the sea water heat exchanger building was submerged andthe emergency power supplies (P/C 1C-2, 1D-2) were flooded, the RHRC pump(D), the RHRS pump (B) and the EECW pump (B) were restored to theiractivatable state by supplying power from the temporarily installed cable, which
received electricity from the power source (P/C 1WB-1) in the radioactive waste
treatment building, which in turn received electricity from external power supply
system and a high voltage power supply vehicle, which was urgently procured
outside the power station. These pumps were activated one by one at 20:17 p.m.
on March 13.
Subsequently, the RHR pump (B) was activated at 01:24 a.m. on March 14. Also,
as a result of S/C cooling using the RHR pump (B), the S/C water temperature
gradually dropped to a temperature lower than 100°C at 10:15 a.m. on the same
day.
Furthermore, in order to cool down the reactor water as well as the S/C water at
an early stage, the implementation procedures were created based on thepredetermined emergency operation procedures. At 10:05 a.m. on the same day
the procedure to pour the S/C water onto the reactor via the LPCI line was initiatedusing the RHR pump (B), while at the same time temporarily cooling the reactor
via the circulation line, through which the reactor water was allowed to flow into the
S/C through the SRV, and the S/C water was cooled using the RHR heatexchanger (B), then again poured back into the reactor via the LPCI line (S/C →RHR pump (B) → RHR heat exchanger (B) → LPCI line → reactor → SRV → S/C).
Through this operation, it was confirmed that the reactor water temperature
dropped to a temperature lower than 100°C at 17:00 p.m. on the same day, andthus cold shutdown was confirmed.
As described above, although the reactor’s cooling function was temporarily lost,
App.-1-6

water was continuously poured onto the reactor. As a result of the reactor water
sampling conducted after the incident, the level of iodine-131 was lower than thedetection threshold. Therefore, it did not damage the fuel.
d. State of the “trapping” function
When the alarm went off detecting the state of “reactor water level low (L-3)”
when the reactor automatically shut down, the Primary Containment Isolation
System (hereinafter referred to as PCIS) and the SGTS properly worked.
Therefore, the PCV was isolated, and the negative pressure in the reactor building
was maintained. Although the PCV pressure increased up to approximately
282kPa [gage] (at the S/C side), it did not reach the PCV maximum allowable
working pressure of 310kPa [gage].
In addition, no abnormal change was observed in the stack radiation monitor or
the MP value. It was therefore confirmed that no radiation affected theenvironment outside the building.
Moreover, assuming that the PCV pressure was going to increase and that it
would take a while to recover the reactor heat removal function, a PCV
pressure-resistant vent line (the state in which one action for operating the outlet
valve at the S/C side was left undone) was created.
e.

State of the spent fuel pool cooling system
With regard to systems required for cooling the SFP, the SFP water level wasmaintained higher than the overflow through the FPC system, and the SFP watertemperature was maintained at approximately 38°C before the occurrence of theearthquake. However, the SFP cooling through the FPC could not be continued
because the coolant could not be supplied to the FPC heat exchanger due to thedisability of some pumps for the following reasons: the FPC pump tripped(“skimmer surge tank water level low” or “pump suction pressure low”) due to theearthquake; the auxiliary sea water system (hereinafter referred to as SW) pump
(A, B, C) of the closed cooling water system located near the outside intake wasflooded by the tsunami; and the reactor building closed cooling water system(hereinafter referred to as RCW) pump (A, B, C) located at the lower ground floor
one in the sea water heat exchanger building was submerged by the tsunami.
Consequently, the SFP water temperature increased up to approximately 62°C.
Therefore, the operation to pour water into SFP using the fuel pool make-up water
system (hereinafter referred to as FPMUW) was conducted at 16:30 p.m. on
March 14, and the water circulation operation using the FPC pump (B) was
initiated at 20:26 p.m. on the same day to cool the SFP. Subsequently, at 00:42
a.m. on March 16 the SFP cooling operation was conducted using the RHR pump(B). By 10:30 a.m. on the same day, the SFP water temperature returned toapproximately 38°C, which was the temperature prior to the occurrence of theearthquake.
As a result of the above countermeasures, although the SFP system temporarily
lost its cooling function, it satisfied the operational limits (SFP water level: near
overflow water level, water temperature: 65°C or lower) stipulated by the Safety
Preservation Rules of Nuclear Facilities.

f.

State of the “power supply” function
Although all the power supplies in the station were in usable conditions
App.-1-7

immediately after the automatic shutdown of the reactor, because the annex of thereactor building was submerged by the tsunami, the emergency power supplies(M/C 1C and 1HPCS) became disabled. Moreover, because the sea water heatexchanger building was flooded by the tsunami, the emergency power supplies(P/C 1C-2 and 1D-2) became disabled.
At that time, the power supply was cut off from MCC 1C-1-8 because theemergency power supply (M/C 1C) was disabled, thereby shutting down theAC120V station vital panel board 1A, thus causing disability to some recorders in
the MCR.
In addition, all the emergency DG systems (systems A, B and HPCS) were in
usable conditions immediately after the automatic shutdown of the reactor.
However, after being flooded by the tsunami, all emergency machinery coolingsystem pumps became unactivatable. Furthermore, because the annex of thereactor building was also flooded by the tsunami, the DG main unit and its
accessory equipment (including pumps, a control panel and MCC) were also
flooded, thus disabling all the DG units.
During the subsequent restoration the AC120V station vital panel board 1A wasrestored to a usable state by receiving electricity from the cable temporarily
installed on the temporary supply panel board of Unit 2 (conducted on March 12).
Furthermore, with regard to the disabled emergency power supply (P/C 1D-2), theRHRC pump (D) and the RHRS pump (B), all of which were required for coolingthe reactor and the SFP, received electricity from the power supply (P/C 1WB-1) of
the radioactive waste treatment building via the temporarily installed cable. As well,the EECW pump (B) received electricity from a high voltage power supply vehicle(conducted on March 13 and 14).
Subsequently, the temporary power supply for the EECW pump (B) was
switched from the high voltage power supply vehicle to the emergency power
supply (P/C 1D-1) (switched on March 30). In addition, expecting to lose theexternal power supply system, the procedures to receive electricity from theemergency power supplies of Reactors 2 and 3 (M/C 2D and M/C 3D,
respectively) were devised as a standby power supply system for the DG (B) of
the usable emergency power supply (M/C 1D) (conducted on April 21).
In addition, even if the external power supply systems failed, the emergency
power supplies (M/C 2D and 3D) can receive electricity from DGs (B) of Reactors
2 and 3 because they are in usable condition.
Moreover, Unit 1’s DG (B) was also restored on July 15. Thus all the emergency
power supplies required for the reactor and the SFP have been secured.

App.-1-8

Table Appendix 1-1

Fukushima Daini Nuclear Power Station Reactor Unit 1
Situation in Chronological Order After the Occurrence of the
Earthquake

Friday, March 11, 2011
14:46
Earthquake occurs.
14:48
The reactor automatically shuts down. (The alarm goes off detecting
the state of “earthquake-accelerated large trip”.) All control rods are
fully inserted.
14:48
One of Tomioka circuits shuts down (number 2 trips, the system
continues to receive electricity from number 1).
15:00
The subcriticality of the reactor is confirmed.
15:22
The first tsunami wave is confirmed. (Since then the tsunami has been
continuously confirmed until 17:14 p.m.)
15:33
The CWP (C) is manually shut down.
15:34
DGs (A), (B) and (H) are automatically activated. Immediately after theactivation, they shut down due to the tsunami.
15:36
The MSIV is fully closed manually.
15:36
The RCIC system is manually activated (Subsequently, the systemstarts and stops randomly.)
15:50
All Iwaido circuits shut down.
(Number 2 shuts down. Number 1 was not operated before theoccurrence of the earthquake due to inspection.)
15:55
The reactor depressurization is initiated (the SRV automatically
opens). (Subsequently, reactor pressure is controlled throughautomatic and manual switches.)
15:57
The CWPs (A) and (B) automatically shut down.
17:35
The alarm goes off detecting the state of “D/W pressure high.” There isan entry describing the state of “MSIV reactor water level low (L-2)” in
the alarm typer at 15:37 p.m. of the same day. Based on these facts,
one cannot deny the possibility that the cause of the pressure increase
could be a leakage of reactor coolant in the PCV. It is therefore
determined that the specified events (leakage of reactor coolant)
stipulated in Article 10, Clause 1 of the Special Law for Nuclear
Emergency has occurred. (As a result of the inspection on the relevant
parameters subsequently conducted, no reactor coolant leakage
is observed. Therefore, at around 18:33 p.m. on the same day, it isdetermined that the situation does not fall under the above event.)
17:53
The D/W cooling system is manually activated.
18:33
Based on the fact that the activation of the sea water pump that has
the function to remove heat from a reactor cannot be confirmed, it isdetermined that the specified event (loss of the function to remove
heat from a reactor) stipulated in Article 10, Clause 1 of the Special
Law for Nuclear Emergency has occurred.
Saturday, March 12, 2011
00:00
MUWC alternative water pouring is initiated.
03:50
Rapid depressurization of the reactor is initiated.
(This is because the situation has turned into the operation prohibition
status.)
04:56
Rapid depressurization of the reactor is complete.
App.-1-9

04:58
05:22

05:58
06:20
07:10
07:37
07:45
10:21
10:30

18:30

The RCIC is manually shut down (operation prohibition due to thereactor pressure decrease).
Because the S/C temperature has exceeded 100°C, it is determinedthat the specified event (loss of the pressure suppression function)
stipulated in Article 15, Clause 1 of the Special Law for Nuclear
Emergency has occurred.
The alarm goes off detecting the abnormality in control rod 10－51 PIPThe S/C cooling operation using the FCS coolant (MUWC) is
conducted.
The D/W spraying operation using MUWC is conducted. (From thispoint forward, it has been conducted according to the situation.)
The S/C spraying operation using MUWC is conducted. (From thispoint forward, it has been conducted according to the situation.)
The S/C cooling operation using the FCS coolant (MUWC) is
terminated.
Creation of the PCV pressure-resistant vent line is initiated.
The alarm for the abnormality in control rod 10－51 PIP is cleared.
(Subsequently, it repeatedly goes off and becomes cleared several
times.)
Around 13:38 One of Iwaido circuits begins receiving electricity. (Therestoration of Number 2 has been completed.)
Creation of the PCV pressure-resistant vent line is complete.

Sunday, March 13, 201105:15 (appx.) Two Iwaido circuits begin receiving electricity (The restration of
Number 1 has been completed.)
20:17
The RHRS pump (B) is manually activated.
(Electricity is received from P/C 1WB-1 through the temporarily
installed cable.)
21:03
The RHRC pump (D) is manually activated.
(Motor replacement/receiving electricity from P/C 1WB-1 through thetemporarily installed cable.)
Monday, March 14, 2011
01:24
Due to the manual activation of the RHR (B) pump (the initiation of the
S/C cooling mode) and the activation of the RHR (B) pump, it isdetermined that the specified event (loss of the function to remove
heat from a reactor) stipulated in Article 10, Clause 1 of the Special
Law for Nuclear Emergency has occurred.
01:44
The EECW (B) is manually activated.
(motor replacement/receiving electricity from the high voltage power
supply vehicle)
03:39
The RHR (B) S/C spray mode is initiated.
10:05
The water pouring operation is conducted under the RHR (B) LPCI
mode.
10:15
Because the S/C water temperature has dropped to lower than 100°C,
it is determined that it has recovered from the specified event (loss ofthe pressure suppression function) stipulated in Article 15, Clause 1 of
the Special Law for Nuclear Emergency.
16:30
The water pouring operation through the FPMUW system is initiated tocool down the SFP.
App.-1-10

17:0020:26
22:07

Because the reactor temperature has dropped to lower than 100°C,
the reactor cooling operation is terminated.
Circulation operation of the FPC (B) is initiated.
Because the radiation level that exceeds 5μGy/h is measured at the
MP (No. 1), it is determined that the specified event (increase in thesite boundary radiation dose) has occurred. (It can be surmised thatthe increase in the radiation dose was caused by radioactive
substances discharged into the atmosphere upon the occurrence of
the accident at Fukushima Daiichi.)Tuesday, March 15, 201100:12
Because the radiation level that exceeds 5μGy/h is measured at the
MP (No. 3), it is determined that the specified event (increase in thesite boundary radiation dose) has occurred. (It can be surmised thatthe increase in the radiation dose was caused by radioactive
substances discharged into the atmosphere upon accident at
Fukushima Daiichi.)Wednesday, March 16, 2011
00:42
The RHR (B) SFP cooling operation is initiated.
10:30
The SFP water temperature of approximately 38°C is confirmed. (Itreturned to the temperature before the occurrence of the earthquake.)App.-1-11

(3) Unit 2
a. Station behavior upon occurrence of the earthquake
The reactor was running at normal rated heat output when an earthquake
centered in Sanriku-coast occurred at 14:46, March 11, 2011. The reactor
automatically shut down its operation at 14:48 with a “large earthquake
acceleration trip” alert. Immediately thereafter, full-insertion of all control rods (CR)and the reactor’s subcriticality were confirmed. It was also confirmed that
equipment necessary for the reactor’s cold shutdown and cooling of SFP were in asound and stable condition.
However, due to the tsunami after the earthquake (1st wave of the tsunami wasvisually confirmed at 15:22, on the same day), these equipment for cold shutdown
and SFP cooling became disabled because of water intrusion, etc. Also, S/C
cooling was not possible due to loss of the reactor heat removal function, and S/Cwater temperature gradually rose up to over 100°C.
Later, to enable a part of the equipment for reactor cold shutdown and SEP
cooling, inspection and repair of submerged equipment were conducted. Also,
power supply was obtained from a temporary power source. After recovery of thereactor’s heat removal function, S/C cooling was performed, and the S/C watertemperature fell below 100°C. Thereafter, by RHR1 system, the reactor
maintained its cold shutdown temperature below 100°C until 18:00, March 14. At
the same time, SFP had been continuously cooled down, and the station had
maintained its stable condition.
b. State of the “shut down” function
The earthquake triggered a “large earthquake acceleration trip” alert at 14:48(operational set point in vertical direction in reactor building underground 2nd floor:
100 gal), which immediately led to full-insertion of all control rods. The reactor
automatically shut down as designed and became subcritical at 15:01.
c.

State of the “cooling” function
Shortly after automatic shutdown of the reactor, a void (coefficient) in the core of
the reactor decreased due to the sharp decline of the reactor output. The waterlevel of the reactor dropped down to “reactor vessel low water level (L3).” After that,with the water supply from the reactor’s feedwater system, the water level wasrecovered and did not drop to the point at which the ECCS pump and RCIC would
automatically start.
At 15:34, March 11, Main Steam Isolation Valve (MSIV) was fully closed
manually, and the reactor’s pressure control was implemented by SRV. Thismeasure was taken to prepare for the situation where the condenser fails tocondense main steam, following CWP shutdown due to the Tsunami, and in thecase of losing turbine ground seal steam, following shutdown of the auxiliary boiler
due to impact of the earthquake.
With MSIV fully-opened, RCIC was manually activated at 15:43, and water
injection into the reactor was conducted. After RCIC’s automatic shutdown at
15:46 by “reactor vessel low water level (L8)”, the reactor’s water level was
adjusted by manual start-up and automatic shutdown of RCIC.
Based on indication of the operation/ shut off lamp displays and the fact that sea
water heat exchanger was flooded by the tsunami, it was determined that RHRC
pump (A,B,C,D), RHRS pump (A, B, C, D), EECW pump (A,B) and HPCSC pumpcould not be started (it was confirmed later at the station that pumps were
inoperable due to water exposure on a part of the monitors and emergency powerApp.-1-12

source (P/C 2C-2, 2D-2). Thus, all ECCS pumps became unusable, and thefunction of removing residual heat from the reactor was lost.
Initially, water injection into the reactor was performed by RCIC. However, with
opening of SRV, reactor pressure lowered. Accordingly, at 4:50, March 12, water
injection by MUWC was started based on operation procedure manual which was
implemented as a severe accident countermeasure. RCIC automatically shut
down at 4:53, on the same day, due to lower steam pressure for the RCIC turbine
caused by depressurization of the reactor. Subsequently, the reactor’s water level
was adjusted by alternative water injection with MUWC.
With operation of RCIC and opening of SRV, temperature and pressure inside
PCV increased. However, as cooling with RHR pump (A,B) was not possible, an
alarm signal was triggered at 18:50 on March 11, indicating “high D/W pressure”(design value: 13.7kPa [gage]).
As a result, all ECCS pump’s auto-start signals were issued. However, as RHRC
pump (A,B,C,D), RHRS pump (A,B,C,D), EECW pump (A,B) and HPCSC pumpwere disabled, manual shutdown was conducted after startup. After this, theauto-start prevention measure (hold pull of control switch) was implemented.
Later on at 5:32 on March 12, S/C water temperature rose over 100°C. The
temperature rose up to approximately 139°C at 7:00 a.m. on March 14.
To cool down S/C, from 6:30, March 12, MUWP, which is cooling water, wasinjected into S/C using the cooling water drain line from FCS’s cooler to S/C.
Alternative injection into the reactor by MUWC was changed into D/W spray from
7:11, and changed into S/W spray from 7:35 as necessary to perform alternative
cooling of PCV.
Inspection and repair of RHRC pump (B), RHRS pump (B) and EECW pump (B)were conducted along with alternative injection into the reactor by MUWC, PCV
alternative cooling and S/C cooling by FCS cooling water (MUWP). The sea water
heat exchanger building and emergency power supply unit (P/C 2C-2, 2D-2) were
submerged in water. Therefore, temporary cables obtained offsite were used tofeed emergency power. Power was supplied from the radioactive waste treatmentbuilding (P/C 1WB-1) receiving power from the external power system. Power was
also received from No.3 heat exchanger building emergency power (P/C3D-2) byinstallation of temporary cables. By reception of this power, RHRC pump (B),
RHRS pump (B) and EECW pump (B) recovered their function and started
sequentially from 3:20 on March 14.
Later on, RHR pump (B) was started from 7:13, March 14. Also, S/C cooling
performed by RHR pump (B) resulted in gradual decline in water temperature. Asof 15:52, S/C water temperature fell below 100°C.
To promptly cool down the reactor water as well as S/C water, an implementing
procedure manual was created using a previously formulated accident operation
manual as reference. From 10:48 on the same day, S/C water was injected into
the reactor by RHR pump (B) via LPCI line. At the same time, the reactor water
was flowed into S/C via SRV, and the S/C water was cooled by RHR heatexchanger (B) to re-inject into the reactor via LPCI line again. Such circulation line
(S/C → RHR pump (B) → RHR heat exchanger (B) → LPCI line → Reactor →SRV → S/C) was implemented as an emergency cooling measure. It was
confirmed that this measure cooled down the reactor temperature to below 100°C
at 18:00, the same day.
The above indicates that despite a temporary loss of the reactor’s cooling
function, water injection into the reactor was continued. Later sampling of theApp.-1-13

reactor water showed that iodine 131 was below detection limit. Thus, no fueldamage was incurred.
d. State of the “trapping” function
With “reactor vessel low water level (L3)” alert triggered at the reactor’s
automatic shutdown, PCIS and SGTS operated normally, and PCV was isolated,
and reactor building subatomospheric pressure was maintained. PCV pressure
increased up to approximately 279kPa [gage], but it did not reach the maximum
operating pressure of 310kPa [gage].
It was confirmed that there was no abnormal change in stack radiation monitor
or MP values, or radiation impacts to outside.
Assuming PCV pressure’s tendency to increase and the long recovery time forthe reactor’s heat removal function, a line configuration for PCV pressure vent was
implemented (one action remained for an operation to open the outlet valve on
S/C).
e.

State of the spent fuel pool cooling system
As for the equipment necessary for cooling of SFP, FPC had maintained SFP
water temperature at about 32.5°C and water level higher than the level ofoverflow water prior to the earthquake. Due to the impact of the earthquake, FPC
pump tripped (“low water level of skimmer surge tank” or “low pump pressure”).
The tsunami flooded SW pump (A,B,C) in non-safety service water system near
the outside water intake. RCW pump on the ground floor of the sea water heatexchanger building was also under water and unusable. For these reasons,cooling water was not supplied to the FPC heat exchanger, and SFP cooling by
FPC was not possible.
This raised SFP water temperature up to about 56°C, but cooling of SFP by
RHP pump (B) was started from 1:28 on March 16, and by 10:30, the temperature
was down to about 32.5°C, the same level as that before the earthquake.
The above indicates that while cooling function was also temporarily lost on SFP,
operational limits (SFP water level; near the level of overflow. Water temperature;
65°C or lower) specified under Safety Preservation Rules of Nuclear Facilities
were met.

f.

State of the “power supply” function
Immediately after automatic shutdown of the reactor, the facility’s power system
was usable. However, as the sea water heat exchanger building was submerged
due to the tsunami, the emergency power source (P/C 2C-2 and2D-2) became
unusable.
Emergency DGs were all usable right after the reactor’s automatic shutdown,
but after the Tsunami reached the facility, it was not possible to start RHRS pump
(A,B,C,D), EECW pump (A,B) and HPCSC. As a result, all emergency DGs
became unusable.
Through the recovery afterwards, out of the power loads of disabled emergency
power (P/C 2D-2), the following secured power by temporary cable installation(performed on March 14): Power for RHRC pump (B) and RHRS pump (B)necessary for cooling the reactor and SFP was supplied from the power source(P/C 1WB-1) of the radioactive waste treatment building. EECW pump (B)received power from the emergency power source (P/C 1WB-1) of the No.3 heatexchanger building.
As the above measure enabled use of RHRC pump (B), RHRS pump (B) andApp.-1-14

EECW pump (B), emergency power (M/C 2D) became available via DG (B) even if
external power source was lost. From April 2, DG (HPCS) became usable, thus
emergency power source for cooling of the reactor and SFP has been secured.

App.-1-15

Table Appendix 1-2

Fukushima Daini Nuclear Power Station Reactor Unit 2
Situation in Chronological Order After the Occurrence of the
Earthquake

Friday, March, 11, 2011
14:46
Earthquake occurred
14:48
Automatic shutdown of reactor (”large earthquake acceleration trip”
alert was triggered and all control rods were fully inserted.)
14:48
Tomioka line No. 1 shutdown (No. 2 was tripped, power continued tobe received from No. 1.)
15:01
Reactor’s subcritical status confirmed
15:22
The first tsunami wave was confirmed (thereafter, tsunami waves wereconfirmed intermittently until 17:14)
15:34
DG (H) automatically started/immediately thereafter it shut down due
to tsunami impact
15:34
MSIV fully closed manually
15:35
RHR (B) manually started (shut down at 15:38)
15:35
CWP (C) manually shut down, CWP (A) (B)automatically shut down
15:41
DG (A) (B) automatically started/thereafter shut down due to tsunami
impact15:41
Depressurization of the reactor commenced (SRV automatically
opened) (thereafter, valve was repeatedly opened and closed tocontrol reactor pressure)
15:43
RCIC manually started (thereafter, starts-stops occurred as thecircumstances demand)
15:50
Iwaido line fully shut down (No. 2 shutdown, No.1 had been shut down
for checkup before earthquake)
18:33
Because startup of a sea water pump for equipment having capability
to remove heat from the reactor could not be confirmed, it wasdetermined that an event specified under Article 10, Clause 1 ofSpecial Law for Nuclear Emergency had occurred (loss of reactor heatremoval capability).
18:50“High D/W pressure” alert was issued.
20:02
Drywall cooling system manually startup
Saturday, March 12, 2011
04:50
Alternative water injection started using the make-up water
condensate system (MUWC)
04:53
RCIC automatically shut down (due to lower reactor pressure)
05:32
Because S/C temperature had risen above 100°C, it was determinedthat an event specified under Article 15, Clause 1 of Special Law for
Nuclear Emergency had taken place (loss of pressure suppression
capability)
06:30
S/C cooling implemented using coolant (MUWP) from flammablecontrol system (FCS).
07:11
D/W spraying implemented using MUWC (thereafter implemented as
circumstances demanded)
07:35
S/C spraying implemented using MUWC (thereafter implemented as
circumstances demanded)
07:52
S/C cooling using FCS coolant (MUWP) shut down
10:33
Configuration of PCV hardened venting line commenced
App.-1-16

10:58Configuration of PCV hardened venting line completed
13:38 (appx.) Power received from Iwaido line No.1 (No.2 restored)
Sunday, March 13, 2011
5:15 (appx.) Power received from Iwaid line No.2 (No.1 restored)
Monday, March 14 2011
03:20
EECW (B) manually started up(Power received from P/C 3D-2 via temporary cable)
03:51
RHRS (B) manually started up(Power received from P/C 1WB-1 via temporary cable)
05:52
RHRC (B) manually started up(Power received from P/C 1WB-1 via temporary cable)
07:13
RHR (B) manually started up (S/C cooling mode commenced)
With startup of RHR (B), it was determined to cancel the occurrence ofa special event (loss of capability to remove heat from reactor) under
the provision of Article 10, Clause 1 of Special Law for Nuclear
Emergency.
07:50
RHR (B) S/C spraying mode commenced
10:48
Coolant injection into reactor was started in RHR (B) low pressure
coolant injection (LPCI) mode
15:52
Because S/C temperature had fallen below 100°C, it was determinedthat the station recovered from the event specified under Article 15,
Clause 1 of Special Law for Nuclear Emergency (loss of pressure
suppression capability)
18:00
Reactor temperature had fallen below 100°C, and cold shutdown ofreactor achieved.
22:07
As radiation dose measured exceeded 5μGy/h at MP (No.1), it wasdetermined that a special event (increase in radiation dose in siteboundary) under the provision of Article 10, Clause 1 of Special Law
for Nuclear Emergency took place (the cause of the dose increase issurmised to be due to the effects of radioactive material released into
the atmosphere, following the accident at Fukushima Daiichi Nuclear
Power Station).
Tuesday, March 15, 2011
0:12
As radiation dose measured exceeded 5μGy/h at MP (No.3), it wasdetermined that a special event (increase in radiation dose in siteboundary) under the provision of Article 10, Clause 1 of Special Law
for Nuclear Emergency took place (the cause of the dose increase issurmised to be due to the effects of radioactive material released into
the atmosphere, following the accident at Fukushima Daiichi Nuclear
Power Station).
Wednesday, March 16, 201101:28
RHR (B) commenced cooling of SFP
10:30
SFP water temperature was confirmed to be about 32.5°C(recovered to the temperature before earthquake)App.-1-17

(4) Unit 3
a. Station behavior upon occurrence of the earthquake
The station was in operation at a constant rate thermal power when theearthquake occurred at 14:46, March 11, 2011. The earthquake’s epi-center was
offshore of Sanriku. At 14:48 on the same day, the reactor shut down due to “large
earthquake acceleration trip” alert. Immediately thereafter, all control rods were
fully inserted, and the reactor’s subcriticality was confirmed. It was also confirmedthat equipment necessary for the reactor’s cold shutdown and cooling of SFP weresteadily operating in sound condition.
While equipment for the reactor’s cold shutdown and cooling of SFP became
partially disabled, cooling of the reactor was implemented from March 12, using
RHR1 system which was operatable without impacts of the tsunami. Cold
shutdown of the reactor was confirmed on the same day.
b. State of the “shut down” function
Due to the earthquake, “large earthquake acceleration trip” alert (reactor
building underground 2nd floor, operational set point value in horizontal direction:
135 gal) was issued at 14:48. Immediately thereafter, all control rods were fully
inserted. The reactor automatically shut down as designed, and it was insubcritical state at 15:05 on the same day.
c.

State of the “cooling” function
Immediately after the reactor’s automatic shutdown, void in the reactor coredecreased, following drastic decline of reactor output. Water level of the reactor
decreased to “reactor vessel low water level (L3).” With water supply from thereactor’s feedwater system, the water level of the reactor was later recovered anddid not drop to the point at which the ECCS pump and RCIC would automaticallystart. On March 11, at 15:37, Main Steam Isolation Valve (MSIV) was fully closed
manually, and the reactor’s pressure control was implemented by SRV. Thismeasure was taken to prepare for the situation where the condenser fails tocondense main steam, following CWP shutdown due to the Tsunami, and loss ofturbine ground seal steam, following shutdown of auxiliary boiler due toearthquake impact.
With full opening of MSIV, Reactor Core Isolation System (RCIC) was manually
started at 16:06 on the same day, and water was injected into the reactor.
Due to the fact that the sea water heat exchanger building was flooded by thetsunami along with indication by operation on/off lamp, it was determined that
RHRC pump (A,C)、RHRS pump (A,C) and EECW pump (A) could not be started(it was confirmed later at the station site that these pumps were not usable
because the motors and emergency power source were partially flooded). For thisreason, it was not possible to start LPCS pump and RHR pump (A).
As for the emergency power source (P/C 3D-2) and its load, RHRC pump (BD),RHRS pump (B,D) and EECW pump (B) as well as HPCSC pump and HPCSS
pump, the amount of sea water exposure to the sea water heat exchanger building
was smaller compared with other pumps. Based on this reason, these pumps were
presumed to be usable as there was little impact on this equipment.
Also, RHR pump (B,C) and HPCS pump were usable because there was nowater intrusion in the 2nd underground floor of the reactor ward of reactor building.
Water injection into the reactor was initially performed by Reactor Core Isolation
Cooling System (RCIC). However, from 22:53, March 11, alternative water
injection by MUWC introduced as a severe accident countermeasure, was alsoApp.-1-18

implemented. Later, lower reactor pressure due to SRV opening reduced steampressure for RCIC turbine operation. This led to manual shutdown of RCIC at
23:11 on the same day. Thereafter, alternative water injection using MUWC had
been performed. However, at 00:06, on the same day, water injection and cooling
was implemented by the usable RHR pump (B). At 12:15, March 12, the reactor’swater temperature fell below 100°C, and its cold shutdown was confirmed.
At 19:46, March 11, “high D/W pressure (design value: 13.7kPa [gage])alert wasissued due to temperature and pressure rise inside PCV, following RCIC operationand SRV opening. This was followed by the automatic start-up signal for all ECCS
pumps. However, HPCS pump, LPCS pump and RHR pump(A,C) were notautomatically started because the automatic start prevention measure was inplace (hold pulling control switch) due to the disabled cooling system (RHRC (A，C), RHRS (A,C) and EECW(A)). RHR pump (B) was in operation for cooling of S/C
at “high D/Wpressure” alert (started at 15:36, March 11).
Based on the above, it is found that fuel was not damaged as the reactor’scooling function was maintained. Later sampling of the reactor water confirmedthat iodine 131 was below detection limits.
d. State of the “trapping” function
With “reactor vessel low water level (L3)” alert issued at automatic shutdown of
the reactor, Primary Containment Isolation System (PCIS) and Stand-By Gas
Treatment System (SGTS) were in normal operation, and PVC isolation andmaintenance of the reactor’s subatomospheric pressure were conducted. PCV
pressure rose up to about 38kPa [gage] at D/W, but it did not reach PCV’s
maximum operating pressure, 310kPa [gage].
Also, it was confirmed that there was no abnormal change in stack radiation
monitor and MP values, or radiation impact to the outside.
In case of rising PCV pressure, line configuration of PCV hardened venting line
was implemented (one action remained for an operation to open the outlet valve
on the S/C).
e.

State of the spent fuel pool cooling system
The following describes the condition of equipment necessary for cooling of
Spent Fuel Pit (SFP). Before the earthquake, Fuel Pool Cooling & Filtering System(FPC) had maintained SFP’s water level above overflow level and kept SFP’s
water temperature about 34°C. Yet, with the impact of the earthquake, FPC pump
tripped (“low water level of skimmer surge” tank or “low pump pressure”). At thesame time, due to the Tsunami impact, SW pump (A, B, C) in non-safety service
water system placed near water intake outside was flooded and became unusable.
Also, RCW pumps (A, B, C) in the ground floor of the sea water heat exchanger
building were submerged and disabled. For these reasons, cooling water was not
supplied to the FPC heat exchanger, thereby disabling SFP cooling by FPC.
These events raised SFP water temperature up to about 51°C. However, from
17:42, March 15, cooling water of FPC heat exchanger was switched from RCW to
RHRC. By this measure, SFP cooling by FPC was implemented. At 22:30, March
16, SFP’s water temperature was restored back to about 34.0°C, the same level
prior to the earthquake.
Based on the above, it was found that while SFP suffered temporary loss ofcooling capability, it met operational limits (SFP water level: about overflow level,water temperature; below 65°C) specified under the Safety Preservation Rules ofApp.-1-19

Nuclear Facilities.
f.

State of the “power supply” function
Immediately after the reactor’s automatic shutdown, all power systems in thefacility were usable. However, as the sea water heat exchanger building was
flooded by the tsunami, emergency power (P/C 3C-2) became unusable.
Also, emergency diesel generators (DG) were all (A system, B system and
HPCS system) usable right after reactor’s automatic shutdown. However, after thetsunami reached the facility, it was not possible to start RHRS (A,C) pump and
EECW (A) pump. As a result, DG (A) became unusable.
Since DG (B) and DG (HPCS) were usable, emergency power (M/C 3D and3HPCS) were receivable by DG (B, HPCS) even in case of losing external power
supply.
Based on the above, it is found that emergency power necessary for cooling of
the reactor and SFP were secured.

App.-1-20

Table Appendix 1-3

Fukushima Daini Nuclear Power Station Reactor Unit 3
Situation in Chronological Order After the Occurrence of the
Earthquake

Friday, March 11, 2011
14:46
Earthquake occurred
14:48
Automatic shutdown of the reactor (”large earthquake accelerationtrip” alert was issued), and all control rods were fully inserted
14:48
Tomioka line No.1 shutdown (NO. 2 tripped, power continued to bereceived by No. 1)
15:05
Reactor’s subcriticality confirmed
15:22
The first wave of tsunami was confirmed (thereafter, tsunami waves
were confirmed intermittently until 17:14)
15:34
CWP (C) Manual shutdown
15:35
DG (A) (B) (H) automatic startup/Immediately after that DG (A)shutdown due to tsunami impact
15:36
RHR (B) Manual startup (S/C cooling mode commenced)
15:37
MSIV fully closed manually
15:38
CWP (B) Manual shutdown
15:46
Depressurization of the reactor commenced (SRV automatically
opened) (thereafter, valve was repeatedly opened and closed tocontrol reactor pressure)
15:50
Iwaido line fully shutdown (No.2 shutdown, No. 1 had been shut down
for checkup before the earthquake)
16:06
RCIC manually started (thereafter, starts-stops occurred as thecircumstances demand)
16:48
CWP (A) Manual shutdown
19:46”High D/W pressure” alert was issued (RHR (B) automaticallyswitched from S/C cooling mode to LPCI mode)
20:07
RHR (B) Switched from LPCI mode to S/C cooling mode
20:12
D/W cooling system manually started
22:53
Alternative water injection using MUWC commenced
23:11
RCIC manual shutdown (due to lower reactor pressure)
Saturday, March 12, 201100:06
RHR (B) Commenced preparation for SHC mode configuration
01:23
RHR (B) manual shutdown (to prepare for SHC mode)
02:39
RHR (B) Manual startup (S/C cooling mode commenced)
02:41
RHR (B) S/C spraying mode commenced
07:59
RHR (B) Manual shutdown (S/C cooling mode and S/C spraying mode
shutdown)
09:37
RHR (B) Manual startup (SHC mode operation commenced)
12:08
Configuration of PCV hardened venting line commenced
12:13
Configuration of PCV hardened venting line completed
12:15
Water temperature of the reactor fell below 100°C, and reactor cold
shutdown achieved
13:38 (appx.) Power received by Iwaido line No.1 (No. 2 restored)
Sunday, March 13, 201105:15 (appx.) Power received by Iwaido line No. 2 (NO. 1 restored)

App.-1-21

Monday, March 14, 2011
22:07
As radiation dose exceeded 5μGy/h at MP (No. 1), it was determinedthat a special event (increase in radiation dose in site boundary) under
the provision of Article 10, Clause 1 of Special Law for Nuclear
Emergency took place (the cause of the dose increase is surmised tobe due to the effects of radioactive material released into theatmosphere, following the accident at Fukushima Daiichi Nuclear
Power Station).
Tuesday, March 15, 201100:12
As radiation dose exceeded 5μGy/h at MP (No. 3), it was determinedthat a special event (increase in radiation dose in site boundary) under
the provision of Article 10, Clause1 of Special Law for Nuclear
Emergency took place (the cause of the dose increase is surmised tobe due to the effects of radioactive material released into theatmosphere, following the accident at Fukushima Daiichi Nuclear
Power Station).
17:42
FPC heat exchanger cooling water was switched (RCW → RHRC)
Wednesday, March 16, 2011
22:30
SFP water temperature was confirmed to be about 34°C (recovered tothe temperature before the earthquake)App.-1-22

(5) Unit 4
a. Station behavior upon occurrence of the earthquake
During the constant thermal output rated operation, due to the earthquake
occurred at 14:46 on 11 March 2011 with epicenter off the coast of Sanriku region
the reactor automatically shut down by "trip due to high seismic acceleration" at
14:48 on the same day. Right after the event, ensuring that all control rods were
fully inserted and the reactor was subcritical, all facilities necessary for cold
shutdown of the reactor and SFP cooling were confirmed integrity and in stablecondition.
However, because of the tsunami after the quake (the first wave was observed
to reach at 15:22 on the same day) the facilities necessary for cold shutdown and
SFP cooling were submerged in water and unavailable.
In addition, S/C cooling was unavailable due to the loss of heat removal functionof the reactor, and then the S/C water temperature gradually rose and exceeded
100°C.
After those events, in order to recover part of the facilities for cold shutdown and
SFP cooling, the submerged facilities were checked and repaired, and power
supply from the temporary source was started. By cooling S/C after the recovery ofheat removal function of the reactor, the S/C water temperature dropped under
100°C. From that time on, by using RHR1 system the water, temperature of thereactor was lowered to less than 100°C and kept in cold shutdown state by 7:15 on
15 March, SFP has been continuously cooled down, and the station is currently
kept stable.
b. State of the “shut down” function
The earthquake caused "high seismic acceleration trip" (on the second floor of
the reactor building, the horizontal trip value setting: 150 Gal) to be issued at 14:48,
and then all of the control rods were fully inserted immediately.
The reactor automatically shut down as designed, and it became subcritical at
15:05 on the same day.
A drift alarm on control rod 10-19 was issued at 12:43 on 13 March, which was
cleared at 20:19 on 14 March but was issued again at 21:07 on 14 March. Control
rod position was readable both with the indicator of "fully inserted" and the displayof position. When the alarm was issued, the control rod status was presented as"fully inserted" with the indicator lit while the position display was unlit.
When the alarm was issued, no significant change was observed in SRNM
indication and the reactor remained subcritical. The control rod status waspresented as fully inserted. The drift alarm was continuously issued, and in orderto stop it the countermeasure was taken by isolating the control rod (valve out) at
16:56 on 15 March. From that time on, no significant change has been observed in
SRNM and the reactor remains subcritical.

c.

State of "cooling" function
Just after the reactor automatically shut down, the reactor water level dropped
to "reactor vessel low water level (L-3)" as voids inside the core decreased due tothe steep drop of reactor output. Later, the reactor water level recovered by supply
from the reactor feedwater system without dropping to the level to automaticallystart ECCS pump and RCIC.
For the possibility of stop of CWP and subsequent outage of main steam
condensation by the condenser due to damage from tsunami, and for theApp.-1-23

possibility of loss of steam in the turbine gland seal caused by stop of auxiliary
boiler due to the quake, MSIV was fully closed by manual operation at 15:36 on 11
March to control the pressure of the reactor by SRV.
Along with full close of MSIV, RCIC was manually started at 15:54 on the same
day to inject water to the reactor. And then, after the automatic stop of RCIC due to"reactor vessel high water level (L-8)" at 16:11 on the same day, the reactor waterlevel was controlled by manual start and automatic stop of RCIC.
Judgeing from the fact that the seawater heat exchanger was submerged inwater due to the tsunami and from the lamps to indicate operating/halt, RHRC
pumps (A, B, C, D), RHRS pumps (A, B, C, D) and EECW pumps (A, B) were
estimated as inoperable (some of the motors and power supplies (P/C 4C-2, 4D-2)
were confirmed at the site on the later day that they were inoperable because ofsubmersion). Under that situation, LPCS pumps and RHR pumps (A, B, C) were
inoperable, and therefore the function to remove the residual heat of the reactor
was lost.
HPCSC pumps and HPCSS pumps were estimated that they were available
because of less damage from water submersion as there was less amount ofseawater submersion in the pump area inside the seawater heat exchanger
building in comparison to other pumps. In addition, as there was no inundation on
the second basement floor of the reactor building, HPCS pumps were in operable
condition.
Injection of water to the reactor was initially done by RCIC, but because thesteam pressure to drive RCIC turbine decreased due to the reactor pressure drop
caused by opened SRV, after the automatic stop of RCIC at 0:16 on 12 March,alternative water injection was started according to the operation manual by
MUWC which was installed as a countermeasure to severe accidents. From thattime on, the reactor water level was adjusted by start/stop of HPCS pumps whichwere operable without damage from the tsunami.
The temperature and pressure inside PCV increased along with the operation of
RCIC and opening of SRV. Because the cooling by RHR pumps (A, B) was notavailable, "high D/W pressure" (setting: 13.7kPa [gage]) was issued on 19:02 on
11 March.
The signal to automatically start all of the ECCS pumps was generated alongwith that alarm, but the water injection to the reactor was done by RCIC and theautomatic start was prevented (the control switch was pulled and hold) because
the cooling system (RHRC, RHRS and EECW) were not operable, each ECCS
pump was not automatically started.
After those events, S/C water temperature exceeded 100°C at 6:07 on 12 March.
The S/C water temperature has reached the maximum of 137°C (12:30 on 14
March).
In order to cool down S/C, the injection of cooling water (MUWP) to S/C wasstarted at 7:23 on 12 March by use of cooling water drain lines from FCS cooler to
S/C, and in addition to that, the alternative cooling of PCV was performed at 7:35
by switchover to S/C spray from alternative water injection by MUWC.
In parallel with the alternative water injection to the reactor by MUWC, thealternative cooling of PCV and the cooling of S/C by FCS cooling water (MUWP),RHRC pump (B), RHRS pump (D) and EECW pump (B) (the motor was replacedfor RHRC pump (B)) were checked and repaired. Because the seawater heatexchanger building was submerged in water and its emergency power supplies(PC 4C-2, 4D-2) were inundated, high voltage power supply vehicles andtemporary power cords were emergently prepared from outside the site and used.
App.-1-24

By installing temporary power cords and providing power from the emergency
power supply (P/C3D-2) of the heat exchanger building of reactor 3 whose power
was supplied from the electric system outside and by providing power from thehigh voltage power supply vehicles, RHRC pump (B), RHRS pump (D) and EECW
pump (B) were recovered to operable conditions, and then they were started one
by one from 11:00 on 14 March.
After that, RHR pump (B) was started on 15:42 on 14 March.
As a result of S/C cooling by RHR pump (B), the S/C water temperature started
gradually dropping and fell under 100°C at 07:15 on 15 March.
Furthermore, in order to quickly cool down the reactor water in addition to thecooling of S/C water, the implementation procedure was improvised by referencing
the predefined emergency operation manual. Along with the start of S/C water
injection to the reactor by RHR pump (B) from LPCI line at 18:58 on the same day,the emergency cooling procedure with the circulating line was performed byflowing the reactor water into S/C via SRV, cooling S/C water by RHR heatexchanger (B), and injecting the water to the reactor from LPCI line again (S/C →RHR pump (B) → RHR heat exchanger (B) → LPCI line → the reactor → SRV →S/C). With this configuration the reactor water temperature dropped under 100°C
and the reactor was confirmed in cold shutdown state at 7:15 on 15 March.
The mentioned above suggests that the water injection to the reactor was
successfully continued although the cooling functions of the reactor was
temporarily lost, and no fuel damage occurred because the level of iodine-131 wasfound lower than detection limit in the reactor water sampled in a later
investigation.
d. State of the “trapping” function
In accordance with the issuance of "reactor vessel low water level (L-3)" on theautomatic shutdown of the reactor, PCIS and SGTS operated correctly to isolate
PCV and maintain the negative pressure of the reactor building. The pressure of
PCV has reached the maximum value of about 245kPa [gage] (S/C side), but notreached its maximum operating pressure of 310kPa [gage].
No unusual changes in the stack radiation monitor or MP values were observed
and no outside radioactive influence was confirmed.
Because there was a rise trend in the pressure of PCV and it could possibly takelong time to recover the heat removal function of the reactor, the line for PCV
pressure control vent was configured (so that it could vent in an action by opening
the outlet valve in S/C side).
e.

State of the spent fuel pool cooling system
The facilities for SFP cooling maintained the water level of SFP more than its
overflow level by FPC, and the water temperature of SFP at around 35°C before
the earthquake occurred. However, because of the trip of FPC pumps due to the
influence of the quake ("skimmer surge tank water level very low" or "pump suction
pressure low"), the inundation of SW pumps (A, B, C) of the non-safety service
water system near the water intake outside the building and the submersion of
RCW pumps (A, B, C) on the first basement of the seawater heat exchanger
building, all of them became inoperable and unable to supply water to FPC heatexchanger, and therefore SFP cooling by FPC became unavailable.
As a result, the water temperature of SFP has reached the maximum of 62°C,
but by switching cooling water of FPC heat exchanger from RCW to RHRC forApp.-1-25

SFP cooling at 16:35 on 15 March, the water temperature of SFP dropped toaround 35°C at 17:00 on 16 March as low as before the earthquake.
The mentioned above suggests that although SFP temporarily lost its cooling
function, it successfully satisfied the operation limits and conditions (SFP waterlevel; close to the overflow level, water temperature; 65°C or less) defined by the
Safety Preservation Rules of Nuclear Facilities.
f.

State of the “power supply” function
The distribution systems were all available just after the automatic shutdown of
the reactor, but the emergency power source (P/C 4C-2, 4D-2) became
unavailable because the seawater heat exchanger building was submerged due tothe tsunami.
Just after the automatic shutdown of the reactor all of the emergency DGs (Asystem, B system and HPCS system) were operable, but after the tsunami
reached, because RHRS pumps (A, B, C, D) and EECW pumps (A, B) became
unable to start, the emergency DGs (A, B) became also unavailable.
In a later recovery procedure, among the loads of the unavailable emergency
power source (P/C 4D-2), RHRC pump (B) and RHRS pump (D) were allocatedpower to cool down the reactor and SFP, by the installment of temporary cords andthe power supply from the emergency power source (P/C 3D-2) of the seawater
heat exchanger of the reactor 3, and to EECW pump (B) by the high voltage power
supply vehicle prepared from outside the site (performed on 14 March).
By this procedure, the emergency DG (B) was in operable condition, and theemergency power source (M/C 4D) became ready to receive power from theemergency DG (B) even if the external power source was lost.
Later, the temporary power source for the EECW pump (B) was switched fromthe high voltage power supply vehicle to the emergency power source (P/C 4D-1)(switched on 29 March).
The emergency DG (HPCS) was in operable condition from the time of theautomatic shutdown of the reactor, and therefore the emergency power supplies
enough to cool down the reactor and SFP had been secured.

App.-1-26

Table Appendix 1-4

Fukushima Daini Nuclear Power Station Reactor Unit 4
Situation in Chronological Order After the Occurrence of the
Earthquake

Friday, March 11, 2011
14:46
Earthquake occurred.
14:48
Reactor automatically shut down ("trip due to high seismic
acceleration" was issued), all of the control rods were fully inserted.
14:48
One of Tomioka line was shut down (line 2 tripped and continued
receiving power from line 1)
15:05
Confirmed that the reactor was subcritical.
15:22
Reach of the first wave of tsunami (tsunami waves were observed
intermittently from this time to 17:14).
15:33
CWP (C) was stopped by manual operation.
15:34 (appx.) Emergency DGs (A) (B) (H) automatically started. / DG (A) (B)stopped right away due to damage from the tsunami.
15:35
CWP (A) (B) automatically stopped.
15:36
MSIV was fully opened by manual operation.
15:36
RHR (B) was manually started (automatically stopped at 15:41).
15:37
RHR (A) was manually started (automatically stopped at 15:38).
15:46
Started depressurizing the reactor (SRV was automatically opened)(from this time on, the reactor pressure was controlled by automaticand manual operations).
15:50
All of Iwaido line shut down. (Line 2 shut down. Line 1 had been shut
down for checkout before the earthquake occurred.)
15:54
RCIC was started by manual operation (from this time on, RCIC wasstarted or stopped as required).
18:33
Because the start of seawater pump of the facility for heat removal of
the reactor was not confirmed, it was judged that there occurred aparticular event (loss of the heat removal function of the reactor)
defined by Article 10, Clause 1 of Special Law for Nuclear Emergency.
19:02"D/W pressure high" alert was issued.
19:14
D/W cooling system was started by manual operation.
Saturday, March 12, 2011
00:16
RCIC automatically stopped (due to the drop of the reactor pressure).
00:16
Alternative water injection by MUWC was started.
06:07
Because the temperature of S/C exceeds 100°C, it was judged that a
particular event (loss of the function to control pressure) occurred as
defined by Article 15, Clause 1 of Special Law for Nuclear Emergency.
07:23
Cooling down of S/C was performed by use of FCS cooling water(MUWP).
07:35
S/C spray was performed by use of MUWC.
11:17
Water injection to the reactor was switched from MUWC (alternative
injection) to HPCS.
11:44
Started configuration of PCV pressure control vent line.
11:52
Completed configuration of PCV pressure control vent line.
13:38 (appx.) Received power from one of Iwaido lines (Line 2 was recovered).
13:48
Water injection to the reactor by HPCS was stopped (from this time on,this procedure was done as required).

App.-1-27

Sunday, March 13, 201105:15 (appx.) Received power from two lines of Iwaido lines (Line 1 wasrecovered).
12:43
Drift alert was issued on control rod 10-19.
Monday, March 14, 2011
11:00
EECW (B) was started by manual operation (receiving power from thehigh voltage power vehicle).
13:07
RHRS (D) was started by manual operation (receiving power via
temporary cords from P/C 3D-2).
14:56
RHRC (B) was started by manual operation (the motor was
replaced./receiving power via temporary cords from P/C 3D-2)
15:42
RHR (B) was started by manual operation (started S/C cooling mode).
Because RHR (B) started, it was judged to cancel the particular event(loss of the heat removal function of the reactor) defined by Article 10,
Clause 1 of Special Law for Nuclear Emergency.
16:02
RHR (B) spray mode was started.
18:58
RHR (B) started water injection to the reactor in LPCI mode (stopped
at 19:20) (from this time on, it was started/stopped as required).
20:19
Drift alert on control rod 10-19 was cleared.
21:07
Drift alert on control rod 10-19 was issued (from this time on, the alert
was continuously issued).
22:07
Because greater radiation dose than 5μGy/h was detected at MP(No.1), it was judged that a particular event (rise of radiation dose atsite boundary) occurred as defined by Article 10, Clause 1 of Special
Law for Nuclear Emergency. (the rise of radiation dose was estimateddue to the radioactive release to the air caused by the accident in the
Fukushima Daiichi Nuclear Power Station.)
Tuesday, March 15, 201100:12
Because greater radiation dose than 5μGy/h was detected at MP(No.3), it was judged that a particular event (rise of radiation dose atsite boundary) occurred as defined by Article 10, Clause 1 of Special
Law for Nuclear Emergency. (the rise of radiation dose was estimateddue to the radioactive release to the air caused by the accident inFukushima Daiichi Nuclear Power Wtation.)
07:15
Because the temperature of S/C dropped under 100°C, it was judgedthat it recovered from the particular event (loss of the function tocontrol pressure) defined by Article 15, Clause 1 of Special Law for
Nuclear Emergency.
07:15
The water temperature of the reactor dropped under 100°C, and thereactor was in cold shutdown.
16:35
Cooling water of FPC heat exchanger was switched (from RCW to
RHRC).
Wednesday, March 16, 2011
17:00
SGF water temperature was confirmed as around 35°C (recovered to
the water temperature before the earthquake occurred).

App.-1-28

Summary of the events on Onagawa Nuclear Power Station(1) Influence of the earthquake and tsunami
All of Onagawa's reactors from 1 to 3 automatically shut down due to theearthquake off the Pacific coast of Tohoku. Some damage was found in part of the
facilities due to the influence of the quake and tsunami, but there was no influence
to the cooling function of the reactors and spent fuel pools because the external
power and the emergency power supply were secured and the multiplicity andredundancy of the cooling facilities effectively worked, and all of the reactors
quickly halted into a cold shutdown state.
The series of events on Onagawa are shown in Figure Appendix 1-2 with theevent tree.

App.-1-29

Events
occurred
Earthquake andTsunamiReactorshutdown
Reactorscram

Electric power
DC power
AC source
Ensuring long-term cold shutdown
Core coolingsourcerecoveryRHR
RHR
PCV vent
Reactor
Emergency IC, turbineDC power External
RPV
External
recovery (before
DG
driven coolant power source, depressuri coolant
sourcepower
core
injection
injection
sourcezationemergency
damage)system(Including (including
DG, power
RCIC,HPCI) interchange alternative alternative
for which AC
sources) sources)
source is not

Status of reactor core

Degraded core coolin

Cold
shutdown,core damage,PCV damage
etc.

Depressu Reactor
rization of coolant
injection
nuclearreactor

Reactor
Final statusbuilding control
Cold shutdown, Core damage, PCV damage etc.
PCV vent Hydrogen SGTS,
ventilation
control(Nitrogenreplacement etc.)

Reactor container control
PCV
coolant
injection

Cold shutdown
Cold shutdown
Cold shutdown (No RHR)
RCIC

PCV damage

Cold shutdown
Cold shutdown (No RHR)
PCV integrity and PCV leakage (excessive
temperature)

PCV damage

Unit 2, 3

Cold shutdown (No power source)(Continuation)

PCV leakage (excessive temperature) and hydrogen
combustion in the building
PCV damage (hydrogen explosion)

PCV damage (over-pressurization)[Tsunami](Continuation)

PCV damage (over-pressurization)

Core damage

PCV damage (MCCI etc.)

Core damage

PCV damage (DCH etc.)(Small capacity for delay)

Cold shutdown
Cold shutdown
Cold shutdown (No RHR)
RCIC

PCV damage

Unit 1[Starting transformer shutdown](成功)[Tsunami][Earth quake][Tsunami]

Diagram Appendix 1-2

Onagawa Nuclear Power plant disaster identification events treeApp.-1-30

a. Impacts of earthquake
The seismic intensity observed on site of power station during the Tohoku - Pacific
Ocean Earthquake was slightly lower than Level 6. Even on a seismographarranged for safety purpose on the 2nd basement level in the Unit 1 reactor
building, 567.5gal was observed.
The maximum acceleration observed on each level of Onagawa Units 1~3 reactor
buildings was almost equivalent, while some levels indicated the maximum
acceleration that exceeded the maximum response acceleration for basic design
earthquake ground motion Ss based on the Examination Guide for Seismic Designof Nuclear Power Reactor Facilities (as revised in September 2006).
With regard to the response spectrum for site ground recorded in the instrumental
earthquake data, every location indicated almost equivalent response spectrum,
while some values exceeded the response spectrum for basic design earthquake
ground motion Ss (reflecting the impact of ground status above the seismograph).
As a result of earthquake response analysis based on the current instrumental
earthquake data and the evaluations of deformation of earthquake resisting wall in
Onagawa Units 1~3 reactor buildings as well as the shear force applied on theearthquake resisting wall of each level, maintenance of the functions of reactor
buildings was confirmed.
5 circuits supply power to Onagawa Power Station as external power sources(No.1 and No. 2 lines of Ojika Main Line [275 kV system], No.1 and No. 2 lines of
Matsushima Main Line [275 kV system], Tsukahama Branch Line [66 kV system]).
While only No. 2 line of Matsushima Main Line was available immediately after the
Earthquake due to actuation of system protection circuit accompanying theaccident of power transmission line in the jurisdiction area of TEPCO, No.1 line of
Ojika Main Line was recovered at 20:12 of March 12, and then, No. 2 line of Ojika
Main Line at 20:15 on the same day. This was followed by No.1 line of Matsushima
Main Line at 10:47 of March 17 and Tsukahama Branch Line on 15:41 of March
26.
b. Impacts of tsunami
The maximum height of tsunami confirmed on a current meter immediately after
the Earthquake was O.P. + approx. 13m* (at 15:29 of March 11) at maximum. This
height did not exceed the altitude of Onagawa site (O.P.+ approx. 13.8 m*)(Before
the Earthquake: O.P.+ approx. 14.8m).
While a trace of sea water intrusion was observed due to runup in a part of plantsite along the coastline, the waves did not reach the main buildings.
*： This value reflects the diastrophism (Approx. -1m：based on the value of top
line report) in vininity of Onagawa announced by Geospatial Information
Authority of Japan after the Earthquake.
(2) Unit 1
a. Plant behavior after the Earthquake
Upon the Earthquake occurred at 14:46 of March 11 during the constant rated
thermal power operation, the seismic acceleration high signal was transmitted andcontrol rods were fully inserted, which resulted in the automatic shutdown of thereactor.
The starting transformers were shut down due to short circuit / earth fault inside
non-essential metal clad switchgear 6 – 1A that occurred in the Earthquake, which
led to temporary loss of non-essential on-site power. While this caused aApp.-1-31

shutdown of entire pump units in the Condensate and Feed Water System, prompt
actuation of RCIC enabled water feed to the reactor.
Because the on-site non-essential power was lost temporarily through the startingtransformer shutdown, control of the reactor pressure was achieved via SRVs
instead of condensers by fully closing MSIVs. After reactor depressurization via
SRVs, RCIC was shut down and water was fed to the reactor by using the control
rod drive hydraulic control system (hereafter “CRD”).
Cooling of the reactor was carried out without problem by RHR and the cold
shutdown was achieved at 0:58 of March 12.
b. Status of “Shutdown” function
Under the impact of the Earthquake, seismic acceleration high signal(“O.P.15.00m vertical direction seismic sensor motion trip” ：Set point 100gal) wastransmitted and all control rods were fully inserted (14:46, March 11, 2011). Thenthe reactors were shutdown automatically as designed and subcriticality wasconfirmed at 15:05 on the same day.
c. Status of “Cooling” function
The starting transformers were shut down due to short circuit / earth fault inside
non-essential metal clad switchgear 6 – 1A that occurred in the Earthquake, which
led to temporary loss of non-essential on-site power. While this caused a
shutdown of entire pump units in the Condensate and Feed Water System, prompt
actuation of RCIC enabled water feed to the reactor.
Because the on-site non-essential power was lost temporarily through the startingtransformer shutdown, control of the reactor pressure was achieved via SRVs
instead of condensers by fully closing MSIVs. After reactor depressurization via
SRVs, RCIC was shut down and water was fed to the reactor by using the CRD.
Cooling of the reactor was carried out without problem by RHR and the cold
shutdown was achieved at 0:58 of March 12.
d. Status of “Confinement” function
PCIS motions accompanying the reactor vessel low water level (L – 3) signal werenormal and no abnormal value was observed on stack monitor, radioactive waste
discharge water monitor and MP. There was no radioactive impact on the external
environments.
e. Status of spent fuel storage pool
While the Earthquake caused FPC shutdown at 14:47 of March 11, it was
restarted at 19:30 of the same day upon confirming that there was no abnormality
in the equipment. During the shutdown, no significant elevation was seen on thetemperature of spent fuel storage pool. The cause of FPC shutdown seems to be
the actuation of level switch for “skimmer surge tank level low low” due to theseismic vibration, or, the drop of FPC pump suction pressure through temporaldrop of spent fuel storage pool level due to the seismic vibration.
f. Status of “Power Source” function
With regard to the power source, the station was receiving the external power fromNo. 2 line of Matsushima Main Line via starting transformers. However, at 14:55
on March 11, on-site power was lost due to shutdown of the starting transformers.
Accordingly, power was supplied to emergency bus by DG (A) and DG (B) based
App.-1-32

on the design.
The cause of the starting transformer shutdown was the earth fault / short circuit
occurred inside the non-essential metal clad switchgear 6 – 1A (this resulted in fire
later), which actuated the overcurrent relay of the starting transformers.
Thereafter, integrity of the starting transformers was confirmed based on theappearance inspection and the measurement of the insulation resistance and thestarting transformers were restored at 2:05 on March 12. After the restoration of
the starting transformers, power supply of non-essential bus was recovered on by
one excluding the non-essential metal clad switchgear 6 – 1A.
The emergency power supply needed to cool down the reactor and the spent fuelstorage pool until the restoration of the starting transformers was secured.

App.-1-33

Table Appendix 1-5: Chronological report for Onagawa Unit 1 after the Earthquake
Before the Earthquake：Constant rated thermal power operation
March 11, 2011 (Friday)
14:46
East Japan Great Earthquake occurred(Seismic intensity observed on site: Slightly lower than Level 6)
Vertical direction seismic acceleration high; Reactor automatic shutdown
14:47
Full insertion of all control rods was confirmed
DGs (A), (B) automatic startup
FPC pump (A) automatic shutdown
14:55
DGs (A), (B) load operation started
14:59
RCIC manual startup
15:00
RHR pump (A) manual startup (For S/P cooling operation)
15:01
RHR pump (C) manual startup (For S/P cooling operation)
15:05
Reactor subcriticality was confirmed
15:05
RHR pump (B) manual startup (For S/P cooling operation)
15:12
RHR pump (D) manual startup (For S/P cooling operation)
15:55
RHR pump (A), (C) automatic shutdown
16:15
RHR pump (A) manual restart (For S/P cooling operation)
Approx.17:10 Reactor depressurization started (SRV was used)
18:29
RCIC turbine automatic shutdown (via L-8)
Approx.19:30 FPC pump (A) manual startup (Fuel pool cooling)
20:20
CRD pump (A) manual startup (Water feed to reactor)
21:56
RHR pump (A) manual shutdown (SHC preparation [for flushing])
23:46
RHR pump (A) manual startup (SHC mode)
March 12, 2011 (Saturday)
0:57
Reactor coolant temperature reached 100 C
0:58
Reactor status “Cold shutdown”
2:05
After the power supply (restoration) of starting transformers, non-essential
buses were recovered excluding M/C6-1A which was damaged by fire.

App.-1-34

(3) Unit 2
a. Plant behavior after the Earthquake
The reactor was in start-up operation from 14:00 on March 11 for the 11th
periodical inspection. After the Earthquake occurred at 14:46 on the same day,seismic acceleration high signal was transmitted. Then, all control rods were fully
inserted immediately and the reactor was shut down.
Because the Unit 2 was in the status immediately after starting reactor operationand the status immediately before the Earthquake was of subcriticality with reactor
water temperature lower than 100 C, cold shutdown was implemented at 14:49
on March 11 by operating the reactor mode switch “Shutdown.”b. Status of “Shutdown” function
The reactor was in start-up operation from 14:00 on March 11 for the 11th
periodical inspection. After the Earthquake occurred at 14:46 on the same day,seismic acceleration high signal (“R/B bottom horizontal direction seismic
acceleration high trip”：set point 200gal) was transmitted. Then, all control rods
were fully inserted immediately and the reactor was automatically shut down as
designed. Status of subcriticality was maintained.
c. Status of “Cooling” function
Because the Unit 2 was in the status immediately after starting reactor operation
for the 11th periodical inspection and the status immediately before theEarthquake was of subcriticality with reactor water temperature lower than 100 C,
cold shutdown was implemented at 14:49 on March 11 by operating the reactor
mode switch “Shutdown.”
Under the impact of Tsunami accompanying the Earthquake, the seawater
intruded from the cooling water intake canal of the sea water pump area entered apart of reactor building via underground trench, which resulted in the function lossof 2 systems (RCW-B and HPCW). However, because RCWA system was sound,
there was no negative impact on the cooling function of reactors by RHR.
d. Confinement
No abnormal value was observed on stack monitor, radioactive waste discharge
water monitor and MP. There was no radioactive impact on the external
environments.
e. Status of spent fuel storage pool
While the Earthquake caused FPC shutdown at 14:47 of March 11, it was
restarted at 20:29 of the same day upon confirming that there was no abnormality
in the equipment. During the shutdown, no significant elevation was seen on thetemperature of spent fuel storage pool.
The cause of FPC shutdown seems to be the actuation of level switch for“skimmer surge tank level low low” due to the seismic vibration, or, the drop of
FPC pump suction pressure through temporal drop of spent fuel storage pool level
due to the seismic vibration.
Under the impact of Tsunami accompanying the Earthquake, the seawater
intruded from the cooling water intake canal of the sea water pump area entered apart of reactor building via underground trench, which resulted in the function lossof 2 systems (RCW-B and HPCW). However, because RCWA system was sound,
there was no negative impact on the cooling function of spent fuel storage pool byApp.-1-35

FPC similar to the case with reactor cooling function by RHR.
f. Status of “Power Source” function
With regard to the power source, the station was receiving the external power fromNo. 2 line of Matsushima Main Line via starting transformers and on-site powerhas been secured constantly.
Under the impact of vibration caused by the Earthquake, generator loss of fieldsignal was transmitted, which resulted in the automatic start-up of DGs (A) / (B)and the high pressure core spay system DG (hereafter “DG(H)”) followed bystand-by status with no load operation. Thereafter, the seawater intruded from thecooling water intake canal of the sea water pump area entered a part of reactor
building via underground trench, which resulted in the function loss of 2 systems(RCW-B and HPCW). While this event triggered the automatic shutdown of DG(B)and DG(H), DG(A) was sound and, even if the external power was lost, theemergency power supply needed to cool down the reactor and the spent fuelstorage pool could be secured.
g. Loss of function in RCW-B, RSW-B and HPCW systems
Under the impact of vibration caused by the Earthquake, generator loss of fieldsignal was transmitted, which resulted in the automatic start-up of DGs (A) / (B) /(H) followed by no load operation. Then, at 15:34, RCW pump (B) shut down
automatically and the RCW pump (D) that was started up as a back-up
immediately also shut down automatically. This led to the loss of cooling watersupply for emergency DG (B) and it was shut down automatically at 15:35. Further
at 15:41, HPCW pump shut down automatically, which resulted in the loss ofcooling water supply for emergency DG (H) and it was shut down automatically at
15:42.
Inspection of the site revealed that the sea water intruded into the RCW heatexchanger (B) area, HPCW heat exchanger area and the staircase that accessed
the elevator in the uncontrolled area on the 3rd basement level of the reactor
building (submergence depth: approx. 2.5m), and that the RCW pumps (B) / (D)and HPCW pump were submerged and the sea water intruded also into the RCWheat exchanger (A) area (submergence depth: approx. 0.5m). Therefore, the task
to drain sea water in the submerged area outside the building was started at 20:25
by using temporary pumps. The draining process was completed at 10:30 on
March 16.
Further, the patrol after the Earthquake revealed that the sea water system(hereafter “RSW”) pump (B) area in the outdoor sea water pump area wassubmerged and the RSW pumps (B) / (D) installed in this area might have been
submerged, too.
We inspected the motors of RCW pumps (B) / (D), RSW pumps (B) / (D) and
HPCW pump affected by sea water intrusion (or suspected sea water intrusion) as
well as the submerged actuator of the motor operated valves in the factory. As aresult of an overhaul inspection, rusts were observed on all the components, whileinsulation resistance value of some components satisfied the criteria. Accordingly,
these pumps were judged as incapable to satisfy the functions necessary tosecure safety of the reactor facility.
This event belongs to the type of events for which reporting is mandatory based on
the Rules for the Installation, Operation, etc. of Commercial Power Reactors,
Article 19-17, Item 1, and No.3.
To protect the area against back wave of tsunami, level instrumentation for CWP
App.-1-36

automatic shutdown (hereafter “this level instrumentation”) was installed
additionally in the RSW pump (B) section of the sea water pump area (in 2002).
However, consideration in selecting the location for installation as well as water
shielding measure were insufficient to protect the area from the impacts of the
waves that push forward.
Therefore, the following process was estimated: Under the impact of Tsunami
accompanying the Earthquake, the seawater intruded from the cooling water
intake canal entered the sea water pump area via this level instrumentation box,
causing submergence of RSW pump (B) area and water intrusion in a part ofreactor building via underground trench. This event resulted in the function loss of
RCW-B system, RSW-B system and HPCW system.
While this type of level instrumentation is installed also in the Unit 1 and the Unit 3,
there was no damage on the RCW system etc. because they are installed in the
different area (dust extractor area).
Although there was a trace of sea water intrusion also into the dust extractor area,
the cover of this level instrumentation was not detached. Therefore, we estimatethat the sea water must have entered from the opening of the dust extractor
installed in front of this level instrumentation.
Following countermeasures were implemented to prevent recurrence of the loss offunction in RCW (B), RSW (B) and HPCW systems:
 This level instrumentation was detached and water shielding work was carriedout to its opening.
The location to install this level instrumentation is to be changed to an area lesssusceptible to sea water intrusion.
 Repair work was conducted on the section from sea water pump area to thepipe penetration to trench and the cable tray penetration.
 As mid-term / long-term countermeasures, we will implement the improvementof water sealing performance of the building doors and the installation of tideembankment and tide wall as a part of countermeasures to prevent
submergence by tsunami. These measures are implemented within theframework of emergency safety countermeasures for Onagawa Nuclear Power
Station complied based on the instructions of Minister of Economy, Trade andIndustry dated March 30, 2011, “On the Implementation of Emergency Safety
Measures at Other Power Plants drawn from the 2011 Accident at Fukushima
Dai-ichi and Dai-ni Nuclear Power Stations (Minister’s Instructions, Releasedon March 30th)”
With regard to the submerged pump motors and motor operated valve actuators,
repair work (cleaning and maintenance etc.) was conducted based on the result ofoverhaul inspection.

App.-1-37

Table Appendix 1-6: Chronological report for Onagawa Unit 2 after the Earthquake
Before the Earthquake： “start-up” immediately before the Earthquake for the 11th
periodical inspection
March 11, 2011 (Friday)
14:00
Reactor mode switch “Refueling” → “Start-up” (Reactor status“Start-up” )Control rod withdrawal started
14:46
East Japan Great Earthquake occurred(Seismic intensity observed on site: Slightly lower than Level 6)
R/B bottom horizontal direction seismic acceleration high; Reactor
automatic shutdown
14:47
Full insertion of all control rods was confirmed
Emergency DGs (A), (B) and (H) automatic startup* Based on generator loss of field signal
FPC pump (B) automatic shutdown
14:49
Reactor mode switch“Start-up”  “Shutdown”(Reactor status “Cold shutdown”)
15:34
RCW pumps (B) and (D) automatic shutdown (due to pump
submergence)
15:35
Emergency DG (B) automatic shutdown (due to RCWs (B) and (D)shutdown)
15:41
HPCW pump automatic shutdown (due to pump submergence)
15:42
Emergency DG (H) automatic shutdown (due to HPCW shutdown)
20:29
FPC pump (A) manual startup (Fuel pool cooling)
March 12, 2011 (Saturday)
12:12
RHR pump (A) manual startup (SHC mode)App.-1-38

Table Appendix 1-7: Chronological report on RCW-B system, RSW-B system and
HPCW for Onagawa Unit 2
March 11, 2011 (Friday)
14:00
Reactor start-up
14:46
Earthquake occurred (Seismic intensity observed on site: Slightly lower
than Level 6)
Reactor automatic shutdown
Emergency DGs (A), (B), (H) automatic startup (No-load operation)
14:49
Issue of great tsunami alarm
Approx.15:21 The 1st wave of tsunami (An operator confirmed the time visually)
15:34
RCW pump (B) automatic shutdown
RCW pump (D) automatic shutdown (Shutdown immediately after started
up as back-up)
15:35
Emergency DG (B) shut down automatically by “RCW differential
pressure low” signal
15:41
HPCW pump automatic shutdown
15:42
Emergency DG (H) shut down automatically by “HPCW differential
pressure low” signal
Approx.16:00 The operator who inspected the reactor building confirmed intrusion ofwater in the RCW heat exchanger (B) area on the lowest basement level,
stairs (2 points) on the 3rd basement level to access the HPCW heatexchanger area and in the RCW heat exchanger (A) area (uncontrolledarea).
16:01
RSW pump (B) manual shutdown (due to submergence of RCW B
system)
16:06
HPSW pump manual shutdown (due to submergence of HPCW system)
Approx.20:12 As a result of analysis of the intruded water, no radioactivity was detected
and the water was found as the sea water.
20:25
Temporary pumps were installed and work to drain the sea water flown
into the 3rd basement level of the reactor building (uncontrolled area) tooutside the building was started.
March 16, 2011
10:30
Draining of the sea water from the building was completed.

App.-1-39

(4) Unit 3
a. Plant behavior after the Earthquake
Upon the Earthquake occurred at 14:46 of March 11 during the constant rated
thermal power operation, the seismic acceleration high signal was transmitted andcontrol rods were fully inserted, which resulted in the automatic shutdown of thereactor.
Under the impact of Tsunami, a “sea water pump area level low low” signal wastransmitted, which resulted in the shutdown of CWP. At the same time,
submergence under the sea water intruded in the sea water pump area of the heatexchanger building caused the shutdown of Turbine Building Closed Cooling Sea
Water System (hereafter “TSW”) pumps. Due to the loss of cooling water supply
caused by this event, all of reactor feed water pumps were manually shut down.
Then, RCIC was started up to feed water to the reactor, and reactor pressure
control was carried out by SRVs upon fully closing MSIVs, because thecondensation of main steam by using condenser failed.
After the shutdown of RCIC accompanying the reactor depressurization, water
was supplied to the reactor by CRD. Thereafter, cooling of the reactor was
conducted by RHR without problem and a cold shutdown of the unit was achieved
at 1:17 on March 12.
b. Status of “Shutdown” function
Upon the Earthquake occurred at 14:46 of March 11 during the constant rated
thermal power operation, the seismic acceleration high signal (“R/B bottom verticaldirection seismic acceleration hightrip”：Setpoint 100gal) was transmitted andcontrol rods were fully inserted, which resulted in the automatic shutdown of thereactor as designed. Status of subcriticality was confirmed at 14: 47 on the sameday.
c. Status of “Cooling” function
Under the impact of back wave of Tsunami, a “sea water pump area level low low”
signal was transmitted, which resulted in the shutdown of CWP as designed. At
the same time, submergence under the sea water intruded in the sea water pumparea of the heat exchanger building caused the shutdown of TSW pumps. Due tothe loss of cooling water supply caused by this event, all of reactor feed water
pumps were manually shut down. Then, RCIC was started up to feed water to thereactor, and reactor pressure control was carried out by SRVs upon fully closing
MSIVs, because the condensation of main steam by using condenser failed.
To feed water to the reactor after the shutdown of RCIC accompanying the reactor
depressurization, multiple options could be considered; i.e. the ECCS by using
suppression pool water as water source, the MUWC by using condensate storagetank as water source and so on. We selected MUWC using condensate storagetank as water source to feed water to the reactor with giving considerations to thedecrease of decay heat through plant shutdown and the maintenance of reactor
water quality.
Cooling of the reactor was carried out without problem by RHR and the cold
shutdown was achieved at 1:17 of March 12.
d. Status of “Confinement” function
No abnormal value was observed on stack monitor, radioactive waste discharge
water monitor and MP. There was no radioactive impact on the external
App.-1-40

environments.
e. Status of spent fuel storage pool
While the Earthquake caused FPC shutdown at 14:47 of March 11, it was
restarted at 15:23 of the same day upon confirming that there was no abnormality
in the equipment. During the shutdown, no significant elevation was seen on thetemperature of spent fuel storage pool.
The cause of FPC shutdown seems to be the actuation of level switch for“skimmer surge tank level low low” due to the seismic vibration, or, the drop of
FPC pump suction pressure through temporal drop of spent fuel storage pool level
due to the seismic vibration.
f. Status of “Power Source” function
With regard to the power source, the station was receiving the external power fromNo. 2 line of Matsushima Main Line via starting transformer, and on-site power has
been secured constantly. Further, all of the emergency DGs were sound and, even
if the external power was lost, the emergency power supply needed to cool down
the reactor and the spent fuel storage pool could be secured.

App.-1-41

Table Appendix 1-8: Chronological report for Onagawa Unit 3 after the Earthquake
Before the Earthquake：constant rated thermal power operation
March 11, 2011 (Friday)
14:46
East Japan Great Earthquake occurred(Seismic intensity observed on site: Slightly lower than Level 6)
R/B bottom vertical direction seismic acceleration high; Reactor automatic
shutdown
14:47
Full insertion of all control rods was confirmed
14:57
Reactor subcriticality was confirmed
15:26
RCIC manual startup (Water feed to reactor)
15:28
RSW pump (D) manual startup (S/P cooling operation)
15:30
RCW pump (B) manual startup (S/P cooling operation)
15:30
RHR (B) manual startup (S/P cooling operation)
15:43
RSW pump (C) manual startup (S/P cooling operation)
15:44
RHR (A) manual startup (S/P cooling operation)
15:45
RCW pump (A) manual startup (S/P cooling operation)
16:40
Reactor depressurization started (by using SRV)RCIC turbine shutdown(by L-8)
16:57
RCIC manual startup (Water feed to reactor)
21:44
RHR pump (A) manual shutdown (SHC preparation)
21:45
RCIC turbine manual shutdown
21:54
Water feeding by MUWC (Water feed to reactor)
23:51
RHR pump (A) manual startup (SHC mode)
March 12, 2011 (Saturday)
1:17
Reactor coolant temperature lower than 100 C (Reactor status “Coldshutdown”)

App.-1-42

Summary of Events in Tokai Daini Nuclear Power Station(1) Impacts of earthquake and tsunami
The Earthquake attacked the Tokai Daini Nuclear Power Station during its
constant rated thermal power operation. At 14:48 on March 11, the reactor shut
down automatically due to turbine trip caused by large vibration of turbine bearing
under the impact of the Earthquake. Immediately after the Earthquake, all threecircuits for external power were lost. However, the power source for emergency
components was secured because of the start-up of 3 emergency DG units.
Immediately after the automatic shutdown of the reactor, its water level was kept
normal by RCIC and HPCS and the reactor pressure was controlled by SRV.
Decay heat after the reactor shutdown was removed through cooling of RHR
suppression pool.
Thereafter, under the impact of tsunami, sea water pumps for cooling Emergency
DG2C were automatically shut down, which resulted in unavailability ofEmergency DG2C. Nevertheless, emergency power could be secured byoperation of the remaining 2 emergency DGs without causing any disturbance tothe cooling function for the reactor and SFP. At 0:40 on March 15, the cold
shutdown of the reactor was achieved.
The development of events in Tokai Daini Nuclear Power Station is illustrated as
an event tree in Figure Appendix 1-3.

App.-1-43

Events
occurred
Earth quake
andTsunamiReactorshutdown

Directpower
sourceDC power
sourceReactorscram

AC source
External
powersourceEmergency
DG

Electric
power
recoveryHigh pressure External
RPVcore coolingpower
depressuriz
systemsource,ation(RCIC,HPCS) emergency (Including
DG, power alternativeinterchange sources)
Core cooling

Status of reactor
core

Ensuring long-term cold shutdown
Reactorcoolant
injection(including
alternative
sources)

RHR

RHR
recoveryPCV vent
Cold shutdown,(before core core damage, PCV
damage)damage etc.

(Successful)
External power source

Cold shutdown
Cold shutdown(NoRHR)
PCV damage(over-(Integrity)
RCIC/HPCS(Successful)

Cold shutdown[2/3 integrity]
DG-2D,HPCS[Earth quake]

Figure Appendix 1-3 Fukushima Daini Nuclear power plant disaster identification events treeApp.-1-44

Impacts of earthquake
Seismic intensity observed on site (Tokai in Tokai Village) was slightly lower thanLevel 6, and the maximum accelerations recorded on a seismograph arranged on
the 2nd basement level of the reactor building was 214gal in the North-South
direction, 225 gal in the East-West direction and 189 gal in the vertical direction.
It was confirmed that the maximum acceleration observed on each level of thereactor building was maintained below the maximum acceleration for the basic
design earthquake ground motion Ss based on the Examination Guide for Seismic
Design of Nuclear Power Reactor Facilities (as revised in September 2006). With
regard to the response spectrum for the instrumental earthquake data, while somerecords indicated that they locally exceeded the response spectrum for the basic
design earthquake ground motion Ss, the analysis result revealed that the data
maintained the level below the response spectrum for the basic design earthquake
ground motion Ss in most frequency bands including those where natural
frequencies for the important components in terms of seismic design centered
upon.
Immediately after the Earthquake, 3 external power circuits (275kV systems and
154kV system) were lost. Thereafter, 154kV system was restored on March 13
and one circuit of 275kV systems was restored on March 17. Another 275kV
system was recovered on April 27. With this, all circuits of the external power wererecovered.
Impacts of tsunami
Status of flood caused by tsunami during the Earthquake was inspected at the
intake area. As a result, it was confirmed that, at around 15:35, the intake area
located at altitude* +3.3m (approx.) (altitude before the Earthquake) suffered
flooding with approx. 1m depth (depth of submergence is estimated based on theimages from monitor cameras in every case).
Further, it was confirmed that the submergence of approx. 2m occurred in theintake area also at around 16:51. This was followed by submergence of several
times (depth <1m) in the intake area.
Based on the results of on-site investigation including the investigation of flood
traces, the height of tsunami in the Tokai Daini Nuclear Power Station is estimatedas altitude* +5.3m (approx.) (altitude after the diastrophism investigation).
Because this height did not exceed the height of Tokai Daini Nuclear Power
Station site (altitude* +8.0m (altitude before the Earthquake)), the tsunami didn’t
reach the main buildings of the power station.
The tsunami reached the point around the sea water pump area in the plant site
along the coastline (altitude* +3.3m (altitude before the Earthquake)). While there
was no water intrusion from above the North-South side walls of the sea water
pump area because they had altitude * +6.1m height (altitude before theEarthquake) as a countermeasure against tsunami, some water intrusion from
penetration hole was observed in the north pump area, since the area was underconstruction for sealing the side wall penetrations.
*：This altitude is based on Tokyo Peil (T.P.) (i.e. It does not reflect the groundsubsidence caused by diastrophism).
(2) Status of “Shutdown” function
Under the impact of the Earthquake, vibration of turbine bearing increased and“turbine bearing vibration high” signal was transmitted, which resulted in a turbine

App.-1-45

trip and, as a consequence, an automatic shutdown of the reactor.
All control rods were fully inserted normally (at 14:48 on March 11) and the reactor
core was maintained in subcriticality status.
(3) Status of “Power Source” function
While 3 external power circuits were lost upon occurrence of the Earthquake, 3
emergency DG units (2C, 2D and HPCS) started up automatically and the power
supply to the emergency power bus was started.
Emergency DG sea water pump (2C) shut down due to intrusion of sea water into
the north pump tank under the impact of tsunami. This event caused an
unavailability of Emergency DG2C, which led to the power loss of the Emergency
AC Power Bus 2C.
Because the power on the Emergency AC Power Bus 2D and HPCS Bus could beensured continuously, power supply to emergency components could bemaintained.
With regard to emergency DC power, power charges to the batteries as well as a
power supply to the DC power load were implemented from the emergency powerbus. While the Emergency AC Power Bus 2C was in power loss, the DC powersource connected to this bus was switched to the power supply from the soundemergency AD power bus and the power charge to the batteries was conductedwithout problem. Therefore, the DC power was not lost from site.
(4) Status of “Cooling” function
Due to the water level fluctuation immediately after the reactor shutdown, an
automatic start-up of HPCS and RCIC as a part of ECCS occurred. This secured
the function to inject coolant to the reactor in high pressure status, which
contributed to the maintenance of normal water level of the reactor. Thereafter, themaintenance of the reactor water level was achieved by RCIC (at first CST, andthen, suppression pool was used as water source) and the pressure control for thereactor was implemented by SRVs.
To remove decay heat after the reactor scram, RHR was started up manually andthe cooling of suppression pool was started.
Excluding the Emergency DG sea water pump (2C) installed in the north pumptank, no negative impact on the function was observed on the RHR sea water
pumps (A) / (C) and the sea water pumps (A) / (C), although they were submerged
up to the bottom level of the generators.
(5) Status of “Confinement” function
Water level fluctuation (level drop) immediately after the reactor scram caused
normal actuation of the containment isolation system, leading to isolation ofreactor containment.
Similarly, water level fluctuation (level drop) immediately after the reactor scramcaused automatic isolation of the reactor building and the normal swathing ofreactor building ventilation system from normal ventilation system to the Standby
Gas Treatment System (SGTS).
(6) Status of spent fuel pool cooling
Sloshing by the Earthquake actuated an alarm for the spent fuel pool level (“FUELPOOL LEVEL HI/LO”). At the same time, the water level decreased by approx.
20cm from the normal level, because the flooding from the pool occurred.
App.-1-46

To address this event, water was filled in the spent fuel storage pool by using CST
water.
While the water level dropped, spent fuels stored in the spent fuel storage pool
were sufficiently covered by water continuously (fuel top + approx. 7m).
While the Fuel Pool Cooling and Clean-up System was shut down due to loss ofexternal power, cooling was resumed by power supply from the sound Emergency
DG2D.
(7) Cause of sea water intrusion into north pump tank under the impact oftsunami and countermeasures
With regard to the emergency sea water pump tanks, separation walls ofaltitude*+6.1m height (altitude before the Earthquake) had been installed toprotect the area against tsunami of altitude* + 5.7m height (approx.) (altitude
before the Earthquake). At the time point of March 11, a construction work forwater sealing for pump tanks was going on.
The height of the tsunami observed in the current disaster was altitude* + approx.
4.8～5.3m (altitude after the diastrophism investigation). While this height did notexceed the height of the separation walls of altitude* +6.1m (altitude before theEarthquake), sea water intruded in the north pump tank from following 2 points
under water sealing work.
The water sealing work on the south pump tank had been completed then.
① Opening between the north pump tank and ASW strainer area (discharge
channel)② Unsealed structure of the cable pit
Therefore, blockage was implemented to these 2 points by placing of concrete.
*：This altitude is based on Tokyo Peil (T.P.).

App.-1-47

Table Appendix 1-9: Chronological report for Tokai Daini Nuclear Power Station
after the Earthquake
Before the Earthquake：Constant rated thermal power operation
March 11, 2011 (Friday)
14:46
The Earthquake occurred (Seismic intensity observed on site: Slightly lower
than Level 6)
14:48
The reactor shut down automatically with turbine stop valve closure due toautomatic shutdown of turbines caused by large vibration of turbine bearing.
14:48
Full insertion of all control rods was confirmed
14:48
PCIS actuation
14:48
HPCS automatic startup
14:49
RCIC automatic startup
14:52
HPCS injection valve automatic closure and RCIC
automatic shutdown (by Reactor Water Level High (L-8))
15:01
Suppression pool cooling operation started by Residual Heat Removal A
System (Manual)
15:10
Reactor subcriticality was confirmed
15:36
RCIC manual startup (Water feed to reactor)
16:40
Suppression pool cooling operation started by Residual Heat Removal B
System (Manual)
19:01
Emergency DG sea water pump 2C automatic shutdown
19:21
Suppression pool cooling operation by Residual Heat Removal A System shut
down (Manual)
19:22
Emergency DG2C manual shutdown
21:52
Reactor depressurization started (by using SRV intermittently)
March 12, 2011 (Saturday)
11:37
Control of reactor water level was switched from RCIC to HPCS
13:11
RCIC manual shutdown (as a result of reactor pressure drop)
March 13, 2011 (Sunday)
19:41
Power supply from external back-up power (154 kV) to the on-site powersources one after another
March 14, 2011 (Monday)
23:43
Shutdown cooling mode operation started by Residual Heat Removal A System
March 15, 2011 (Tuesday)
0:40
Reactor coolant temperature lower than 100 C (Reactor status “Coldshutdown”)
March 17, 2011 (Thursday)
15:47
Tokai Nuclear Power Station Line No.1 (275 kV) was charged
March 22, 2011 (Tuesday)
22:10
Emergency DG2C stand-by (Inspection of Sea Water Pump 2C completed)
April 27, 2011 (Wednesday)
16:29
Tokai Nuclear Power Station Line No.2 (275 kV) was charged

App.-1-48

Appendix-2

Agenda for Future Review

In this report, we summarized the results of investigation into the initial phase of theaccident in Fukushima Daiichi Nuclear Power Station, the analysis of the accident
causes and the countermeasures considered effective based on the lessons welearned from the accident.
However, because there are many aspects that require further collection / analysis
of information regarding the international approaches for improving safety, further
approaches to control the impacts of this accident, and the consultations withnational and local governments, we still have a series of agenda that are notincluded in the scope of the current review. Therefore, we would like to identify theitems requiring examination in future by nuclear industry community for further
improvement of safety in nuclear power stations.
In the current review, we did not cover the analysis of root cause of the accident
including the organization factors, because it was impossible for TEPCO staffs incharge of each area to share their time for interview that may last for long time due totheir intensive efforts and time pressure they are exposed to for controlling this
accident. If additional information that are not accessible to us at this moment
become unveiled in future, we want to proceed with our examination by integrating
the new information.
Agenda for future review are shown below:
 Countermeasure against the fire associated with earthquake
Industry-wide examinations have been made and various countermeasures
have been implemented to secure plant-specific safety against the fire so far.
However, we want to proceed with our examination on items, if any, that need tobe addressed based on the findings from the current earthquake.
 Countermeasure against the flood associated with earthquake
With regard to the flood, assessments of impacts on safety-related components
have been made and protection measures have been examined since theNiigata-oki Earthquake. However, we want to proceed with our examination on
items, if any, that need to be addressed based on the findings from the current
earthquake.
 Countermeasure against the facility destruction by terrorist attacks
Since 9.11 in 2001, The United States have promoted diverse commitments toprevent terrorist attacks on the nuclear power stations. It is necessary for Japanto promote examinations on how to secure functions of nuclear power stations(“shutdown,” “cooling” and “confinement”) to address the large-scale destructionof facilities by terrorist attacks.
 Reinforcement of disaster preparedness measures
In the current accident of Fukushima Daiichi Nuclear Power Station, the extent
of discharge / diffusion of radioactive materials exceeded the scope of impacts
estimated in the conventional disaster preparedness plans. Radical review ofthe disaster preparedness plans are needed with reflecting the findings of thecurrent disaster. Realization of elaborate countermeasures should be sought forfrom the standpoint of nuclear power station owners with reflecting the contents
to be reviewed.
App.-2-1

 Reconsideration of approaches for safety culture
Industry-wide serious reconsideration is needed on the cause of inability toprevent this accident including the aspects such as “Why was preventive
measure not implemented in anticipation of tsunami in the scale far beyond theestimation?” and “Why was countermeasure not taken in anticipation of longtime power loss?” The results of the reconsideration should be reflected in theapproaches to nurture the safety culture with the purpose to prevent recurrenceof the accident.
 Approaches to PSA
While industry-wide arguments have been accumulated on the seismic
probabilistic safety assessment (PSA) so far, the current accident calls an
enhanced attention to the importance of PSA-based approaches to natural
phenomena. Accelerated review and establishment of various types of PSA
evaluation methods are necessary to implement appropriate evaluation of eachnuclear power plant facility. Further, identification of the points to be reinforced
and the countermeasures to address the points are desired.
 Approaches to human resources development
It is impossible for the industry to learn lessons directly from this accident as to“what kind of strategy should be in place to develop appropriate humanresources.” From the standpoints of improving safety of nuclear power stations
and the nuclear disaster preparedness, it is necessary for the industry to
examine possible policies for development of human resources in future so as
to establish a system to enable a systematic human resource recruitment /education with wide outreach. Further, at least the educations to hand down thelessons from this accident as well as the leaning of the new countermeasuresare essential and it is imperative to establish a mechanism to keep this cycle
running on continuous basis.
 Approaches to the agenda related to multiple reactor siting
At present, the scope of impact and problems associated with specific measuresagainst the accident in multiple-reactor site are yet to be identified except thehydrogen explosion occurred in the Unit 4 caused by the hydrogen generated in
the Unit 3 and migrated to the Unit 4 via an exhaust line.
Based on the experience of explosion in the Unit 4, we plan to screen out thesimilar type of systems from the standpoint to ensure independency as one of
the lessons learned from this accident.
If further information concerning the multiple-reactor siting is obtained in theprocess of the accident investigation in future, we want to proceed with ourapproach to extract the lessons on the multiple-reactor siting and examination
on specific countermeasures.
We consider that the manpower deployment should be reviewed depending on
the necessity through the trainings based on the assumption of simultaneous
accidents in multiple units.
 Approaches to the findings related to accident handling
While many kinds of works are going on currently to terminate the accident, we
receive various lessons from each of these works. For instance, with regard to
the handling of retained water, there are various lessons in a series of its
App.-2-2

process including the generation of the polluted water, prevention of its leakage
to the environment, manageme and disposal. Approaches to the issues related
to the accident handling should be continued on mid-term / long-term basis.
 Containment filter vent
Based on the experience of this accident, necessity and importance ofcontainment ventilation are recognized to prevent damage of the containment
through pressurization. However, with regard to the containment ventilation
particularly under the severe accident condition, impact of discharged
radioactive materials on surrounding environments is a concern.
To address this issue, some overseas countries (Europe) install containment
filter vent to suppress discharge of high amount of radioactive materials to theatmosphere as one of the countermeasures for severe accidents. This practicehas yet to be introduced to Japan.
In the current accident, the discharge route of radioactive materials has notbeen identified yet. Therefore, it is unknown at present whether or not the
installation of a containment filter vent could have prevented discharge of thehigh amount of radioactive materials. Nevertheless, appropriateness of
installing a containment filter vent should be examined as a part ofcountermeasure to mitigate impacts on the surrounding environment.

App.-2-3

Appendix-3

Comparison with Government Report / NRC
Task Team Report

In conducting the current review, we, as the industry, extracted the items that need tobe addressed from the wide range of issues to prevent recurrence of the accident interms of both direct factors and indirect factors based on the clearly defined facts at
this stage without eliminating the guessing as much as possible. Further, we
manifected our continued commitment to the review by identifying the agenda
requiring further examination in future besides the lessons perceived fromdevelopment in the accident of Fukushima Daiichi Nuclear Power Station.
Meanwhile, in the report of Japanese Government to the IAEA Ministerial
Conference on Nuclear Safety held in June 2011, 28 lessons were compiled that
require commitments by operators, national government and local authorities. To
clarify the industry’s attitude to these 28 items, a comparison between these items
and the lessons compiled by the industry is indicated in Appendix-3.1.
Further, in “Recommendation for Enhancing Reactor Safety in the 21st Century”
compiled by US NRC task force as a report of investigation and review on theaccident in Fukushima Daiichi Nuclear Power Station, 12 recommendations arelisted including the requests to regulatory bodies. We also made comparison withthese recommendations and listed the comparison result in Appendix-3.2 to identify
any additional issues to be addressed.

App.-3-1

Comparison with the training of 28 items of Government Report to IAEA and the measure against this report on the first nuclear power plant
accident of appendix-3.1 Fukushima
Group nameThe
preventive
measure ofasevereaccident

Items(1) Measure tempering toanearthquake
andtsunamiContents

Additional requirements in attachment data

This earthquake was a very large-scale earthquake due to the interlocking ofmultiple epicenters. As a result, in the first nuclear power plant of Fukushima, theacceleration response spectrum of the earthquake motion observed on the reactor
building basic board exceeded with some periodic belts as against to theacceleration response spectrum of the basic earthquake ground motion of a design.
Due to the earthquake, damage was observed for external power source.
Although the major damage caused by an earthquake is not checked forimportant facility to safety or important safety related equipment. Detailed situationis still not known and further investigation about it is required.
The assumed height of the Tsunami that struck the Fukushima nuclear power
station, was exceeded to a large scale of 14~15m against the construction permit
design and valuation. As a result of this tsunami there is major damage to sea water
pumps etc., and cause of this damage is due to the non- possibility of assurance /reservation of emergency diesel power supply and assurance/ reservation forreactor cooling function. In the manual, intrusion of tsunami was not assumed but
only the measures against an undertow were defined. Thus, assumption of thegenerating frequency of tsunami or height is insufficient, and correspondence
against extensive tsunami attack is insufficient.
From the view point of design, in the nuclear power station for the aseismic
design, range of activity period of active fault is considered within 120000~130000
years（as per the old guidelines it is 50000 years. Recurrence cycle of a big
earthquake and residual risk should be considered. On the other hand, the design
to tsunami is performed based on the past tradition and clear trace of tsunami. For
attaining safety objectives, there were no efforts by considering appropriate
recurrence cycle.
Due to this, while assuming earthquake, multiple epicenters interlocking should
be considered and at the same time external power source resistance should be
improved. About tsunami, the suitable generating frequency of the tsunami inconsideration of sufficient recurrence cycle for achieving a safety objective andsufficient height are assumed from a viewpoint of preventing a severe accident. For
the tsunami having appropriate height, safety design of construction which prevents
flood influence on the site, should be executed with consideration of destructive
ability of that Tsunami. From the view point of defense in depth, risk existence
should be recognized by observing exceeded tsunami’s coverage on site as against
planned design for tsunami. Counter measures that can preserve important safetyfunctions should be taken by considering site flood or destruction ability of running
wave.

Comparison with the measures of this report
About evaluation of an earthquake and tsunami, thefollowing evaluation conditions are added based on theknowledge acquired from this data.
・ As per the present view, Ss measures for standard
earthquake are considered to be an almost satisfying
decision, in order to be assure further safety, interlocking of
two or more epicenters should be considered.
・ After the valuation of Tsunami expected as per earthquake,consideration should be done for interlocking of multiple
epicenters as wave source at plate barrier and, examine thenecessity of setting sliding quantity of epicenter fault, on alarge scale.
Related to the external power supply, on the basis of NISA
direction document [About reliability reservation of the external
power supply of a nuclear power plant and a reprocessing
facility], the measure (circuit connection, tower tempering,measure against switching station flood) for reliability
reservation is performed.
Valuation of assumed tsunami height is done and,prevention should be done against intrusion of tsunami withinsite. Moreover, following multi level counter measures shouldbe performed. Assurance of functions of safety installation
should be finalized.
Counter measures should be performed against the attack of
the tsunami inside site. Examples are as follows.
・ Care should be taken for the equipments of safety important
power source installations. While preparing for the flood bytsunami, coastal levee (storm surge barrier) or breakwater
should be installed, or from the view point of multilevelsecurity, protection barrier should be installed in theimportant system.
・ Protection to tsunami, such as a tank which may serve as
flotsam and may interfere with emergency correspondence(protective barrier etc.). Disposition of the heavy industrial
machine for flotsam removal.
Moreover, counter measures should be taken for preventing
intrusion within the building. Illustrative examples are as
follows.
・ Regarding to the Tsunami which exceeds site height, as per
the flood height, tsunami preventive measures such as
improvement of the external gate of the building or thesealing of air supply opening and penetration should beexecuted.

App.-3.1-1

Group name

Items

Contents

Additional requirements in attachment data

Comparison with the measures of this report
・ Regarding to the Tsunami which exceeds site height, for theflood prevention of safety important installations, countermeasures should be improved for water tightness of theground stage gate and the gate which is passes through themain ground stage.
・ For early re-establishment, portable water discharging
pumps should be deployed in building for discharging
entered flooded sea water.
・ Regarding to the impact of wave power of tsunami,
attention should be paid to the places where direct impactof tsunami is observed.

The major factor of this accident is that a required power supply was not assured.
This cause indicated that the, power supply diversity is not done so as to overcome
vulnerability for common cause failure due to the external events; setup of powerboard etc. was not good so as to bear severe environment. Moreover, battery life
ended till the returning of AC source, it indicates that the, objective of battery is notclear. Time needed till the restoration of external power source was not known
properly.
For this, power supply can be assured for a long time in severe situation, byinstalling multiple power air cooled diesel generator, combustion turbine etc,
planning for the diversification of power source by deploying of power source car,distributing frame having high environmental tolerance installation of battery
charging generator.

(2)
Power
assurancesourceMoreover, tsunami counter-measures should be taken against
seawater pumps.
■Written part in this report is
4.6 Preparation against earthquake・tsunami■Counter-measure Examples
Preparation against earthquake tsunami (1)~⑨
From the view point of power source assurance, external・ For external power supply reception, fromthe view of reliability improvement, power supply should be assured, and when it is useless,counter-measures such as connection of equipment should be installed for supplying power from DG ineach machine and all power lines, emergencies. However, these preparations carried out loss oftransmission tower (power lines) switch function this time.
yard soaking etc. were indicated.
Therefore, the reliability of an external power supply shouldbe improved。
・ Entrepreneur is asked about the measurerequired for the power supply reservation in① External power supply is based on the NISA directions[reliability reservation of the external energizer of aan emergency. These measures are such
nuclear power plant and a reprocessing facility.] and, it
as, assurance of high capacity ofis corresponding to the measures (line connection,
rechargeable battery or battery charging
steel tower strengthening, and measure against
from the existing backup power source,
switching station flood) for power source reliability
safety important equipment or power
assurance.
supply boards having Higher evaluation so
Following details are based on the assumption, whenthat entire functioning will not be lost byflood・decentralized geometry which can be emergency DG functioning is failed.
installed on height, earthquake-proof (2)As backup power supply of emergency DG for emergency,
diversification of power source should be done by powertempering of switching station equipment,
source car or high capacity power source installation
maintenance of the organization which(including necessary fuel). Regarding power panel,
supplies fuel oil required for backup power
preparations are done for tsunami attack by tsunami
reservation etc.
counter-measures. Moreover, the power source car is
installed on the heights which are not subject to tsunami【2011.9.11 attached Government Report】
influence. Separate consideration is done for direct power
As a future measure, rechargeable battery
supply and measures at the time of loss is also considered.
having high capacity or earthquake-proof
tempering of the fuel tank of backup power, (3)Rechargeable battery reserves charge route from powersource car and it is prepared for tsunami attacks with floodetc. are planned.
measures.
Reliable power supply reservation is achieved by these
multilevel measures.
■Written part in this reportApp.-3.1-2

Group name

Items

(3) Assurance of secured
cooling function of reactor
and containment

Contents

Additional requirements in attachment data

In this accident, as per the function loss of seawater pump, last heat escaping place(last heat sink) is lost. The nuclear reactor cooling function is operated byinjection, but depletion of water source for injection or prevention of core damage
due to the loss of power was not possible. Container cooling function was notoperated properly. After that, reactor decompression also will take time. As an
accident management counter-measure, for the reactor injection, installation ofheavy machines like fire truck was not done for the injection after decompression.
In such a way reactor and container has lost cooling functionality and, aggravationof accident has increased.
Due to this, alternate cooling function of reactor or container should be assured
by the diversification of an alternative injection function, diversification of water
source for injection or increase of capacity, introduction of an air cooling system,
and assurance of an alternate long lasting heat sink.

・ Entrepreneur is asked about the necessary
measures required for the reservation ofmeans of injection. Intake pit or large raw
water
tank
Reinforcement
againstearthquakes, countermeasures necessary
for water source assurance of injection toreactorandcontaineror,checkreinforcement against the component ofcontainment spray ring, Installation ofpump for absorption of reservoir orseawater pit or, pump which can perform
external injection to reactor which is not
requiring power supply and steam
generator, Maintenance of injection
equipment (example: DG drive pump,high-pressure piping, etc.), etc.,
・ The entrepreneur is asked also about thenecessary measures which are required,for the assurance of the heat sink used forremoving decay heat and components.
These measures are such as an intake pit
installation for cooling in seawater,
disposition of intake pump spare parts,
diversificationofaintakeparts,Development and maintenance of an aircooled system etc.
【2011.9.11 attached government report】
As a future measure, earthquake-proof
strengthening of a large-scale fresh water tanketc. is planned.

(4)Assurance of secured
cooling function of usedfuel pool.

This time, since cooling of the used fuel pool became impossible by loss of thepower supply, in parallel to accident correspondence of a nuclear reactor, thecorrespondence which prevents the severe accident by the cooling function loss ofa used fuel pool is also necessary. Until now, risk of major accident of used fuelpool was considered minor compared with the risk of reactor accident. Because ofthis, measures like alternate injection etc. are not considered.
For this reason, at the time of power supply loss, secured cooling is assured byintroducing the alternative cooling function of a natural circulation cooling system oran air cooling system, and alternative injection function so that cooling of a usedfuel pool can be maintained.

The entrepreneur is asked about the measurerequired for assurance of the secured cooling
function of a used fuel pool such as,
earthquake-proof strengthening of coolingsystem piping of a spent fuel storage pool,assurance of the power supply fromemergency power source to the water gauge
and thermometer of a used fuel pool,
Strengthening of check of the cooling pumpetc. of a used fuel pool,, Strengthening of thecondition monitoring of a used fuel pool (ITVetc.),introduction of dry cask storage, etc.
【2011.9.11 attached government report】
As a future measure, earthquake-proof
strengthening of cooling piping of a used fuelApp.-3.1-3Comparison with the measures of this report
4.7.1 Complete blackout and direct power source loss
■Counter-measure examples
Preparation of power source (1)~⑥
In this accident, external power supply lost due to theearthquake and the seawater pump was flooded by tsunami, it
resulted in complete blackout and loss in seawater functioning.
Regarding RCIC, HPCI, IC, reactor core cooling system can beused at the time of complete blackout, but this functionality islost due to the loss or depletion of direct power supply.
For this reason, the following measures are considered as
an example of measure. And these measure examples can beconsidered as multilevel measures by combining them with theconventional equipment.
① Injection cooling function reservation of reactor iscorresponding to the assurance of injection means toreactor, by portable power pump which is not present
in existing power source, assurance of water source
for injection having seawater, assurance of backup
driving source necessary for SRV operations because
during injection, secured reactor decompression isnecessary for SRV etc.
(2)It corresponds about assurance of the last heat sink by thesea water system restoration by early restoration of aseawater pump, or spare-parts disposition.
(3)Assurance of alternate last heat sink is corresponding withescaping of in atmosphere through containment vent.
■Written part in this report
4.8.1 Injection to reactor
4.8.2 Seawater cooling loss
■Counter-measures example
Heat sink loss correspondence(1)、(2)、⑧~⑪

For assurance of the secured cooling function of a used fuelpool, maintenance of water level is very important whencooling function is lost, used fuel should not be exposed. For
that purpose, assurance of an alternative injection means isimportant. For example, assurance of injection with fire
truck/pump car and assurance of corresponding injection routeand water source.
Moreover, in order to check the validity of an alternative
injection measure, enhancement of the monitoring function ofpool water level and pool water temperature is also necessary.
For example, power source supply to level instrumentation andthermometer or status monitoring enhancement of pool using
ITV etc. of battery drive measures should be considered.
Thus, there is possibility of securing the soundness of usedfuel by performing injection to a used fuel pool and maintaining

Group name

Items

Contents

Additional requirements in attachment datapool, etc. are planned.

Comparison with the measures of this reportpool water level. For further improvement in reliability, it ismentioned as an example of a measure also for the policy
which maintains a cooling function.。
■Written part in this report
4.11 Soundness assurance of used fuel■Measure example
Soundness assurance of used fuel (1)~(3)(5)Accident management(AM)measurescomplianceThis accident results in a severe accident. For minimizing the possibility of severe
accident or mitigating the influence of severe accident, the accident management
measures are introduced also in the Fukushima nuclear power plant. From the view
point of this accident situation, part of alternate injection of reactor from the Fire
Working Water System was working, and various correspondences of powersource or reactor cooling function assurance were fail and accident management
measures were insufficient. Moreover, it was fundamentally considered as theentrepreneur's independent measure, and the demand for laws and regulations
was not carried out, but maintenance contents were lacking. Furthermore, accident
management guidelines were designed in 1992,and they are not reexamined,
improved and reinforced.。
For this reason, accident management measures are performed for preventing
severe accidents effectively and efficiently. These measures are, self control safety
precautions are re-modified by an entrepreneur. This is done on the demand oflaws and regulations. Probabilistic valuation techniques are applied. Design
requirement items are re-modified.

(6) Correspondence tothe subject for multiple
furnaces.

In this accident, the accident occurred simultaneously at multiple furnaces, andresources required for accident correspondence distributed. Moreover, as two
nuclear reactors are having shared equipments or the physical interval between
them was very small, etc., emergency correspondences of adjoining reactor wasaffected by the progress of one reactor accident
for this reason, when multiple nuclear reactors are located in one plant, and incase accident occurred in one reactor, operations of other reactor should be
separated and executed independently. Also, Engineering independency of eachnuclear reactor should be ensured and care should be taken so as, impact ofreactor under accident should not extend till adjoining reactor. Moreover, it isrecommended that, chief person should be assigned for reactor safety assurance
and he should execute independent accident correspondence.

The entrepreneur is asked about thenecessary measures for the correspondence
for the subject in the location of same site ofmajority basis, assurance of issue period
necessary for safety assurance, thoroughness
isolation of building period, assurance(Rationalization of arrangement of a reactor
building and a turbine building) of theengineering independency in the location ofmultiple units etc.

This time, the difficulty generated in accident correspondence, as the used fuel pool
was located in the high position of the reactor building.。Moreover, the contaminatedwater of the reactor building was not able to prevent expansion of the contaminatedwater between a turbine building and a building.
For this reason, even though occurrence of a serious accident is considered inbasic design like an arranging nuclear power generation institution, hereafter
cooling etc. can be carried out certainly., and suitable arrangement of the institution
and building which can prevent expansion of the influence of an accident isadvanced. In that case, the additional measure for having an equivalent function istaken against the existing institution.

The entrepreneur is asked for rationalization ofarrangement of a reactor building and aturbine building, etc.

(7)Consideration on
basic designs of nuclear
power plant arrangement.

App.-3.1-4

About the state of regulation, it is jurisdiction of a country andas a subject it is not raised in this report of industrial world.
However, the validity of PSA use would be recognized fromthis accident, and here onward use of PSA will be considered.
About the example of measure, measures are examined in
multiple ways. Handling of valid measures regarding thesevere accident measures such as preventing outbreak ofaccident, is strongly recommended in [4.5 chapter measurescollection].

【2011.9.11 attached government report】
Hereafter the verification of the policy formaking engineering in-dependency, of eachnuclear reactor in multiple furnace locations ina more positive way, is scheduled.

【2011.9.11 attached government report】
Materialization of examination is planned byconsidering the institution of a nuclear power
plant, suitable arrangement of a building, etc.
in the basic design for nuclear reactor
establishment etc.

At present, the level or problem of influence to concrete
accident correspondence by multiple furnace location, are notin sight other than the influence on work with the affair in which
the hydrogen which occurred at No. 3 started surroundings
lump explosion from the exhaust line to No. 4, and thecontiguity number machine by hydrogen explosion. If theinformation more than the former about multiple furnace
location will come out in the process of accident investigation
here onward, consideration can be done that examination of aconcrete measure is advanced. For the time being, training will
be conducted by considering occurrence of a simultaneous
accident at two or more furnaces. In it, reexamination of aprocedure, the necessity for reexamination of staff disposition,etc. are checked.

In an establishment plant, about arrangement of apparatus,
designing should be done in such a way that it reflects this
accident perception. For the existing plant, following concrete
examples of a measure are raised for filling the gap between
ideal conditions and present conditions.
It is necessary to think on counter measures in case of
examining injection route to used fuel pool, and when pool is atnatural height.
・ Injection of water by fire truck, pump truck etc, as well aswater injection route and water source should be ensured.

Group name

Items

Contents

Additional requirements in attachment data

Comparison with the measures of this report
The counter-measures such as the isolation from water forthe compartments or breakwater or sealing the opening,should be carried out with respect to the installation location or
installation conditions.
・ Build sea wall as well as breakwater，also build protective
wall for important installations.
・ Evaluate the tsunami impact in case of site heightexceeding，building outdoor, air supply opening aperture &penetration with respect to flood height，implement flood
prevention counter-measures related to seal improvementetc
・ Evaluate the tsunami impact in case of site heightexceeding and perform counter measures like flood
prevention of safety equipments, improve isolation fromwater of openings leading to basement and basement
openings and limit the impact of flood
・ Deploying portable drainage pump to drain sea water inside
the building for early restoration
・ Have consideration of tsunami wave impact power in caseof sea facing door etc have chances of direct tsunami
impact.
・ Have flood prevention measure like water prevention wallaround sea water pump.
・ Deploy motor spare parts for early restoration &implementation
・ Deploy movable sea water pump or waterproof pumpinstallation■Items mentioned in this report
4.6.2 Tsunami flood prevention within the premises
4.6.3（1）Waterproofing of important facilities
4.6.3（2）Flood counter-measures of sea water system
4.11 Ensure soundness of spent fuel■Counter-measures
Provisions against earthquake・tsunami (1) – (2)Ensure soundness of spent fuel (1) – (2)(8)Ensuring
Waterproofing
important facilitiesof

One of the reason of this times accident was，lot of important equipment facility likesea water pump facility for auxiliary cooling， Emergency Diesel Generator，
switchboard were flooded in the tsunami，As a result there were difficulties inensuring electricity supply and cooling system.
As a result from the viewpoint of achieving objective level of safety，tsunami
exceeding the design time assumption. Ensure necessary safe functionality evenin case of flood striking due to exceeding design time assumptions of river sideadjacent location. Basically, installation of doors based on destructive force oftsunami or flood, blocks in the piping or flood pathway, using emergency water
pump installation, ensuring water proofing of important equipment facility

App.-3.1-5

Moving and distributing important safety
equipments to upper floor or upland,demanding the counter measures to ensure
water proofing of important facilities to theoperators.。

For Tsunami, Firstly doing scientifically reasonableassumptions to evaluate tsunami height ，For that 1 [ensure
necessary sight height] is the top priority counter measures.
But from multi layered protection viewpoint ， considering
assumptions for the case of the attack of tsunami exceedingsite height， In case push force of tsunami is more powerfulthen investigation of avoiding destruction of important safety
facility.
Considering the importance 2. Perform defense of breakwateretc, moreover, assumptions based on tsunami height being
exceedingly high, places where the sea water can infiltrate likebuilding aperture, pathways places where equipments loss offunction can occur. 3. Counter measures like Installation ofGroup name

Items

Contents

Additional requirements in attachment data

Comparison with the measures of this reportwaterproof doors, Sealing of apertures etc. Also making
assumptions that prevention can not be done at building
infiltration and doing important safety to each instrument. 4.
Doing waterproofing of each compartment, ensure the safetyfunctions, more over, still if the water enters water proof
compartments necessary assumptions to be done. emission offlooded water as early as possible 5. Deploy emission pump.
Again, about emergency light etc 6. In case of tsunami
reaching, crossing upland and also moving in, and impacts
certainly and the existing power supply fails, in that case backup is expected to work.
Like this multi layered counter measures counter measures
for tsunami attack. Assuming these counter measures, power
supply failure or heat sink failure and other common factored
failures or functionality failures prevention and expansion ofcertain accident prevention.
■Items mentioned in this report
4.6.2 Flood prevention from tsunami within the premises
4.6.3 flood prevention measures for building
4.7.1 Complete loss of AC power supply and loss of DC
power supply■counter-measures
Provisions for earthquake・tsunami 1-3, 5Power supply preparation 1-3, 5, 6
Heat sink failure measures 12(9)Reinforce
thecounter-measuresforprevention of hydrogen
explosion.

Responding
to incidents
insevereaccident

In case of the present accident, it was seen that there was hydrogen explosion ineach of unit 1 of nuclear reactor building at 15:36 on March 12, and in unit 3 ofnuclear reactor building at 11:01 on March 14. Moreover, there was an explosion
which seemed to be due to hydrogen in unit 4 at 15:06 on March15.
That is, no help can be received from the initial explosion for unit 1 and theconditions were such that the explosions occurred in continuation, and that was
A major thing in case of the present accident. In order to maintain the soundness ofthe containment vessel in boiling water reactor, as per the design standards, innerpart of containment vessel was inactivated, and burnable gas control system issetup. However, it was not assumed that the there will be leakage of hydrogen gas
in the nuclear reactor building and will lead to conditions that can create explosion,
hence, hydrogen counter-measures were not implemented in nuclear reactor
building.
Hence, in order to precisely reduce the escape of generated hydrogen should,
along with the hydrogen related counter measures for containment vessel, it isnecessary to reinforce the hydrogen explosion prevention counter-measures forburnable gas control system setup in nuclear reactor, which is used in case of
severe accidents and completion of facilities in order to escape the hydrogenoutside.

App.-3.1-6

・ With respect to BWR, it is demanded thatthe implementation of decision and practiceof measures with an objective of hydrogen
explosion prevention and blowout panel
opening means should be ensured.
For BWR, implementation of themeasures for prevention of hydrogenaccumulation in the burnable gas control
system in nuclear reactor building, isdemanded. Based on the analysis of theconditions which occurred in Fukushima
Daiichi(leakage),theimportantcounter-measures such that hydrogendetector device within the building or
setup in nuclear reactor building forhydrogen vent etc should be reinforced.
・ For PWR, the power supply for the annulus
exhaust system for the electric car forescape of the hydrogen at the time ofleakage, power supply for the igniter for theelectric car (hydrogen ignition device)
should be ensured.
The important counter-measures such as
the installation of passive catalytic

Based on the design standards up till now, with respect tothe hydrogen which can generate due to acid reaction of
nuclear reactor coolant with fuel cladding tube, it is done suchthat there is prevention of hydrogen generated by reactor
cooling by emergency reactor cooling system or in case ofboiling water reactor (BWR), the deactivation is done due tonitrogen inclusion in nuclear reactor containment vessel andhence even if hydrogen leaks by some chance, theatmosphere inside the nuclear reactor containment vessel
remains below combustible limits.
However, in case of BWR, due to circumstances which wereout of design conditions, there was hydrogen leakage from thesame container to the inside of the nuclear reactor building.
For this, (1) [the counter-measures for releasing the hydrogen
stagnated in the nuclear reactor building etc or reducing its
concentration](2) [Based on the analysis of path where hydrogen leakage
had occurred, setting up of the hydrogen gas detector system,
which will carefully monitor the accumulation of hydrogen gas
inside the nuclear reactor building] are proposed.
Moreover, in order to prevent the collection of hydrogen
inside the nuclear reactor building, at the time of containment
vessel venting corresponding to severe accident, the following
counter-measures : (3)[In case junction is present in theGroup name

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Additional requirements in attachment datahydrogen supply device on containment
vessel should be reinforced.
[Refer 2011.9.11 government report]
About the mid-range efforts hereafter・ For BWR, it is planned that the hydrogenvent device should be setup at the top of
the nuclear reactor building. Hydrogendetector should be setup inside the nuclear
reactor building. For PWR, it is planned to
set up a device which will reduce theconcentration of hydrogen inside thecontainment vessel of passive catalytichydrogen reunion device etc , which does
not use power supply.

(10)Reinforceofcontainment vessel ventsystem(11)Strengthening
environment
corresponding toaccidentthe
the

In this times accident, at the time of occurrence of severe accident there were
problems in the operability of containment vessel vent system. Also containment
vessel vent system could not handle functionality of radioactive substance removal
management, hence accident management could not make effective use of countermeasures. Also could not do the isolation of vent line hence due to connecting
pipes etc there was a chance of bad impact on other areas.
Hence here after, it will be ensured to have independent operability of containment
vessel vent system. reinforcing of radioactive substance removal management
and hence reinforcing of containment vessel vent system

At the time of this accident, the radiation dose of the central room was increased,
once the conditions were such that the operators could not even enter and it wasdifficult to do work for a long time, and hence presence in the central control room
was reduced a lot. Moreover, the radiation dose rose even in nuclear power station
emergency station which becomes central location for emergency
counter-measures implementation. It became more and more difficult to perform theactivities corresponding to this accident due to worsening of communication
environment or illumination conditions,Hence, the protection of central control room or emergency station from theradiation, exclusive air heating and ventilating system on the site should be
reinforced. The communication which will not depend on AC power supply, setup pf

App.-3.1-7

・ Installation of Filter etc at vent, design ofrupture disk, evaluation of operating
condition or review, installation of
accumulator
at
vent
AO
valve,
independence of vent exhaust line which
was assumed at accident, reinforcing
leakage prevention at adjacent Unit,seeking necessary counter measures atreinforce of containment vessel vent to theoperator.
・ [2011.9.11 Government Report]
By doing the broader study of domestic
technical knowledge, reinforce radioactive
material and work out reinforcement ofcontainment vessel vent system

・ Ensure that the emergency power supply isavailableforincampusPHScommunication
system,ensurecommunication tools such as transceiveretc., ensure portable illumination device,
maintaining radiation shielding function ofcentral control room corresponding to air
heating and ventilating system installationfor emergency electric supply car andcompletion of such infrastructure of theenvironment corresponding to the accident

Comparison with the measures of this reportcontainment vessel vent line, prevention of circulation ofhydrogen from that exhaust pipe](4)[Prevention of circulation of hydrogen in the other units
sharing this exhaust pipes] are proposed and the prevention of
accumulation of hydrogen inside the nuclear reactor building isdone as shown in the diagram.
Thus, the explosion due to hydrogen accumulation in the
nuclear reactor building is prevented by implementing thesecounter-measures and the severity of the accident can beprevented.
Furthermore, if the counter-measures for prevention ofgeneration of hydrogen are certainly implemented, it ispossible to prevent hydrogen explosion, and the setup of
burnable gas control system inside the nuclear reactor building
for BWR is not essential.
■Items mentioned in this report
4.9 hydrogen counter-measures■Examples of counter-measures
Hydrogen counter-measures (1) ~ (4)Contentment vessel vent has post accident over pressure
prevention and heat release functionality as severe accident
counter measures. To ensure the actual vent functionality, as
line configuration counter measure 1. Preparation for existingsource failure, ensuring necessary backup for operation ofcontainment vessel vent valve (driving source - electricity /Pump)
2. [Optimization of operating conditions for vent]
Investigation about leakage prevention to adjacent unit throughexhaust line (wrap around).
3. Moreover, after the core damage of containment vessel
investigation of counter measure of cooling using substitutespray.
■ Items mentioned in this report
4.8.3 containment vessel vent
4.9 hydrogen counter-measures■Counter measures
Measures for heat sink failure 3 - 5
Hydrogen counter measures 3
For livable conditions inside the central control room, setup
of central control room ventilation fan containing charcoal filter
for leaving behind raw materials and while designing shielding,ensuring that the conditions are livable even at the time of
accident is done.
However, due to complete loss of AC power supply thecentral control room ventilation fan containing charcoal filter for
leaving behind raw materials could not function and hence, thefollowing counter-measures (1)[ Ensure the availability of
power supply to the central control room ventilation fan so as toperform functions in case of emergency. Moreover, complete

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Comparison with the measures of this report

illumination systems should be reinforced. Even if severe accident occurs, theactivities corresponding to the accident should be able to be continued and such anenvironment should be reinforced.

situation etc is demanded.
・ Reinforcement of communication system(diversification of electric supply etc）andreinforce the functions ofemergency
room (ensure isolation, shielding, important
personnel skills etc) The aseismic
reinforce for administration building,countermeasurestoreinforcetheenvironment corresponding to accident
should be implemented.

the operation procedures for this.]Also, (2) [Investigate the main cause for rise of radiationdose in the central control room, reinforce thecounter-measures for radiation protection of the central controlroom] are proposed.
Hence it is possible to improve the environment
corresponding to the accident, by implementing thecounter-measures for improving the livability of the central
control room at the time of accident.
Regarding communication system, which is an importantdevice for support in unusual conditions of class 3 importanceclassification, it is ensured that the authenticity is maintained atleast equal or above the general industry facility.
However, if natural phenomenon much beyond theassumptions occur, similar to this time, it is feared that such
communication systems become unusable and hence
regarding communication facility within the power plant as wellas with the outside, the following counter-measures(1) Ensure a self reliant backup supply system which is notaffected by earthquake, tsunami etc.
(2) Implement the facilities such that they are not cut off
easily due to earthquake, tsunami etc.
(3)Deploy multiple means of communication
Are proposed.
Also, from a viewpoint that the information sharing with theactual site should take place smoothly at the time of accident(4) Necessary number of communication devices should be
set up including the actual site.
(5)It is proposed that the counter-measures such as
mastering the operation by conveying the definition of themethod of practical use and practice etc.
Such important counter-measures for maintaining theenvironment at the time of accident have been proposed andbecause of these counter-measures, the communication within
the power plant as well as with the outside is maintained even
if unusual natural phenomenon exceeding all the assumptions
occur.
Emergency station は, In case of complete loss of AC power
supply for long duration in case of severe natural phenomenon,such as earthquake or tsunami etc and severe conditions suchas release of large amounts of radioactive material in theenvironment, the functions of emergency station become
important, hence arrangements should be made such thatthese functions should be successful after assuming these
conditions.
Hence, (1) the structure should be resistant to theearthquake or tsunami.
(2)Ensure an emergency power supply system which is
independent from the plant.
(3)Ensure monitoring important plant parameters and[Refer 2011.9.11 government report]
Along with shifting the PHS etc devices within
the site, to higher elevation, reinforcing thefunctions of emergency counter-measureaseismic
reinforceofbuilding
oradministration building is planned.

App.-3.1-8

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Comparison with the measures of this reportmeans for communication with the central control room, actual
site.
(4)Radiation protection counter-measures.
(5)Taking into consideration the things such as ensuringlivability of necessary person corresponding to emergency
conditions and that of important materials and equipments,
complete the emergency station.
Regarding the emergency station, all the aspects areextensively considered, if the environment corresponding to
the accident is reinforced by implementing thesecountermeasures, the activities corresponding to the accident
can be carried out smoothly.
■Items mentioned in this report
4.10.2 Central control room air conditioning, shielding
4.10.7 Communication
4.10.4 Emergency station■ Example of counter-measures
Arrangements for emergency situations. (6), (7),(13),(22)~(28),(30)~(33)(12) Reinforcement ofradiation
exposureadministration system atthe time of accident

In this time accident, there were lots of individual dosimeters or radiation reading
devices immersed in water and become unusable. Hence adequate radiation
measurement control was difficult; personnel engaged in radiation work weredifficult to get to the site. Also in air radioactive material measurement was alsodelayed.
Risk of internal exposure was increasing.
Hence individual dosimeter or radiation protection equipment at the time of accident
should be kept in abundance. At the time of accident, system of expansion ofradiation control personnel to be done.
Radiation control workers’ exposure
measurement should be fast.
Radiation exposure control system should be reinforced at the time of accident
instead of just fast moving system or equipment completion system.

App.-3.1-9

・
Ensure seeking of more individual
dosimeter based on no of people atemergency work at the time of accident
・ Seek system completion for expandable
radiation control personnel at the time of
accident.

Based on this times accident, in case of severe accident
occurrence, support activities need to be done in high radiation
environment. There could be a situation in which radiation
control may become essential in area where in normal timeradiation control not required. Considering the factor ofsevere accident, severity of natural phenomena, power failureof power plant, can lead to considerable confusion.
In such kind of situation, adequate personnel doing radiation
control to prevent excess radiation, to report 1. Necessary
equipment need to be ensured at radiation control. 2. Ensure
adequate radiation control personnel. 3. Policy to avoid
unwanted radiation exposure is under proposal.
Basically about 1, considering complete power failure,preparationofradiationmeasurement
instrument,
accommodating the radiation measurement instrument foralternate power source, based on accident review the no ofprotection equipments, 2 is about, completion of radiation
control support system, 3 is about information sharing onradiation etc.
Apart from these counter-measures, the completion of
emergency station which has taken measures for radioactive
protection, as a completion of support environment at the timeof accident, is demanded, also, it is pursued to takemeasures to prevent each type of accident and furthermore,considering occurrence of accident and then taking countermeasures to reduce radiation exposure■Items mentioned in this report
4.10.5 Radiation control / Work management■Example of counter-measures
Preparations corresponding to emergency (1) ~ (5)Group name

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Comparison with the measures of this report(13)Reinforce practice for
dealing with an accident

In case of severe accidents, to deal with nuclear power station accidents, effective
practice for implementing convergence of related facilities was not much in place till
now. For e.g., this time, at the time of accident, time was needed to establish
cooperation between Nuclear Emergency Response Headquarters, local nuclear
emergency response headquarters and emergency station inside the power plant,
cooperation between self defense force, police, fire department etc which play animportant role in dealing with an accident, it is possible to prevent it byimplementing precise practice regarding these points also.
Hence, in case severe accident occurs, then, to smoothly carry out emergency
assembling of skilled persons for dealing with an accident, and to ensure theunderstanding of the conditions that occur inside and outside of the power plant andsafety of citizens, practice should be done regarding how to deal with severe
accident.

・ It is pursued that workers should carry out
the practice to deal with emergency
conditions with assumptions of occurrenceof severe accident due to short timebreaking of coolant material etc, as well as
its prolonged duration and severe
conditions.
・ Important
counter-measuresaredemanded from the workers regarding
practice for dealing with terrorism.

It is assumed that, when the severe accident takes place,then the activities to deal with the accident are mainly focusedon various operations on the actual site, also the operations on
actual site are executed under severe working conditions
assumptions and hence, following counter-measures(1) The knowledge and abilities required from eachnecessary person should be sorted out and practice
should be done based on this.
(2) Execute practice of activities on actual site related toaccident management.
(3) It is proposed to execute counter-measures by carryingout practice in modeled severe conditions such asnight time work, heavy equipments etc.
Also, in order to be able to properly deal with events as they
progress, it is proposed that the counter-measures as below
should be executed:(4)High level executions such as blind practice, real timepractice etc is carried out.
(5)Accessing the actual site or prior confirmation of therequired time for the actual work(6)Practical usage of simulator etc,
Thus, without forgetting the lessons learned this time,
accident management practice at the time of occurrence ofsevere accident should be completed and carried out
continuously; due to which it is thought that it is possible toperform activities much more smoothly to deal with accident.
Moreover, there is a plan to conduct investigation afterwards
regarding the nuclear reactor emergency practice to be
executed by the country as well as self governingorganizations, but the workers should deal appropriately on thebasis of this analysis result and should cooperate positively.

[Refer 2011.9.11Government Report]
The country will seek that workers should
carry out the practice to deal with emergency
conditions with assumptions of occurrence ofsevere accident due to short time breaking of
coolant material etc, as well as its prolonged
duration and severe conditions.

■Items mentioned in this report
4.10.1 Practice■Examples of counter-measures
Arrangements related to emergency (14) ~ (19)(14)Reinforce
theinstrumentation system of
nuclear reactor as well as
containment vessel

It was difficult to ensure important information such as water level or pressure of
nuclear reactor, source of release of radioactive material, release quantity quickly
and accurately, if the instrumentation system of nuclear reactor and containment
vessel does not work under severe accident conditions.
Hence, reinforce the instrumentation system of the nuclear reactor andcontainment vessel which performs sufficient functions at the time of severe
accident.

・ It is demanded that the business operators
should reinforce development as well ascompletion of instrumentation system of
nuclear reactor and containment vessel,
which performs sufficient functions at thetime of severe accident, and that of spent
fuel pool and the instrumentation system
for understanding the situation.
[Refer 2011.9.11 Government Report]
Planning the preparation and development of
the instrumentation system of the nuclear
reactor and containment vessel which
performs sufficient functions at the time ofApp.-3.1-10

Regarding the instrumentation system of the nuclear reactor
and containment vessel, inspection within the estimated scopefor fluctuations was possible till now; irrespective of taking
measures at the time of accident after knowing about theaccident, the design is capable of monitoring importantparameters by distributing over a sufficient scope using proper
methods andHowever, the supply of the meter is lost due to loss offunctionality of DC power supply system and total loss of AC
power supply for long duration, the important parameters, themeasurement or recording of important parameters was notpossible and hence, the following things(1)[Install temporary storage battery or connection cable inthe vicinity of central control room for electricity backup.]

Group name

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(15)Focusedmanagementof
equipments for dealingwithemergency
andcompletion of one part of
the forces.

Contents

In case of accident this time, around the J village, all the concerned persons
corresponding to the emergency or accident, concentrated the materials andequipments and are providing earnest support from behind. Initially, though people
were there in the surrounding area, the mobilization of the part of rescue teamforces, supporting the activities of accident management and emergency support
materials and equipments, could not be carried out fast as earthquake or tsunami
disaster had occurred.
For this, the completion of the rescue team supporting the management ofemergency support materials and equipments and their practical use, is promoted
so that the support for emergency conditions can be smoothly carried out even incase of severe conditions.

App.-3.1-11

Additional requirements in attachment data

Comparison with the measures of this report

severe accident as well as instrumentation
system of spent fuel pool.

(2)[Reconsider conservation of electric power of the meter or
non failure of electricity as per requirement] are proposed.
Also, under the conditions at the time of occurrence ofsevere accident, it was not assumed that there will becircumstances which lower the range of measurement of thewater level meter and hence, the following measure is
proposed.
(3)[Consider the occurrence of severe accident, developmentof the meter system with wide range of measurement ofreactor water level or investigate the means to understand thecondition of the nuclear reactor from containment vessel meter,even if the reactor meter became non functional.]
Also, there was no measurement requirement till now, for thethings such that hydrogen concentration of the nuclear reactor
building and hence, support can not be given to those items
which were other than the requirements for measurements.
Considering this, the following action is proposed.
(4) [Establish a measurement system for the parameterssuch as concentration of hydrogen inside the nuclear
reactor building.]
Furthermore, the means for confirming authenticity ofmeasurement system for important parameters such as
nuclear reactor water level etc, under the circumstances ofoccurrence of severe accident and hence the following action
is proposed(5)[Considering the occurrence of severe accident,
development of measurement system to reinforce diverse
environmental qualifications, measurement methods etc」
■Items mentioned in this report
4.10.3 Measurement at the time of accident■Examples of counter-measures
Preparations to deal with emergency(8)~(12)・ In order to carry out restoration rapidly at
the time of accident, sufficient devices toprocess rubble etc should be deployed.
Ensure sufficient protection clothes,protection mask under high radiation dose,the procedures for doing transactions
between the enterprises during a period of
dealing with accident, should be defined.
・ Complete the management system ofcollection of all the emergency support
equipments including robot or unmanned
helicopter. Reinforce the completion of one
part of the rescue team forces having highlevel of skills of supporting emergency
conditions by practically using theseequipments.

About the diversification of means of transport by assuming
conditions such as blocking of access route or simultaneous
occurrence of accident in multiple units, and about the
preparation of materials and equipments, the lessons from theaccident this time, are、
(1)Bringing of important heavy guns and materials andequipments and their deployment(2)Establish the means of air transport as well as transport
through sea, instead of land transport, completion of setup(3)Finalizing of means for setup of finance between enterprises
and reconfirmation.
(4)Establish equipments or devices (robot, unmanned
helicopter etc) for remote operation under high radiation
conditions, of the reactor in case of emergency. Cooperation
for the investigation committee concerning the development
and practical use of such equipments, as well as, for actual
bringing in of the equipments, perform setup of cooperation
between the enterprises and related equipments.

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Comparison with the measures of this report
Investigation is done to promote completion of one part of
the rescue forces, while using the existing resources from thecountry or self governing body. Also, it is also considered that
there can be conditions when transportation by normal means
is not possible. The cooperation is planned beforehand suchthat requests for cooperation of self defense force etc is
possible.
■Items mentioned in this report
4.10.9 Provision for emergency counter-measures (heavygun, rescue), cooperation set-up in case of emergency■Examples of counter-measures
Preparations to deal with emergency (20), (21),(41)~(43)(16)Support for complex
circumstances of nuclear
reactor disaster and large
scale natural calamity

SupportnuclearhazardThis time, along with a large scale natural calamity, there was a nuclear reactor
accident leading to extreme difficulties with respect to contact, communication,assembling of people, provision of goods etc. Also, due to prolonged period ofreactor accident the measures for evacuation of citizens have been forced toprolong though it was initially assumed for a short duration.
For this, support for the conditions when large scale natural calamity as well as
nuclear reactor accident simultaneously occurred, the environment should beestablished which will ensure appropriate means of communication and methodof supply of materials. Also, it should be assumed that the nuclear reactor accident
can occur for a long time and reinforce the support for mobilization of important
persons from every field related to support for accident or disaster.

forPresently, due to basic disaster prevention plan, support will
be done as a country, self governing bodies, but for that it isbeing investigated how enterprises will support. Corresponding
to the complex conditions, it is necessary to review this basic
disaster prevention plan. Afterwards, it is important to do thereview as per the investigation done by the country along withthe support from the enterprises.
However, this report proposes to carry out the reinforcementof arrangement of communication equipments and materials
and equipments. Hereafter, we aim to support, investigationof added counter-measures and means of communication withthe country, in case of wider investigations.
Battery supply which is independent of the nuclear reactor
facility and does not have any impact of earthquake, tsunami
etc, is allowed.
・ The facility should be done such that the communication
setup for server and converter etc which are used in case of
emergency, cable communication circuit and wireless relay
base, should not cut off due to natural calamities such as
earthquake, tsunami etc.
・ Deploying various communication equipments such as
exclusive circuit, satellite phone and, wireless devices
etc，and clarify the method of use of these devices.
Assuming the conditions such that, due to natural calamity(earthquake, tsunami), when the access route inside the power
plant from outside is blocked, the consolidation of the device toremove rubble is done. The diversification of methods oftransportation of people and materials and equipments isimportant , hence considering the cooperation with the related
facilities, basically, the following support is thought of :・ Identifying and deploying important heavy equipments otherthan foil holder which is deployed for severe accident
counter measures.
・ Completion of establishment and setup of means of airApp.-3.1-12

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Comparison with the measures of this reporttransport , sea transport instead of land transport
・ Based on the support for this accident, identification anddeployment of important materials and equipments as wellas confirmation of necessary quantity
・ Completion of procedures for finance transactions between
the enterprises reconfirmation and setup in case of
emergency.
・ Establish equipments or devices (robot, unmanned
helicopter etc) for remote operation under high radiation
conditions, of the reactor in case of emergency.
Cooperation for the investigation committee
concerning
the development and practical use of such equipments, aswell as, for actual bringing in of the equipments, perform
setup of cooperation between、the enterprises and related
equipments.

(17)Reinforcing
environment monitoring

Currently, emergency environment monitoring is the duty of local government but,as the environment monitoring equipment damaged by earthquake and tsunami,
forced to perform evacuation by base facility for emergency response and countermeasures, hence adequate environment monitoring could not be done, tocompensate this by taking cooperation of Ministry of education etc agencies
monitoring activity was performed.

■Items mentioned in this report
4.10.7 communication
4.10.9 Preparation of emergency counter-measures (heavyequipment, rescue), cooperation setup for emergency■Examples of counter-measures30~○
33,⑳,○
41~○
43
21,○
Preparation with respect to emergency ⑬,○
As the basic plan for Emergency Preparedness, in case of
nuclear power disaster monitoring, site boundary and its
internal is by operator and outside the site boundary is by local
government is decided primarily.
On this times nuclear power disaster, site boundary andwithin premises of power plant monitoring done by operator,
similarly local government was doing environment monitoringoutside the site boundary, here towards cooperation support
as counter measure was assigned to operator
Industry supported environment monitoring done by local
government, re completion of support structure for Industry
along with operators was needed, contents of that completion
was thought as bellow
・ Monitoring equipment necessary to accommodate inemergency, reconfirmation of instrument and deploying
necessary numbers.
・ Reviewofnecessary
personforsupportat
emergency ,completion of rules related with that dispatch
Important utility need to be reinforced on constant set
monitoring host which is based on this time natural disaster –earthquake, tsunami, the contents of reinforce are thought as
bellow・ Reinforce of electric and communication line.
・ Tolerance reinforce against earthquake, tsunami
Alternative monitoring procedure for Constant set monitoring
host etc need to be defined. The contents of the same areApp.-3.1-13

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Comparison with the measures of this reportconsidered as following
・ Review of measurement alternate procedure for Constant
set monitoring host instrument
・ Deploy important equipments and its determine its numbers■Items mentioned in this report
4.10.8 Environment monitoring■Preparation against Counter – measures side emergency
34-39(18) Definition of roles of
local
and
centraladministrative agencies

During the initial period of the accident, ensuring means of communication was
difficult. From this it is seen that, starting from local and central agencies, thecommunication, cooperation between the related administrative agencies was not
sufficient, also the sharing of roles as well as responsibility relationship was notalways clear. Basically, regarding the relationship between nuclear reactor disastercounter-measures headquarters and local nuclear emergency response
headquarters, relationship between government and Tepco, relationship between
Tepco and the actual nuclear power station site, sharing of roles among the internal
departments of the government etc, the structure of rights and responsibilities had
some uncertain things. Particularly, the mutual understanding between thegovernment and Tepco was not sufficient.
Hence, for this, we recommend, starting with nuclear reactor disastercounter-measures headquarters, doing review and defining responsibility
relationship and sharing of roles among the related administrative agencies etc,
defining and consolidating set-up of roles and responsibilities related to information
communication.

[Refer 2011.9.11 Government Report]
Regarding the television conference used at
the time of nuclear reactor disaster, the plan isto promote consolidation of the government
administrative agencies and all the electric
supply enterprises so that emergency
instructions as well as information collection
can be carried out clearly and rapidly, byconnecting with the nuclear power station.

As for the items necessary to be investigated, considering
the organization / command, order within the enterprises,
instructions were given for the following counter measures.
Among the support operations in case of emergency, one isto define the time for doing judgment regarding the things
which exert bad impact for the convergence of the accident.
Preparations should be made such that analysis is done fastand operations can be carried out rapidly.
About the basic conditions at the time of accident or thejudgement, they can be clear from the analysis of Fukushima
daiichi nuclear power plant accident and interview of theconcerned people taken by investigative commission, if new
facts are identified, investigation should be done to decide
whether the counter-measures and lessons should be added
or not.
Furthermore, regarding nuclear reactor disaster prevention
counter-measures, country, local self governing body, related
administrative agencies and enterprises should implement as
one body , for relation between the other agencies, enterprises
should take measures based on result of analysis of this
accident, and analysis of investigative commission.
■Items mentioned in this report
4.10.6 organization / command order■Examples of counter-measures
Arrangements corresponding to emergency (29)(19)Reinforce
thecommunication related tothe accident

Regarding sharing of information with the citizens in the surrounding area, in theinitial period when accident has occurred, difficulties came due to damage ofmeans of communication owing to the occurrence of large scale earthquake
disaster. After this, even information communication with the citizens of thesurrounding area or with the self governing bodies could not be achieved at proper
timing. Moreover, there was no sufficient, easy-to-understand explanation regarding
thinking of International Commission on Radiological Protection(ICRP) related toradiation protection, or health impact of radiation for the citizens in the surroundingarea. Also, regarding official announcement of information to the nationals, theprecise facts have always been announced to the public till now but the review ofrisks etc have never been described in details official announcement of information
to the nationals and hence, there was anxiety regarding future happenings.
Hence, we strengthen sharing of accurate information with the citizens of thesurrounding area, giving proper explanation regarding impact of radiation.
Regarding the information announcement when accident was progressing,App.-3.1-14

AT the time of this accident, it was under the jurisdiction to
give instructions regarding evacuation of citizens in thesurrounding area, hence this topic is not raised in this reportwhich is for the industry.
However, press was carried out on enterprise side also, andlessons should be extracted from the press conditions thistime. The examples of counter-measures related to official
announcement of environment monitoring data etc, arespecified.
It is important to define the means of communication or thecontents of the important information to be communicated togeneral people, related administrative agencies, persons
related to the actual site actual site. Following things arethought of regarding this.

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Comparison with the measures of this reportattention should be given to the point that information should include the descriptionof future risks etc.

Contents of information to be communicated, themethod of deciding this, sorting out necessary persons
 Means for transmission (press, HP etc) and study ofofficial announcement timing■Items mentioned in this report
4.10.8 Environment monitoring■Examples of counter-measures
Arrangements related to emergency (40)(20)Strengthening of
dealing with aid from eachnation and information
sharing with the public

After the occurrence of this accident, the proposals regarding aid of materials andequipments from each nation could not be dealt sufficiently as there was no system
in place within the government which will associate this with the news in the
country. Also the communication was not done beforehand with the neighboring
countries and with the community about the release of water contaminated with low
radiation levels. Thus there was no sufficient information sharing with theinternational community.
Hence, a list of stock of materials and equipments should be prepared beforehand
for international cooperation. In case of accident, the contact points of each nation
should be clearly kept in advance. The things such as strengthen the setup of
information sharing by improving the international reporting system, enable
evidence based reports, provide information more rapidly and accurately should bedone and a proper structure should be formed for effectively dealing withinternational world.

There is no topic raised regarding the government support
offered by each country and information sharing in thisindustrial report.
However, the investigation is being done regarding the relatedindustries such as national nuclear power industrialorganizations such as WANO and INPO and the nuclear
reactor related administrative agencies.

(21)Prediction of accurate
understanding of releaseof radioactive material

System for Prediction of Environmental Emergency Dose Information network
system (SPEEDI) could not obtain information about the source of release ofradiation, hence prediction of impact of radiation based on the original source of
information, could not be done. Due to such constraints, And hence according toSPEEDI, with certain assumptions, and hence guessing the spreading tendency of
the radioactive material, the action to execute evacuation was must. Also, regarding
the calculation results of SPEEDI, things from the initial stage till the necessary
information was officially announce.
Hence, strengthen the measuring equipment which will clearly obtain theinformation about the source of release at the time of accident. Also, while SPEEDI
etc provide effective action plan in various situation , the result of actions should
also be officially announce from initial stage.

Regarding the activities of SPEEDI, it is under thejurisdiction of government and hence no topic is raised in thisindustrial report.
Counter-measures of strengthening the environment
monitoring utilities of the nuclear power plant which are
implemented by the enterprises are specified which are
not about the source of release at the time of accident, but
used to understand the impact of radioactive materialStrengthening important utilities for posting constantmonitoring, based on this natural disaster(earthquake,tsunami), is important.
The contents to be strengthened are thought as below:
・ Strengthening of electric supply and transmission line.
・ Strengthening the tolerance with respect to earthquake,tsunamiAlso, to strengthen the means of communication with theoutside, following counter-measures are specified.
counter-measures.
■Items mentioned in this report
4.10.8 environment monitoring
4.10.7 communication■Examples of counter-measures
Arrangements related to emergency (37),(38),(31)(22)Define the wide area

In case of this accident, from the start of accident occurrence, evacuation zone andApp.-3.1-15

About the way of having a complete system for disasterGroup name

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Contents

evacuation or radiation
protection standards at
the time of nuclear
reactor disasterthe zone where people should be inside their houses, was set , and with thecooperation from all the related people such as police as well as local self
governing bodies etc, the evacuation operation initially for the citizens from thesurrounding areas, was carried out rapidly. On the other hand, due to the prolonged
duration of the accident, the evacuation as well staying inside the residence also
got prolonged. After that, as per plan, the evacuation zone and emergency
evacuation preparation zone were set and according to that the activities could be
carried out very fast as per the guidance of ICRP and IAEA. Furthermore, the scopeof the protection area set at the time of this accident, was considered as the area
which should be improved as per the protection counter-measures and wasincreased to about 8~10km.
For this, based on the experience of this accident, a structure should be setup to
define the scope of wide area of evacuation at the time of accident and guide forradiation protection standards.

prevention, it is under the jurisdiction of the government andhence no topic is raised in this industrial report.
However, accompanying the review of plan of disasterprevention, the review of support program at the Tepco side, isto be considered as next investigation topic. Also, the review
which needs adjustment with the government as well as self
governing bodies, we process by cooperating as enterprises
and will be investigated later by checking the related points.

(23) Reinforce safety
regulation administration
systemSafety regulations as a regulatory agency the primary by Nuclear and Industrial
Safety Agency, METI, monitor regulatory administrative agencies the primary by the
Nuclear Safety Commission, Cabinet Office, and the implementation ofenvironmental monitoring by provincial and local governments involved in an
emergency, ensure nuclear safety divided by the relevant administrative
organizations, and their whereabouts were unclear have the primary responsibility
to ensure that adequate safety work is carried out to the public on disasterprevention. In addition, the current system, when a large nuclear accident such as
this, in order to respond swiftly to mobilize the power had not forced the issue.
For this, Nuclear and Industrial Safety Agency is made independent of Ministry of
economy, and we should start the investigation for implementing environment
monitoring and nuclear safety administration regulations including Nuclear Safety
Commission and all the ministries.

Regulatory framework with respect to safety it is under thejurisdiction of the government and hence no topic is raised inthis industrial report.
However, if there is a chance, we would like to participate ina discussion about the method for the desirable regulation,
based on the lessons from this accident.
About the way of having an organization from Tepco side, forsupport for the accident, the following counter-measures are
specified.
Among the support operations in case of emergency, one isto define the time for doing judgment regarding the things
which exert bad impact for the convergence of the accident.
Preparations should be made such that analysis is done fastand operations can be carried out rapidly.

Ensure basic
safetyAdditional requirements in attachment data

Comparison with the measures of this report■Items mentioned in this report
4.10.6 organization/command,order■Examples of counter-measures
Preparation related to emergency 29○(24) Maintaining andreinforcing the legal
system, standards andguidelines

The current accident raised many issues regarding the legal system and the related
standards and guidelines for nuclear reactor safety and nuclear reactor disasterprevention. Moreover, based on the experiences of the current accident, the IAEA
was expected to reflect these experiences in the standards and guidelines.
Therefore, the review and maintenance of the legal system concerning thenuclear reactor safety and nuclear reactor disaster prevention and the related
standards and guidelines is under progress. In this case, the countermeasures forhigh ageing of the existing facilities will have to be re-examined not only from theviewpoint of structural authenticity, but also from the viewpoint of the latest findingsthat include the advancements in system concepts. Furthermore, the orientation ofregulation system for technical demands based on latests laws and new findings
related to existing authorized facilities has become clear. Also, maximum
contributions have to be made by provuiding related data so that the IAEA canreinforce the standards and guidelines.

(25) Ensuringwell-qualified personnel

In case of an accident along the lines that happened this time, along with experts
who can respond to severe accidents, it is also necessary to have experts in theApp.-3.1-16

Discussion on the reviewed guidelines is a basically regulator's
task and is not involved in the task of reporting to the industry.
However, he participats in planning of the guidelines review
such as positive recommendation of the items which arenecessarily reflected in industrial field, discussion in advance,
and independently backfeed for the measures which arenecessary. Furthermore, the establishment of industry
guidelines are expected in the future, to ensure a constantlevel of voluntary support for the private sector.

・ Aim for reinforcing the cooperation within
the country as well as with educational

Until now, each company was engaged in ensuring the talent
for its own compnay and the work of the entire industry as aGroup name

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Additional requirements in attachment data

Comparison with the measures of this reportfor nuclear reactor saftey
and nuclear reactor
disaster prevention

fields of nuclear reactor safety, nuclear reactor disaster prevention,
crisis-management, and radiation medical treatment, etc. and to handle such
events using the latest and best knowledge. Moreover, it is also necessary to not
only focus on the current accident, but it is extremely important to develop andpromote personnel who are competent and well-qualified in nuclear reactor safety
and nuclear reactor disaster prevention in order to ensure long and mid-term
nuclear reactor safety work.
Therefore, in addition to reinforcing the development of qualified personnel ineducational establishments in the fields of nuclear reactor safety, nuclear reactor
disaster prevention, crisis management, radiation medical treatment, etc., the talentdevelopment activities of nuclear reactor enterprises and regulatory bodies alsoneed to be reinforced.

establishments in order to build up a
network of nuclear reactor experts.
Reinforce the development of qualified
personnel in regulatory bodies andspecialized agencies, arrange for active
employment of experts including
public-private exchange, etc., andregularize the methods of experts in on-siteand off-site response.

whole was limited to some activities related to operating
in-charge systems that were implemented by the Nuclear
Power Technical Society. On the basis of the response
conditions in the current accident and from the viewpoint of
improving the nuclear power station safety and uclear reactor
disaster prevention, it is necessary to study the importantmeasures for developing qualified personnel for the entire
industry and to build a systematic and broad-range talentdevelopment system.

(26)
Ensuring
self
reliance and diversity ofsafety system

For ensuring the authenticity of safety system, multiplicity was been pursued so far,but there was lack of response to avoid common cause failure, and sufficient self
reliance and diversity could not be ensured.
Therefore, for accurate handling of the common cause failure and further
improvement of authenticity, ensuring self reliance and diversity of safety system isreinforced.

[Refer 2011.9.11 Government Report]
For ensuring self reliance and diversity in thetypes and installation sites of emergency
generators and sea water cooling systems, theplan is to reinforce ensuring self reliance anddiversity of safety system in addition to theaccurate handling of the common cause
failure and further improvement of authenticity.

Based on the lessons learned from this accident,
counter-measures are examined from the point of multilayered
protection, and concrete counter-measures are presented bytaking into account self reliance and diversity.
Loss of power and loss of heat sink are considered to be thecommon factors for failure. Although examples of concrete
counter-measures against such factors are given, they are thecounter-measures that have taken multiplicity and diversity into
consideration.
To be more specific, the counter-measures are described in‘4.12 Summary of Counter-measures’ as follows.
－Injection / Cooling・
Flood prevention measures such as bulkhead etc
around the sea water pump・
Deployment of sea water pump motor spare parts
・
Flood prevention counter-measures such as corespray system etc of the establishment area related to thesafety system
・
Deployment of movable sea water pump・Preparation of backup power for SRV drive and extra
air container
・
Deployment of backup power for vent operation anddriving source
・
Improvement in the authenticity of existing injection
system by deployment of backup power supply car or mass
power supply
・
Flood prevention counter-measures of severe
accident counter-measures system
・
Improvement in the authenticity with the flood
counter-measures of severe accident counter-measuressystem・Portable power pump and hose etc that are
independent of existing systems
・Ensuring water supply
・Heat escape in air through containment vessel vent
－Power supply
・Install sea wall and breakwater in important safety

App.-3.1-17

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Contents

Additional requirements in attachment data

Comparison with the measures of this reportequipments.
・
Flood prevention counter-measures of the importantsafety equipments in the establishment area
・
Flood prevention counter-measures such as
improvement in the sealing of the opening such as air supply
opening and penetration corresponding to the flood height・Water-tightness of the placement location ofreceiving transformer and switchyard or flood
counter-measures of the equipments
・Water-tightness of the installation area of the direct
current power supply system or flood counter-measures of theequipments
・
Deployment of backup power supply car or mass
power supply, Improvement in the emergency procedures
・ Improvement in the direct power supply charging route withthe backup power■Items mentioned in this report
4.12 Summary of counter-measures■Examples of counter-measures
Provisions against earthquake / tsunami (1)(2)(5)(7)⑨Corresponding to the loss of heat sink (1)~(3),⑥,⑧,⑩~⑫
Power supply preparations (1)~(4)(27) Effective utilization of
the Probabilistic Safety
Assessment Methodology(PAS) in the risk
management

Bring about
a culture ofsafety(28) Bringing about
culture of safetya

It was found after systematic examination of the efforts taken to reduce the risks ofthe nuclear reactor power generation facility that PSA has not been effectively
used. In addition, quantitative evaluation of the unusual events such as massive
tsunami are very difficult also by using PSA, but with more uncertainty, the efforts
that increases the authenticity has not been taken sufficiently by clearly specifying
the uncertainty of such risks.
Hereafter, apply the PSA again actively and promptly based on the opinion
related to the uncertainty and develop the safety improvement measures including
effective accident management counter-measures based on that.

All those involved in nuclear power generation should inculcate a safety culture.
According to (IAEA), 'Nuclear Safety Culture' means, 'The organization andindividual should be prepared with the attitude of an integrated recognition andtemperament to give priority according to the importance of nuclear safety issues'.
According to our terms, it is a starting point, duty and responsibility of those
involved in nuclear power generation. There is no constant improvement in safety, if
safety culture is not adopted.
However, when compared with this time accident, nuclear operator of our country
are those who bear the primary responsiblity to ensure the safety along with theorganizations and individuals, everybody have looked for the new opinions, which
confirms that whether it means vulnerability of own plant, and when it was foundthat it is affecting the hope of reducing the risk related to public safety of ourplant,the sincere efforts are taken fto improve the safety measures.
Moreover, as personnel who assume the responsiblity for ensuring nuclear safety

App.-3.1-18

Till now the deterministic plan and operations have been
conducted, and it has been concluded that the accident will notoccurr by confirming that the accident will not occur with
sufficient conservative conditions. As for this review, it isnecessary to evaluate by the PSA though the probability is very
low and it is also necessary to improve safety by checking forany event that cliff edge. One of it is the stress tests conductedunder the guidelines of the country, and as a result, upcoming
new challenges are also taken into consideration. Regarding
the efforts to PSA, it is necessary to work on the agenda for the
future.

・ The country and enterprises need to workon establishing safety objectives of theorganizations,promotion,
evaluation,
improvement of activites for building asafetycultureinindividuals
andorganizations, reinforcing the cooperation
with educational establishments andworking towards developing qualified
personnel for regulatory bodies.
[Government Report dated 2011.9.11]
Both organizations and individuals involved innuclear reactor enterprises and safety
regulations, are sincerely working on

Following the JCO accident and concealment of troubles by
Tokyo Electric Power Company, the nuclear power industry is
working honestly towards reinforcing the activites fordeveloping a safety culture. The industry needs to reflect
sincerely on the reasons in to why the accident at Fukushima
Daiichi Nuclear Power Plant could not be prevented and toreview the methods related to safety culture. The industry
should work on the future problems in having an ideal safety
culture.

Group name

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Contents

Additional requirements in attachment datafor both organizations as well as individuals, the people involved in setting thenuclear power regulations for the country must show sensitivity and agility
towards latest findings and opinions, without downplaying even the smallest of
doubts related to ensuring safety.
Therefore, it is necessary to return the basic focus back on the absolutenecessity for pursuing defense in depth for ensuring nuclear reactor safety in the
future. The personnel involved in nuclear reactor safety must nevel neglect learningand gaining expert knowledge related to safety and must work towards building acomplete safety culture by always working to find out any weak points in the nuclear
reactor safety and to find room for improving the safety.

restructuring the professional attitude towards
understanding and gaining new knowledge.
Steadfast acquisition of a nuclear reactor
safety culture by all organizations andindividuals is the starting point for personnel
involved in nuclear reactor safety and is an
obligation
andresponsibility.
Constant
improvement in nuclear reactor safety inplaces where there is no safety culture is thestarting point for ensuring future safety of ourcountry and it needs to be confirmed andimplemented through renewed and various
formats.

App.-3.1-19

Comparison with the measures of this reportAppendix-3.2 Comparison of Recommendation of 12 items of NRC Japan Task Force with the countermeasures of this report
Item
Clarification of regulatory
framework

Guarantee of protection

Recommendation(1) A logical, systematic and consistent
regulatory framework should be established
to coordinate the defense in depth and risk
consideration in a right way for adequate
protection.
(2) NRC should re-evaluate the protection ofeach active reactor's SSC for earthquake andflood of design standards, and ask business
operator to raise the standards if needed.

Detailed requirements for electric power-

Comparison with the countermeasures of this report
This is related to the concept of regulations and it is not raised as an issue inthis report for industry.

For the evaluation of earthquake and Tsunami, the following evaluation conditions
 According to the existing requirements and guidelines of RC, earthquake andflood hazards on the premises should be re-evaluated and whenever required, are added based on the knowledge acquired from this data.
the important security design standards and SSC which must be protected from  Standard earthquake motion Ss design is considered as almost satisfactory as
updated hazards should be updated.
per the present way of thinking, but working of multiple earthquake centers is
considered for further safety assurance.
 A walk down preventive measure for earthquake and flood should be At the time of evaluation of Tsunami height which arrives along with theimplemented, along with the specification and improvement in the plant specificearthquake, hereafter the working of multiple earthquake centers is consideredvulnerability, the appropriateness of monitoring and preservation of the
as a wave source at plate boundary and the necessity of setting the slip amountprotection function such as temporary waterproof barrier and sheet should beof earthquake source fault to a larger value is also verified at the same time.
verified until the long-term activity to update the design standards for outside
events is completed.
A possible Tsunami height is evaluated, and protection is provided for the
intrusion of Tsunami on the premises. Further, the following several measures
are taken and security for the functions of safety equipment is provided.

Measures are taken for the intrusion on the premises due to invasion of
Tsunami. The concrete example is as follows.
 Attention is ensured for the arrangement of power-supply system important tosafety, and provided with the intrusion due to Tsunami, the storm surge barrier(flood barrier) or breakwater is setup or the protection barrier is setup on
important equipment from the viewpoint of multi-tiered protection.
 Protection against Tsunami such as tank responsible for problem like a sea
wrack at emergency response (protection barrier etc.). Deployment of heavymachinery to remove a sea wrack.
Further, measures are taken for the protection against intrusion in thebuilding. The concrete example is as follows.
 For a Tsunami which exceeds the assumption, intrusion prevention measuresuch as improvement in the sealing of opening section and penetration sectionof the outside door or air supply opening of a building as per Tsunami height is
implemented.
 For a Tsunami which exceeds the assumption, measures are taken such as
improvement in water-tightness of the door necessary to prevent the intrusion inequipments important to safety, and the impact of instruction is determined.
 For the early restoration, a portable drainage pump is arranged to pump out
the sea water entered in the building About the impact of wave power of Tsunami, places where there is a direct
impact of Tsunami at the outside door facing the sea are considered.
Also, measures for flooding from tsunami are taken for the sea water pump.
■ Describing section in this report
4.6 Preparation for earthquake or Tsunami■ Example of countermeasureApp.-3.2-1

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report

Preparation for earthquake Tsunami  to ,, ,(3) As a part of long-term re-examination, apossibility of capacity building to prevent or
mitigate the earthquake fire and overflow
stream must be evaluated.
Strengthening mitigative
capacity(4) For the design standards and non-design
standards outside events, SBO mitigative
capacity of all the active reactors and newreactors should be strengthened.

-

Fire protection measures are already implemented for the fire. And for theinternal overflow stream, investigation is in progress as a part of earthquake
response of Kashiwazaki Kariwa. But, combining with the earthquake, hereafter
it is necessary to continue working on the investigation of measures forcomposite events.
From a power supply security viewpoint, firstly when the secured external power Rulemaking to revise 10 CFR 50.63 should be started, and for the businesssupply was not good, the equipment was already installed to supply the power by
operators of active reactors as well as new reactor, (1) Response time of
emergency DG. However, this time the equipment did not work due to loss ofminimum 8 hours should be verified for the station blackout, (2) The
equipments, procedure and training required to implement the response time of function.
72 hours during “station blackout extension” for the reactor core and cooling of Therefore, firstly it is necessary to improve the reliability of external power supply.
 The external power supply is based on NISA instructions document "Aboutalready used fuel pool, or soundness of reactor cooling system and primary
ensuring reliability of external power supply for nuclear power station andcontainment as required, should be established, and (3) External traffic
reprocessing plant", and corresponds to the measures (circuit connection,
infrastructure function related to the large scale natural disasters should include
suspension tower strengthening, switching station intrusion measure) to ensure
the capacity to acquire equipment in the given time for response capacity
the reliability of power supply.
building under significantly reduced conditions, and persistent cooling capability
Next, preparation is done by assuming the loss of function of emergency DG.
of the reactor core and already used fuel pool on the premises or the external As a backup power supply of emergency D/G, diversification of power supply is
resource to support the soundness of reactor cooling system and primary
provided by deploying (including the required fuel) the power supply car or power
containment system as required, should be planned and prepared in advance.
supply with large capacity, and for the power panel, preparation is done for According to 10 CFR 50.54(h)(2), currently provided equipment should be
Tsunami invasion by taking intrusion countermeasures. Further, the power supply
rationally protected from outside events of design standards and together the
car is placed on the elevated location.
other requirements are revised and implemented, hence respond to events ofAnd
for DC power supply, separately countermeasures are taken for the loss.
multiple reactors by adding equipment as required.
 Battery secures the charging route from backup power supply and also ready for
Tsunami invasion by intrusion countermeasures.
A good power supply security is achieved by taking these several countermeasures.
Following countermeasures are taken to secure the functions of cooling system.
 Security of injection measures to the reactor by portable power pumpindependent of the existing power supply, security of water source for injection
including sea water, and depressurization of a good reactor by SRV is necessary
in injection, hence a good injection cooling function to the reactor is secured by
securing the backup drive force necessary for SRV operation.
 Security of alternate last heat sink corresponds to heat escape to the air bycontainer vent.
 The last heat sink security corresponds to the sea water system restoration by
early restoration of sea water pump or arrangement of spare parts.
About the time factor, it is not necessary to set up the time. Training is conductedby considering the time from loss of function of equipment of safety system to thecore damage, and it is cleared through the confirmation of whether the alternative
measure is ready before the core is damaged.
Further, basically all the necessary equipments are placed inside the power station.
Refer to item (7) for cooling of spent fuel pool.

App.-3.2-2

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report■ Describing section in this report
4.7 Preparation for power supply
4.8 Heat sink loss response■ Recommended example of countermeasure
Preparation for power supply  to Heat sink loss response , ,  to(5) Pressure vent design should bedemanded to BWR provided with Mark type I
and Mark type II container.

(6) Hereafter with the disclosure of additional
information by investigation of an accident
occurred in Fukushima Daiichi Nuclear
Power Station, a vision should be determined
for mitigation and hydrogen control in thecontainer and in the building as a part oflong-term re-examination.

Not only for the installation of pressure vent, but also to secure a good vent
 A reliable pressure vent should be installed in BWR provided with Mark I typefunction, as an example of countermeasure for line configurationand Mark II type container.
 In case of loss of existing drive source, the security of backup drive source(power supply or pump) which is required to operate the container vent valve isincreasing.
And the vent operating procedure will be considered hereafter, by taking into
consideration the possibility of over temperature failure.
Investigation will proceed for “Optimization of operating conditions of vent” and“Preventive measures for the leakage (wrap) to neighbouring unit from ventexhaust line”.
 Further, investigation is carried out as a countermeasure for cooling by container
alternate spray after core damage.
■ Describing section in this report
4.8.3 Container vent
4.9 Hydrogen measure■ Example of countermeasure
Heat sink loss response ,  to 
Hydrogen measure ―
For the hydrogen which may generate due to the oxidizing reaction between
reactor coolant and fuel cladding, based on the past design basis events, theatmosphere in the reactor containment is made below the fuel limit by
preventing the hydrogen generation by core cooling such as emergency corecooling system or by creating inert gas environment by nitrogen sealing toreactor containment in the boiling water reactor (BWR) even though thehydrogen is generated suddenly.
However, when hydrogen is generated in the same container of BWR anddesign conditions are exceeded, the hydrogen leakage occurred inside thereactor building. Hence,  “Countermeasure to release or reduce the hydrogenretained inside the reactor building” and  “Installation of hydrogen gas
detector to properly monitor the retention of hydrogen in the reactor building,along with the investigation of a channel where the hydrogen leaked inside thereactor building” are recommended.
Moreover, during container vent in severe accident management response, “Prevention of hydrogen wrap from the ventilation pipe when a branch pipe isplaced in the container vent line” and  “Prevention of hydrogen wrap to otherunits sharing the vent stack” are recommended to prevent the wrap of hydrogen
inside the reactor building, and tried to prevent the accumulation of hydrogen
inside the reactor building.
In this way, for the hydrogen which may generate in the reactor core,
explosion of reactor building due to accumulation of hydrogen can be prevented
by taking the above countermeasures.
Further, the hydrogen explosion can be prevented by ensuring the action onhydrogen generation prevention and release countermeasures, and hence it isApp.-3.2-3

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report
not necessary to install flammable gas control system in the reactor building in
BWR.

(7) Condenser capacity and instrumentationof spent fuel pool should be strengthened.

■ Describing section in this report
4.9 Hydrogen countermeasure■ Example of countermeasure
Hydrogen countermeasure  to 
In the security of a reliable cooling function of a spent fuel pool, it is extremely
 Enough safety system instrumentation which can withstand natural
phenomenon of design standards, and can be monitored from the control room important that the water level in the pool is maintained even if the cooling
function is lost and the spent fuel is not exposed. For that, securing alternativeof important parameters (water level, temperature and area radiation level) of
injection measure is important. For example, countermeasures such as
spent fuel pool should be provided.
injection by fire extinguisher car or pump car and together the security of Safety system AC power supply should be provided to the makeup system of
injection route and water source can be considered.
spent fuel pool.
Moreover, it is necessary to intensify the monitoring function of pool waterlevel and pool water temperature to check the validity of alternate injection
 Technical specifications should be revised so that the demand to provide
measure. For example, power supply to the level instrumentation andoperable one house emergency power supply train for the makeup andthermometer from emergency power supply or monitoring enhancement of pool
instrumentation of spent fuel pool is fulfilled, irrespective of the operation modeusing battery drive ITV can be considered as countermeasures.
of a reactor, when there is irradiated fuel in the spent fuel pool.
In this way, soundness of the spent fuel can be secured by maintaining thepoolwater level by injection to the spent fuel pool, but this can also be raised as Already installed equipment to spray the water on the spent fuel pool which isan example of countermeasure to get further improvement of reliability or aconfirmed after considering the earthquake should be provided including themeasure to maintain the cooling function.
connection where the access to water supply is easy on the same plane (byIn Fukushima Daiichi Nuclear Power Station, it is good enough from theusing the portable pump or pump train) as external building.
viewpoint of fuel damage prevention that to certainly supply the water bytemporary equipment, than making a power supply of a permanent makeup
water system when all the power supply is lost. (Therefore, it is not necessary torevise the technical specifications.)■ Describing section in this report
4.11 Soundness security of spent fuel■ Example of countermeasureSoundness security of spent fuel  to (8) The ability to respond during emergency
in the facilities such as plant emergency
operation procedure (EOP), severe accident
management guidance (SAMG) and EDMG
must be strengthened and integrated.

 Technical guidelines (supplement 1 "Requirements for emergency response
ability" for NUREG-0737 in GL 82-33) for plant emergency operation proceduremust be revised so as to (1) include the plant emergency operation procedure,
severe accident management guidance and EDMG in an integrated format, (2)specify precise command and control strategy related to the implementationand (3) grant the rights and provide training to the staff with suitable
qualifications to make a decision during emergency.
 Section 5.0 "Administrative control" of standard technical specifications torefer to technical guidelines for plant emergency operation procedure approvedfor plant in each reactor operation design must be revised, and the technicalspecifications of each plant must be revised so as to comply with thosechanges.

 Countermeasure is taken such as installation of equipment by considering all
power supply loss or heat sink loss, and measures are also raised for making
the procedure for that.
 As a countermeasure related to the organization/command or order, for thethings which get badly affected by accident convergence due to delay in the
judgment, the timing of judgment is clarified, and external adjustment is carriedso that the operation can be done quickly. And a countermeasure to buildup
communication facility is also raised. On the emergency station, securingcollection means of the plant parameters required for command is also raised
as a countermeasure.
About the qualification, it is not demanded officially, but seeking to secure aknowledge and technique as a part of training.
■ Describing section in this report
4.7 Preparation for power supply
4.8 Heat sink loss response
4.10.1 Training
4.10.4 Emergency station
4.10.6 Organization/command or orderApp.-3.2-4

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report
4.10.7 Communication■ Example of countermeasure
Preparation for power supply , 
Heat sink loss response , , , ,  to 
Preparation for emergency time

Strengthening emergency
response(9) NRC should request the summary to
respond to long-term SBO and multiple unit
events in facility emergency
countermeasures.

 The following points should be implemented until rule making activity iscompleted.
 Determination of workers to fulfill all the positions required to respond to multiple
unit events and its implementation.
 Addition of guidelines in the documented emergency plan for the implementation
method of multiple unit dose evaluation (including the discharge from spent fuelpool) using the dose evaluation software and approach specialized for the facility
given by business operator.
 Implementation of periodical training and practice for multiple units and long-term
SBO scenario. Exercise related to the identification and acquisition of
non-institutional resources within the possible range (simulate).
 Assurance about the fact that facilities and equipment for emergency response will
properly deal with the scenario of multiple units and long-term SBO.
 Power supply measures to communication equipment required for thecommunication within the plant (wireless communication between the response
team and the plant etc.) as well as outside the plant (cellular phone, satellite phoneetc.) during long-term SBO.
 Maintenance of capacity of emergency response data system throughout theaccident occurrence period.

to , 2○4, 2○8, 2○9, 3○1 to 3○3

 Countermeasure is taken such as installation of equipment by considering all
power supply loss or heat sink loss, and measures are also raised for making
the procedure for that.
 For communication equipment strengthening or emergency response forsystem, instrument, monitoring, measures cited in the countermeasures. Itsvalidity will be checked through training.
 Implementation of training is raised as a countermeasure. Whether the
preparation to respond to multiple units is done will be checked through training,
and as a result, re-examination such as strengthening of system will be carriedout if required.
 Provision of disaster control measures and building a cooperative system aretaken as measures for resources.

■ Describing section in this report
4.7 Preparation for power supply
4.8 Heat sink loss response
4.10.1 Training
4.10.7 Communication
4.10.8 Environmental monitoring■ Example of countermeasure To secure the monitoring capability of multiple units, initiative to bring the
Preparation for power supply , emergency response data system to the last state needs to be completed
Heat sink loss response , , , ,  to 
before June 2012.
Preparation for emergency time(10) As a part of long-term review, NRC
should search for additional themes toemergency response, which are related tomultiple unit events and long-term SBO.

 Based on the insights taken from Fukushima Daiichi Nuclear Power Station
accident, analysis should be carried out for emergency responder about therequirements and guidelines related to the current security equipment.

 About long-term SBO or multiple unit events or both, to secure the existence As a countermeasure related to the organization/command or order, for thethings which get badly affected by accident convergence due to delay in the
judgment, the timing of judgment is clarified, and external adjustment is carriedso that the operation can be done quickly. And a countermeasure to buildup
communication facility is also raised. On the emergency station, securingcollection means of the plant parameters required for command is also raised
as a countermeasure.
 About the qualification, it is not demanded officially, but seeking to secure aknowledge and technique as a part of training.

of proper level authority and monitoring system in a proper plant, evaluation
should be carried out for command and control formulation as well as
qualifications of the decision-maker.
 Emergency Response Data System (ERDS) is evaluated by using thefollowing methods.
 Alternative methods (such as satellite connection) are determined to send the■ Describing section in this report
data of emergency response data system, which is independent of the fixed-line
4.10.1 Training
infrastructure which may not be useful during large scale natural disaster.
4.10.7 Communication
 Currently received data set in various locations determines whether the■ Example of countermeasureconditions necessary in the updated evaluation are fulfilled.
 During emergency, whether the emergency response data system should
continuously send the data so that the operator need not perform it isdetermined.
App.-3.2-5

to , 3○0 to 4○3

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report

Preparation for emergency time(11) As a part of long-term review, NRC
should search for emergency response
related to decision-making, radiation
monitoring and promotion of citizens.

 Investigation should be carried out by emergency response resources within
the plant about whether it is necessary to effectively implement the emergency
response such as transportation capacity of local equipment, when there is no
support from outside the plant or the support is delayed since the infrastructure
outside the plant is destroyed due to the extensive natural disaster or there areother priorities for response resource.

 Insights obtained from the implementation of emergency response in
Fukushima should be evaluated.

 Validity (including consideration about independence from AC source,
availability of real-time internet) of radiation monitoring within the plant and in
the range of emergency planning zone in real-time should be investigated.

 Training about radioactivity, its safety and correct dose method KI should be
implemented in a local community around each nuclear power station.

to , 2○4, 2○8, 2○9, 3○1 to 3○3Country and the local government have carried out the monitoring at the timeof accident and publicity of its result, and power will cooperate with that system.
In response to this accident, investigation for accident response such as
monitoring is proceeding in the country, but voluntary measures are taken
regarding cooperation for the power.
Basically, countermeasures are implemented for the following items.
- Monitoring equipment required in advance for emergency, reconfirmation ofequipment and their deployment as required- Re-examination of the supporting staff necessary during emergency andupgrading the rules related to their dispatch- Strengthening of power supply and transmission line
- Buildup resistance for earthquake and Tsunami- Re-examination of the test procedure by alternative measures such as
temporary monitoring equipment- Information to send, its compilation method and arrangement of therequired staff- Review of means of communication (press, HP etc.) or announcementtiming■ Describing section in this report
4.10.8 Environmental monitoring■ Example of countermeasure

Efficiency improvement of
NRC program(12) NRC should strengthen the regulatory
monitoring for a safety record (that is, reactor
oversight process) of a business operator, bykeeping the focus on requirements to
defense in depth which conform to therecommended defense in depth framework.

 To include the consideration of defense in depth completely, annual
self-assessment of reactor oversight process and reorganization of reactor
oversight process every 2 years should be implemented.

(Note) Text written in Italic is the countermeasure which may relate to both regulations as well as business operator.

App.-3.2-6

Preparation for emergency time 3○4 to 4○0This is not raised as an issue in this report for industry as it is a regulator's
approach.
Further, hereafter in the peer review carried out by WANO or JANTI, EP or SAM
will also be tackled as review items.

Appendix -4

MARK-I Containment

Various arguments have been raised concerning the MARK-I Containment. We willsummarize the industry’s opinions on the MARK-I Containment as below.
With regard to the details of the arguments, please access General Electric Co.
website (http://www.ge.com/jp/docs/1307504328207_NEI_Report.pdf) to read the
explanations.
Primarily, MARK-I containment has been designed to have sufficient strength bybeing compliant to various design requirements including the seismic resistance.
Further, the recent engineering findings have been integrated in the facilities at eachstage. Accordingly, the safety performance of the MARK-I containment is judged as
equivalent at least to the other containment models.
The points raised as targets of arguments are the 4 items as below:
1. The containment is vulnerable to earthquakes
2. Because of small containment capacity, the capacity for absorbing the energy
discharged from the core in the event of accident is low.
3. The containment is likely to be damaged when dynamic load is applied to theinner wall of its suppression chamber in the event of disasters such as anearthquake.
4. Containment Failure Probability is high when a severe accident is assumed.
Our opinions on these points are indicated below.

1. Earthquake-resistant performance of MARK-I Containment
MARK-I Containment has following characteristics:
 A suppression chamber is arranged on the side of RPV.
 Its center of gravity is low, because low containment can be made.
 Its center of gravity is low, because the position to arrange RPV in thecontainment is low.
In MARK-I Containment, height of the facility can be made low by arranging the
suppression chamber on the side of PRV. At the same time, it contacts the groundwith wider area.
Accordingly, the MARK-I is considered as an advantageous containment type interms of aseismic design.
A comparison of MARK-I Containment and MARK-II Containment is indicated in
Figure Appendix-4-1.

App.-4-1

Figure Appendix-4-1

App.-4-2

2. Capacity of MARK-I Containment
The energy discharged in the event of an accident is proportional to the thermal
output of the reactor. Comparing the proportion of reactor containment capacities
that absorb the energy, the proportion of MARK-I Containment is higher than thoseof MARK-II containment or PCCV. While the capacity of the MARK-I Containment issmaller, it shows an equivalent level of energy absorption to the energy absorption inother models in terms of absorption of energies discharged from the core in theevent of an accident.
Table Appendix-4-1 indicates the average containment space volume and theaverage reactor thermal output in MARK-I Containment and the other types ofcontainments.
Table Appendix-4-1
Containment type
MARK-I
MARK-I (Revised)
MARK-II
MARK-II (Revised)
RCCV

Containment space volume versus thermal output
Averagecontainment spacevolume [m3]
6558
12500
9775
14406
13355

Average reactor
thermal output[MW]
1681.82581.2
3293
3293
3926

Containment spacevolume ÷ thermal
output [m3/MW]
4.10
4.95
2.96
4.37
3.40

3. Dynamic load to suppression chamber pool
Following two conditions are considered to generate pressure on internal wall of the
suppression chamber in the arguments.
(1) Earthquake(2) Steam discharge from SRVs
(1). Earthquake
As indicated below, dynamic load by an earthquake is not a MARK-I
Containment-specific problem.
Following 2 items are the key points in the arguments:
 Sloshing of pool water caused by an earthquake generates a dynamic load on
the internal wall of suppression chamber.
 Sloshing of pool water caused by an earthquake results in exposure of steam
exhaust pipes from SRVs to the suppression chamber space. If this event
occurs concurrently with a loss of coolant accident, steam from RPVs isdischarged directly to the suppression chamber space. This may elevate thepressure inside the suppression chamber.
The dynamic load generated by sloshing of pool water under the impact of anearthquake has been confirmed to have no negative impact based on the
structural analysis. With regard to the SRV exhaust pipes, the exhaust pipes
should not come above the water level, even if the water level of the suppression
pool is the lowest and considering the point where amplitude of the pool watersloshing is the maximum. Therefore, it is confirmed that the event raised in thearguments never occurs.

App.-4-3

(2). Impact of steam discharge from SRVs
A countermeasure has been implemented against the dynamic load through the
SRV steam applied on internal wall of the suppression chamber. Therefore, itdoes not constitute a problem specific to MARK-I Containment.
When an internal pressure of RPV elevates, SRVs are actuated to discharge
RPV steam into the water of suppression chamber pool with the purpose toprevent damage on the RPV.
In this occasion, the possibility of applying a local dynamic load on suppression
chamber has been pointed out due to local discharge of the steam.
To address this issue, an improvement was conducted so that the steam exhaust
pipe should be located on the cross type nozzle instead of a straight nozzle. In
this configuration, the steam is distributed and discharged into the water(quencher type). Based on this modification local pressure can be avoided.
4. Security of containment integrity based on the assumption of a severe accident
Each operator carried out the probabilistic safety assessment (PSA), identify andimplement valid countermeasures against the severe accidents. Integrity of thecontainment in the event of severe accident is secured based on this process.
Following two factors are identified as causes of the containment damage: Pressure that exceeds maximum operating pressure of the containment
 Temperature that exceeds maximum operating temperature of the containment
Following four countermeasures have been examined and implemented to maintainintegrity of the containment by eliminating the aforementioned damage factors.
1. Containment vent2. Alternative coolant injection
3. Containment spray
4. Alternative coolingApp.-4-4

Reference -1

Plant Overview

Plant overviews for the Units 1, 2 and 3 of Fukushima Daiichi Nuclear Power Station
are indicated in Table Reference -1, Figure Reference1-1 and Figure Reference 1-2.
Further, the outlines of severe accident countermeasures already implemented in
the Units 1, 2 and 3 are indicated in Table Reference-2 and from Figure References
1-3 through Figure Reference 1-8.

Ref.-1-1

Table Reference 1-1
Plant
Reactor typeContainment type
Thermal output(MWt)Power output (MWe)Fuel assembly #Control rod #Safety relief valve
Safety valve
Emergency
condenser(Heat exchanger #)Cooling system atisolation( Pump #)Emergency corecooling system( Pump #)Containment coolingsystem / Residual
heat removal system
Containment coolingsytem( Pump #)Shutdown coolingsystem( Pump #)Emergency DG(Unit #)
Containment
capacity

Plant overview

Unit 1
BWR3
Mark-I
1380

Unit 2BWR4Mark-I
2381

Unit 3BWR4Mark-I
2381

460
400
97
4
3

784
548
137
8
3

784
548
137
8
3

2

—

—

—

1

1

HPCI:1
CS:4
ADS

HPCI:1
CS:2
LPCI:4ADS

HPCI:1
CS:2
LPCI:4
ADS

—

2

2

4

—

—

2

—

—

2

2

2

DW:3410 m3
WW:2620 m3
SP:1750 m3

DW:4240 m3
WW:3160 m3
SP:2980 m3

DW:4240 m3
WW:3160 m3
SP:2980 m3

Ref.-1-2

Table Reference 1-2
Function
Reactor shutdown
function

Countermeasures implemented for severe accident

Countermeasure for severe accident
Recirculation pump trip (RPT)
Alternative control rod insertion (ARI)

Coolant injection toreactor andcontainment

Function to remove
heat from containment

Function to support
safety features

Alternative coolant injection(Coolant injection to reactor /containment by using condensate watermakeup system/ Fire extinguishingpump etc.)
Automatic actuation of reactor
depressurization(Addition of ADS interlock)
Alternative heat removal(Positive use of DW cooler / Reactor
coolant cleanup system)
Restoration of failed component ofresidual heat removal system(Procedure)
Pressure resistant reinforced vent
Additional installation of emergency
D/Gs(2 dedicated DGs for each plant)
Interchange of power source(480V is made available by adjacent
plant)
Restoration of failed component ofemergency D/Gs
(Procedure)

Ref.-1-3

Unit 1Units 2/3













—



























What is the countermeasure against severe accident (=accident management)?
A countermeasure against severe accident (=accident management) means acountermeasure to prevent development of an event that may lead to a severe
accident (=a large scale accident that may result in a serious damage of reactor
fuels) as well as a countermeasure to mitigate the impacts of a severe accident even
if the event should develop into a severe accident.
In May, 1992, the Nuclear Safety Commission made recommendations on theaccident management for severe accidents, and in July, the government requested
each electric utility to implement an accident management. In theserecommendations, electric utilities are requested to make voluntary commitments to
reduce accident risks and to enhance safety, although it is admitted that no
additional safety regulations are necessary because the safety in the domestic
nuclear power stations has been sufficiently guaranteed by currently available safetymeasures.
Upon receiving these recommendations, electric utilities have implemented their own
accident management measures and a report summarizing the contents of thesemeasures was submitted to the government in May 2002. The Nuclear Safety
Commission reviewed the report and evaluated that the countermeasures developedby electric utilities were appropriate.
In the accident management, a situation is assumed where abnormality (orabnormalities) occurred and the entire emergency core cooling system (ECCS)
failed. In such a situation, some special countermeasures are taken such as pumps
normally used for fire extinguishing are used to inject coolant to the core for coolingthe fuels. In the accident management, it is necessary to make most of thecomponents originally used for some other functions to address an abnormal
situation so as to prevent development of the event and to mitigate its impacts.
Accident management is also a requirement in IAEA standards.
IAEA NS-G-2.15 SEVERE ACCIDENT MANAGEMENT PROGRAMMES FOR
NUCLEAR POWER PLANTS
1.4. Accident management is the taking of a set of actions during the evolution of abeyond design basis accident:(a) To prevent the escalation of the event into a severe accident;(b) To mitigate the consequences of a severe accident;(c) To achieve a long term safe stable state [4].
The second aspect of accident management (to mitigate the consequences of asevere accident) is also termed severe accident management. Accident
management is essential to ensure effective defence in depth at the fourth level

Ref.-1-4

High pressure injection system

Main turbine
To suppression
pool

Feed waterpump
Condensate(3 motorpump
operated units)(3 units)

Emergency condenser (IC)Condenser

Circulating
water
pump
(2 units)

Sea

Heat exchanger

Heat exchanger

Reactor shutdown
cooling system

Condensate
storage tank

Borated water
storage tank

Condensate watermakeup system(2 units)
Containment
coolingsystem heatexchanger

Standby liquid control
system (SLC)

Containment cooling system (CCS)–B system

Control rod drive
hydraulic control system(2 units)

Core spray system(CS)-B system

Containment
coolingsystem heatexchanger

Containment cooling system (CCS)–A system

Core spray system(CS)-A system

Figure Reference1-1 Plant overview（Unit 1）

Feed water pump
(3 motor operated
units)

Highpressure
injectionsystemMain turbine
Condensate pump
3 high pressure 3 low pressure
units
units

Condenser

3 motor operated units

To suppression
pool

Circulating
water
pump
(2 units)

To
suppression
pool

Condensate
storage tank

Residual heatremoval system
heat exchanger

Borated water
storage tank

Condensate watermakeup system(2 units)

Standby liquid control
system (SLC)

Residual heatremoval system
heat exchanger

Condensate watermakeup system(2 units)

Residual heat removal
system (RHR)

Figure Reference1-2

Core spray system (CS)

Plant overview（Units 2 / 3）

Ref.-1-5

Sea

Emergency
condenser B

Reactor pressure vessel

Reactor containment

PLR pump

Figure Reference1-3

Emergency condenser

Ref.-1-6

Emergency
condenser A

Condensate water Fire extinguishingsystemmakeup system

Diesel pump

Condensate
water makeup
systemPressure vesselCore spray system

Dry wellMotor operated
pump

Filtered watertank

Fire extinguishingsystemMotor operated
pump (Stand-by)

PCV spray(S/C)

Containment
coolingsystemCondensate
watermakeupMotor operated
pumpsystemCondensate
storage tank

Condensate watermakeup system

Pressure control
chamberCore spray system

Containment cooling system

Figure Reference 1-4

Alternative coolant injection system（Unit 1）

Pressure vessel

Condensate watermakeup system

Pressure vessel head spray

Fire extinguishingsystemDiesel pump

Dry wellMotor operated
pump

Filteredwater tank

Fire extinguishingsystemMotor operated
pump (Stand-by)

PCV spray (D/W)
Low pressure
injection

PCV spray (S/C)

Residual
heat removal
systemCondensate
storage tank
Motor operated
pump
Condensate
Condensate water
water
makeup system
makeupsystemResidual heatremoval system
Pressure control
chamber

Figure Reference 1-5

Alternative coolant injection system（Units 2/3）

Ref.-1-7

StackRupture disc

Small valve
Electromagnetic valve

Electromagnetic
valve
Small valve
Electromagnetic valve

CylinderLarge valve

CylinderLarge valve

Electromagnetic valve

Figure Reference1-6

Pressure resistant reinforced vent system（Unit 1）

Stack
Rupture disc

Small valve
Electromagnetic valve

Electromagnetic
valve
Small valve

Electromagnetic valve

CylinderLarge valve

CylinderLarge valve

Electromagnetic valve

Figure Reference1-7

Pressure resistant reinforced vent system（Units 2 / 3）

Ref.-1-8

Starting transformer

Common bus (6.9KV)

Normal bus (6.9KV)Emergency bus (6.9KV)Emergency bus (480V)

Transformer

Breaker(M/C)

Battery
cell

Breaker(MCC)

Spare battery
charger
Dedicated battery charger
125V DC bus
Unit 1 (3, 5)

Unit 2 (4, 5)<A> Route: Interchanges 6.9KV AC power source.
(Operation at M/C is possible only if DC
power is available)<B> Route: Interchanges 480V AC power source.
(MCC is manually operated. MCC isnormally open and its access is controlled
by key lock.)

Figure Reference 1-8

Power source interchange

Ref.-1-9

Ref.-1-1