Examination of Accident at Tokyo
Electric Power Co., Inc.’s Fukushima
Daiichi Nuclear Power Station and
Proposal of Countermeasures
This report has become final version on 25 Jan 2012.
Revision 1 and 2 will be updated after completingtranslates.

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 coolant 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 coolant 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
Members of the Examination Committee
Deliberation Members of the Examination Committee on Accident at
Fukushima Daiichi Nuclear Power Station
Report Study Members of the Examination Committee on Accident at
Fukushima Daiichi Nuclear Power Station
Report Review Team of the Examination Committee on Accident at
Fukushima Daiichi Nuclear Power Station
Abbreviations
Glossary
Chapter 1 Purpose
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
2.1.2 Overview of Great East Japan Earthquake and Tsunami
2.1.3 Overview of accident due to Great East Japan Earthquake
2.1.4 Impact of earthquake
2.1.5 Impact of tsunami
2.2 Course of accident at Unit 1
2.2.1 Conditions from earthquake to arrival of tsunami
2.2.2 Situation of nuclear reactor building from tsunami to hydrogen
explosion
2.2.3 Condition of nuclear reactor building after hydrogen explosion
2.2.4 Main events after accident
2.2.5 Status of spent fuel pool
2.3 Course of accident at Unit 2
2.3.1 From occurrence of earthquake to arrival of tsunami
2.3.2
From arrival of tsunami to malfunctioning of pressure
suppression chamber
2.3.3 Major events thereafter
2.3.4 Condition of spent fuel pool
2.4 Course of accident at Unit 3
2.4.1 From occurrence of earthquake to arrival of tsunami
2.4.2 From arrival of tsunami to hydrogen explosion of the reactor
building
2.4.3 After hydrogen gas explosion in the R/B
2.4.4 Major events thereafter
2.4.5 Condition of spent fuel pool
2.5 Course of accident at Unit 4
2.6 Course of accident at Unit 5
2.7 Course of accident at Unit 6
2.8 Status of dose rate around power station
2.9 Evaluation of core conditions of Unit 1 to Unit 3 at time of accident
2.9.1 Evaluation of core conditions of Unit 1 at time of accident
2.9.2 Evaluation of core condition of Unit 2 at time of accident
2.9.3 Evaluation of core conditions of Unit 3 at time of accident
TOC-1

1
3
3
4
6
7
9
1-1
2-1
2-1
2-1
2-3
2-6
2-11
2-14
2-18
2-18
2-19
2-24
2-25
2-25
2-37
2-37
2-37
2-42
2-43
2-54
2-54
2-54
2-58
2-58
2-59
2-69
2-74
2-78
2-80
2-81
2-81
2-82
2-83

Chapter 3

Analysis of Causes of Accident Events and Extraction of
Problems
3.1 Flow of accident events
3.2 Extraction of subject from accident event progress
3.2.1 Cause analysis from accident event progress (event tree)
3.3 Arrangement of problems confirmed from functional viewpoint3.3.1 Causal analysis of the accident in Units 1 to 3 and managementof issues
3.3.1.1 Problems by function
3.3.1.2 Problems related to common factors and failures
3.4 Causal analysis of the accident at Unit 4 and management of issues
3.5 Management of events at Units 5 and 6
3.6 Comparison of course of events at Fukushima Daiichi Nuclear
Power Station with other stations
3.7 Causal analysis
Chapter 4 Lessons Learned and Measures to Take
4.1 Countermeasures against natural hazards
4.2 Securing power supply
4.3 Measures against loss of heat sink systems
4.4 Measures against hydrogen leakage
4.5 Preparation for emergencies (training and drills)
4.6 Countermeasures against earthquakes and tsunamis(example measures)
4.6.1 Assumption of earthquake and tsunami strength
4.6.2 Protection of the site from tsunami
4.6.3 Protection of buildings
4.7 Preparation for power supply (example measures)
4.7.1 SBO and loss of DC power supplies
4.8 Examples of measures addressing heat sink loss
4.8.1 Water injection in reactors
4.8.2 Loss of seawater cooling
4.8.3 Containment vent
4.9 Hydrogen measures
4.10 Emergency preparedness (especially training)
4.10.1 Training
4.10.2 Air conditioning and shield at Main Control Room
4.10.3 Measurement during accidents
4.10.4 Emergency operation facility
4.10.5 Radiation control and working control
4.10.6 Organization and command or order
4.10.7 Communication
4.10.8 Environmental monitoring
4.10.9 Preparing disaster countermeasures (heavy equipment
and rescue), emergency cooperative framework
4.11 Security of integrity of spent fuel4.12 Summary of countermeasures
Chapter 5 Timeline of Accident to the Present
Chapter 6 Conclusion

TOC-2

3-1
3-1
3-4
3-4
3-16
3-16
3-23
3-29
3-32
3-35
3-39
3-43
4-1
4-1
4-1
4-2
4-2
4-3
4-4
4-4
4-5
4-5
4-7
4-7
4-9
4-9
4-10
4-11
4-13
4-14
4-14
4-15
4-15
4-17
4-18
4-20
4-20
4-21
4-23
4-24
4-25
5-1
6-1

Appendix 1 Comparison of the Development of the Accident at the
Fukushima Daiichi Nuclear Power Station and at Other
Stations (Detailed Version)
Appendix 2 Agenda for Future Review
Appendix 3 Comparison with Government Report and NRC Task Team
ReportAppendix 4 MARK-I Containment
Reference 1 Station Overview

TOC-3

app-1-1
app-2-1
app-3-1
app-4-1
ref-1-1

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 house electricity system due to flooded water of tsunami
Units 1-3

Component

Starting
transformer

Cable

D/G

Emergency highvoltage switchboard(M/C)

Regular highvoltage switchboard(M/C)

Emergency Powercenter(P/C)

Regular Powercenter(P/C)

STr(1S)

Installation
site

Unit 1
Installation
floor

Transformer
Above groundyard

UnclearCondition

Immersed inwater

Component

STr(2S)

OF Cable(Switchyard STr(1S))
DG 1A

－

Underground

Unclear

T/B

B1FL

×

PartialOF Cableappearance (Switchyard is goodSTr(2S))
Submerged
DG 2A

DG 1B

T/B

B1FL

×

Submerged

－

－

－

－

M/C 1C

T/B

1FL

×

M/C 1D

T/B

1FL

×

－

－

－

－

M/C 1A

T/B

1FL

×

M/C 1B

T/B

1FL

×

－
Immersed inwater
Immersed inwater－
Immersed inwater
Immersed inwater

Unit 2
Installation
Availability
Installation floorsite
for useTransformer
Above groundyard

UnclearCondition

Component

Immersed inwater
Damage to theattachment
such asinsulator

STr(3SA)－

Under repair

×

Submerged

DG 3B

T/B

B1FL

×

Submerged

－

－

－

－

－

－

×

Submerged

M/C 3C

T/B

B1FL

×

Submerged
Submerged

DG 2B

Shared pool

1FL

×

－

－

－

M/C 2C

T/B

B1FL

M/C 2D

T/B

B1FL

×

Submerged

M/C 3D

T/B

B1FL

×

M/C 2E

Shared pool

B1FL

×

Submerged

－

－

－

－

－

M/C 2A

T/B

B1FL

×

Submerged

M/C 3A

T/B

B1FL

×

Submerged

M/C 2B

T/B

B1FL

×

Submerged

M/C 3B

T/B

B1FL

×

Submerged

M/C 2SAM/C 2SA
Building

1FL

×

Submerged

M/C 3SA

C/B

B1FL

×

Submerged

Submerged

M/C 3SB

C/B

B1FL

×

Submerged

P/C 3C

T/B

B1FL

×

Submerged

P/C 3D

T/B

B1FL

×

Submerged

－

－

－

－

－

P/C 3A

T/B

B1FL

×

Submerged

M/C 2SB

T/B

B1FL

×

C/B

B1FL

×

Submerged

P/C 2C

T/B

1FL



P/C 1D

C/B

B1FL

×

Submerged

P/C 2D

T/B

1FL



－

－

－

－

－

P/C 2E

Shared pool

B1FL

×

P/C 1A

T/B

1FL

×

Immersed inwater

P/C 2A

T/B

1FL



P/C 2A-1

T/B

B1FL

×

P/C 1B

T/B

1FL

×

Immersed inwater

P/C 2B

T/B

1FL



－

－

－

－

Base portion is
immersed inwater
Base portion is
immersed
Submerged
Base portion is
immersed inwater
Submerged
Base portion is
immersed in
water
－

－

－

－

－

－

－

－

－

－

－

－

P/C 2SB

T/B

B1FL

×

Submerged

P/C 3SB

－

－
×

-

-

－

－

－
Immersed in
water
－

-

-

－

－

－

C/B

B1FL

125V DC
BUS-1B

C/B

Component
Starting
transformer

STr(3SB)Cable

OF Cable(Switchyard STr(3SB))

Cannot bechecked(Note 1)

B1FL

×P/C 1C

1FL

Unclear

Underground

B1FL

Immersed in
water

－

Above
ground

－

T/B

×

T/B

Transformer
yard

T/B

Unclear

1FL-

Condition

OF Cable(Switchyard STr(3SA))
DG 3A

Underground

T/B

P/C 1S

Unit 3
Installation Installation Availability
site
floor
for useCannot bechecked(Note 2)Submerged
M/C
Submerged
Cannot be
used
－

－

M/C 1S

125V DC
BUS-1A

Direct current
125V

Availability
for use

－

－

－

－

－

P/C 3B

T/B

B1FL

×

Submerged

－

－

－

－

－

P/C 3SA

C/B

B1FL

×

Submerged

－

－

－

－

C/B

B1FL

×

Submerged

T/B

MB1FL



－

T/B

MB1FL



－

×

125V DC
Submerged
DIST CTR2A

C/B

B1FL

×

125V DC Main
Submerged
bus panel 3A

B1FL

×

Submerged

125V DCDIST CTR3A

C/B

B1FL

×

Submerged

125V DC Mainbus panel 3B

Installation
site

Unit 4
Installation
floor

Availability
for useCondition

Component

Condition

Component

Transformer
yard

Above thegroundUnits 4-6

D/G

Emergency highvoltageswitchboard(M/C)

Regular highvoltage switchboard(M/C)

Emergency Powercenter (P/C)

Regular Powercenter (P/C)

Direct current
125V－

Underground

UnclearUnclearCannot bechecked(Note 1)
Cannot bechecked(Note 2)

STr(5SA)OF Cable(Switchyard STr(5SA))

Unit 5
Installation
Availability
Installation floorsite
for useTransformer
yard－

Above thegroundUnderground





－

STr(5SB)－

OF Cable(Switchyard STr(5SB))

Unit 6
Installation Installation Availability
site
floor
for useCondition

Transformer Above theyardground

Underground－
Associatedequipment(Sea waterpump)
is submerged

－

－

DG 4A

T/B

B1FL

×

Submerged(Underrepair)

DG 5A

T/B

B1FL

×

Associatedequipment(Exciter)is submerged

DG 6A

C/S

B1FL

×

DG 4B

Shared pool

1FL

×

M/C
Submerged
Cannot be
used

DG 5B

T/B

B1FL

×

Associatedequipment(Exciter)is submerged

DG 6B

DG building

1FL

○

－

B1FL

×

Associatedequipment(Sea waterpump)
is immersed

－

－

－

－

－

－

M/C 4C

T/B

B1FL

×

M/C 4D

T/B

B1FL

M/C 4E

Shared pool

M/C 4A

T/B

M/C 4B

T/B

－
－
－

－
－
－

－

－

－

－

M/C 5C

T/B

B1FL

×

Submerged

M/C 6C

C/S

B2FL



－

×

Submerged(Underinspection)
Submerged

M/C 5D

T/B

B1FL

×

Submerged

C/S

B1FL



－

B1FL

×

Submerged

－

－

－

－

－

C/S

1FL



－

B1FL

×

Submerged

M/C 5A

C/B

B1FL

×

Submerged

B1FL

×

Submerged

M/C 5B

C/B

B1FL

×

Submerged

－
－
－

－
－
－

－
－
－

M/C 5SA-1
M/C 5SA-2
M/C 5SB-1

C/B
C/B
C/B

B1FL
B1FL
B1FL

×
×
×

Submerged
Submerged
Submerged

M/C 6D
HPCS DG
M/C
M/C 6A-1
M/C 6A-2
M/C 6B-1
M/C 6B-2
－
－
－

T/B
T/B
T/B
T/B
－
－
－

B1FL
B1FL
B1FL
B1FL
－
－
－

×
×
×
×
－
－
－

Submerged
Submerged
Submerged
Submerged－－－

Submerged
Immersed inwater
Immersed inwater－
Immersed inwater

HPCS DG

C/S

－

－

－

－

－

M/C 5SB-2

C/B

B1FL

×

P/C 4C

T/B

1FL

－

Under repair

P/C 5C

T/B

B1FL

×

P/C 4D

T/B

1FL



－

P/C 5D

T/B

B1FL

×

P/C 4E

Shared pool

B1FL

×

Submerged

－

－

－

－

P/C 4A

T/B

1FL

－

Under repair

P/C 5A

C/B

B1FL

×

－

－

－

－

－

P/C 5A-1

T/B

2FL



－

P/C 4B

T/B

1FL



－

P/C 5B

C/B

B1FL

×

Immersed inwater

P/C 6B-1

T/B

B1FL

×

－

－

－

－

－

P/C 5B-1

T/B

2FL



－

P/C 6B-2

T/B

B1FL

×

－

－

－

－

－

P/C 5SA

C/B

B1FL

×

－

－

－

－

－

－

－

－

－

－

P/C 5SA-1

T/B

B1FL

×

Immersed inwater
Immersed inwater
Immersed in
water

－

－

－

－

－

－

－

－

－

－

P/C 5SB

C/B

B1FL

×

125V DC Mainbus panel 4A

C/B

B1FL

×

Submerged

125V DC Mainbus panel 5A

T/B

MB1FL



125V DC Mainbus panel 4B

C/B

B1FL

×

Submerged

125V DC Mainbus panel 5B

Availability for use : Judgment of employees of Tokyo Electric Power Company after checking the conditionof the equipment on the site
Immersed in the water : Condition with the traces of immersion
Submerged : Water is accumulated: Component that cannot be used
: M/C at the load dispatching source cannot be used, so electricity cannot be received

T/B

MB1FL



－

－

－

－

－

P/C 6C

C/S

B2FL



－

P/C 6D

C/S

B1FL



－

P/C 6E

DG Building

B1FL



－
Immersed inwater
Immersed inwater
Immersed inwater
Immersed inwater

P/C 6A-1

T/B

B1FL

×

P/C 6A-2

T/B

B1FL

×

－

－

－

－

－

－

125V DC
PLANT DISTRCENTER 6A

T/B

MB1FL



－

－

125V DC
PLANT DISTRCENTER 6B

T/B

MB1FL



－

Note 1: Since the radiation dose is highNote 2: As submerging of installation site is assumed

T/B : Turbine building
C/B : Control building
C/S : Reactor combination building

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
2-21

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.

2-22

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
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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.

2-56

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.

2-59

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.
2-62

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.

2-63

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 of Accident Events andExtraction of Problems

Problems of the accident are that the 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 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 us 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 do not 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 established severe
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 Unit 1, in addition to the loss of external power due to the earthquake, loss of
emergency DG due to the tsunami made all the AC power unavailable. Besides,
DC power was lost too. It was verified that the IC worked until the tsunami hit thepower plant. It is assumed that the core was damaged after the tsunami arrived.
Then, it is assumed that hydrogen generated in the core leaked from the container
to the reactor building causing a hydrogen explosion and resulting in the dischargeof radioactivity to the environment.
For Unit 2, because of the loss of external power due to the earthquake and loss of
emergency DG due to the 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 the loss of DC power and worked for a little less than three days until around13:30 on March 14. 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 Unit 3, because of the loss of external power due to the earthquake and loss of
emergency DG due to the tsunami, all AC power was lost. But the RCIC and the
HPCI worked and secured core cooling for one-and-a-half-day until DC power wasran out at around 2:40 on March 13. But, it is assumed, since it took time to
implement alternate water injection after stop of the RCIC and the HPCI andinjected water amount was too little, the core was damaged. After that, it isguessed, hydrogen generated in the core leaked to the PCV and the R/B causing ahydrogen explosion and resulting in the discharge of radioactivity to theenvironment.

3-1

For Unit 4 (under suspension due to periodical inspection), because of the loss ofexternal power due to the earthquake and loss of emergency DG due to thetsunami, all AC power was lost. Besides, DC power was lost too. There were fuelswith 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 the loss of
power. And the 5th floor of the Reactor No. 4 building was damaged by a hydrogen
explosion. Afterward, water injection to the SFP was executed with a concrete
pump vehicle.
In Unit 5 (under suspension due to periodical inspection), fuels were loaded in thereactor and a test for pressure resistance and leakage of the RPV with the control
rods being fully inserted was performed at the time of the earthquake. Although all
AC power was lost due to the earthquake and the tsunami, AC power was securedby the power provided from Unit 6. The seawater pumps in the RHR line weredamaged by the tsunami, but the makeshift seawater pumps were used for the
RHR and the spent fuel pool and the reactor well were cooled by the RHR line.
With this, the reactor got in a state of cold shutdown.
Unit 6 (under suspension for periodical inspection) was in a state of cold shutdownwith 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 theseawater pump and the power panel were lost due to attack of the tsunami. As AC
power was secured with the remaining one Unit (air-cooling type), power supply for
Unit 5, cooling the SFP and for the reactor 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 state of cold shutdown.

3-2

Scram signal

Reactorshut-down

AC power supplyBig
earthquake

Reactorscram

External
power
supplyDC Power
supplyEmergency DC Power
DG
supplyRestoration of
Ensuring of long term cold shut-down
AC power
supplyExternal
RPV
Reactor
RHR
RHR
IC, Turbine
restoratio
driven coolant power supply, depressurizatio waterninjectionninjection
Emergency(Including(Including
system
DG, Power(RCIC,HPCI) supply
alternative
alternativeinterchange sources)
sources)that do not
require AC
power supply

Status ofCoreCore cooling

PCV Vent Cold shutdown, Core damage,(BeforePCV damage etc.
coredamage)Damaged corecooling

Containment vessel control

Depressu Reactor
rization of coolant
reactorinjection

PCV
coolant
injection

Reactor
Final statusbuilding control

PCV Vent Hydrogen SGTS, Other
control
ventilation(nitrogensubstitutionetc.)

Cold shutdown, Core damage, PCV damage etc.

Cold shutdown
Cold shutdown(A)

Cold shutdown (Long term cooling is required)
PCV damage (Overpressure)Same as (A)Same as (A)RCIC/HPCI

Integrity of PCV or PCV leakage (Overtemperature)
Cold shutdown (Long term cooling is required)
Battery iscollected
temporarily(Successful)

１F-3

PCV damage (Overpressure)

Restoration
is notpossible[Earthquake]Radioactive discharge (PCV leakage (Overtemperature) andhydrogen combustion in the building)(Continuation)

PCV damage (Hydrogen explosion)(Not
executed)(Continuation)[Tsunami]

Accident sequence
assumed by 1F-3

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.)

Accident sequence
assumed by 1F-2

Integrity of PCV or PCV leakage (Overtemperature)
Same as (A)Cold shutdown (Long term cooling is required)

PCV leakage (Overtemperature) and hydrogen combustion in the building(Continuation)

1F-2

１F-1,2

Restoration
RCIC (Starts is not
before loss of possible
DC power)

Battery iscollected
temporarily[Tsunami]

PCV damage (Overpressure)

PCV damage (Hydrogen explosion)(Not executed)

PCV damage (Overpressure)(Continuation)

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

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

Same as (A)Cold shutdown (Long term cooling is required)
Integrity of PCV or PCV leakage (Overtemperature)

1F-1
1F-1IC (Operating 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

Core damage(Delay in coolant injection or actual quantity of water injected to thereactor is less )Core damage(RPV low pressure state)(Not executed)

Radioactive discharge (PCV leakage (Overtemperature) andhydrogen combustion in the building)Accident
PCV damage (Hydrogen explosion)
sequence
assumed by 1F-1
PCV damage (Overpressure)
PCV damage (MCCI etc.)

Figure 3.1-1 Disaster identification events tree in Fukushima Daiichi Nuclear Power Station, Units 1 to 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 to supply AC power
2) Inability to remove heat from nuclear reactor
3) Hydrogen leakage to building and hydrogen explosion
Cause analysis of the above-mentioned three factors was executed to extract
the points to examine.
Moreover a common factor which obstructed the work was discovered throughthe cause analysis. These factors were analyzed to extract the points to
examine.
(1) Inability to supply AC power
Figure 3.2.1-1 shows the result of analysis of the cause that AC power could
not to be supplied.
Before the tsunami, the AC power was supplied by an external source before
the earthquake occurred and it was supplied by an emergency DG after theearthquake occurred.
The AC power could not be supplied to the equipment and for operation forreasons including the inability to supply power from the external source,
inability to supply power with an emergency DG, inability to jointly use power
supply and inability to restore power supply at early stage.

3-4

Analysis of factors
Execution phase

Issues that should be examined[Units 1 to 4]
Operation of protective device
by the contact of transmission
and steel tower[Units 1, 2]
Unable to supply
power by external
power supply

Reliability of external power
supply (substation,transmission line, switchyard)
for earthquake and tsunami

Damage to receiving circuit
breaker, Disconnect Switch inthe switchyard[Units 3, 4]
Assumption of tsunami in theswitchyard and flood

switchyard immersed in thewater[Units 1 to 4 (Air cooling type diesel generator in Units 2, 4 are excluded)]Interruption of supply
of AC power supply(Station blackout）

It was notpossible to supply
power byemergency Diesel
Generator

Emergency Diesel generator
was immersed in the water
due to tsunami

Circuit of external power
supply is concentrated in 1
location at the substation
Assumption for tsunami to thebuilding where the diesel
generator is installed and flood[1]
Flood to the installation areaof emergency Diesel
generator equipment

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

Water flooded into the building
from the opening, penetration of
the building

Loss of cooling function of
auxiliary system (seawater
pump)

[6]

For the factor analysis,refer to [6] in the diagram
3.2.1-2[Air-cooling type Diesel Generator of Units 2, 4]
Assumption of Tsunami to thebuilding where emergency highvoltage power panel is locatedand the flood

Power panel was
submerged due to tsunami[2]
Flood to the installation
area of emergency highvoltage power panel

Water flooded into the building
from the opening, penetration of
the building

Power could not be interchanged(Countermeasure for severe
accidents)

Inability to promptly
restore power
supplyPower panel of adjacent
plant was immersed in thewater

AC power supply of adjacent plant could
not be maintained

Difficulty in deployment of
power supply vehicles(Emergency arrangement)

Difficulty in arrival due todamage of roads, traffic

Power panel is notreusablePower panel was
submerged

Access failure due todamage of roads in thepremises, floatingk

For factor analysis, refer to above [2]Multiple routes within thepremises

Securing materials and equipment
to maintain the nighttime, indoor
work environment , Provision ofprocedure/training

Difficulties in the operation caused byenvironment· Nighttime operation· Outdoor work under the warning ofmassive tsunami

Loss ofcommunication
methods[2]

Securing heavy equipment
for ensuring the accessibilityof the roads

Cable laying operation became
difficult (Weight, Distance,
Work environment)

Loss of power ofcommunication facility

Loss of power ofalternative
communication facility[1]

For factor analysis, refer toabovementioned [1]Securing the communication
facility (communication
methods, quantity) and power
supply for communication

Quantity of alternative
communication facility

Communication facility
was immersed in the
water

[4]

Work difficulty due tohigh dose ofradioactivity[3]
Maintaining the number of
persons required to control
the radiation

Ensuring radiation control
staff at the time of
emergency

Difficult y in securing required numberof staff for personal dosimeter,
protective equipment, management
system equipment

Radiation control systemequipment were immersedin the water

Difficulty in ensuring
necessary personnel due toradiation exposure

For the analysis offactors for [3] :[Inability topromptly restore power
supply], refer to following
items

Exchange of materials andequipment between electricalpower

Access within the building
became difficult due to highdose rate
Outdoor debris removal
operation became difficult due
to radioactive contamination

Figure 3.2.1-1 Analysis of causes for interruption of AC power supply

3-5

For the analysis offactors for [4] : [loss ofcommunication
methods], refer tofollowing items

Communication facility
was flooded

Damage of temporary cabledue to explosion of adjacent
plant(After the core damage)For the analysis offactors for [2]:[Submersion of power
panel due to the
Tsunami], refer tofollowing items[5]

For the factor analysis of[5]: [Difficulties in thework] after core damage,refer to these items

a.

Inability to supply power from external source
The power supply from the outside was able to be received from the new
Fukushima substation. After the earthquake, however, the power supply
lost its function with the damage to the substation equipment due to theearthquake, with the damage to the receiving breaker and disconnecting
switch in the switchyard in the power plant due to the earthquake, and withthe movement of protection instrument caused by the power line coming
into contact with the steel tower and such like.
Note that traces where tsunami reached were confirmed at the switchyardof Units 3 and 4.
From these points the following is listed as the points to examine tomaintain the power supply with an external source.
• Reliability of external source (substation, power line and switchyard) inthe event of an earthquake and tsunami• Flooding of switchyard by tsunami• Concentration of external source lines in one place in substation(multiplexing of line)b.

Inability to supply power with emergency diesel generator
The power had been supplied by the emergency DG after the earthquake.
After the tsunami, the functions of the seawater pumps were lost except for
Unit 6 and the functions of the DG were lost because of flooding in thebuilding. Although air-cooled emergency DG of Units 2 and 4 were notflooded by the tsunami, they lost their power supply functions because thehigh-voltage power supply panel was flooded.
From these points the following is listed as the points to examine tomaintain the power supply by emergency DG.
• Flooding in installation area of emergency power generating machine(DG and power supply panel)• Flooding in building from opening and penetration part of building• Flooding of cooling seawater system pump for emergency DG
Note that AC power could be supplied to Units 5 and 6 with the air-cooled
emergency DG of Unit 6.

c.

Inability to jointly use power supply
Joint use of power supply from the adjacent Unit prepared for a severe
accident was not performed for Units 1 to 4 since the power was lost in allthese Units.
Joint use of a power supply could be performed since the AC power had
been supplied to Units 5 and 6 from the emergency DG of Unit 6.

d. Inability to restore power supply at early stage (excluding Units 5 and 6
for which AC power was secured)
Since the AC power was lost, the workers made an attempt to restore thepower supply at an early stage by arranging to promptly procure from3-6

outside the power plant power supply vehicles for emergencies. As a result,however, the power supply could not be restored before the core became
damaged.
Work to restore the power at an early stage was frequently interrupted by
the aftershocks and official tsunami warnings. Moreover night work or work
conducted under an official tsunami warning had a lower 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 and such like. In addition the communicationequipment (paging and security telephone) excluding some pieces ofapparatus were unusable because of flooding or because they lost theirpower supply. It became impossible to use cellular phones and PHS
gradually as their batteries ran out. A means of communications between
worksites, or with MCR or emergency stations were broken off and work
became difficult.
From these points the following are listed as points to examine to restore
the power supply at an early stage• Flooding on power supply panel
Moreover the following are listed as the points to examine to improve theeffectiveness of recovery efforts.
• Securing the materials and equipment to secure a working environment
in the nighttime or room
• Securing heavy equipment to secure accessibility of roads• Having multiple routes in the plant• Securing communication equipment (communication means andamount) and power supply for communication including batteries• Flooding on communication equipment
Furthermore, “Maintenance of procedure and training” is also extracted asa point to examine to improve the effectiveness of work though it is not
limited to the power supply restoration.
After the hydrogen explosion that followed the reactor core damage, there
was a difficult work environment in the plant.
Workers got injured by the hydrogen explosion of the reactor building. In
Unit 1, there was a loss such as the damage of a cable installed for
emergencies to execute alternative water filling using the SLC pump (standby liquid control). 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 systemwere flooded by the tsunami. Hence there were not enough personal
dosimeters and there were not enough for all workers to wear one.
From these points the following are listed as the points to examine.

3-7

•
•
•

Joint use of materials and equipment between electric powercompanies
Securing of radiation control workers for emergency
Flooding on radiation control system(2) Inability to remove heat from nuclear reactor (excluding Units 4, 5 and 6that were stopped)
Figure 3.2.1-2 shows the result of analyzing the reason why heat could not be
removed from the nuclear reactor.
IC (Unit 1), HPCI (Units 1 to 3) and RCIC (Units 2 and 3) were built as heatremoving equipment and as high-pressure system equipment to decompress
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 Units 1 to 3 had a containment vessel vent Unit to prevent
overpressure of containment vessel and some severe accident measureequipment.
It was impossible to remove heat from the nuclear reactor from the view of theequipment and operation for the following reasons: inability to conduct
decompression with SRV, inability to remove heat with CS and RHR, inability
to remove heat with severe accident measure equipment (alternative water
filling), and difficulty in venting containment vessel.

3-8Analysis of factors
Execution phase

Injection by HPCI,RCIC is impossible

Issues that should be examined

Loss of control power
supply[1]

Station Blackout

Injection by IC is notpossible

Loss of power supply
necessary for valve
driving[Units 1, 2]
Storage battery andpower panel were
submerged

Loss of DC power supply

Difficulty in depressurization
by main steam relief safetyvalve

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

D.C. power supply system(Storage Battery, Powerpanel) was flooded[Unit 3]

Loss of power supply
necessary for valve
driving

Depletion ofStorage battery

Difficulty in depressurizing
the reactor by main steam
relief safety valve

Ensuring storage battery
capacity

Ensuring alternative power
supply by the battery installed on
the vehicle

Ensuring materials and equipment (storage
battery etc.), Provision of procedures andtrainings[Units 2, 3]
Inability to remove
the heat from thereactorInability to remove theheat by Core spraysystem and residual
heat removal system

Power supply of low
pressure flooder pump etc.
is not dependable

Station Blackout

Loss of cooling function of
auxiliary system (seawaterpump)

Submersion ofseawater pump

[1]

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

Assumption of tsunami toseawater system and flood

Ensuring means of cooling byalternative methods

Power supply of sea
water pump could not be
maintained

Difficulty in early
restoration of seawaterpump

Power panel issubmerged[2]

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

Securing heavy equipmentfor
ensuring the accessibility of theroads

Access trouble due to damage ofinternal roads, floating wreckage

Multiple routes within thepremises
Operational difficultiescaused by theenvironment· Work in the nighttime∙ Outdoor work under warning of massive
tsunamiFor the analysis ofSecuring material and equipment to factors for [6]: [loss ofmaintain the nighttime, indoor work cooling function
environment , Provision of(seawater pump) ofprocedures and trainings
auxiliary system), refer

Huge workload (laying operation of power
supply cable) and task experience

Providing procedures andtrainings

Loss ofcommunication
methods

Inability to remove the heatby severe accidentscountermeasure system

Unusable condensate watermakeup system(Severe accidentscountermeasures)Containment & Vessel Cooling
Water System could not be used
(Severe accidentscountermeasures)[6]Power panel was
submerged due to
Tsunamito the following items

[4]

[2]

For factor analysis, refer to [4]in the diagram 3.2.1-1

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

Early restoration ofalternative power supply
was not possible[3] [3] in the diagram 3.2.1-1

Power panel was
submerged due to Tsunami[2] [2] in the diagram 3.2.1-1

Early restoration ofalternative power supply
was not possible[3] [3] in the diagram 3.2.1-1

For factor analysis, refer to

For factor analysis, refer to

For factor analysis, refer toSubmersion of sea
water pump

[6]

For analysis of factors, refer
to abovementioned [2]Early restoration of sea water
pump was impossible

Fire Protection System could
not be used(severe accidentscountermeasures)

Power panel was
submerged due to
TsunamiDifficulties in making fireengines ready to use

Difficulties in
depressurization of reactor

Difficulties in the
work

[2]
(Units 1, 2 )Power panel was submerged
due to Tsunami[2]

For analysis of factors, refer to[2] in the diagram 3.2.1-1

For analysis of factors, refer to[2] in the diagram 3.2.1-1(Unit 3)
Exhaustion of DC power
supplyEnsuring capacity of storage
battery

Access trouble due to damage ofinternal roads, floating wreckage

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

Operational difficulties caused by theenvironment· Work in the nighttime· Outdoor work under the warning ofmassive tsunami

Loss of communication
methods

Depletion of fire prevention
water tanks

Securing materials and equipment tomaintain the nighttime and indoor work
environment , Provision of procedures andtrainings[4]

For analysis of factors, referto[4] in the diagram 3.2.1-1

Securing source of water necessary foralternative water injection

Evacuation of field,high dose debris, damageof hose for injection, re-laying due toexplosion of neighboring plant

Reactor container ventdifficulties(After core damage)

Loss of power supply ofMotor Operated Valve

Securing driving source of valvethat constitutes pressure
resistant vent line

Air Operated Valve driving
source failAcquiring materials and equipment in case of loss ofvalve driving source, Provision of procedures andtrainings

Difficulties in work due tohigh dose of radioactivity[5]

Figure 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 to cooling by with 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, the workers opened the valve and confirmed thegeneration of steam. Nevertheless, the extent of the operation performanceis unknown. Actually, we suppose that only a limited level of cooling could
be achieved.
The HPCIs in the Unit 1 and Unit 2 were in standby mode, because they did
not reach the start-up condition before the tsunami arrived. However,
because their DC power was lost under the impact of the tsunami, theirfunctions were lost. The HPCI in Unit 3 started up automatically because
the reactor water level fell. Then, the system must have shut down due tothe fall in reactor pressure, which led to an inability to operate due to lack of
DC power.
The RCIC in Unit 3 was started up manually after the reactor scram. The
system was started up and shut down for adjustment of the reactor waterlevel. While the operation was continued using battery even after thetsunami arrived, the operation must have shut down when the battery
power ran out.
Based on above, the following items are listed as points to examine toensure maintenance of functions of IC, HPCI and RCIC.
• Submergence of D.C. power supply systems (battery cell, power supply
panel)• Security of battery capacityb.

Inability to conduct depressurization with main steam safety relief
valve
In Unit 1 and Unit 2, the DC power necessary to drive SRVs was lost due tosubmergence under the impact of the tsunami. With regard to Unit 3, it
stopped working with other components operated by DC power on March
13 due to exhaustion of DC power, while the DC power was not affected bysubmergence caused by the tsunami.
To achieve alternative coolant injection, it was necessary to depressurize
the reactor by operating the SRVs. Therefore, the workers needed to open
an electromagnetic valve to supply nitrogen as a source to drive the SRVs.
In Unit 3, the workers tried to open electromagnetic valves by using theon-board batteries of fire engines when they came to the site for alternative
water injection. Because the battery cells stored in the station were used torecover the instrumentation in Unit 1 and Unit 2, batteries were collectedfrom the vehicles of employees who were working in the on-site disastercountermeasure headquarters and connected to the instrumentation
panels. Finally, after about 6 hours from the loss of the high-pressure
injection system function, the SRV were opened and rapid depressurizationof the reactor could be achieved.

3-10Based on above, following items are listed as points to examine to maintaindepressurization function by SRVs.
••••

Inability of supplying AC power (See 3.2.1(1))
Submergence of D.C. power supply systems (battery cell, power panel)
Security of battery capacity
Security of materials and equipment to ensure SRV operation in theevent of the loss of DC power and implementation of the procedure andthe training thereofc.

Inability to remove residual heat with core spray system (Units 1 to 3)
and residual heat removal system (Units 2 and 3)
The inability to remove residual heat by using CS and RHR (low pressure
injection mode) is attributable to an inability to supply AC power and thefunction loss of seawater pumps for cooling related auxiliary components.
The inability to supply AC power was already explained in 3.2.1(1). The
loss of cooling function in seawater pumps occurred because of thesubmergence caused by the tsunami that exceeded the estimated highest
level (O.P. 5.7 m). All of the seawater pumps stopped working
simultaneously.
With regard to the recovery of seawater 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 seawater pumps weremaintained; work was interrupted repeatedly due to aftershocks; work
efficiency was poor because of the issue of great tsunami alarm at nightand such like. Besides the seawater pumps, the power panels necessary
for the function of the component cooling system were submerged.
Therefore, early restoration of this system was extremely difficult.
Based on above, the following items are listed as points to examine tomaintain residual heat removal function by CS and RHR.
• Estimation of tsunami and submergence of seawater systems• Security of cooling method by using alternative approachesd.

Inability to remove residual heat by systems used for as severe
accident countermeasure
As systems to serve as a 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 dueto the station blackout and the function loss of auxiliary component coolingsystem.
While it was stipulated to use FPs as an alternative injection system basedon severe accident countermeasures, the function of the motor operated
fire extinguishing pumps was lost due to the power loss. While the pumps
driven by a diesel motor could be operated temporally in Unit 1, they wereshut down later due to a malfunction. It seems that these pumps failed toachieve start-up operation in Unit 2, the cause of this failure is unknown,
making it difficult to judge whether or not this event is attributable to
3-11

component failure. In Unit 3, these pumps operated temporarily and shut
down later. Details of this event are unknown.
Workers tried to inject water into the reactor with fire engines that weremaintained as a part of earthquake preparedness after the earthquake thatstruck Kashiwazaki Kariwa Nuclear Power Station. However, 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; work
was interrupted repeatedly due to aftershocks; work efficiency was poor
because of the issue of great tsunami alarm at night and such like. As aconsequence, 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 theseawater reserved in back wash valve pit in the afternoon on March 12 due
to exhaustion of the fresh water.
Further, workers were injured in the hydrogen explosion in the reactor
building. Hoses and fire engines prepared for alternative injection weredamaged by the hydrogen explosion and made unavailable. Materials andequipment left behind on site caused disturbance in outdoor work later,which became a contributing factor for the prolonged work period.
Based on above, following items are listed as points to examine to secureresidual heat removal function by using systems for severe accident
countermeasure:• Loss of AC power• Loss of seawater pump function• Security of water source necessary for alternative injectione.

Containment vent work
The purpose of the containment venting is to secure the integrity of thereactor containment Unit by relieving elevated containment pressure.
Further, the venting may be used to discharge heat when reactor heat isaccumulated inside the containment due to the loss of safety-related
systems.
In Units 1 to 3 of Fukushima Daiichi Nuclear Power Station, the purpose of
the vent work at the initial phase was to relieve pressure.
In Units 1 to 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 Unit 1, motor-operated valves were opened manually. The
AO valve that constitutes the main line (i.e., 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 the radiation level in this area was high. Besides,
the workers continued to operate in darkness and in a narrow space. These
factors prevented them from accessing the valve. The workers gave up on
3-12

their plan to open the valve manually and decided to use the temporarycompressor for the work. As a consequence, it took a long time until thestart of venting.
While venting was finally achieved on Unit 1 and Unit 3, it took a long time
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 Unit 2, because thecontainment pressure failed to reach the actuation pressure of the rupture
disk continuously for a long time after the reconstruction of the vent line.
Based on above, following items are listed as points to examine to secure
implementation of containment vent:• Security of power to drive valves that constitute the containment ventline
• Security of materials and equipment to achieve prompt line
configuration in the loss of electric power and pneumatic source to drive
the valves•

Improvement of procedure and training(3) Hydrogen leakage to building and hydrogen explosion (excluding Units 5
and 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 to thebuilding is estimated to be attributable to the hydrogen generated from fuelcladding due to core damage. This hydrogen seems to have leaked out to thereactor building via the containment vessel. With regard to the leakage fromthe containment to the reactor building, leakage through the flange gasket or
penetration sealing with the increase of containment pressure is suspected.
With regard to Unit 2, pressure was lost from the S/C and the containment ataround 6:00 on March 15. Further investigation is needed to identify theleakage route from RPV and the containment upon convergence of theaccident.
With regard to Unit 4, generation of a large amount of hydrogen in the fuel poolis unlikely based on the analysis of nuclides obtained by sampling the fuel poolwater in addition to the result of visual observations of the fuels in the poolusing monitor cameras.
Meanwhile, the SGTS exhaust pipe in Unit 4 merged with the exhaust pipe of
Unit 3 before the stack. Therefore, the hydrogen generated in Unit 3 may havemigrated in the Unit 4 building via the SGTS exhaust pipe, resulting in ahydrogen explosion in the Unit 4 reactor building after accumulating there for awhile. Based on the radiation measurement result, the radiation dose was thehighest at the exit of SGTS filter train in Unit 4. The dose gradually deceased
as the measurement point moves closer to the inlet. These results suggest theappropriateness of the above-mentioned scenario. While an AO valve wasinstalled on the SGTS exhaust pipe, the valve was stuck open due to the lossof 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 event of containment venting due to power loss.

3-13

In every plant, prior detection of hydrogen leakage into the reactor building
could not be achieved. Besides, there was no means available for removing
the hydrogen with a loss of AC power.
Based on above, following items are listed as points to examine to preventhydrogen leakage to the buildings and the hydrogen explosion in the event ofhydrogen negation due to core damage and such like:• 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 training

3-14

Factor analysis
Implementation stage

Hydrogen leakage tobuildings and hydrogen
explosion

Prevention ofhydrogengeneration

Hydrogen generated inreactor pressure vessel

Agenda for review

Maintenance of cooling
functionCore damage

Maintenance of confining
function

Prevention ofhydrogenaccumulation

Hydrogen leakage tothe reactor building

Further investigation isnecessary to identify
the leakage route frompressure vessel

Damage ofpressure vessel

Units 1 to 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 to
the reactor building fromthe exhaust pipe

Accumulation ofhydrogen in thereactor building

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

No equipment to remove hydrogenfrom the reactor building

Difficulty of conducting the work to
remove hydrogen from the 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 hydrogenin the reactor building

Procedure and materials andequipment to remove hydrogenwas not accessible

Security of materials andequipment to removehydrogen gas from thebuilding

Implementation of procedureand training

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

3-15

3.3

Arrangement of problems confirmed from functional viewpoint
Fukushima Daiichi Nuclear Power Station Units 1 to 4, located adjacent to eachother, were different in condition when the earthquake and tsunami occurred.
Units 1 to 3 were producing their rated power, while Unit 4 had been stopped toundergo a periodic inspection (with all fuel discharged). In addition, Unit 5 and 6of the power station are located away from Units 1 to 4. These requirements
caused differences in the progress of phenomena.

3.3.1

Causal analysis of the accident in Units 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 vehicle) 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 the accident,
classified by functions of the Units (Units 1 to 3) producing rated power, andthe results of arrangement.

3-16

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

Related facility, etc.
Safety protection system,
control rod and control rod
Recirculation pump trip (AM)
Alternative reactivity control
Alternative control rod insertion
Boric acid water injection
Sub-criticality maintenancesystemfunction
External power supply

Power supply function

Emergency diesel generator
6.9 kV high pressure power
supply480 V low pressure power
supply125 V DC power supply
Power supply interchange(AM)

Supportsystems

Alternative power supply
function

Power source car (emergency
arrangement)

Function loss or function degradation(Normal operation when earthquake occurred)

-

-

-

(Not required as scram was successful)(Not required as scram was successful)

-

-

-

(Not required as scram was successful)

-

-

X

Loss of external power supply due to the earthquake

X

Function loss due to submergence

X

Function loss due to submergence

X

Function loss due loss of 6.9 kV high pressure power
supplyFunction loss due to submergence

X
X

X

Function loss due to submergence
Power supply interchange unavailable from adjacent
plantsDifficulty in reaching

Difficulty in connection work

Power supply interruption
Function loss due to submergence

Containment vessel coolingseawater system

X

Cooling seawater system

X

Function loss due to loss of AC power supply
Function loss due to submergence

Auxiliary cooling function

Reactor component coolingwater system

Function loss due to loss of AC power supply
Function loss due to loss of DC power supply
Function loss due to loss of component coolingseawater system

X

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

Function to prevent overpressurization of reactor
coolant pressure boundaryHigh pressure core cooling
function

Main steam relief safety valve(safety valve function)-

High pressure coolant injection
systemX

Emergency condenser (mainbody)

(Temporary
operation)

Emergency
condenser(support
system)Demineralized
water supply
systemFire protection
systemCool
down

Low pressure core cooling
function(Controlled with emergency condenser)X(Temporary
operation)
X

Control rod drive water system(AM)

X

Deterioration in function due to loss of DC power
supply (remote operation not possible)
Isolated with isolation signal transmission (Open
halfway due to loss of AC power supply)
Function loss due to loss of AC power supply
Unable to use electric pump due to loss of external
power supply
Diesel driven extinguishing pump shutdown

Difficulty in power supply recovery operation
Function loss due to loss of AC power supply

Core spray system

Makeup water system (AM)Difficulty in reaching due to damaged roads and traffic jams
Unable to connect due to submergence of DC power supply
panels of Unit 1
Difficulty in laying cables (weight, isolation, work environment)Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of AC power supply
Cables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building
Submergence of containment vessel cooling seawater pump
due to the tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of cooling seawater pump due to the tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 125 V DC power supply panels due to tsunami
Submergence of cooling seawater pump due to the tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 125 V DC power supply panels due to tsunami(1) Reliability of switchyard circuit breaker(2) Submergence of the emergency diesel generator
Submergence of 6.9 kV high pressure power(3)
supply panels
Same as (3)Submergence of 480 V low pressure power supply
panels(5) Submergence of 125 V DC power supply panels
Submergence of power supply panels of the(6)adjacent plant(7) Procurement of alternative power supply from
(4)

Same as (3), (4)(8) Alternative power supply cable laying(9) Communication means at a loss of AC power
Submergence of containment vessel cooling(10)seawater pumpSame as (3)(11) Submergence of component cooling seawater
Same as (3)Same as (5)Same as (11)Same as (3)Same as (5)-Function loss due to loss of DC power supply

Function loss due to loss of AC power supply

Boric acid water injection
system (AM)

Main steam relief safety valve(relief valve function /Receiving circuit breaker of Ookuma Line 2L in the switchyard of
Unit 1/2 damaged due to the earthquake
Submergence of emergency diesel generator due to the tsunami
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
Submergence of 480 V low pressure power supply panels due totsunamiSubmergence of 125 V DC power supply panels due to tsunami
Loss of station power of adjacent plant (Unit 2) due to tsunami-

Submergence of 125 V DC power supply panels due to tsunami

Alternative injection function(high pressure)

Reactor depressurization
function

ProblemCause analysis



X

X

X

Function loss due to loss of DC power supply
Function loss due to loss of component coolingseawater system
Function loss due to loss of DC power supply

Same as (5)Submergence of 125 V DC power supply panels due to tsunami

Receiving circuit breaker of Ookuma Line 1L damaged due tothe earthquake
Mechanical malfunction (presumed)
Submergence of 480 V low pressure power supply panels due totsunami[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Submergence of 480 V low pressure power supply panels due totsunamiSubmergence of 125 V DC power supply panels due to tsunami
Submergence of cooling seawater pump due to the tsunami
Submergence of 125 V DC power supply panels due to tsunami

Injection disabled as reactor depressurization function Submergence of 125 V DC power supply panels due to tsunami
cannot be used
Submergence of 6.9 kV high pressure power supply panels due
Function loss due to loss of AC power supply
to tsunami
Function loss due to loss of DC power supply
Submergence of 125 V DC power supply panels due to tsunami
Function loss due to loss of containment vessel
Submergence of containment vessel cooling seawater pumpcooling seawater system
due to the tsunami
Injection disabled as reactor depressurization function Submergence of 125 V DC power supply panels due to tsunami
cannot be used
Submergence of 480 V low pressure power supply panels due toFunction loss due to loss of AC power supply
tsunami3-17

Same as (5)Pipe break detection circuit operational due to loss of DC power
Verification of the design of isolation signal for loss
(12)
supplyof DC power supply
Submergence of 480 V low pressure power supply panels due toSame as (4)tsunamiSame as (1)Same as (4)Same as (4)Same as (5)Same as (11)Same as (5)Same as (5)Same as (3)Same as (5)Same as (10)Same as (5)Same as (4)Table 3.3-1 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Station Unit 1 and Sorting of the problems (2/2)
Safety function

Related facility, etc.

Function loss or function degradation
Cause analysis
Injection disabled as reactor depressurization function Submergence of 125 V DC power supply panels due to tsunami
cannot be used
Submergence of 6.9 kV high pressure power supply panels due
Containment vessel cooling
Function loss due to loss of AC power supplyX
to tsunami
system (AM)Function loss due to loss of DC power supply
Submergence of 125 V DC power supply panels due to tsunami
Function loss due to loss of containment vessel
Submergence of containment vessel cooling seawater pumpcooling seawater system
due to the tsunami
Injection disabled as reactor depressurization function Submergence of 125 V DC power supply panels due to tsunami
Alternative injection function
Fire protection system (AM)X
cannot be used(low pressure)
Diesel driven extinguishing pump shutdown
Mechanical malfunction (presumed)
Injection disabled as reactor depressurization function Submergence of 125 V DC power supply panels due to tsunami
cannot be used
Difficulty in ensuring the required coolant injection flow Difficulty in ensuring the capacity of the fire protection water tankrateused as the water source
Obstacles due to the impact of the earthquake and tsunami(insufficient flow
Fire engine (Chuetsu coast
Site evacuation due to explosion in the Unit 1 Reactor Building
rate, difficulty inearthquake response)and high radiation dose
working)seawater injection hose damaged due to explosion in the Unit 1
Difficulty in working
Reactor Building  relaying
Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of AC power supply
Main steam isolation valve
Containment isolation(Normal operation)
Submergence of 6.9 kV high pressure power supply panels due
Function loss due to loss of AC power supply
Containment vessel coolingto tsunami
X
Function loss due to loss of containment vessel
systemSubmergence of containment vessel cooling seawater pumpcooling seawater system
due to the tsunami
Function for removing heat
Submergence of 6.9 kV high pressure power supply panels duefrom containment vessel
Function loss due to loss of AC power supply
to tsunami
Shutdown cooling system
X
Function loss due to loss of DC power supply
Submergence of 125 V DC power supply panels due to tsunami
Function loss due to loss of component cooling
Submergence of cooling seawater pump due to the tsunami
seawater system
Alternative containment vessel
Submergence of 480 V low pressure power supply panels due toX
Function loss due to loss of AC power supply
spray with makeup watertsunamiUnable to use electric pump due to loss of external
Receiving circuit breaker of Ookuma Line 1L in the switchyard of
Alternative containment vessel
power supply
Unit 1/2 damaged due to the earthquakeX
spray with fire protection
Diesel driven extinguishing pump shutdown
Mechanical malfunction (presumed)
Operation not implemented for alternative
system (AM)Priority for feed water and alternative coolant injection tocontainment vessel sprayemergency condenser (presumed)
Submergence of 125 V DC power supply panels due to tsunami(Remote operation of air-operated containment vessel ventvalve not possible)
Block up
Drop in pressure of instrument air due to loss of external powerPressure withstanding
Alternative heat removal(Time required
Difficulty in vent operation
Unable to open the electric valve of cylinder line due to blackout
reinforced vent (AM)function
due to blackout)
and rise in radiation
Difficulty in accessing due to rise in radiation dose
Loss of communication means due to loss of AC power supply
Difficulty in maintaining the air pressure and power supply of AO
Submergence of 480 V low pressure power supply panels due toFunction loss due to loss of AC power supply
tsunamiDrywell cooler (AM)X
Function loss due to loss of component cooling
Submergence of component cooling seawater pump due to theseawater system
tsunamiSubmergence of 480 V low pressure power supply panels due toFunction loss due to loss of AC power supply
Reactor coolant cleanup
tsunamiX
Function loss due to loss of component coolingsystem (AM)Submergence of component cooling seawater pump due to theseawater system
tsunamiRadioactive material release
Submergence of 480 V low pressure power supply panels due toStand-by gas treatment system
X
Unable to operate due to loss of DC power supply
reduction function
tsunamiDeterioration in the seal of flange gasket and penetration due toContainment vessel
X
Leakage generation
loss in the function for removing heat from containment vessel
Other
Hydrogen generation due to the reaction of zircaloy and water
Reactor building
X
Damage due to hydrogen explosion
Hydrogen leakage from containment vessel penetration part
Hydrogen retention in reactor building
Air heating and ventilating
Important functions related to
Submergence of 480 V low pressure power supply panels due toX
Function loss due to loss of AC power supply
system in central control room
safetytsunamiFunction to determine theMonitoring instruments at thecondition of the plant during
X
Function loss due to loss of DC power supply
Submergence of 125 V DC power supply panels due to tsunami
time of an accident
anaccident
Other
Submergence of 480 V low pressure power supply panels due toCommunication equipment
X
Function loss due to loss of AC power supply
tsunamiFunction to determine
Submergence of 480 V low pressure power supply panels due toabnormal conditions
Function loss due to loss of AC power supply
Emergency lightingX
tsunamiFunction loss due to loss of DC power supply
Submergence of 125 V DC power supply panels due to tsunami
: Normal operation; : Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)

3-18

ProblemSame as (5)Same as (3)Same as (5)Same as (10)Same as (5)Same as (5)(13) Depletion of fresh water source(14) Obstacles due to the earthquake and tsunami
Deterioration in work environment due to hydrogen
explosion and rise in radiation dose

(15)

Same as (15)Same as (9)Same as (3)Same as (10)Same as (3)Same as (5)Same as (11)Same as (4)Same as (1)Same as (5)(16) Difficulty in ensuring compressed air
Same as (4) and (15)Same as (15)Same as (9)Same as (4)Same as (11)Same as (4)Same as (11)Same as (4)Heat resistance and pressure resistance of gasket
and penetration seal(18) Hydrogen retention in reactor building(17)

Same as (4)Same as (5)Same as (4)Same as (4)Same as (5)Table 3.3-2 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Station 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
Recirculation pump trip (AM)
Stoppage Alternative reactivity control
Alternative control rod insertion
Boric acid water injection
Sub-criticality maintenancesystemfunction

Power supply function

Problem(Normal operation when earthquake occurred)

-

-

-

(Not required as scram was successful)(Not required as scram was successful)

-

-

-

(Not required as scram was successful)

X

Loss of external power supply due to theearthquake

Emergency diesel generator

X

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

Function loss due to submergence

125V DC power supply
Power supply interchange(AM)

Supportsystems

Cause analysis

External power supply

480V low pressure power
supplyAlternative power supply
function

Function loss or function degradation
Power source car (emergency
arrangement)

X

Function loss due to loss of 6.9 kV high pressure
power supply
Function loss due to partial submergence

X
X

X

Function loss due to submergence
Power supply interchange unavailable fromadjacent plants
Difficulty in reaching
Difficulty in connection work

Emergency diesel generator
facility and cooling seawater
systemX

Seawater system for residual
heat removal system

X

Cooling sea water system

X

Function loss due to submergence
Function loss due to loss of AC power supply
Function loss due to submergence
Function loss due to loss of AC power supply

Auxiliary cooling function

Function loss due to submergence

Reactor component coolingwater system

X

Function loss due to loss of AC power supply
Function loss due to loss of DC power supply
Function loss due to loss of component coolingseawater system
Function loss due to loss of AC power supply
Function loss due to loss of DC power supply

Function to prevent overpressurization of reactor
coolant pressure boundaryHigh pressure core cooling
function

Main steam relief safety valve(safety valve function)

High pressure coolant injection
systemX(Normal operation)
Function loss due to loss of DC power supply

Function loss due to loss of AC power supply
X
Power supply interruption

Alternative injection function(high pressure)

Function loss due to loss of AC power supply
Control rod drive water system(AM)

X

Function loss due loss of component coolingseawater system
Power supply interruption

Reactor depressurization
function

Low pressure core cooling
function
Cool
down

Main steam relief safety valve(relief valve function /automatic depressurization
function)
Automatic depressurization

Core spray system

Difficulty in reaching due to damaged roads and traffic jams
Difficulty in laying cables (weight, isolation, work environment)Intermittent aftershocks and continuous issue of large tsunami
warnings

Function loss due to loss of DC power supply
X
X

X(1) Reliability of switchyard circuit breaker
Submergence of the emergency diesel
generator facility(3) Submergence of 6.9 kV high pressure power
supply panels
Same as (3)(2)(4) Submergence of 480V low pressure power
supply panels(5) Submergence of 125V DC power supply panels(6) Submergence of power supply panels of theadjacent plant(7) No deployment of alternative power supply(8) Alternative power supply cable laying(9) Communication means at a loss of AC power
supplyCables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building
Submergence of seawater pump for cooling the emergency(10) Submergence of cooling seawater system fordiesel generator due to the tsunami
emergency diesel generator
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)to tsunami
Submergence of seawater pump for cooling the residual heat(11) Submergence of seawater pump of the residual
removal system due to the tsunami
heat removal system
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)to tsunami
Submergence of cooling seawater pump due to the tsunami(12) Submergence of cooling seawater pump
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)to tsunami
Submergence of 125V DC power supply panels due to tsunami
Same as (5)Submergence of cooling seawater pump due to the tsunami
Same as (12)Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 125V DC power supply panels due to tsunami

Same as (3)Same as (5)-Reactor core isolation cooling(Operational for Unable to control due to loss of DC power supply
systemapprox. 3 days)
Boric acid water injection
system (AM)

Loss of station power of adjacent plant (Unit 1) due to tsunami

Loss of communication means due to loss of AC power supply
Power supply interruption-

Receiving circuit breaker of Ookuma Line 2L in the switchyard of
Unit 1/2 damaged due to the earthquake
Circuit breaker for Ookuma Line 2L in new Fukushima
substation damaged
Emergency diesel generator submerged in water due to thetsunami (2A) and power source panels submerged (2B)
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
Partial submergence of 480V low pressure power supply panels
due to tsunami
Submergence of 125V DC power supply panels due to tsunami

Difficulty in depressurizing
Function loss due to loss of DC power supply
Injection disabled as reactor depressurization
function cannot be used
Function loss due to loss of AC power supply
Function loss due to loss of DC power supply
Function loss due to loss of seawater system ofresidual heat removal system

3-19

Submergence of 125V DC power supply panels due to tsunami

Same as (5)Same as (5)Submergence of 125V DC power supply panels due to tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Unable to receive power due to loss of external power supply
and unavailability of emergency diesel generator
Submergence of cooling seawater pump due to the tsunami[Recovery not possible] Cables damaged and power supply car
stopped automatically due to explosion in the Unit 1 Reactor
Submergence of 125V DC power supply panels due to tsunami
Deterioration of the steam condensing performance due to riseof pressure and temperature in the pressure suppression
Insufficient voltage in temporarily installed batteries
Submergence of 125V DC power supply panels due to tsunami
Submergence of 125V DC power supply panels due to tsunami
Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 125V DC power supply panels due to tsunami
Submergence of seawater pump for cooling the residual heatremoval system due to the tsunami

Same as (3)Same as (1) - (3)Same as (12)Same as (5)Same as (5)Same as (5)Same as (3)Same as (5)Same as (11)

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

Related facility, etc.

Residual heat removal system
Low pressure core cooling (low pressure coolant injection
function
system)Makeup water system (AM)Function loss or function degradation
Injection disabled as reactor depressurization
function cannot be used
X

X

Function loss due to loss of AC power supply
Function loss due to loss of seawater system ofresidual heat removal system
Injection disabled as reactor depressurization
function cannot be used

Other

Fire engine (Chuetsu coast
earthquake response)(insufficient
flow rate,difficulty inworking)

Main steam isolation valve

X

Alternative containment vessel
spray with makeup water

X

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

Pressure withstanding
reinforced vent (AM)X

Drywell cooler (AM)X

Reactor coolant cleanup
system (AM)X

Communication equipment
Function to determine
abnormal conditions

Emergency lighting

Same as (11)Same as (5)Same as (3)X

Radiation release reduction
Stand-by gas treatment system
functionContainment vessel
Other
Reactor building
Air heating and ventilating
Important functions related
system in central control room
to safety
Function to determine theMonitoring instruments at thecondition of the plant during
time of an accident
an accident

Submergence of 125V DC power supply panels due to tsunami

Same as (3)Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of 6.9 kV high pressure power supply panels due
Function loss due to loss of AC power supplyX
to tsunami
Function loss due to loss of DC power supply
Submergence of 125V DC power supply panels due to tsunami
: Normal operation; : Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)Fire protection system (AM)Function for removing heat
Residual heat removal systemfrom containment vessel

Alternative heat removal
function

Submergence of 6.9 kV high pressure power supply panels due
to tsunami
Submergence of seawater pump for cooling the residual heatremoval system due to the tsunami

Same as (3)Alternative injection function(low pressure)

Block up

Submergence of 125V DC power supply panels due to tsunami

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

Function loss due to loss of AC power supply

Containment isolation

ProblemCause analysis

X
X

X
X
X

Injection disabled as reactor depressurization
Same as (5)Submergence of 125V DC power supply panels due to tsunami
function cannot be used
Unable to use the diesel driven extinguishing pump
Cause unknown(presumed)
Injection disabled as reactor depressurization
Same as (5)Submergence of 125V DC power supply panels due to tsunami
function cannot be used
Difficulty in ensuring the required coolant injection Difficulty in ensuring the capacity of the fire protection water tank (13) Depletion of fresh water source
flow rateused as the 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 3 Reactor Building(15) Deterioration in work environment due toand high radiation dosehydrogen explosion and rise in radiation dose
[Recovery not possible] seawater injection hose damaged due toSame as (15)Difficulty in working
explosion in the Unit 3 Reactor Building → relaying
Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of AC power supply
Same as (8)Interruption in coolant injection because of no fuel(16) Fuel supply for fire engine(Normal operation)
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)Function loss due to loss of AC power supply
to tsunami
Function loss due to loss of seawater pumping
Submergence of seawater pump for cooling the residual heat
Same as (10)function of residual heat removal systemremoval system due to the tsunami
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)Function loss due to loss of AC power supply
to tsunami
Unable to use electric pump due to loss of external 2L receiving circuit breaker of Okuma Line in the switchyard ofSame as (1)Unit 1/2 damaged due to the earthquake
power supply
Unable to use the diesel driven extinguishing pump
Cause unknown(presumed)
Submergence of 125V DC power supply panels due to tsunami
Same as (5)Drop in pressure of Instrument Air due to loss of external power (17) Difficulty in ensuring compressed air
Difficulty in accessing due to rise in radiation dose
Same as (15)Loss of communication means due to loss of AC power supply
Same as (8)Large valves on suppression pool side closed due to explosionin the Unit 3 Reactor Building
Vent unavailable[Recovery not possible] Large valves on suppression pool side
Same as (17)closed due to insufficient compressed air[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(18) Inoperative rupture disk
Unable to receive power due to loss of external power supply
Same as (1) - (3)Function loss due to loss of AC power supply
and unavailability of emergency diesel generator
Function loss due to loss of component coolingSame as (11)Submergence of cooling seawater pump due to the tsunami
seawater system
Unable to receive power due to loss of external power supply
Same as (1) - (3)Function loss due to loss of AC power supply
and unavailability of emergency diesel generator
Function loss due to loss of component coolingSame as (12)Submergence of cooling seawater pump due to the tsunami
seawater system
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)Unable to operate due to loss of DC power supply
to tsunami
Leakage generation
Pressure drop (cause unknown)
Blow out panel opening (no damage to the building) Hydrogen retention prevention
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)Function loss due to loss of AC power supply
to tsunami
Submergence of 6.9 kV high pressure power supply panels due
Same as (3)Function loss due to loss of AC power supply
to tsunami
Function loss due to loss of AC power supply

3-20

Same as (3)Same as (5)Table 3.3-3 Analysis of cause of the accident at Fukushima Daiichi Nuclear Power Station Unit 3 and Determining the problems (1/2)
Safety function
Related facility, etc.
Safety protection system,
Emergency
reactor shut- control rod and control rod drive
systemdown function
Alternative
Recirculation pump trip (AM)
Stoppagereactivity control Alternative control rod insertion
Sub-criticality
Boric acid water injection
maintenancesystemfunction
External power supply

Power supply
function

Function loss or function degradation

Support systems

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

-

-

-

(Not required as scram was successful)

-

-

X

Loss of external power supply due to theearthquake

Emergency diesel generator

X

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

Function loss due to submergence

480V low pressure power
supplyX

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

Power supply interchange (AM)X

Power supply interchange unavailable fromadjacent plants

Function loss due loss of 6.9 kV highpressure power supply
Function loss due to submergence

Damage to external substation facilities due to the earthquake

X

Difficulty in connection work

Reliability of switchyard equipment
Submergence of the emergency diesel
generator
Submergence of 6.9 kV high pressure
power supply panels

Use of DC power supply exceeding the available supply time

Depletion of DC power source(5)

Loss of station power of adjacent plant (Unit 4) due to tsunami (6)Unable to connect due to submergence of AC power supply
panelsDifficulty in laying cables (weight, isolation, work environment)
Power source car (emergency
arrangement)(1)

Submergence of emergency diesel generator due to the(2)tsunamiSubmergence of 6.9 kV high pressure power supply panels(3)
due to tsunami
Submergence of 6.9 kV high pressure power supply panels
due to tsunami
Submergence of 480V low pressure power supply panels due(4)
to tsunami

Difficulty in reaching due to damaged roads and traffic jams

Difficulty in reaching

Alternative
power supply
function

Cause analysis

Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of AC power supply(7)

Same as (3)Submergence of 480V low pressure
power supply panels

Submergence of power supply panels of
the adjacent plant
Procurement of alternative power supply
from outside
Same as (3), (4)(8)

Alternative power supply cable laying-(9)

Cables damaged and power supply car stopped automatically
due to explosion in the Unit 1 Reactor Building
Submergence of seawater pump for cooling the emergency
Function loss due to submergence(10)
diesel generator due to the tsunami
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami
Submergence of seawater pump for cooling the residual heat
Function loss due to submergence(11)
removal system due to the tsunami
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami
Submergence of cooling seawater pump due to the tsunami
Function loss due to submergence(12)

Communication means at a loss of AC
power supply-

Power supply interruption

Emergency diesel generator
facility and cooling seawater
systemX

Seawater system for residual
heat removal system

X

Cooling seawater system

X

Auxiliary cooling
function

Submergence of 6.9 kV high pressure power supply panels
due to tsunami
Function loss due loss of component cooling Submergence of cooling seawater pump due to the tsunami
seawater system
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami
Function loss due to loss of AC power supply

Reactor component coolingwater system
Function toprevent overpressurization of Main steam relief safety valve(safety valve function)
reactor coolant
pressureboundary
High pressure High pressure coolant injection
core coolingsystemReactor core isolation cooling
functionSame as (3)Same as (12)Same as (3)

-

-



Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time
Submergence of 480V low pressure power supply panels due
to tsunami[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
Function loss due to loss of AC power supply
to tsunami
Function loss due loss of component cooling Submergence of cooling seawater pump due to the tsunami
seawater system
Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time
Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time
Injection disabled as reactor depressurization
Use of DC power supply exceeding the available supply timefunction cannot be used
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami
Function loss due to loss of seawater system Submergence of seawater pump for cooling the residual heatof residual heat removal systemremoval system due to the tsunami
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami

Same as (5)Function loss due to loss of AC power supply

X

ReactorMain steam relief safety valve
depressurization Automatic depressurization



Residual heat removal system(low pressure coolant injection
system)Submergence of component coolingseawater pumpSame as (5)Control rod drive water system(AM)

Low pressure
core cooling
function

Same as (3)Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time

X

Core spray system(Normal operation)

Same as (3)Submergence of seawater pump of theresidual heat removal systemBoric acid water injection
system (AM)

Alternative
injectionfunction (highpressure)

Cool down

X

Submergence of cooling seawater pumpfor emergency diesel generator

X

X

3-21

Same as (4)Same as (4)Same as (12)Same as (5)Same as (5)Same as (5)Same as (3)Same as (5)Same as (11)

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

Related facility, etc.
Makeup water system (AM)Fire protection system (AM)

Alternative
injectionfunction (lowpressure)Fire engine (Chuetsu coast
earthquake response)

Function loss or function degradation
ProblemCause analysis
Injection disabled as reactor depressurization
Use of DC power supply exceeding the available supply time
Same as (5)function cannot be used
X
Submergence of 480V low pressure power supply panels due
Function loss due to loss of AC power supply
Same as (4)to tsunami
Injection disabled as reactor depressurizationX
Use of DC power supply exceeding the available supply time
Same as (5)function cannot be used
Injection disabled as reactor depressurization
Use of DC power supply exceeding the available supply time
Same as (5)function cannot be used
Insufficient flow rate of the required coolant
Difficulty in ensuring the capacity of the fire protection water(14) Depletion of fresh water source
injection
tank used as the water source
Obstacles due to the earthquake andObstacles due to the impact of the earthquake and tsunami(15)
tsunami
Deterioration in work environment due toSiteevacuation
due
toexplosion
in
the
Unit
3
Reactor
Building(insufficient flow
(16)hydrogen explosion and rise in radiationand high radiation doserate, delay)
Seawater
injection
hose
damageddue
toexplosion
in
the
Unit
Difficulty in coolant injection
Same as (16)
3 Reactor Building  relaying
Intermittent aftershocks and continuous issue of large tsunami
warnings
Loss of communication means due to loss of AC power supply
Same as (9)Containment
isolation

Main steam isolation valveFunction forremoving heatfromcontainment

Containment vessel coolingsystemX

Alternative containment vessel
spray with makeup water
Alternative containment vessel
spray with fire protection
system (AM)Containment vessel vent (AM)X△△

Alternative heatremoval function
Block up

Operation (generation of isolation signal
related to break before and after the main
steam isolation valve closure)
Function loss due to loss of seawater system Submergence of seawater pump for cooling the residual heatof residual heat removal systemremoval system due to the tsunami
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
due to tsunami
Submergence of 480V low pressure power supply panels due
Function loss due to loss of AC power supply
to tsunami
Unable to use the electric pump due to loss ofSubmergence of switchyards of Units 3/4 due to tsunami
power supply. Diesel pump temporarily
operational
Use of DC power supply exceeding the available supply time
Drop in pressure of Instrument Air due to loss of external
power supply
Difficulty in vent operation
Difficulty in accessing due to rise in radiation dose
Loss of communication means due to loss of AC power supply
High temperature, pitch dark working environment
Submergence of 480V low pressure power supply panels due
to tsunami
Function loss due loss of component cooling Submergence of cooling seawater pump due to the tsunami
seawater system
Submergence of 480V low pressure power supply panels due
Function loss due to loss of AC power supply
to tsunami
Function loss due loss of component cooling Submergence of cooling seawater pump due to the tsunami
seawater system
Function loss due to loss of AC power supply

Drywell cooler (AM)

Reactor coolant cleanup
system (AM)Radioactive
material release
Stand-by gas treatment system
reductionfunction

X

X

X

Unable to operate due to loss of DC power
supplyContainment vessel

X

Leakage generation

Reactor building

X

Damage due to hydrogen explosion

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

Function todetermineabnormalconditions

Communication equipment
Emergency lighting

Same as (3)Same as (4)Same as (1)Same as (5)Same as (1)Same as (16)Same as (9)Same as (4) and (5)Same as (4)Same as (11)Same as (4)Same as (11)Same as (4)Hydrogen leakage from containment vessel penetration part
Hydrogen retention in reactor building

Other

Same as (11)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 seal
vesselHydrogen generation due to the reaction of zircaloy and water-

Multiple barrier
function

Important
Air heating and ventilating
functions related
system in central control room
to safety
Function todetermine theMonitoring instruments at thecondition of thetime of an accident
plant during an
accident-(18) Hydrogen retention in reactor building

X

Function loss due to loss of AC power supply

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

Same as (4)X

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
Submergence of 480V low pressure power supply panels due
Function loss due to loss of AC power supplyX
to tsunami
Function loss due to loss of DC power supply Use of DC power supply exceeding the available supply time
: Normal operation; : Insufficient operation; X: Unable to operate; -: Operation not required(Note) AM: Accident management (Severe accident measures)
X

Function loss due to loss of AC power supply

3-22

Same as (4)Same as (4)Same as (5)3.3.1.1

Problems by function(1) “Stopping” functiona.

Emergency reactor shut-down function of the reactors
The reactors scrammed normally due to the earthquake. The emergency
reactor shut-down function of the reactors has not been lost in the safety
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 the loss of external power
supply.

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” functiona.

Function to prevent over-pressurization in the reactor coolant
pressure boundary
Signs of the loss of reactor coolant by the earthquake were not found fromthe maintenance 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, andperformed its function normally.

b.

High pressure core cooling function(a) High pressure coolant injection system
For water levels of reactors after the scram, Units 1 to 3 were not givenfunctional requests for HPCI operation request, but HPCI was put in astandby state. After that, arrival of the tsunami caused Units 1 and 2 tolose their functions due to the loss of 125 V DC power. In Unit 3, since
the D.C. power supply system avoided being flooded, RCIC and HPCI
could be used. With the reduction of water level in the reactors, the
HPCI automatically started and the water level of the reactors was
retained for a while. However, it then stopped and the function for
injecting coolant into the reactors was lost. This was caused by the DC
power supply running out.

3-23

(b) Emergency condenser (Unit 1 only)
The high pressure in the reactor automatically started the IC after MSIV
was closed. After that, the power was lost due to the tsunami during thereactor pressure control operation by the IC. Since loss of the control
power supply (DC power supply) of IC due to the tsunami prevented theworkers from confirming the open and closed state of the IC valve, andthe indicator lamps of HPCI were off, they judged that startup was
impossible. According to an investigation after that, there is a possibility
that deenergizing the detection circuit of break in pipe, normally madeup of an excitation circuit, could originate an isolation signal,
automatically closing the valves of the IC. An almost simultaneous lossof all AC power supplies may have caused the isolation valve inside thereactor containment, driven by AC power supply, to stop closing with thevalve half opened (the clear opening is unknown). When the 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 the IC’s core cooling
function is unknown according to the opening of the isolation valve
outside the reactor containment. There seems to be a high possibility
that the IC’s function may have been lost until the temporary recovery of
the DC power supply after the tsunami arrived.
As a facility for supplying coolant to the IC, FP and called a makeup
water system purified (hereafter called “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 pump stopped working due to the loss of AC power supply. It is
confirmed that the diesel driven pump ran temporarily but stopped due
to a malfunction. The details of the malfunction are unknown.
(c) Reactor core isolation cooling system (Units 2 and 3)
After the scram, manual startup of the RCIC, automatic stop due toincrease of reactor water level, and manual startup were repeated tocontrol the reactor water level.
In Unit 2, the operation state of the RCIC was not confirmed. At around3:00 on March 12, however, the discharge pressure of the RCIC pump
was confirmed, and the pump is judged to have been running. After that,the water source was switched from CST to S/C to perform theoperation. After 13:00 on March 14, the water level of the reactor tended
to decrease, and the function of the RCIC was judged to have stopped
working at 13:25. As a result, the RCIC of Unit 2 is considered to haveworked for approximately 3 days. It seems that loss of the power supply
set the valve of the RCIC to work “as is” and the valve happened toopen with the timing of the loss of the power supply.
In Unit 3, since the DC power supply was not affected by the tsunami,
the RCIC and HPCI, operable with DC power supply, were usable. Any
unnecessary loads were isolated to increase the operating time of thebatteries. At 11:36 on March 12, the RCIC automatically stopped andthe HPCI automatically started up. At 2:42 on March 13, the HPCI
automatically stopped and lost its function to inject coolant into thereactor. As a result, coolant was injected to Unit 3 for approximately 1.5
days until the battery ran out.
3-24

c.

High pressure alternate injection function (countermeasure for severe
accident)(a) Standby liquid control system
Stopped working due to the loss of the power supplied from the low
voltage power supply of 480 V necessary for the operation of thestandby liquid control pump in Units 1 to 3. Since the power panel of thestandby liquid control system and the main body of the standby liquid
control were not affected by the tsunami, power supply operation fromthe power supply vehicle, arranged urgently after the station blackout,
were carried out. However, because the hydrogen explosion in thereactor building of Unit 1 damaged the cable that had been laid, thepower supply vehicle arranged urgently stopped automatically, and thenthe SLC function was not recovered.
(b) Control rod drive water system
Stopped working due to the loss of the power supplied from the low
voltage power supply of 480 V necessary for operating the drive water
pump in Units 1 to 3.

d.

Reactor depressurization function(a) Main steam relief safety valve (manual relief function andautomatic depressurization function)
The manual relief function and automatic depressurization function
were lost because Units 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 and facilities having the low pressure
core cooling function could not inject coolant into the reactors. However,since the systems having a low pressure core cooling function lost theirpower supplies when all AC power supplies were lost, the function to
inject coolant into the reactor stopped working.
Since Unit 2 required the depressurization of the reactor for fire engines
to inject coolant to the reactor after the RCIC stopped, temporary
batteries were used to ensure the power supply necessary for SRV to
operate, so that the reactor was depressurized.

e.

Low pressure core cooling function(a) Core spray system
In Units 1 to 3, the core spray pump stopped working due to the loss ofthe power supplied from the high voltage power supply of 6.9 kVnecessary for the operation of the core spray pump.
(b) Low pressure coolant injection system (residual heat removal
system) (Units 2 and 3)
The residual heat removal pump (low pressure coolant injection mode)
stopped working due to the loss of the power supplied from the highvoltage power supply of 6.9 kV necessary for the operation of theresidual heat removal pump.

3-25

f.

Low pressure alternate injection function (countermeasure for severe
accident)(a) Makeup water system
In Units 1 to 3, the makeup water pump stopped working due to the lossof the power supplied from the low voltage power supply of 480 V
necessary for the operation of the makeup water pump.
(b) Fire protection system
The fire protection system pump stopped working due to the loss of thepower supplied from the low voltage power supply of 480 V necessary
for the 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 to a malfunction. In Unit 2, a startup
operation seems not to have been performed. However, the details are
unknown, e.g., whether the operation was not done due to a failure. In
Unit 3, pumps ran temporarily but then stopped. The details are
unknown.
Alternate coolant injection was carried out using fire engines prepared
as a countermeasure taken after the Chuetsu Offshore Earthquake.
However, the volume of the fire protection tank used as a water source
was limited, so the required volume of coolant was insufficient.
Therefore, workers prepared to inject seawater. Operations for the
injection of coolant into the reactors had difficulties caused factors suchas obstacles produced by impact of the earthquake and tsunami,
evacuation from the field site due to explosion of the reactor building ofUnit 1, high-dose debris, relaying of seawater injection hoses replacedwith damaged ones, consecutive aftershocks, continuation of large
tsunami warning, and loss of communication methods due to the loss of
AC power supply. In Unit 2, fire engines started injecting seawater after
the reactor depressurization. However, since they ran out of fuel andstopped, injection of coolant to the reactor was suspended.
(c) Containment & vessel cooling water system
The containment & vessel cooling water pump (Units 2 and 3 was put inthe containment & vessel cooling mode for RHR pump) stopped
working, because the power supplied from the high voltage power
supply of
6.9 kV and the 125 V DC power supply necessary for the operation on
the containment & vessel cooling water pump were unavailable or
because the pump could not be started up due to flooding of thecontainment & vessel cooling seawater system (residual heat removal
cooling seawater system in Units 2 and 3) and loss of the power supply.
For the reasons above, problems with the “cooling” function by facility
are as follows.
• Submergence of power supply systems• Exhaustion of seawater pump
• Exhaustion of fresh-water source

3-26• Deterioration of working environments due to hydrogen explosion andincrease 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” functiona.

Reactor containment isolation function
For Units 1 to 3, the MSIV is normally closed by the isolation signal
generated due to the loss of external power supply after the earthquake.
No leakage occurs in the main steam piping because the flow rate of main
steam is zero after the MSIV is closed and because the reactor pressure
has been increased.
The reactor containment isolation function works normally immediately
after the occurrence of a scram, and there are no causes of accidents or
problems.

b.

Heat removal function from the containment
CCS (Unit 1: CCS, Units 2 and 3: RHR containment cooling mode) started
up manually after the scram, and Units 1 and 2 started up manually to cool
the water in the S/C pool. After that, the CCS pump stopped working due tothe loss of the power supplied from the high voltage power supply of 6.9 kVnecessary for the operation of the CCS pump. Unit 3 did not start up due tothe loss of power supply.

c.

Alternative cooling function (facilities for countermeasures for severe
accident)(a) Alternative containment spray by the makeup water system
The makeup water system pump stopped working due to the loss of thepower supplied from the low voltage power supply of 480 V necessary
for the operation of the makeup water system pump.
(b) Alternative containment spray by the fire protection system
The fire protection pump stopped working due to the 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 V DC
power supplies necessary for the operation and the air pressure for
instrumentation was decreased.
For Units 1 and 3, the workers tried to perform a manual release
operation. However, the operations had difficulties because access was
3-27

difficult caused by increase of radiation dosage or by loss ofcommunication methods due to the loss of AC power supply. As a result,vent is considered to have been made because the pressure of thecontainment 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 indicated value of MP radiation
dosage decreased. It was therefore confirmed that there was a
possibility that the vent might not be effective. The cause of this is
assumed to be that the vent valves of the containment were closed
immediately because the residual air pressure for instrumentation was
low and the valves could not be kept open. Considering the vent of thecontainment 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 (Kumaarea), confirmation of evacuation and such like, the reactor was vented
at 9:15.
Although Unit 3 also had difficulties in operation, the Station judged thatthe venting had 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
not clear at the current stage.
The pressure of D/W increased, while the pressure of S/C decreased.
However, the factors causing such behavior are 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 radioactive materials
The SGTS stopped working because the power was not supplied from thelow voltage power supply of 480 V.
e.

Others
The heat removal function from the primary containment vessel (hereafter
called “PCV”) was lost, and so the temperature and pressure of thecontainment atmosphere increased excessively. Hence, the gasket of theflange area and seal of the penetration are assumed to have deteriorated,causing the leakage. It is conjectured that, since leaked hydrogen from thispenetration of the containment was retained in the reactor building, andincrease of hydrogen concentration exceeded the flammability limit, ahydrogen explosion occurred.
3-28

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 seawater pump
• Deterioration of working environments due to hydrogen explosion/increase of radiation dosage• Sealing property in a state exceeding the design conditions of gasket
and seal 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(4) Basic requirements for implementing the operation (facility
environments)a.

Habitability of the Main Control Room
When the accident occurred, the air heating and ventilating system toensure the habitability of the MCR stopped working 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, which work to determine the status of thestation at the time of an accident, stopped working, because DC power for
instrumentation was not supplied to Unit 1 and the power from the low
voltage power supply of 480 V was not supplied to Units 2 and 3.

c.

Functions of seizing an abnormal stateCommunication equipment and emergency lighting, which were functions
to determine abnormal states, stopped working, because power was
supplied neither from the low voltage power supply of 480 V nor 125 V DC
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 and failures

Extracted problems by function in 3.3.1.1 include “without power supplied frompower 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 function” and “auxiliary component cooling function.”

3-29

(1) Power supply functiona.

External power source
All Units 1 to 4 receive power supplied from external power sources via the
Shin-Fukushima Subtation. The power supplies were lost caused byfactors including damage to facilities in the substation due to theearthquake, damage to power receiving breakers and disconnecting
switches within the switchyard of the station due to the earthquake, andoperations of protective devices due to the power transmission lines
coming into contact with steel towers due to the earthquake. In addition to
the damage due to the earthquake, it became difficult for the workers torecover external power supplies to the switchyards for Units 3 and 4 at an
early date because they were flooded by the tsunami.

b.

Emergency diesel generator
After the loss of the external power supplies, Units 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 damage, the high voltage power panel of
6.9 kV was flooded), they stopped working, and the reactors then lost all
AC power supplies.

c.

On-site power
The high voltage power supply of 6.9 kV stopped working because thepower panel was flooded. Therefore, the low voltage power supply of 480 V,
used as the load, also stopped working. The 480 V power panels for Units
1 and 3 were also flooded.
The 125 V DC power for Units 1 and 2 could not be supplied from batteries,
because the 125 V DC power panel was flooded by the tsunami. The 125 V
DC power panel for Unit 3 escaped the flood. After the batteries worked forseveral days, they became exhausted and stopped working.

d. Alternative power supply function
The station blackout prevented the power interchange from adjacent
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 vehicles arranged
urgently after the station blackout due to damage to 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, the hydrogen explosion in the reactor
building damaged the laid cables, and the power supply vehicles stopped
automatically.
For the reasons above, problems on power supply functions by facility arelisted as follows.
• Reliability of external power supplies against earthquakes and tsunami• Reliability of breakers in switchyards when earthquakes occur• Submergence of power supply systems
3-30

•
•
•

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 supplies were lost due to the earthquake,the seawater systems requiring the auxiliary component cooling function
received power from the emergency DG and operated normally. However,since the seawater pump was flooded due to the tsunami, and the high 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 seawater 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 stopped
working at once due to the tsunami, the accident escalated. The facilities ofcountermeasures against severe accidents, which were prepared for
multifailure, could not stop the accident from expanding, because they
stopped working due to the loss of electric power systems and auxiliarycomponent 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 at once.
• Means of preventing tsunami from affecting items (breakwater, or
deployment in rising ground, etc.) have been prepared so that damage bytsunami can be avoided.

3-31

3.4

Causal analysis of the accident at Unit 4 and management ofissues

The 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 are shown in
Table 3.4-1.
After the earthquake occurred, there was a loss of an external power supply, and astation blackout, which was caused by a loss of emergency DG due to the tsunami.
The DC power supply and a seawater pump were also lost.

3-32

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

Related facility, etc.
External power supply

Function loss or function degradation

Cause analysis

X

ProblemLoss of external power supply due to theDamage to external substation facilities due to the earthquake (1) Reliability of switchyard equipmen
1 out of the 2 installed generators is underroutine inspection.
Submergence of 6.9 kV high pressure
Submergence of 6.9 kV high pressure power supply panels
Emergency diesel generator(2)
X
The remaining generator cannot be used as thepower supply panels
due to tsunami
power supply panels are submerged (theemergency
dieselgenerator
isnotsubmergedinPower supply
6.9 kV high pressure power
Submergence of 6.9 kV high pressure power supply panels
function
X
Function loss due to submergence
Same as (2)supplydue to tsunami
Function loss due to submergence (only powe480 V low pressure power
Submergence of 480 V low pressure
Submergence of 480 V low pressure power supply panels due(3)X
supply panels installed in auxiliary public facilities
supplypower supply panels
to tsunami
in operation)
Submergence of 125V DC power supply panels due toSubmergence of 125V DC power
Support
125V DC power supply
X
Function loss due to submergence(4)
tsunamisupply panels
systems
Submergence of seawater pump of theSubmergence of seawater pump for cooling the residual hea(5)Function loss due to submergence
Seawater system for residual
removal system due to the tsunam
residual heat removal system
X
Submergence of 6.9 kV high pressure power supply panels
heat removal system
Function loss due to loss of AC power supply
Same as (2)due to tsunami
Submergence of component cooling
Function loss due to submergence
Submergence of cooling seawater pump due to the tsunami (6)Auxiliary coolingseawater pumpseawater cooling system
X
Submergence
of
6.9
kV
high
pressure
power
supplypanelsfunction
Function loss due to loss of AC power supply
Same as (2)due to tsunami
Function loss due loss of component cooling
Submergence of cooling seawater pump due to the tsunami
Same as (6)seawater system
Cooling seawater system
X
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
Same as (3), (4)and 480 V low pressure power supply panels due to tsunam
Submergence of 6.9 kV high pressure power supply panels
Fuel pool cooling system
X
Function loss due to loss of AC power supply
Same as (3), (4)and 480 V low pressure power supply panels due to tsunam
Function loss due to loss of seawater system of Submergence of seawater pump for cooling the residual hea
Cooling of Cooling function
Same as (5)residual heat removal systemremoval system due to the tsunam
spent fuelResidual heat removal system
X
Submergence of 6.9 kV high pressure power supply panels
Function loss due to loss of AC power supply
Same as (2)pools
due to tsunami
Coolant
Submergence of 6.9 kV high pressure power supply panels
Makeup water supply system
X
Function loss due to loss of AC power supply
Same as (3), (4)injectionand 480 V low pressure power supply panels due to tsunam
SecondaryHydrogen retention in reactor building(7) Hydrogen retention in reactor building
X
Damage due to hydrogen explosion
Wraparound of hydrogen gas in units
Reactor building
Block upcontainment
Inflow of vent gas in Unit 3(8)that share the vent stackvessel: Normal operation; : Insufficient operation; X: Unable to operate; -: Operation not

3-33

(1) Loss of cooling function of the spent fuel poola.

Spent fuel pool cooling function
The FPC, which had been operating right before the earthquake, stopped
because the external power supply 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 480
V low voltage power supply that is necessary for the operation of the FPC
was lost.
The RHR, which had been operating right before the earthquake, stopped
because the external power supply was lost due to the earthquake. After
that, the SFP cooling function was lost because the function of the RHR
seawater system was lost due to the tsunami and the 6.9 kV high voltage
power supply that is necessary for the operation of the RHR was lost.
The MUWC has a function of supplying water to the SFP. However, it
stopped working because the 480 V low voltage power supply that isnecessary 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 seawater pump
• Soaking of the power supply facility(2) Hydrogen explosiona.

Main cause of hydrogen generation
An explosion that is considered to be due to hydrogen occurred in thereactor building of Unit 4 at about 6:00 on March 15.
When the earthquake occurred, Unit 4 had been stopped because it wasundergoing a periodic inspection, and all fuel was taken out from thereactor to the spent fuel pool. Because of this, there was no possibility ofhydrogen generation from the reactor. As a result of observing the fuel inthe pool with a camera, it can be assumed that no damage to the fuel was
found, and that there was no large-scale damage of the fuel from the resultof analyzing nuclides of the fuel pool water taken from the fuel pool of Unit
4. Consequently, the possibility of a large amount of hydrogen being
generated 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 acontainer vent that was carried out in Unit 3, and the hydrogen was
retained. The radiation dose of the SGTS filter train of Unit 4 was measured,and it was found that the radiation dose was high on the exit side and got
lower going toward the entrance 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 vent
stack
3-34

•

3.5

Wrapping around from the container vent line to the building

Management of events at Units 5 and 6

Because the reactors of Units 5 and 6 were not working and there was enoughtime for correspondence, the power supply 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 seawater pump was able to recover.
The course of the events of Fukushima Daiichi Units 5 and 6 is shown in Figure3.5-1 using an event tree.
The workers successfully prevented accidents from escalating in Units 5 and 6 bydirectly connecting a power supply of one air-cooled emergency DG to a safety
component. It is considered that safety can be ensured when the power supply issecure.
Hence, having a variety of cooling types of the emergency DG and having a variety
of installation sites from the viewpoint of soaking can become effective
countermeasures.
However, it should be kept in mind that there was a need to supply power bydirectly 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 ofelectricity to multiple safety components will be simultaneously lost when the
power panel is submerged, it is necessary to investigate a plan for supplying power
to the necessary components from the available power supplies even whensoaking occurs, while ensuring soaking countermeasures in order to increase thesecurity of the dependable power supply.

3-35

Events
occurred
Earthquake andTsunamiReactorshutdown
Reactorscram

DC power
Electric power
AC power supply
Ensuring long-term cold shutdown
Core cooling
Status of reactor coresupplyrecovery
ReactorDC power External
Emergency IC, turbineExternal
RPV
RHR
RHR
PCV vent Cold shutdown, core damage,driven coolant power supply, depressuri coolant
supplypower
DG
recovery (before
PCV damage etc.
injection
supplyemergency
zationinjectioncore
system
DG, power(Including (including
damage)(RCIC,HPCI) interchange alternative alternative
for which AC
sources) sources)
power supply
is notDegraded 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
Temporarily
installedseawater pump

1F-5 sequence

Cold shutdown (long-term cooling is required)

Unit 6 power supply
interchange

Integrity of PCV or PCV leakage (overtemperature)
PCV damage (overpressure)
Cold shutdown (long-term cooling is required)

PCV leakage (overtemperature) and hydrogen
combustion in the building
PCV damage (hydrogen explosion)[Earthquake]

1F-5

Not required

PCV damage (overpressure)[Tsunami]

PCV damage (overpressure)Core damage

Shutdown

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

Integrity 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)

Integrity 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 Disaster identification events tree in Fukushima Daiichi Nuclear Power Station Units 5, 6

3-36

(1) Unit 5
Refer to “2.6 Course of accident at Unit 5” in this article for the behavior of theplant when the earthquake occurred.
a.

Status of “stopping” function
When the earthquake occurred, Unit 5 had been 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 control rods were fully insertedb.

Status of “cooling” function
All AC power source was lost due to the tsunami, and the operations of the
RHR, the low pressure core spray, and the HPCS became impossible. The
pressure of the RPV was reduced a day after the earthquake by opening avalve 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 power
supply 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 seawater pump to take seawater and by alternating cooling of
the spent fuel pool and the reactor by switching system configurations of
the RHR.

c.

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

d.

Status of the spent fuel pool cooling system
The water level of the spent fuel pool was full and the water temperature of
the pool was about 24°C before the earthquake, and it was not a conditionin which cooling of the fuel was impeded during an early stage. Therefore,
the pool had not been cooled before the tsunami arrived. Because all AC
power was lost due to the tsunami, the RHR became unusable. Then, boththe spent fuel pool and the reactor were alternately cooled by building atemporary seawater pump to take seawater and by switching system
configurations of the RHR.

e.

Status of “power supply” function
Because the external power supply 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
and power that is necessary to accommodate the power source to Unit 5
was supplied.

(2) Unit 6
Refer to “2.7 Course of accident at Unit 6” in this article for the behavior of theplant when the earthquake occurred.

3-37

a.

Status of “stopping” function
When the earthquake occurred, Unit 6 had been 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, and it
was not a condition in which cooling of the fuel is impeded during the earlystage. Therefore, the pool had not been cooled when the reactor was
stopped before the tsunami arrived.
The function of two diesel generators was lost due to the influence of thetsunami, and the RHR, the low pressure core spray system, and the HPCS
became unusable because power supply was lost and the seawater pump
was unavailable. On the other hand, one diesel generator continued tooperate and power that is necessary to maintain the function of supplying
water to the reactor was supplied. The pressure and the water level of thereactor were controlled by starting a condensate water transfer pump on
March 13, by properly reducing the pressure with the SRV, and at the sametime by repeating an operation of supplying water from the CST to thereactor on and after March 14.
Then, both the spent fuel pool and the reactor were cooled by building atemporary seawater pump to take seawater and by alternating cooling of
the spent fuel pool and the reactor by switching system configurations of
the RHR.

c.

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

d.

Status of the spent fuel pool cooling system
The water level of the spent fuel pool was full and the water temperature of
the pool was about 25°C before the earthquake, and it was not a conditionin which cooling of the fuel was impeded during the early stage. Therefore,
the pool had not been cooled before the tsunami arrived. Because twodiesel generators were lost due to the tsunami, the RHR became unusable.
Then, both of the spent fuel pool and the reactor were alternately cooled bybuilding a temporary seawater pump to take seawater and by switchingsystem configurations of the RHR.

e.

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

3-38

3.6

Comparison of course of events at Fukushima Daiichi Nuclear
Power Station with other stations

In the accident at this time, a large amount of radioactive material was discharged
from Fukushima Daiichi Nuclear Power Station (Fukushima Daiichi) into theenvironment. However, we decided to learn a lesson also by comparing a stationthat ended up discharging radioactive materials with a station that did notdischarge any even though the tsunami hit. Fukushima Daini Nuclear Power
Station, Onagawa Nuclear Power Station (hereafter called “Onagawa”), and Tokai
Daini Nuclear Power S (hereafter called “Tokai Daini”) were affected by thetsunami. They have been selected as stations that did not discharge radioactive
materials, and we investigated and compared Fukushima Daiichi Units 1 to 3 withthese stations.
As shown in Fig. 3.6-1, the course of events in Fukushima Daiichi Units 1 to 3 andother stations differ from the heading of the AC power supply (the external power
supply or the emergency DG), and the power supply was secured in all the otherstations as a result, which prevented the accident from escalating.
Hence, we found that the main cause of the escalation of the accident inFukushima Daiichi is that the external power supply and the emergency DG werelost due to the earthquake and the tsunami and all of the safety components andthe facilities to counter severe accidents could not function excluding some of thecomponents that did not need a power supply.
As shown in Table 3.6-1, the main reason that the power supply was secured inother stations is considered to be that the tsunami did not reach their sites. The
tsunami did infiltrate the site of Fukushima Daini, and the external power supply,
the power panel, the emergency DG, and such like were damaged. However, theaccident was resolved by using a power supply 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 it impossible to remove decay heat and can cause the accident to escalate.
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 cooling system
was sufficiently effective as long as the power supply could be secured.
Ultimately, cooling by the RHR was secured and the event was settled in all of theabove described stations. In the case of Tokai Daini, tsunami countermeasures
were taken in the seawater pump area for the seawater system pertaining to theultimate heat sink. When the components in which the countermeasures had
already been taken operated, a smooth settlement of the event became possible.
Hence, the importance of tsunami countermeasures for the seawater system(soaking prevention countermeasures) can be seen.
Details of the investigation of each station are attached as Appendix 1.

3-39

Events
occurred
Earthquake andtsunamiReactorshutdown
Reactorscram

DC power
AC power supply
supplyDC power External
Emergenc
supplypower
y DG
supplyElectric
power
IC, turbineExternal
driven coolant power
injection
source,systememergency(RCIC, HPCI) DG, power
for which AC interchange
power supply
is notrequired

Ensuring long-term cold shutdown

Core cooling

RPV
Reactor
RHR
depressuri coolant
zationinjection(Including (including
alternative alternative
sources) sources)

RHR
recovery

Status of reactor core

Degraded core coolin

PCV vent Cold shutdown, core damage,PCV damage etc.
(beforecore
damage)Depressu Reactor
rization of coolant
nuclear injection
reactor

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

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

Cold shutdown

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

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

2F-3,O-2-3
Cold shutdown
2F-1,2,4[Tsunami]

2F-1-4,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)
PCV damage (Overpressure)O-1 RCIC,CRD
Tokai RCIC,HPCS

Cold shutdown

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

Cold shutdown

O-1 D/G（A,B)
Tokai D/G（D,H）

O-1 RCIC
Tokai RCICCold shutdown (No RHR)
PCV damage

O-1
Tokai
Cold shutdown
Cold shutdown
（A)

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

Integrity of PCV or PCV leakage (overtemperature)

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

1F-3
RCIC/HPCI
1F-3(Successful)(1) Working with SBO with which facility can be recovered

Cold shutdown(long-term cooling is required)

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

PCV damage (hydrogen explosion)

PCV damage (Overpressure)
SRV

PCV leakage (overtemperature) and hydrogen
combustion in the building(Continuous)(Not executed)

PCV damage (overpressure)(Continuous)

Reactor core damage

Recovery failed(Quantity of water actually injected to the reactor is less)Submerged due to
1F3 tsunami

Accident sequence
estimated with 1F3

PCV damage (MCCI, etc.)

Reactor core damage
PCV damage (DCH, etc.)
Accident
sequenceestimated with 1F2Integrity of PCV or PCV leakage(overtemperature)

Same as (A)1F-1/2
1F-2

Cold shutdown (Long-term cooling isrequired)

RCIC

PCV damage (Overpressure)[Tsunami][Earthquake]

PCV leakage (overtemperature) and hydrogen
combustion in the building(Continuous)

PCV damage (hydrogen explosion)(Not executed)

PCV damage (overpressure)(Continuous)

Reactor core damage

Recovery failed(Quantity of water actually injected to the reactor is less)

PCV damage (MCCI, etc.)

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

1F-1Integrity of PCV or PCV leakage (Overtemperature)

Cold shutdown (Long-term cooling is required)

IC (operational till tsunami, operation period after
tsunami is not known) Recovery failed

PCV leakage (overtemperature) and hydrogen
combustion in the building

PCV damage (Overpressure)(RPV low pressure)

PCV damage (hydrogen explosion)
Reactor core damage (RPV low pressure)(Water injection delayed or Quantity ofwater actually injected to the reactor is less)(Not executed)

PCV damage (overpressure)

Accident sequence
estimated with 1F1

PCV damage (MCCI, etc.)

Figure 3.6-1 Disaster identification events tree in Fukushima Daiichi Nuclear Station Units 1 to 3,
Fukushima Daini, Onagawa, Tokai Daini

3-40

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

Condition of plant when earthquake occurred

1

2

3

4

5

6

1

2

3

4

Operational

Operational

Operational

Suspended

Suspended

Suspended

Operational

Operational

Operational

Operational

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

External power supply

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

Condition of the plant after tsunami

Emergency DG (main body)
Power panel (M/C,
P/C, DG)

Emergency

Among 4 lines (1 line under construction), 1 line usable*1
No damage

Damaged (except part P/C)

No damage

DamagedNo damage

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

Damaged (except part P/C)

NormalEmergency cooling system seawater
pump (main body)

No damage

DamagedRHRS
No damage[Loss of power]

No damage to
RHR (B) only
[Loss of power]

RHRS
No damage

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

RCIC

RCIC

RCIC

RCIC, HPCS

RCIC
Emergency cooling system(Including Emergency condenser (IC))

IC (operationaltime unknown)

Containment vessel
vent
MUWC alternative
injection
Expected height of tsunami (Society ofConstruction Engineers)
Height of tsunami at arrival
Site height
Principal building surrounding
immersion depth [Inundation height]

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

Emergency

Severe accident
countermeasureequipment

Specifications of each plant

Fukushima Daini(Operational for
approximately 3
days)(Operational for
approximately 1day),(Suspended)

HPCI(Operational for
approximately half a
day)

○

?

○

-

-

-

×

×

×

×

×→○

○

DG

T/B B1F

Power
panel

T/B 1FC/B B1F

Power
panel

T/B 1F

Availability of Air-cooling DG (Y/N)

RCIC

N

(Vent line configuration complete)
○

○

○

○

O.P.+5.7m

O.P.+5.2m

Approximately +13m*2

Approximately +9m *2
O.P.+12m
Approximately 2.5m or less (almost zero except surroundings of Unit 1)[O.P. approximately +12 to +14.5m*4]

O.P.+10m
Approximately 1.5 to 5.5m
[O.P. approximately +11.5 to +15.5m*3]
T/B B1F
T/B B1F
T/B B1
Common pool
Common pool
1F
1F
T/B B1F
T/B B1F
T/B 1FT/B 1FT/B B1F
Common pool
Common pool
B1F
B1F
T/B B1F
T/B B1F
T/B B1F
T/B 1FC/B B1F
T/B 1F
2SA building 1F
YYN(DG2B)(DG4B)

O.P.+13m
Approximately 1.5m or less
[O.P. approximately +13 to +14.5m]
T/B B1

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

C/S B1F
Hx/B 1F

C/S B2F
Hx/B

C/S B1F
Hx/B

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

T/B B1F

C/B B1F
C/B 1F
Hx/B

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

C/B B2F
C/B 1FNNNY(DG6B)NN

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

6 lines

4 lines

Water-tightening (pump area, building)

Seawater system installed outside the building

Although the seawater system was installed in the building, electrical powersystems 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 the conventional reactor building), Hx/B: Heatexchanger building*1: Damage to transmission tower, switchyard and such like is damage due to the earthquake, *2: Height of tsunami in tide station installation position of both the power plants. For instrument damage, the actual height of tsunami in tide station is unknown, *3: O.P.
approximately +16 to +17m [immersion depth approximately 6 to7m] locally in the southwest of the said area, *4: O.P. approximately +15 to +16m [immersion depth approximately 3 – 4m] locally from the south of Unit 1 till the seismic isolation significant buildings

3-41

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 the plant after tsunami

Condition of plant when earthquake occurred
External power supply
Emergency DG (main body)
Power panel(M/C,P/C,DG)

No damageEmergency
NormalM/C 6-1A Fire

Emergency cooling system seawater
pump (main body)

RHRS, ECWS
No damageEmergency cooling system(Including Emergency condenser (IC))
Containment vessel
Severe accident
ventcountermeasure
MUWC alternativeequipment
injection
Expected height of tsunami (Society ofConstruction Engineer)
Height of tsunami at arrival
Site height
Main building surrounding immersion
depth [Inundation height]
DG
Deployment of
power panel
Power(M/C,P/C),
panel
emergency DG(building, hierarchy) Normal Power
panel
Availability of Air-cooling DG (Y/N)
Diversity in external power supply(diversity in no. of lines, switchyard)
Emergency

Specifications of each plant

Onagawa
1
2
3
Operational
Start-up
Operational
Among 5 lines, 1 line usable
No damage** In Unit 2, as the RCW (B) system and HPCW system were flooded, DG (B) and DG (H) could not be used.

Water-tightening (pump area, building)Operational
Among 3 lines, all unusable*1
No damage
No damage

No damage
RSW (B) system: Damaged
RSW (A) system and HPSW systemno damage

RCIC, HPCITokai Daini

No damage
RSW, HPSWNo damage

RCIC, HPCS

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

-

-

-

-

-

-

○

-

O.P.+13.6m

Altitude + 4.9m

O.P.+13.0m
OP+14.8m** Crustal movements around the power station due to the earthquake has been estimated to beapproximately -1m, currently it is 13.8m

Altitude + 4.8-5.3m

No immersion

T.P.+8.0m** Value before earthquake
No immersion

C/B B3F

R/B 1FR/B 1FN

C/S B1F
C/S B2FC/S B1F
C/S 1F (direct current power supply panel)
C/S B2FC/S B1FN

T/B B1F, C/B B2F

R/B B1F

R/B B1F

T/B B1F

C/B B1F

S/B B2F

5 lines

3 lines

Sea water system installed outside the building

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 the conventional reactor building), Hx/B: Heatexchanger building*1: Damage to transmission tower, switchyard and such like is damage due to the earthquake

3-42

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 due to thefailure of power-supply sources, which were necessary to activate the safetysystems.
According to the same analysis, other nuclear power stations averted a similar
deteriorating situation largely due to the securing of power supplies.
Nuclear power 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 supplies 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
why the situation was exacerbated.
In short, the real issue was the lack of preparing for such a state of emergency.
Stricter safety measures could have given different results. A worst-case scenario
and countermeasures for it should have been developed. For example, “if atsunami reaches the station premises, water may come into the buildings andimportant safety equipment including power panels may be submerged. With thisin 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 a situationin case such enhancement is not sufficient.”
The major cause for aggravation of Fukushima Daiichi situation was the loss of
power-related facilities and equipment such as external power supplies,
emergency DGs and power panels, as well as the lack of preparedness for dealing
with the functional loss of those facilities caused by the tsunami.
While securing a power supply could have mitigated the emergency to a certain
extent, cooling functions would also have needed to be secured for recovery of thesafety systems. During the disaster, Fukushima also lost component coolingsystems as a result of the lack of seawater pump functions. Therefore, stringent
emergency measures are needed for power supplies as well as for coolingsystems.
We have classified identified issues into the following five categories. Chapter 4
will describe these issues and required preparation and measures:• Countermeasures against earthquakes and tsunamis (natural hazards)• Securing of power supply• Measures against loss of heat sink systems• Measures against hydrogen leakage• Preparation for emergencies

3-43CategoryIssues and requirements

Countermeasures against •earthquakes and tsunamis •Seismic adequacy of external power supplies
Tsunami countermeasures for switchyards
Tsunami countermeasures for seawater systems
Protection of seawater pump room fromsubmergence
Tsunami countermeasures for emergency DGs
Air/water tightness of opening of the buildings
Only one incoming panel to receive external power
Reliability of power supplies in case of tsunami
Submergence of power supply systems
Exhaustion 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 seawater pumps
Ensuring entire facilities and equipment so as it
cannot be put out of commission by a single event
Setting up heat sink systems that will not benullified by tsunamis
IC isolation signal in the event of the loss of DC
power supply
Securing of materials and equipment to cope withvalve drive failure
Establishing emergency procedures and training
Loss of freshwater sources
Earthquake protection and tsunami
countermeasures for FP piping
Procurement of fuel for fire engines
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 containment vessel gasket andpenetration 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 and monitoring
Protection of radiation control facilities fromtsunami damage
Obstructions created by earthquakes and tsunamis
Communication methods in the event of the loss of
AC power••

Securing of power supply

Measures against loss ofheat sink systems

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

Measures against
hydrogen leakage•••

Preparation for
emergencies

•
•
•
•
•
•

3-44

CategoryIssues and requirements
•
•
•
•
•

Materials and equipment to support nighttime andindoor operations
Materials and equipment to secure road access
Securing of more than one access route within thepremises
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

3-45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 supplies as a result of destruction ofmost safety equipment, expect for few of them, by the tsunami, as well as the failureof backup measures (severe accident measures) that were expected to control thesituation.
From this accident, we have learned that the following five preparations andcountermeasures 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 and drills)

4.1

Countermeasures against natural hazards

This document describes the damage caused by the earthquake and subsequenttsunamis.
The size of the earthquake was mostly within the predicted values for the facility
design. The post-evaluation conducted by TEPCO utilizing acquired seismic
waves showed that the safety devices and equipment would not have been
inoperable. However, the combination of several 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-evaluatedbased on this newly recognized phenomenon.
Power supplies for the station were damaged by the tsunami. Calculation based on
outdated knowledge did not anticipate that the height of a tsunami would exceed
ground level at the station. Therefore, the possible consequences of seawater
infiltration were not carefully considered and sufficient countermeasures were nottaken.
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 though theprobability of that possibility is minimal, we must take steps to protect existing
properties and bring temporary facilities and equipment to bear in order to improveendurance and resolve vulnerability issues.

4.2

Securing power supply

In the event the power supply for a station is cut off, there are external power
supplies that may be tapped to supply electricity. If for some reason these external
power supplies 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 the earthquake,and subsequently all AC power supplies became inoperable because the power
panels and emergency DGs were submerged as a result of the tsunami. Although
4-1some DC sources managed to escape water-damage, battery life was exceeded
before the power supply was reestablished. As a result, most facilities ceased tofunction and the situation became critical. On the other hand, an air-coolingemergency DG for Fukushima Daiichi, Unit 6, was not affected by the tsunami andcontinued to operate. Decay heat at this unit was relatively low because it wasunder suspension and there was adequate time to cool down. The fact that thisunit 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 and 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 watertight inorder to protect them from possible tsunamis, and using power-supply vehicles toprovide 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 the 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 seawater pumps caused by the tsunami also contributed to thefailure of some DGs.
Since the power failure caused severe damage, which was functional loss ofvarious pieces of equipment, the seawater pump failure was not very conspicuous.
However, loss of seawater pumps may lead to the simultaneous functional failureof various other pieces of equipment (such as cooling water destination pumps andheat exchangers), and result in further aggravation of the situation.
Therefore, seawater pumps must be protected from tsunami damage andmeasures to immediately restore their functionality are also important. A reactor
cooling system must also be secured in case the seawater 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 of hydrogen at Fukushima Nuclear Power Station, Units 1, 3
and 4 destroyed the roofs and walls of these R/Bs and made it harder to deal withthe situation.
Under such circumstances, the cooling down of the reactors and maintenance ofthe 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.

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A PCV is usually used to contain hydrogen. PCVs also act as recombiners in which
nitrogen is added in order to inactivate the hydrogen.
However, in situations where the high temperatures and pressures exceed thedesign parameters, hydrogen may leak from penetrations or the flange gasket
sealing of PCV and then accumulate inside of R/Bs. As the containment vessel
ventilates, hydrogen then may enter the pipes connected with exhaust stacks. Thispossibility should also be kept in mind.
Under the presumption that hydrogen is susceptible to leakage, countermeasures
should be established to stop the accumulation, leakage and wraparound, andprovisions for controlled burns should be developed. Power outages also need to
be assumed whenever these measures are taken.

4.5

Preparation for emergencies (training and 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 unit
accidents, as well as having to deal with other emergency operations that included
the need to arrange for temporary hoses and cables, and radiation monitoring
equipment. Few of those workers had ever had such experience and that was oneof the reasons why 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 and 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 and 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 such training.

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Examples of emergency measures are as follows:

4.6

Countermeasures against earthquakes and tsunamis (example
measures)

4.6.1

Assumption of earthquake and tsunami strength

Although the strength of the earthquake on March 11, 2011 exceeded the
maximum response acceleration value against basic design earthquake groundmotion, Ss, all the emergency functions, “stop,” “cool” and “confine” atFukushima Daiichi were functioning during and right after the quake. Also it was
assumed that the function of safety component was secured until the tsunami
descended upon the site.
Seawater 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 switchgear, power center and D.C. power supply systems.
Road damage and debris caused by the quake and tsunami interfered with
operations to connect external power supplies. As a result, the Station Blackout
lasted for hours and coolant injection into reactors took a long time.
Findings and lessons learned• The combination of multiple hypocenters created a massive earthquake (inmagnitude, size of affected area and length of shift of a geological fault).
Such interrelated earthquakes had not been anticipated.
• Despite the strength of the quake, the earthquake response spectrum
located on the foundation of the R/Bs showed about the same value as the
response spectrum for the basic design earthquake ground motion, Ss.
Essential safety facilities and equipment continued functioning.
• Large shift of a geological fault triggered huge tsunami as big as theearthquake. The combination of such waves created a massive tsunami thatexceeded the predicted tsunami water level and descended upon the station,
causing extensive damage.
Measures to take
Given the above findings, the following measures should be taken:• The current basic design 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 geological shift at a hypocenter should also be set to larger value
as appropriate.
Measures to deal with obstacles that interfere with emergency operations suchas the handling debris are described in “4.5.4 Preparation for emergencies(heavy equipment and rescue efforts), and emergency cooperative system.”

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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 theseawater systems and the loss of the power supply including emergency power.
Tanks that were flushed away by the wave blocked roads, causing further delayin the preparing of injecting coolant by fire engines.
Findings and lessons learned• Station equipment and facilities may submerge depending on their locationand tsunami may cause seawater to prevent safety equipment from properly
functioning.
• Debris and 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 that will
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 the safety of the facilities.
• Barriers, such as protection walls, that prevent structures from becoming
floating wreckage and blocking access of emergency vehicles needs to beinstalled. Heavy equipment needs to be available in order to remove debris
and wreckage.
4.6.3

Protection of buildings

Buildings were flooded by seawater from the tsunami, causing safety 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 and pipe pass-through slots).
Most emergency power panels in Units 1 through 5 and DC main busbar
panels in Units 1, 2 and 4 were submerged. (There was no inundation in Unit
6.)
Emergency DGs also became inoperable in Units 1 through 5 due to thesubmergence of the DGs themselves or their power panels. Air-cooled
emergency DGs were installed in Units 2, 4 and 6, and the ones in Units 2 and4 became inoperable due to submergence of power panels. (The water-cooled
emergency DG in Unit 6 also became inoperable due to the inoperability of theseawater cooling system stemming from damage as a result of the tsunami.)
The framework of main building including outside walls and columns did not
suffer significant damage from tsunami. However, inundation into the buildings
from openings caused the loss of power-supply equipment resulting in loss of
all DC/AC power supplies and ultimate heat sinks.

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Findings and lessons learned• Insufficient preparation to prevent inundation and inadequate endurance towave power allowed seawater 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, seawater overflowed into important facilities andequipment (such as electrical facilities) from entrance doors and otheropenings including pipe penetration and resulted in those facilities andequipment becoming inoperable.
• Recovery operations were delayed due to accumulated seawater inside thebuildings.
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) dependingon the inundation height, must be taken for an unanticipated size of tsunami.
• Impact of inundation must be minimized by the water tightening of doors to
key safety facilities for an unanticipated size of tsunami.
• Portable drain pumps need to be equipped in buildings in order to be able todrain seawater and ensure speedy restoration.
• Doors facing the ocean and that may receive direct impact from a tsunami
must be reinforced.
(2) Protection of seawater systems against inundation
At Fukushima Daiichi, seawater systems ceased functioning due to theunanticipated size of the tsunami, but the system at Units 5 and 6 wasreestablished by using substitute pumps. At Fukushima Daini, all seawater
systems except the one at Unit 3 ceased functioning, but the system waseventually brought back on line by using replacement motors. At Tokai DainiPower Station, all seawater pumps remained operational except one because
a floodwall was under construction around the seawater pump area. No
stations experienced any blockage or damage of intake channels due to debris
and wreckage.
Findings and lessons learned• Fukushima Daiichi lost all seawater pumps due to the unanticipated size ofthe tsunami.
• Specific measures must be taken so as not to lose the operation of all of
seawater pumps at the same time due to tsunami.
Measures to take
Given the above findings, the following measures should be taken:• Protection against inundation including the building of floodwalls aroundseawater pumps must be initiated.
• Extra motors must be reserved to ensure a speedy recovery.
• Portable seawater pumps should be held in reserve or waterproof pumps
should be installed.

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4.7

Preparation for power supply (example measures)

4.7.1

SBO and loss of DC power supplies

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 right away
as intended and the required power supply for the station was reestablished.
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 supplies were lost. Power from
D.C. power supply system that were not submerged ceased later due to theexceeding of battery life. As a result, Fukushima Daiichi lost all power supplies.
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
the closest station also went down.
Fukushima Daini, Units 1, 2 and 4, also lost their seawater cooling systems.
However, core cooling was maintained by RCIC followed by an alternate cooling
operation. Meanwhile, the seawater system was reestablished and the radiation
cores went into a cold shutdown state. Thus, the safety of power station can beensured through the use of emergency equipment and facilities provided thepower supply is secured.
In order to recover all the power supplies, Fukushima Daiichi tried to bring power
supply vehicles 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 speedy recovery effort,
submergence of power panels resulted in an extended power blackout period.
Findings and lessons learned• Emergency DGs were installed to cover the loss of external power supplies.
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 D.C. power supply system were submerged and lost functionality.
Other D.C. power supply system that were not affected by the tsunami ran
out of power and became inoperable because they could not recharge. Suchlong-term needs to be considered.
• Severe accident response facilities (power panels to bring electricity fromother stations) were submerged by the tsunami and was unusable. Thesefacilities need to be protected from inundation.
• As an emergency response to the loss of all the power supplies, power
supply vehicles were sent to Fukushima Daiichi, but were delayed becauseof the adverse conditions brought on by the earthquake and tsunami. The

4-7

mobility of power supply vehicles 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 and tsunami-proof power supply vehicles orlarge-capacity batteries (gas turbines or diesel generators) as backup
power supplies and the establishment of emergency procedures (e.g.,
steps to obtain fuel such as light oil and how to connect power supplies).
• Protection of external power supplies against inundation- Review location and arrangement of incoming electricity transformers
and switchyards, and make them watertight or protect against possible
water damage as needed.
• Protection of emergency DGs and related facilities against tsunamis- Review location and arrangement of emergency DGs, power-supply
facilities (high and low voltage) and cooling systems for emergency DGs,
and make them watertight or protect against possible water damage asneeded.
• Protection of D.C. power supply systems against inundation- Review location and arrangement of D.C. power supply systems, andmake them watertight or protect against possible water damage asneeded.
- Establish DC power supply (battery) charging routes from backup power
supplies.
• 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 supplies, 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 toaddress the reliability and performance of multiple power systems.
Expansion of battery capacity, which is one of the challenges addressed, is alogical strategy, but feasibility is questionable due to required space for such asystem and its limited effectiveness. Therefore, in order to secure DC power
supply, the first step will be to secure power supplies to feed the DC power
supply. The second step will be to establish methods to charge batteries assoon as the power supply 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.”

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4.8

Examples of measures addressing heat sink loss

4.8.1

Water injection in reactors

In this accident, the electric-driven water injection system of ECCS could not be
activated due to the loss of power supply while the steam-driven water injection
system could not continue to operate due to the 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 the driving air
pressure of SRV (ADS function, relief valve function). In addition, MUMC, one of
the alternative water injection systems, could not operate because the pump wasinundated and the FP diesel fire pump failed. Furthermore, the alternative
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 damage was
unknown because they could not function due to the loss of power supply,
except some of the equipment was known to be damaged by seawater flooded
into the building.
Findings and lessons learned• The Station was unsuccessful in dealing with the situation because all
ECCSs stopped working due to complete loss of power supply for a longtime caused by the earthquake and tsunami, and because the facilities forpreventing severe accidents could not be used. Therefore, it is necessary to
examine the measures 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 be performed to depressurize thereactor. However, it is difficult to reduce the pressure due to the loss of the
supply which drives the valve for reactor depressurization. It would takesome time before the alternative low-pressure injection system can be used.
Therefore, it is necessary to examine the measures to cope with loss ofdriving source of facilities 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 be considered
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 the loss of existing driving supplies for a long periodof time.
- To improve the power supply reliability for the existing injection system bydeploying a backup power supply vehicles or large-capacity power
supply- To secure alternative means of injection to reactors using portable power
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•

•

pumps 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 whena station blackout occurs. In these means, water will be supplied to the steam
generators through the turbine-driven auxiliary water feed pumps with steam 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 the function ofwater injection to reactors in the BWR; therefore, it is important to ensure themeans of water supply to steam generator and steam discharge means. In orderto achieve continuous heat removal for a long time, the following measures aresuggested.
• Improve the power supply reliability for the existing water injection system bydeploying a backup power supply vehicle 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).
• 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. explained that a fact that“damage to the FP by the earthquake caused difficulty in responding to theaccident” was not yet certain. Therefore, the need to examine FP will be decided
after the investigation results of Tokyo Electric Power Co., Inc. become available.
4.8.2

Loss of seawater cooling

As the tsunami was stronger than expected in the Station’s design, the seawater
pump motor and ancillary equipment were damaged and could not operate. The
heat removal route of “reactor → residual heat removal system → reactor
building closed cooling water system → reactor building closed cooling seawater
system → seawater” for final heat relief (final heat sink) lost 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 for
4-10cooling by seawater pumps could not be used; therefore, ECCS and ancillary
reactor equipment could not operate even without the power loss.
Findings and lessons learned• It is necessary to examine the measures for inundation caused by tsunami
because the seawater pumps will no longer be used in an unanticipatedtsunami. (Refer to the section of Tsunami for measures)• It is necessary to examine the means to recover inundated seawater pumps.
• It is necessary to examine a way to secure the heat removal route in theevent that the seawater pumps could not be recovered early.
Measures to takeIn light of those described above, the following measures are suggested.
• Alternatives to seawater pumps- Deploy redundant seawater pump motors and develop a replacement
procedure after loss of functionality- Ensure the power supply which can back up the minimum functions
required for seawater pumps• Have measures for early recovery of seawater pumps- Have the cleaning and drying materials available for seawater 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 seawater
system → seawater” as final heat sink loses its function.
4.8.3

Containment ventIn this accident, the route for heat removal to the final heat sink (sea) failed tofunction due to the loss of seawater 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 to lower
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 of theloss of power supply to the metering instruments.
• The remote control was disabled due to the loss of power supply to the ventvalve and the cylinder pressure. Although alternative manual operation was

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•
•

planned, some time was needed to examine and establish the ways for
implementation.
In addition, due to the increase in building radioactive dose and torus room
temperature, it was extremely difficult to access the vent valve area.
The evacuation of the surrounding residents was finally confirmed.

Findings and 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 higharound the vent valve area when the reactor core was damaged, and sometime was needed 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 materials 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 damage tocontainment while performing the vent.
Measures to takeIn light of those described above, the following measures are suggested in caseof 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 damage to the reactor core
In addition, in the PWR, there should be a secondary cooling system with steam
generator even if the heat removal to the final heat sink fails due to the loss ofseawater cooling. This system should 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 is believed
not necessary to make a change in the design of the rupture disk. However, themeasures to lower the set value for operation of rupture disk, or to remove therupture disk, will have a wider choice in response to accidents as thesemeasures might depressurize the containment early; and whether thesemeasures should be adopted will be determined based on judgment of electric

4-12

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 respectively in the Unit 1 reactor building at 15:36 on March
12, and in the Unit 3 reactor building at 11:01 on March 14.
Due to these explosions, it was difficult to lay power cables and connect the hoses.
Also, an explosion, possibly caused by hydrogen, occurred in the Unit 4 reactor
building at around 6:00 on March 15.
Findings and 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
the reactor buildings. It is necessary to consider measures to prevent such asituation.
• In the case of containment vent using the reinforced pressure-enduring ventline, it is necessary to examine the measures to prevent the wraparound ofhydrogen from SGTS exhaust pipe, or to prevent the wraparound ofhydrogen through the junction 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 release or reduce emission of hydrogen which will bedeposited 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 hydrogen gas detectors
to appropriately 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 wraparound of hydrogen from such
exhaust pipes.
• If an exhaust pipe is shared, it is necessary to examine the measures toprevent the wraparound 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 demand 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.

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4.10 Emergency preparedness (especially training)
4.10.1 Training
Because of the severe work environment of the incident response site in this
accident where there was hardly any lighting or communication methods, andhigh-dose of radiation with scattered wreckage of building, it was very difficult toperform fieldwork such as cable laying from power supply vehicles and join up,
laying alternative injection hose and so forth. Usually the fieldwork needs to takea lot of time and trouble, but we tackled it within a relatively short time thanks to
the workers’ efforts. The fact that fieldwork would require this time had not been
assumed in the countermeasures training for severe incidents so far.
As this accident is a multi-unit and coincident accident that is not presumed in
the conventional training, prompt decision-making and response were requiredwith a limited number of workers on a moment-to-moment basis.
Findings and lessons learned• The countermeasures training for severe incidents should include field
operation training 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.
• Workers should get training for various situations on a routine basis in orderto take actions in line with time in the event, and are encouraged tounderstand how his or her incident response activities can affect the courseof events.
Measures to takeIn light of those described above, the following measures are suggested.
• Assemble knowledge and 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 workers with the field trainings for the fieldwork presumably
required as countermeasures for severe incidents so as to take theworkers’ proficiency in procedures and methods to a higher level.
- Simulate the fieldwork during nighttime or without communication
methods, and the fieldwork with full range of protective gear such asprotective clothing and full-face mask. Also consider an impact of
obstacles on approaching the field.
• Training based on proceeding with time in the event- Provide a training having effectiveness with a focus on reliable
cooperation and decision under the same condition as real such as
blind training and real-time training in addition to training based on a
scenario.
- Confirm time required for such as approaching the field, wearing theprotective gear, and performing fieldwork, and then ensure how his or
her activities can affect the course of events.
- Make use of simulator etc. to enable each personnel to take actions inline with time in the event.

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The training mentioned above is preferably provided on a regular basis to remind
the lessons learned and to maintain or improve the proficiency continuously.
4.10.2 Air conditioning and shield at Main Control RoomIn this accident, workers could not enter MCR or stay at MCR for a long time
because of the high radiation dose.
And MCR ventilation system having a charcoal air filter for iodine removal did notwork due to the loss of electric power for a long time; therefore it is necessary towear a full-face mask even in MCR.
These conditions made MCR habitability lower, and this hindered incident
response activities.
Findings and lessons learned• Loss of electric power for a long time disabled the MCR ventilation system.
In response to this situation, we need to consider a countermeasure.
• Due to the high radiation dose, workers could not enter MCR or stay at MCR
for a long time. In response to this situation, we need to consider acountermeasure.
Measures to takeIn light of those described above, the following measures are suggested.
• 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 at MCR.
4.10.3 Measurement during accidents
In this accident, instrument power supply was lost, because D.C. power supply
system had stopped working in addition to the fact that there was a station
blackout for a long time. Therefore, important parameters such as reactor waterlevel, pressure, and temperature were not measured, records for theseparameters were lost, and the status of a reactor like the injection status was notdetermined. To restore the instruments, workers needed to connect batteries
collected from vehicles to the MCR instrument board as temporary power supply.
And from the viewpoint of instrument data reliability, the indicated value of thereactor vessel level instrumentation system varied from instrument to instrument.
According to comparative result of a water level instrumentation installed
temporarily to confirm the reliability of instrumentations, the actual water level ofreactor might be less than the measurement limit of the water level
instrumentation in the fuel zone. Therefore, 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 ofhigh temperature in the containment vessel.
As the accident progressed, workers were asked to make unexpected
measurements such as water level measurements due to evaporation of poolwater at the spent fuel pool, hydrogen-concentration measurement to determinehydrogen leakage and accumulation from the containment vessel at the reactor
building. In fact, there was no means of measurement, so it became difficult to
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respond to the incident. Moreover, inside the building was exposed to highradiation and this prevented the workers from approaching the instruments sothat restoring the measurement system became difficult.
Findings and lessons learned• (Dependable supply of power) Losing MCR monitoring function due to theloss of electric power for a long time was beyond the scope of assumption. In
response to this situation, some measures must be considered.
• (Enlargement of measurement specification) Because the reactor water level
was less than the measurement limit of the instrumentation during a severe
accident, we need to consider the outbreak of a severe accident in terms ofthe 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 inconventionally assumed events such as hydrogen-concentration in thereactor building. In response to this situation, some measures must 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, some measures must be considered.
Measures to takeIn light of those described above, the following measures are suggested.
• (Considering dependable supply of power) Consideration for restoring
instrumentation early in case of the 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 specifications) Consideration of
the means to measure important parameters such as reactor water level tounderstand the plant status during a severe accident.
- Consider how to measure important parameters during a severe accident
in order to understand the plant status such as water level of the reactor.
- 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,
and measurement system for them. This is not requested in theconventionally assumed accident.
- Establish a parameter measurement system such ashydrogen-concentration in the reactor building.
• (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
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and a variety of measurement means in view of the outbreak of a severe
accident.
4.10.4 Emergency operation facility
Emergency Operation Facility is a facility to give countermeasure instructions
during an accident and placed within Examination Guide for Safety Design for
Nuclear Power Facilities. However, in this accident the available communication
methods between Emergency Operation Facility and MCR were only hotline andland line after station blackout. The communication environment became worse
so that the communication between Emergency Operation Facility and the field
was hardly established, which hindered discussions on countermeasures andposed a problem for giving instructions. And personnel went to theadministration building, which was restricted area, to get valve drawings while
there were aftershocks so as to study the procedure of vent operation for thecontainment vessel after the 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 and take off radiation
protective clothing in the earthquake-proof principal building where Emergency
Operation Facility was located, the concentration of radioactive materials in the
air of building was beyond the limits by law to the day of April 3, so that it did notprotect workers from the risk of internal dose even in the building and it did notprovide enough space for relaxation for workers who have been working over along time.
However, the earthquake-proof principal building could withstand the earthquake
and tsunami with independent emergency electric power supply unit and air
conditioning system. Though it had problems as mentioned above, it isnoticeable that it functioned as a base for incident response activities.
Findings and lessons learned• Emergency Operation Facility is a facility to give countermeasure
instructions during accident. It inevitably ensures the communication
methods to MCR or the field and enables people 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 prepared at Emergency Operation Facility andpersonnel should be capable to get them easily if required.
• Emergency Operation Facility is a facility where workers stay and use whenan emergency arises including radioactive release. It needs adequate
radiation protection including preventing the internal retention of radioactive
materials and considering the external environment when used.
Measures to takeIn light of those described above, the following measures are suggested.
• Emergency Operation Facility is expected to use under the severe natural
phenomenon such as an earthquake, tsunami, and under the severe
conditions such as station blackout for a long time, releasing large amount ofradioactive materials into the environment so that it should be prepared to

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provide its functionality considering above mentioned situations. Especiallyconsider the following points.
- Building designed and located to withstand an earthquake and tsunami- Station capable of using in case of the loss of external power supply for along time and ensuring independent emergency electric power supply- Collecting the plant parameters required for the countermeasure
instructions and ensuring the communication methods to MCR and thefield- Radiation protection countermeasures considering large volume ofradioactive material release- Ensuring habitability for long-stay workers- Ensuring emergency material for severe environment- Scrutinizing and ensuring judgment material required for emergency
countermeasures
4.10.5 Radiation control and 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, there were not enough APDs for all workers. Only someworkers could use APD. And the exposure control system did not work due tothe loss of power, so we had to manually record the irradiated dose. Charcoal
filter masks which used as iodine adsorbent were also in short supply; as a result,
the risk of internal exposure increased. Holding-counter type WBC for measuring
internal exposure became unavailable partly because the backgroundradioactive materials on site increased. Workers took measurements using
mobile type WBC and measurements in other power stations, but it was notenough. It caused confusion because of the shortage of all sorts of radiation
measuring instruments and radiation protective clothing, and also loss of related
functions.
In the middle of the confusion, some people were exposed to radiation beyond
the dose limit. We confirmed that multiple workers involved in emergency work
were exposed beyond the dose limit of 250 mSv. And two female workers whostayed in the earthquake-proof principal building were also exposed beyond thedose limit of 5 mSv in 3 months as the total of external exposure and internal
exposure. And 3 workers from the contracted company were exposed beyond
170 mSv, ignoring the alarm from the APD. Especially two of them, wearing low
shoes, got their feet wet in radiated water. Since we suspected possible beta ray
burns because there were radioactive materials on their shoes, they were taken
to a hospital and then to the National Institute of Radiological Sciences (NIRS).
In the earthquake-proof principal building, since no buffering area was specified
to put on or take off radiation protective clothing, the concentration of radioactive
materials in the air of the building was beyond the legal limits by April 3.
In this accident, because of the vent operation for the containment vessel,
radioactive material was released to the environment due to the hydrogen
explosion, and scattered wreckage with a high dose of radiation, radiation
control service including measurement of the environment increased abruptly.
Thus, it was very difficult to control the radiation.

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Findings and lessons learned• Due to the tsunami and loss of electric power for a long time, it was beyond
the scope of assumption that personal dosimeters like APD would become invery short supply, that radiation protective clothing like masks would run
short due to the rapid progress of the accident, and that WBC would become
unavailable due to the high radiation dose.
• Because radioactive material possibly is released to the environment in
activities such as the vent operation for the containment vessel during an
accident, internal exposure by iodine might be beyond the assumed scenario
if there is no radiation protective clothing like masks. A base for incident
response activities also might have radiation pollution, and as the accident
progresses, some areas of the workplace might see a higher dose rate or be
polluted locally. Hence, radiation control should manage both external
exposure and internal exposure depending on the situation. And working
control should assume such a situation beforehand.
• During an accident, the amount of radiation control work might increaseabruptly so we 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 the
National Institute of Radiological Sciences. It is an anecdotal experience andwe should continuously promote understanding associated with emergency
exposure medical treatment.
Measures to takeIn light of those described above, the following measures are suggested.
• Prepare materials and equipment such as radiation measuring instruments
and radiation protective clothing so as to do adequate radiation control. The
following measures are listed for example.
- Ensure diversification of electric power supplies like battery-operated
ones for radiation measuring instruments and enough electric power
supplies to use for a long time. Place the required number of batteries inan area unaffected by tsunami. And make a plan arranging to share therequired radiation measuring instruments beforehand.
- Ensure mobile WBC such as WBC on-vehicle, or arrange to use WBC ofother facilities.
- Ensure the required amount of the radiation protective clothing
considering the rapid progress of an accident.
• Make the Emergency Operation Facility, which becomes a base for incident
response activities during an accident, available on the assumption thatradioactive materials exist in the environment. Make sure that all workers
know the latest information regarding dose rate and pollution in theworkplace with the progress of an accident so that they can fully recognize
the importance of exposure dose reduction.
• Reinforce the system to let workers act more quickly to handle the radiation
control work which might increase abruptly. For example, request assistancefrom radiation management workers outside the station, and have workers,
who do not have the knowledge of radiation control, calculate a radiation
dose. Such an support system enables radiation management workers tofocus on more important operations.
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•

Prepare to arrange an emergency exposure medical treatment team in caseof an accident.

4.10.6 Organization and command or order
Originally the 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 for government authorization and to negotiate with the local
government, when the vent operation became inevitable. External adjustment
took time because, for example, we needed to wait until the evacuation had
been completed.
And when doing seawater injection at Unit 1, workers proceeded based on theconfirmation and agreement of the company president. After that, as a result of
discussion between the head office and the power plant, the Station decided totemporarily stop seawater injection, but it was continued based on the judgmentof the director of the power plant. There was confusion in the chain of commandor order.
Findings and lessons learned• Regarding emergency measures, in the confusion during emergencies
adjustments need to be made with the associated organizations. Therefore,
there is a possibility of delay or confusion in making decisions.
Measures to takeIn light of those described above, the following measures are suggested.
• Clearly articulate the timing of judgment for the emergency measures that
affect badly an accident, and proceed with external adjustment for them at
an early date preparing to conduct prompt operations.
4.10.7 Communication
In this incident response activities, due to water exposure by the tsunami andstation blackout for a long time, internal PHS or paging devices, which were aform of ordinary communication methods in the field, countermeasures
headquarters, and MCR, were hardly used.
Because field workers put on a full range of protective gear such as protective
clothing and a full-face mask they had to work in a severe environment andwithout communicating with one another. Therefore, it was very difficult to do theincident response activities because it took time to confirm the field status and toprovide directions for fieldwork.
On the other hand, regarding communication between the power station andoutside parties, in fact the leased line network to internal and externalorganization was available in this accident. But there is always a risk it will be
disrupted like the internal network in the power station due to an earthquake,tsunami or such like.
Findings and lessons learned• The communication methods within the power station and to the outsideshould be built and maintained by means which are free from the influenceof an earthquake or tsunami, and which are capable of using a power supply
even when a station blackout occurs for a long time.

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•

One communication method to be used between field workers and anothercommunication method to be used among field workers, countermeasures
headquarters and MCR should be prepared, considering use with a fullrange of protection gear and the severe environment during an accident.

Measures to takeIn light of those described above, the following measures are suggested.
• Countermeasures for communication methods within the power station andto the outside which are free from the influence of an earthquake or tsunami,
and which are capable of using power supply even when a station blackout
occurs for a long time.
- Prepare backup power such as battery which is free from 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 switches, and cable communication lines
or wireless staging base so that they cannot easily be disrupted bynatural disasters such as an earthquake and tsunami.
- Deploy multiple communication devices such as leased lines, satellite
phones, and wireless lines, and clarify the way they operate.
• Countermeasures for facilitating communication between field workers, oramong workers, MCR and Emergency Operation Facility.
- Deploy the required number of communication devices including thoseused in the field.
- Clarify the way to operate 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 worker 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 the siteboundary and border line which business worker takes charge of, measurementat MP within the site boundary and on border line became unavailable due to theloss of external power supply caused by tsunami. Therefore, until the external
power supply was restored, the environmental monitoring was continued by useof monitoring vehicle, temporary MP and survey meter etc. And themeasurement results were released on the Internet as applicable, though it waslimited due to manual creation.
In the case of local government’s environmental monitoring, environment
monitoring equipment and facilities required were not available due to the effectof power outage, communication line disconnection, and local governmentbuildings swept away by the tsunami. And also, the environmental monitoring
activities had a major obstacle because persons in local government had tofocus on responding to the earthquake and tsunami that occurred in a wide area,and because the onsite countermeasures headquarters had to move from theoffsite center to Fukushima prefectural government. Moreover, related
government offices, who were supposed to help local government to executeand support monitoring based on local government’s request, were engaged in
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other earthquake responses such as investigating the great number of missing
persons that covered a wide range, so it was difficult to build the environmental
monitoring system just after the earthquake.
On the other hand, business workers except those at Tokyo Electric Power Co.,
Inc. dispatched workers based on an atomic energy cooperation agreement andsupported the environmental monitoring outside the site boundary executed by
the local government utilizing a monitoring vehicle.
As mentioned above, there were many problems in monitoring facilities andsecuring workers, but monitoring activities have been done as much as possible.
However, some mass media criticized the Station for the delay in releasing
monitoring data, and low reliability of offshore monitoring data, and doubted thatsome of monitoring-vehicle measurement data, which were complementarily
measured every 2 minutes by Tokyo Electric Power Co., Inc., were not released.
Findings and lessons learned
Since the entity in charge of environmental monitoring differs depending onwhether it is inside or outside the site boundary, we only describe the monitoring
executed by business workers.
• Due to the effect of power outage by tsunami, the permanent monitoring
facility of business workers became unavailable, so alternative means had to
be used.
• Although accepting workers based on the atomic energy cooperationagreement and working with environmental monitoring activities conductedby the local government to do monitoring activities continuously, it took along time to build the system to do the environmental monitoring under thecondition where extensive natural disasters and nuclear accidents occurredat the same time.
• The monitoring data obtained by business workers was released on theInternet or by the mass media if applicable, but from the viewpoint ofpromptly releasing it and ensuring the reliability of the released data, theStation failed to satisfy the general public and the mass media.
Measures to take• Public-private partnership such as cooperation and support in theenvironmental monitoring conducted by the local government, andprivate-private partnerships such as cooperation among businesses should
be reconstructed. Specific details are shown below.
- Reconfirm the monitoring equipment and facilities that are required to beshared in an emergency, and deploy the required number.
- Reconsider the workers to be dispatched in an emergency, and clarify
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 equipment and facilities by use of alternative means,
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•

and deploy them
Required information and means of transmitting it to the general public,
related organizations, and workers involved should be clarified. Specificdetails are shown below.
- Streamline the contents of transmitted information, how to coordinate
them, and required workers- Consider transmitted means (newspapers, websites, etc.) and timing of
release4.10.9 Preparing disaster countermeasures (heavy equipment and rescue),emergency cooperative framework
In this accident, extensive natural disasters and nuclear accidents occurred at
the same time. Therefore, it was very difficult to contact and communicate withpeople, do an emergency call up, and procure goods. The Station was not able
to quickly and sufficiently accept the materials and equipment for use inemergency measures or rescue teams who support the accident management
activities as the initial reaction to the accident. Also, the high radiation level at thefield became an obstacle to human responses. In addition, activities had to be
conducted with conditions such as looking out for aftershocks and the following
tsunami, darkness, high contamination status, ensuring electric power supply inanticipation of worsening of environmental conditions such as contaminatedwater, arranging materials and equipment and temporary work, removing
blockages so as to let people pass through on and off site. These conditions
greatly influenced the following incident response activities.
Findings and lessons learned• Due to the earthquake and tsunami, routes to access were damaged within
and outside the power plant. So it took time to bring in materials andequipment for use in incident response and to call up supporting workers.
This hindered smooth incident response activities. Materials and equipmentthat facilitate working in constrained conditions for various sorts of work were
expected.
• Preparations for a multi-unit and coincident accident like this accident were
not enough, so that protective clothing, dosimeters and masks for use inincident response were not deployed in an appropriate manner. And having
workers work under high radiation for a long time was not assumed
specifically. Because there had been no preparation for work under these
conditions, incident response was not able to proceed promptly.
Measures to takeIn light of those described above, the following measures are suggested.
• On the assumption that routes to access are blocked within and outside thepower plant due to natural disasters (earthquake, tsunami), preparing heavyequipment to remove scattered wreckage diversifying workers and means oftransport for materials and equipment are required. Considering cooperation
with related organizations, specific details are shown below.
- Clarify and deploy the required heavy equipment except for the wheel
loader which was deployed as severe accident countermeasures.
- Establish a means for sea transportation and air transport instead of landtransport, and deploy the system.

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•

We need to ensure the required materials and equipment for use in incident
response, and to reconstruct a system to share them among workers on an
emergency basis. Considering cooperation with the related organizations,specific details are shown below.
- Clarify the required materials and equipment based on the incident
response of this accident, confirm required number of them, and deploy
them (including temporary lights, temporary air conditioning, andtemporary drainage equipment).
- Reconfirm a system to interchange among business workers on an
emergency basis and deploy means of interchange.
- Focus on a remotely-operable facility and equipment (robot, unmanned
helicopter) for Nuclear Emergency Preparedness that can work under ahigh radiation level, participate in review sessions associated with its
development and operation, and deploy a cooperative system withworkers and related organizations for actual installation.

4.11 Security of integrity of spent fuelAll external power supplies were lost due to the earthquake. However, allemergency DGs were activated excluding one that was undergoing an inspection.
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 the functionof the seawater pump was lost due to the tsunami.
Further, we consider that there is no exposure of fuel in the spent fuel pool.
Findings and lessons learned• When the cooling function of the spent fuel pool was lost, it is extremely
important to keep the water level of the spent fuel pool at a safe level, andmaintaining and strengthening the water level of the spent fuel pool and themonitoring function of the pool water temperature are also important.
• There is a need to prevent the pool water level from falling and the spent fuelbeing damaged followed by the loss of the spent fuel pool’s cooling function.
Hence, in the wake of this accident, we consider that new countermeasures
have to be additionally prepared in order to maintain the cooling function of
the spent fuel pool at the time of a station blackout in a number of units
including adjacent units and when the function of the seawater pump is lost.
Measures to takeIn light of those described above, the following measures are suggested.
• It is necessary to secure an alternative coolant injection means to secure thewater level of the spent fuel pool.
- Supplying water by fire engines, pump trucks, and such like 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.)
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•

It is necessary to investigate the maintenance of the cooling function of the
spent fuel pool water as a continuous cooling means after the water levelhas been secured.
- Securing the alternative cooling function of the spent fuel pool water.

4.12 Summary of countermeasures
As described in Chapter 3 “Analysis of Causes of Accident Events and Extractionof Problems,” to prevent reoccurrence of the accident it is important to increase thesafety margin by assuming that an accident will escalate and by preparing for it bytaking appropriate countermeasures.
The profoundness of the system of countermeasures is summarized and outlinedin Table 4.12-1 by combining the countermeasures that have already existed andthe countermeasures that are exemplified here as new countermeasures.
In the causal analysis, the main cause of the accident escalating was the loss ofthe power supply, and it was described that the safety components stopped
working due to the loss of the power supply. Therefore, we decided to confirm thestatus of the system of countermeasures regarding the power supply. Further,
there was a loss of a heat sink (including the auxiliary component cooling system)
as a main factor that meant the safety components other than the power supply
stopped working. We also confirmed matters from that viewpoint. Furthermore, we
confirmed the situation about hydrogen because the hydrogen explosion largely
affected the discharge and diffusion of radioactive materials in addition to hindering
the Station’s response to the accident.
The countermeasures are outlined, and basically the following approach can betaken.
First, the Station can attempt to resolve an assumed major accident at the stage ofdesigning the countermeasures with the safety components (the seawater pump,
the emergency DG, the ECCS, etc.) that have been already installed. Reviewing
what happened this time, we shall further protect these safety components thathave been already installed.
The existing safety components have multiplicity and diversity. Severe accident
countermeasures (accident management) were prepared independently at eachnuclear power station in the 1990s by assuming a case where the functions of allthese safety components was lost. In the Fukushima Daiichi accident, most of theaccident management could not be applied due to various constraints such as theloss of the power supply and high contamination. Because of this,countermeasures for smooth application shall be considered.
Because this accident management did not sufficiently function, we shall prepare
severe accident countermeasures that will be added this time.
Therefore, as shown in the confirmation table, selectively treating thecountermeasures means further protecting the exisiting safety systems andadditional severe accident countermeasures.
In such way, a layered protection system can be prepared by adding severe
accident countermeasures to the existing safety components and to the accident
management components. We consider this will become an effective
countermeasure for preventing the accident from progressing.

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- Power Supply
The power supply shall be secured surely against an earthquake and tsunami bylayered countermeasures.
• The power is supplied by an external power supply during a blackout in theconventional design. However, the power supply is supplied by anemergency DG when external power supply is also lost. This power supply
protects these functions.
• Assuming a case where these functions are lost, the power supply issupplied by an alternative means such as a power supply vehicle.
• Valves and such like are individually operated by deploying a storage batteryor such like.
- Heat sink (coolant injection and cooling)
Layered countermeasures shall ensure the core is cooled without fail when an
accident occurs.
• The seawater system and the ECCS component that are expected toprevent an accident from escalating in the conventional design shall beprotected.
• Spares for the seawater system are deployed as countermeasures when theprotection is breached. These are protected by the facility of severe accident
countermeasures (accident management) against the loss of the function of
the ECCS.
• Considering a case where 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
the injection of water.
• Furthermore, countermeasures shall be taken in the BWR for certainly
performing a container vent that is a final cooling means when an accident
escalates.
- Countermeasures for a hydrogen explosion
An explosion shall be prevented by countermeasures for each stage of the event
such as prevention of hydrogen generation, prevention of leakage, and reductionof discharge.
• The generation of hydrogen due to damage to the fuel is prevented in theconventional design by securing the cooling of the core. Also, an explosion isprevented in the BWR by confining the generated hydrogen in a container
having a nitrogen 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 the building by considering that hydrogen leaks from thecontainer because hydrogen easily leaks.
• Countermeasures are taken to prevent the leaked hydrogen fromwraparound other units from a vent line.

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Specifically for investigating (adapting and combining) other layeredcountermeasures, it is effective to consider 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,• protection against possible common causal factors shall not be decreased
and new common causal factors shall not be induced,• independency of barriers shall not be decreased,• preparation for human error shall be maintained, and• functions that are planned in a plant’s design standard shall be secured.
In order to determine priority among individual countermeasures, the following
criteria shall be used.
• Means such as the UHS and the power supply 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”
of layers have higher priority.
• Qualitatively, an individual step is considered to become more important as
the number of steps decreases.
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 possible internal and external hazards and failures.
By combining various countermeasures at this time with the countermeasures thathave been considered at the stage of designing and constructing, the layeredcountermeasures shall be taken that are sufficiently effective for securing safety
and for effectively preventing an accident where a large amount of radioactive
material 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 be
adopted when each company implements countermeasures.
- Power supply• Establishment of a tide embankment and breakwater for a facility that is
important for safety• Prevention of inundation of the area where components that are importantfor safety are provided• Prevention of inundation, including improving sealing capability at openings
and penetration of an air supply port, depending on the inundation height• Water tightening of the sites where the transformer for receiving and theswitchyard are arranged, or countermeasures for inundation of thecomponents
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•
•

•
•

Water tightening of the sites where the D.C. power supply system isarranged, or countermeasures for inundation of the components
Deployment of a power supply vehicle or a power supply having a large
capacity (a gas turbine or a diesel generator) and preparation of its
emergency procedure
Preparation of a DC power supply charging route by the backup power
supplyImprovement of reliability by countermeasures for inundation of the facility
that accommodates a power suuply between the Units (severe accident
countermeasures)- Heat sink (water injection and cooling)• Measures for preventing inundation of a bulkhead and such like around theseawater pump
• Disposition of spares for the seawater pump
• Prevention of inundation of areas where safety components such as the
emergency core cooling system are installed• Prevention of inundation of the facility for severe accident countermeasures• Disposition of a movable coolant injection pump
• Preparation of the backup power supply for driving the SRV, spare air
cylinders, etc. (BWR)• Security of the core cooling through the SG by an Atmospheric Dump Valve(PWR)• Disposition of a backup power source for operating a vent and a driving
source (BWR)• Improvement of reliability of the existing coolant injection system bydeployment of a backup power supply vehicle and a power supply having alarge capacity• A portable power pump, hose, and such like that do not depend on theexisting 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 resistantstrengthened vent line
• Prevention of wraparound of hydrogen between the Units sharing an
exhaust pipe
Further, countermeasures are combined by considering the safety of the plant as awhole and selecting countermeasures for implementation. Various combinations ofcountermeasures 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.
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Table 4.12-1 Checklist processed in terms of defense in depthFirst layer
Multiple levels of protection(normal operation)

Second layer(Anomaly in the expected operation)

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

Purpose Prevention of abnormal operations and failures
Control abnormal operations and detect failures
High quality in conservative design, construction andControl, limitation and protection systems
ECCS and accident procedures
operation- Not considering the flooding.
- Assess the expected tsunami height by basic design - Backup with emergency DG at a loss of external
earthquake ground motion Ss. Tsunami with the height power supply. Install 100% emergency DG in 2 systems - Ensure power supply by the emergency D/G and DC
battery
serially.
greater than the site height is not expected.
Supply the power by external power supply if the power - Assure impartiality for power supply by installing the
Beforesteam-driven equipment
generation of the plant is stopped.
earthquake - Evaluated that seismic capacity of transmission tower - The water immersion through the openings, such as
is enough. Implementation of measures to improve the building ventilation port, by tsunami are not considered.
seismic reliability are in process

Mandatory measures

Source- Set up the storm surge barrier and breakwater for the - Measures to prevent water immersion into theimportant equipment for safety reasons
installation areas for important equipment for safety
reasons
Proposed- Measures to prevent water immersion through themeasuresopenings like air supply opening depending on theheight of water by improving seal for penetration area

- Water tightness for receiving voltage converter andswitchyard, or anti-flood measures for equipment- Water tightness for an installation location of DC
power supply system, or anti-flood measures forequipment- Place the transportable drainage pump

- Install the 2 series of 100% seawater systems or the 4 - Install 2 series of safety systems such as ECCS.
series of 50% seawater systems. Ensure the diversity. - Install systems (IC, HPCI, RCIC) that function only
- Assess the expected tsunami height by the latest
with DC power supply.
knowledge of construction engineer. The measures for - Enable handling of transient by operational action- Multiplicity, diversity and impartiality has been
without safety system equipment such as ECCS etc.
these has already been carried out.
Before- The loss of functionality of all equipment at the same ensured.
earthquake
time due to the immersion of water into seawater pump - Although seawater system has unwanted HPCI and
RCIC in the cooling of oil and bearing, if the DC power
and power panel is not expected.
supply is cut off, it will fail to operate.

Fourth layer(Severe accident)

Mitigation of SA
AM included the protection ofcomplementary measures and PCV- Improve the flexibility of power between units as AM- Long term power outage, by the estimating that the
AC power supply is recovered since about last 8 hours, measure. However, the power loss of multiple units atthe same time is not expected.
is not predicted.
- After a loss of AC power supply, perform the corecooling as per the DC powered equipment and expect
the recovery of AC power supply

Complementary measures and accident management- Deployment of the power source vehicles or large
power (gas turbine or diesel generator), proceduredevelopment during emergency- Root development of the DC power supply withbackup power supply- Improvement of reliability with the anti-flood measures
for the power interchange facilities between the unit

- No development of injection means other than AM- Develop the severe accident measures (accident
management). Use the system as per necessity as an measure.
alternate injection system- Loss of electric power, inundation are not assumed- No alternative sources of various types of valves driveCore- Measure to prevent water immersion such as
bulkheading around the seawater pump

Heat sink(Injection/cooling)Proposedmeasures

Existing
state *- Place the spare parts for seawater pump motor- Wash the seawater pump motor and place the drymaterial- Release heat to atmosphere by containment vent- Place mobile injection pump
- Prepare SRV drive backup power supply and spare (guarantee the containment integrity)- Deploy the backup power supply and drive source for
air cylinders etc.
- Ensure the injection means by deploying the backup vent operation - - Optimization of operating conditionsof vent
power vehicles or high-capacity power supply- Measures to prevent water immersion into AM system - Ensure the injection, the route for injection by fireengines, etc. and river head
- Portable pumps and hoses that are independent ofexisting power supply- Ensure river head (also include seawater as lastresort)- Release heat to atmosphere by containment vent(emission of heat from S/C)
- Deploy the backup power supply and drive source forvent operation- Hydrogen leakage can occur from containment if core
- Prevent hydrogen generation by core cooling by
is damaged.
ECCS etc.
- Set well below the flammability limit even though there - Reverse flow of gas between the units is not
is a hydrogen leakage by encapsulating the nitrogen in considered.
the PCV and by setting inactivated environment.

- Measures to prevent the water immersion in the
installation area related to emergency core coolingsystem- Install waterproof pumps

Hydrogen (H)- Reduce the emission of retained hydrogen in the- Release and reduction of accumulated hydrogen
reactor building- Prevent the circulation from pressure-resistantstrengthening vent line
- Prevent the circulation of hydrogen between units thatshare the stack.

Proposedmeasures

SFP

Injection,
cooling,source, waterlevel- Install the equipment with sufficient cooling capability - Set the height of discharge pipe above the required - Loss of cooling means is not expected.
for decay heat of maximum storage capacity.
water level and maintain the required water level eventhough there is a leakage in pipe due to siphon break.
Before- Contains the supply function of river head of seismic
earthquake
S class. Maintain the water level by supplying the waterfrom water supply system.
Proposedmeasures- Ensure the route for injection by fire engines, etc. and - Ensure alternative cooling function (or early recovery
river head.
of auxiliary cooling system)

4-29

― 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](1) Preparation of backup power and reserve air cylinder for SRV drive, etc.
[Seawater cooling failure](2) Heat relief into atmosphere from container vent[Container Vent](3) Consideration of provisional/temporary power supply to the monitoring
instruments necessary for determining 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 spray

1[Measures to prevent flooding of building](1) Depending on the height of the flooding level, thesemeasures are to prevent flooding of the outer door of
the building, open the air supply vent and such like,and improve the seals corresponding to penetration
part(2) Measures to prevent flooding of equipment
important for safety(3) Deployment of portable drainage pump(4) Consideration of wave power with regards to outer
door that is directly affected by tsunami

Reactor Building[Injection to nuclear reactor]
11(13) Flooding countermeasures for layout areasuch as countermeasures for severe
accidents, core spray system.

External power supply(Normal (275KV)

Centralcontrolroom

Used Fuel Pool

External power supply(Backup (154KV)ReactorContainment Vessel

10[Injection to nuclear reactor](11) Injection by portable power pump or hose when
there is no pre-existing power.
(12) Ensure source of water (as a last resort, include
seawater).

Fuel
Component
cooling
waterpump

Pump forpressurizedwater injection

Filtered water
storage tank

OutletCirculating water pump

Cooling water(seawater)

Emergency Diesel
Generator (Backup
power source)

Pressure Control
Pool

Condensate
storage tank

Diesel fire extinguishingpump

9[Earthquake, tsunami anticipation](10) Basic earthquake ground motion
established as Ss consideringinterlocking of multiple epicenters(11) Review of setup conditions at the timeof assessing the tsunami height

Condenser

Water

Feed water pumpControl rods Recirculation pumpFire engine

4[Injection into nuclear reactor](7) Improvement in reliability of existing injection
system by deployment of generator car for
backup or high capacity power source[Failure of seawater cooling](8) Power supply that can provide backup forthe minimum essential functions of seawaterpump

Outlet(seawater)

Seawater pump for coolingthe Emergency Diesel
Generator

Seawater

8[Seawater cooling failure](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

3[Station blackout and direct current failure](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 of reliability due to flooding
countermeasures for interchange facility between
power supply equipment (countermeasuresagainst sever accidents)

Steam

Used fuel coolingsystem (Normal)

Reactorpressure
vesselElectrical panel

27[Flooding countermeasures for seawater](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 flooding-prevention pump

Figure 4.12-1 Examples of Countermeasures 1/3
4-30

5[Station blackout and D.C. power source failure](6) Water-tightness and flooding countermeasures forlayout area of emergency D/G, power source
equipment, emergency D/G cooling equipment.

[On-site flooding countermeasures 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 tanks and such like that
may become an obstacle in an emergency by drifting.
Setting up of heavy machinery for removal of driftingmaterial

– Hydrogen Countermeasures 14,Emergency Preparedness 12, 15, 16, 17, 18, 20, 21, Integrity Assurance of Used Fuel 13, 19 [Radiation control and work management]
12(1) Diversification of power supply sources for the radiation
measurement instrument (battery cell type), assuranceof long-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 baseisolation, providing information on site dose rate,
consolidation of support system for radiation control
personnel(5) Promote understanding of emergency treatment forradiation exposure, consolidation of system

21[Communication](30) Independent communication facility backup power source thatis not affected by earthquake or tsunami(31) Communication equipment that will not be broken by natural
disasters(32) Setting up of multiple pieces of communication equipment andclarification 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 pieces 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

13[Integrity assurance of used fuel](1) Used fuel pool level, Supply of power to watertemperature thermometer from emergency
power source, enhancement of condition
supervision(2) Assurance of alternate cooling functions
OutsideImportantbase-isolatedbuilding
Countermeasure
point during
emergency

14[Hydrogen countermeasures](1) Release of accumulated hydrogen,measures for reduction(2) Setting up of hydrogen gas detector(3) Prevention of bifurcation to branch pipe
from the containment vessel vent pipe
(4) Prevention of hydrogen being diverted
between facilities providing stack(5) Drill in anticipation of hydrogen leak,accumulation

Reactor Building
Centralcontrolroom

Used Fuel Pool
Reactor Containment Vessel

Steam

Used fuel coolingsystem (Normal)

Reactorpressure
vesselPump forPressurizedWater
injection

Fuel
Component
coolingwater
pumpCondenser

Water

Feed water pumpControl rods

Recirculation pump

Circulating water pump

OutletCooling water(seawater)

Emergency Diesel
Generator (Backup
power source)

Fire engine
Filtered water
storage tankPressure Control Pool

Diesel fire
extinguishing pump

Condensate
storage tank
Outlet(seawater)

Seawater pump forcooling the Emergency
Diesel Generator

20[Countermeasure points at the time of emergency](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 roomand collection of plant parameters(25) Radiation protection countermeasures(26) Assurance of livability(27) Assurance of emergency material(28) Careful examination and assurance of material necessary for emergency
countermeasures[Organization and command, orders](29) Preparedness of rapid operation by early coordination outside the company
and clarification of judgment timing

Seawater[Integrity assurance of used fuel](3) Assurance of water injection by fire-engine,
injection route, water source

19

Figure 4.12-1 Examples of Countermeasures 2/3
4-31

15[MCR air-conditioning, shielding](6) Assurance of power source
responsible for functioning of
emergency MCR ventilation system,
maintenance of operation procedures(7) MCR radiation prevention
countermeasures[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
16[Measurement at the time of
accident](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

17[Communication](13) Familiarization of facilities and operation ofcommunication equipment between workers andcentral control room
18
[Drill](14) Identify the required knowledge and skills and execute
drill(15) Onsite activity drill for severe accident
countermeasures(16) Onsite activity drill for simulated situations likeobstacles, multiplexing, conditions where normal
communication means cannot be used and night-timework(17) High efficiency drills such as blind drills and real-time
drills which place emphasis on the coordination andjudgment in conditions 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 systemat the time of emergency](20) Deployment and removal of heavy machinery forelimination of rubble(21) Development of facilities that can be operated remotely
for atomic power disaster prevention, cooperation towards
introduction coordination system

- Heat Sink Failure Response (PWR) 1, 2, 3 Other countermeasures (preparedness
against earthquake, tsunami, preparation of
power source, hydrogen countermeasures,etc.) are the same as the examples given for
BWR.

[Water injection to reactor](1) Assurance of water injection means byorganization of backup generator or highcapacity power source[Water injection to nuclear 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

External power
supplyMotor driven fire
extinguishingpump

To fire service
water system

Primary containment
vesselContainment spray

Diesel fire
extinguishing pump

Main steam To Steam turbine

Main feed waterContainment
spray pump
Motor driven auxiliary feed water pump

DieselControl
PanelHigh pressure injection pump

Reactor vessel
Reactorcoolant pump

Diesel generator
Containment sump

Turbine driven auxiliary feed water pumpComponent cooling water pump

Seawater pump

Next

[Water injection to reactor]
3(4) Assurance of source of water for water injection to SteamGenerator (includes seawater as a last resort)

Figure 4.12-1 Examples of Countermeasures 3/3
4-32

Chapter 5

Timeline of Accident to the Present

The Great East Japan Earthquake which occurred at 14:46 on March 11, 2011 struck
the Fukushima Daiichi and Daini Nuclear Power Stations, a long-term nuclear
accident of an unprecedented scale.
In Units 1 to 3 of the Fukushima Daiichi Nuclear Power Station, workers remained
unable to inject water into the RPV for a period, exposing the fuel in the cores andleading to a 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
was damaged, radioactive materials in the fuel rods were released into the RPV, andhydrogen and radioactive materials 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 wet wells several times.
After the wet well venting of the Units 1 and 3, an explosion thought to be caused byhydrogen 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 thereactor building of the Unit 4, destroying the upper portion of the building. Around6:00 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 thereason for that is unclear.
Cooling of the spent 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 spent fuel caused the waterlevel to continue to drop. To address the water level, helicopters and spray vehicles
were used to inject water into the spent fuel pools, though ultimately a concrete
pump truck 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
workers.
On March 11, the Nuclear Emergency Response Headquarters set the area within a
3 kilometer radius from Fukushima Daiichi as an evacuation zone and the area within
a 3 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 on March 15, the area within a 20 to 30
kilometer radius as an indoor evacuation zone. On April 21, the area within a 20
kilometer radius was established as a no-entry zone, with only restricted entry
allowed.
Although the Local Nuclear Emergency Response Headquarters was initiallyestablished off-site, it was subsequently moved to the Fukushima Prefectural Office5-1when 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 a level
3 due to the Station’s determination that there was insufficient water in theemergency core cooling system on March 11. On March 12, this was raised to level 4
due to the state of radioactive material emissions, and on March 18, it was raised tolevel 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 that of Chernobyl, and the level was raised tolevel 7.
As a result of the station blackout at Fukushima Daiichi, the internal Personal
Handy-phone System (PHS) and Safety Parameter Display System (SPDS) 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 Environmental Emergency Dose Information(SPEEDI) could not acquire emission source data or otherwise perform its primaryfunction of making quantitative predictions, including how the concentration ofradioactive materials in the atmosphere 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
be provided by the Self-Defense Forces because the power supply vehicles werebeyond their weight capacity. Before daybreak on March 12, 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 switchyards for Units 1 and 2,
and on March 24, MCR lighting was restored for Unit 1, and on March 26 for Unit 2.
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 (MAAP)
composed of severe accident analysis code was used to assess the status of thereactor core, and found 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 nowater was being injected into the nuclear core at that time, melting of the fuelprogressed, and when water injection commenced around 06:00 on March 12, themelted fuel had already migrated 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 13:25 on
March 14, when it was determined that the RCIC shut down, approximately 75 hours
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 02:42
on March 13 when it was determined that the HPCI shut down, approximately 40
minutes after the earthquake struck, followed by damage to the core approximately
two hours 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 fuel5-2

region) initially planned, a review was made and it was decided to establish a“recycled water injection cooling” to process contaminated water that had been
accumulated in the reactor building for reuse as nuclear reactor injection.
Additionally, the reliability of the injection water was ensured (with measures toaddress abnormalities and multiple water injection methods), a hydrogen explosion
was avoided through filling the containment vessel with nitrogen, and “stablecooling,” the second step of the road map, was reached.
Currently, the temperature of each part of the RPV has been stably shifting, 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 spent fuel pools, a heat exchanger was used for circulating
cooling at all Units, 1 to 4, and by August 10, “more stable cooling,” step 2 of the roadmap, was achieved.
Seawater and underground water from the tsunami, as well as water injected into the
nuclear reactor and spent 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 sothat the water level would not overflow. To carry out treatment of the accumulated
water and inject it into the nuclear reactor in a stable, efficient manner, a liquid waste
concentrate system was put into service on August 7 as a second line of treatment
equipment to increase the desalting capacity, and the operation moved forwardsmoothly. On October 9, the augmentation of the salt treatment equipment with aliquid waste concentrate system 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 caused 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, electrical
workers having the primary responsibility for construction and operation of nuclear
power stations have worked with electrical engineers directly involved with thedesign and construction of Power Stations, voluntarily summarizing the results of
their impartial analysis of the causes of the accident, their deductions of the mostsignificant lessons to be learned, and countermeasures they have considered.
Japan Nuclear Technology Institute (JANTI) is composed of many companies
involved in the nuclear power industry as well as electrical workers, and through thisassociation, power engineers and Plant engineers have been able to come together
to hold discussions. The positions of power production and Plant construction wereseparated, and the abilities of experienced specialists were employed to objectively
study the issues from an engineering perspective to further delve into the results thatwere assembled by the power and Plant specialists. These efforts were also
reflected in this report.
Furthermore, specialists of the technical analysis subcommittee of the Atomic
Energy Society of Japan (AESJ) have provided a review of this report, providing
additional perspectives.
Central to this investigation have been the cause analysis into why it was notpossible to avoid a 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 due to the tsunami, the cooling, control and monitoring
functions of the internal electrical equipment, particularly the emergency AC and DC
electrical panels were lost due to incoming water, and that due to the time required toestablish a reactor core cooling means 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 have already been implemented for the present toprevent incidents caused by tsunamis, and through this investigation, multilayeredcountermeasures have been deduced, including those already implemented inPower Stations throughout Japan and those planned for implementation.
In this investigation, however, it was not possible to directly interview the workers
working at the time of the accident. By conducting an investigation with the focus onfinding room for improvement, if only to alleviate the burden on the Station workers,
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 disseminationand disclosure, and a system for instructions and orders were based solely on
information obtained from the industry. In relation to these matters, however, there
are likely to be many issues that ought to be reconsidered and many lessons to belearned when the perspectives of people in the local communities and others aretaken into consideration, and those issues should be the focus of investigations
going forward.

6-1

When engineers design facilities, they must ensure that certain conditions, including
regulations, are satisfied. The thinking of the people who set those conditions issusceptible 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
the importance 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 critical to take yet another look at whether any of a variety of methods areneeded 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 rest with the organization” in the IAEA’s Fundamental
Safety Principals. Moreover, it is surely the responsibility of the engineer tocontinuously and repeatedly question safety without being under the illusion thatsatisfying regulatory requirements is an adequate discharge of one’s duties, and weshould demand that nuclear engineers recall what their purpose is as an engineer.
Whether such investigations will be conducted with due seriousness and without
getting caught up in the past, and whether the results of those investigations will bereflected in Stations to improve safety is truly something to be decided by the abilityof the nuclear power industry and each and every nuclear engineer. It is hoped thatthe industry and engineers will return to the roots of their calling as engineers andmove 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.
When others view the report from their various perspectives, they will likely noticemany oversights and consider measures superior to those offered here. We hopethat the reader will subject this report to the strictest of scrutiny and point out all such
shortcomings.
In closing, as members of the Examination Committee on Accident at Fukushima
Daiichi Nuclear Power Station, 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 Comparison of the Development of the Accident
at the Fukushima Daiichi Nuclear Power Station
and at Other Stations (Detailed Version)
Section 3.6 “Comparison of course of accidents at Fukushima Daiichi Nuclear Power
Station with other Stations” summarizes a comparison of Fukushima Daiichi Units 1
to 3 and Fukushima Daini, Onagawa and Tokai Daini. Details for each Station areprovided below.

Summary of the events at the Fukushima Daini Nuclear Power Station
Although the functionality of the seawater cooling systems was lost at the
Fukushima Daini Units 1, 2 and 4 due to the tsunami, RCIC operation ensured thecapability of reactor core cooling. Subsequently, a make-up water system wasused to provide an alternative means for injecting water into the nuclear reactors
and containment vessels. As a result, cooling of the reactor cores and SFP fuelwas maintained for roughly three days without venting the containment vessels.
Meanwhile, the workers restored the seawater system, and finally restored theresidual heat removal functionality when the RHR equipment began working again.
The workers shifted to a cold shutdown. Because they secured the power supply, it
was possible to secure extra time by utilizing safety equipment for severe
accidents even without the functionality of the seawater system, and it waspossible to shift 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 workers used the B system tosecure the residual heat removal (RHR) function.
The course of events at Fukushima Daini is shown in an event tree in Appendix
1-1.

app-1-1

Events
occurred

Reactorshutdown

Earthquak Reactor
e and
scram
tsunamiDC power
supplyAC power supply

DC power External
supplypower
supplyCore coolingElectric power
recovery

Ensuring long-term cold shutdown

Reactor coolant
Emergency IC, turbine-driven External
RPV
coolant injection
DG
power supply, depressuri injection
system (RCIC,(including
emergency
zation
HPCI) and HPCS
alternative
DG, power(including sources,
for which ACinterchange alternative continuous
power supply is

RHR

2F-3

RHR
recovery

Status of reactor core
PCV vent(beforecore
damage)Cold
shutdown,core damage,PCV damage
etc.

Cold shutdown

2F-3 3/12 12:15

Cold shutdown

2F-1 3/14 17:00
2F-2 3/14 18:00
2F-4 3/15 07:15

2F-1,2,4
SRV[Tsunami]

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 Station disaster
identification events tree
app-1-2

(1)Effects of the earthquake and tsunamia.

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 beless than the maximum response acceleration for basic earthquake groundmotion 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 the periodic band, the Station confirmed as generally being
equivalent or lower.
The Station used the seismological record to analyze the seismologicalresponse of the critical safety equipment, and it is believed that the Station
secured the safety functions during and subsequent to the earthquake.

b.

Effects of the tsunami
A study on the tsunami caused by the Great East Japan Earthquake thatstruck on March 11 showed that the inundation height and inundation areas on
the ocean side and primary reactor building areas were as follows (groundmovement due to the earthquake is not included):1) Inundation height(a) Ocean side area (site O.P. +4 m)• Approximately +7 m* (inundation depth, approximately 3 m)*

There were local levels that were higher such as on the south side of the Unit 1
heat exchanger building.

(b) Primary reactor building area (site O.P. +12 m)• Approximately +12 to 14.5 m* (inundation depth, approximately
2.5 meters or lower)*

Locally, O.P. + approximately 15 to 16 m from the south side of the Unit 1building to the base-isolation structure(Inundation depth: approximately 3 to 4 m)

2) Inundation areas(a) Although the entire ocean side area was affected, flooding was notfound in the primary building area beyond the slope of the ocean sidearea(b) Inundation concentrated on the roads from the southeast side of the
primary building area to the base-isolation structure, and the building
environments of Units 1 and 2 as well as the south side of the building ofUnit 3 were inundated (there was no flooding near the building of Unit 4)(2)

Unit 1a.

Station behavior upon occurrence of the earthquake
During the constant operation at the rated thermal output, Unit 1 automaticallyshut down at 14:48 on March 11, 2011 due to the “high seismic accelerationtrip” caused by the earthquake occurred at 14:46 on the same day (theepicenter was Sanriku-oki). The maximum acceleration observed at the powerapp-1-3station was 305 gal at the second floor of the basement of the Unit 1 reactor
building. All reactors automatically shut down because their protection system
activated properly as designed. Immediately after the automatic shutdown of
the reactor, the following facts were confirmed: the reactor was subcritical; allcontrol rods were fully inserted; and all the facilities required for cold shutdownof the reactor and cooling of SFP were in normal and stable conditions.
However, such facilities required for cold shutdown of the reactor and coolingof SFP became inundated and disabled by the tsunami which occurred after
the earthquake (the first wave of the tsunami was visually confirmed at 15:22
on the same day, March 11).
In addition, due to loss of the reactor’s heat removal function, S/C could not becooled down. Therefore, the S/C water temperature gradually increased toover 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 workers
inspected and repaired the flooded facilities, and power was supplied from a
temporary power supply. After recovering the heat removal function of thereactor, the workers cooled the S/C water to a temperature lower than 100°C.
From then until 17:00 on March 14, the reactor’s water temperature wasmaintained at a temperature lower than 100°C (i.e., in a cold shutdown state)through the RHR1 system, while at the same time the SFP system wascontinuously cooling down. Currently, the Station is in a stable condition.
b.

State of the “Stopping” function
Upon the occurrence of the “high seismic acceleration trip” at 14:48 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, andthe reactor became subcritical at 15:00 on the same day.
At 05:58 on March 12, the alarm went off, detecting the abnormality in the
Position Indicate Probe (hereafter called “PIP”) of the control rod 10-51.
Although the alarm condition was cleared once at 10:30 on the same day, thissituation was repeated several times. There are two ways to indicate thecontrol rod position. One is to indicate the “full insertion” state. The other is toindicate the position of the rod itself. When the alarm went off, while the PIP’s“full insertion” indicator lamp was not on, the position indicator lamp wasindicating that the control rods were fully inserted. When the alarm condition
was cleared, the “full insertion” indicator lamp was on.
Furthermore, when the alarm went off, no significant change was observed in
the indication of the Start-up Range Neutron Monitor (hereafter called“SRNM”) and the reactor’s subcritical state remained unchanged. Although thealarm condition was cleared at 12:02 on March 13, the control rods wereisolated (bulb-out) at 15:18 on the same day to avoid activation. Since then, no
significant change has been observed in the SRNM indication and thereactor’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, theapp-1-4

reactor 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
from the reactor feed water system.
*1 ECCS pumps (common among Units 1 to 4 of this Power Station)• HPCS pump
• Low Pressure Core Spray (hereafter called “LPCS”) pump
• RHR pumps (A, B, C) Low Pressure Coolant Injection (hereafter called“LPCI”) mode
*2 Automatic-initiation water levels (common among Units 1 to 4 of this Power
Station)• HPCS and RCIC………………L-2• LPCS and RHR (LPCI) ………L-1
MSIV was completely closed manually at 15:36 on March 11 and the reactor
pressure was controlled through SRV in preparation for the following
situations: the circulating water pump (hereafter called “CWP”) may shut down
due to a tsunami, and may thus cause malfunction of the main steam
condenser; and the turbine gland sealing steam may become lost due to theauxiliary boiler shutdown caused by earthquakes. Furthermore, upon fullclosure of the MSIV, the RCIC system was manually activated at 15:36 on thesame day in order to pour water onto the reactor. After the RCIC system
automatically shut down at 15:40 on the same day due to the “reactor waterlevel high (L-8)” the reactor water level 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 (the workers later confirmed at the scene that somemotors and emergency power supplies (P/C 1C-2, 1D-2) were unusable due tosubmersion) because the seawater heat exchanger building was flooded by
the tsunami and the operation/shut down indication lamp indicated that thesepumps shut down. All the ECCS pumps therefore became unactivatable, thus
causing loss of the reactor’s residual heat removal function. Moreover, theemergency power supplies (M/C 1C and 1HPCS) became unusable because
the annex of the reactor building was submerged by the tsunami, thus making
the LPCS pump, RHR pump (A) and HPCS pump unusable.
*3 Pumps for the emergency equipment cooling system:• Residual Heat Removal Cooling System (hereafter called “RHRC
pumps”) pumps (A, B, C, D)• Residual Heat Removal Cooling Sea Water System (hereafter called“RHRS”) pumps (A, B, C, D)• Emergency Equipment Cooling Water (hereafter called “EECW”) pumps
A, B)• HPCS Cooling Water System (hereafter called “HPCSC”) pump
• HPCS Cooling Sea Water System (hereafter called “HPCSS”) pump
Initially, the workers poured water onto the reactor using the RCIC system.
However, they introduced the alternative method using MUWC at 00:00 on
March 12 as a countermeasure for severe accidents and implemented thisapp-1-5along with the RCIC system for injecting water. Furthermore, rapid reactor
depressurization was initiated at 03:50 on the same day because therelationship between the reactor pressure and the S/C water temperatureturned the situation into the operation prohibition status because of the heat
capacity limitation. The workers manually shut down the RCIC system at 04:58
on the same day due to the decrease in the pressure of the steam to drive the
RCIC turbine caused by the rapid depressurization of the reactor.
Subsequently, they adjusted the reactor water level through the alternative
water injection method using MUWC.
At 17:35 on March 11 the alarm went off after detecting the state of “D/W
pressure high” (set point: 13.7 kPa [gage]). There was an entry (system A)
describing the state of “MSIV reactor water level low (L-2)” in the alarm typer
that was recorded at 15:37 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
to shutdown of the 120 V AC Station vital panel board 1A that was caused by
the tsunami. 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, after the automatic activation, the workers
manually shut down the RHR pumps (B, C) because the RHRC pumps (B, D),
the RHRS pumps (B, D) and the EECW pump (B) were unusable. After thisincident, the workers undertook a preventive measure for automatic activation(the control switch is maintained at the “pull” position).
Subsequently, the S/C water temperature increased to over 100°C at 05:22 on
March 12. It further increased up to approximately 130°C (at 11:30 on March
13). At 06:20 on March 12 in order to cool down the S/C, the workers injected
the coolant (MUWC) into the S/C from the cooler of the Flammability (Gas)
Control Diagram (hereafter called “FCS”) via the coolant drainage line. At
07:10 on the same day the workers began using the alternative method using
MUWC. D/W spraying was initiated at 07:10 and the cooling method wasswitched to the S/C spraying at 07:37. Thus the workers used PCV alternative
cooling by alternating these two methods according to the situation.
Furthermore, along with the implementation of the alternative water injection
method using MUWC, the PCV alternative cooling method and the S/C coolingmethod using the FCS coolant (MUWC), the workers inspected and repaired
the following pumps: the RHRC pump (D); the RHRS pump (B); and theEECW pump (B). (Motors were replaced for the RHRC pump (D) and theEECW pump (B).) Additionally, because the seawater heat exchanger building
was submerged and the emergency power supplies (P/C 1C-2, 1D-2) were
flooded, the workers restored the RHRC pump (D), the RHRS pump (B) andthe EECW pump (B) to an activatable state by supplying power from thetemporarily installed cable, which received electricity from the power supply(P/C 1WB-1) in the radioactive waste treatment building. This in turn received
electricity from an external power supply system and a high voltage power
supply vehicle, which had been urgently procured outside the Power Station.
These pumps were activated one by one at 20:17 on March 13.
Subsequently, the RHR pump (B) was activated at 1:24 on March 14. Also, asa result of S/C cooling using the RHR pump (B), the S/C water temperatureapp-1-6

gradually dropped to a temperature lower than 100°C at 10:15 on the same
day.
Furthermore, in order to cool down the reactor water as well as the S/C waterat an early stage, the Station created implementation procedures based on thepredetermined emergency operation procedures. At 10:05 on the same day
the workers began the procedure to inject the S/C water into the reactor via
the LPCI line using the RHR pump (B). At the same time, they temporarily
cooled the reactor via the circulation line, through which the reactor water wasallowed to flow into the S/C through the SRV, and the S/C water was cooledusing the RHR heat exchanger (B), then again injected 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, the Station confirmedthat the reactor water temperature dropped to a temperature lower than 100°C
at 17:00 on the same day, and thus a cold shutdown was confirmed.
As described above, although the reactor’s cooling function was temporarily
lost, water was continuously injected into the reactor. As a result of the reactor
water sampling conducted after the incident, the level of iodine-131 was lower
than the detection threshold. Therefore, it did not damage the fuel.
d. State of the “Confining” 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 (hereafter called “PCIS”) and the SGTS properly worked. Therefore,
the PCV was isolated, and the negative pressure in the reactor building wasmaintained. Although the PCV pressure increased up to approximately
282 kPa [gage] (at the S/C side), it did not reach the PCV maximum allowable
working pressure of 310 kPa [gage].
In addition, no abnormal change was observed in the stack radiation monitor
or the MP value. The Station 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, the workers
created 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).
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
water temperature was maintained at approximately 38°C before theearthquake. However, the SFP cooling through the FPC could not be
continued because the coolant could not be supplied to the FPC heatexchanger due to the disability of some pumps for the following reasons: theFPC pump tripped (“skimmer surge tank water level low” or “pump suction
pressure low”) due to the earthquake; the sea water system (hereafter called“SW”) pump (A, B, C) of the closed cooling water system located near theoutside intake was flooded by the tsunami; and the Reactor Auxiliary Cooling
Water System (hereafter called “RCW”) pump (A, B, C) located at the lower
app-1-7

ground floor in the seawater heat exchanger building was submerged by thetsunami.
Consequently, the SFP water temperature increased up to approximately 62°C.
Therefore, the workers injected water into SFP using the Fuel Pool Make-Up
Water System (hereafter called “FPMUW”) at 16:30 on March 14, and thewater circulation operation using the FPC pump (B) was initiated at 20:26 on
the same day to cool the SFP. Subsequently, at 0:42 on March 16 the SFP
cooling operation was conducted using the RHR pump (B). By 10:30 on thesame day, the SFP water temperature returned to approximately 38°C, which
was the temperature prior to the earthquake.
As a result of the above countermeasures, although the SFP systemtemporarily 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
All the power supplies in the Station were in usable conditions immediately
after the automatic shutdown of the reactor. However, because the annex of
the reactor building was submerged by the tsunami, the emergency power
supplies (M/C 1C and 1HPCS) became disabled. Moreover, because theseawater heat exchanger 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 the
120 V AC Station vital panel board 1A, which was its load, 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 120 V AC Station vital panel board 1A
was restored to a usable state by receiving electricity from the cable
temporarily installed on the temporary supply panel board of Unit 2 (conductedon March 12). Furthermore, with regard to the disabled emergency power
supply (P/C 1D-2), the RHRC pump (D) and the RHRS pump (B), all of whichwere required for cooling the reactor and the SFP, received electricity from thepower supply (P/C 1WB-1) of the Radioactive Waste Disposal Building via thetemporarily installed cable. Also, the EECW pump (B) received electricity froma high voltage power supply vehicle (conducted on March 13 and 14).
Subsequently, the workers switched the temporary power supply for the
EECW pump (B) from the high voltage power supply vehicle to the emergency
power supply (P/C 1D-1) (switched on March 30). In addition, expecting tolose the external power supply system, the Station devised procedures toreceive electricity from the emergency power supplies of Units 2 and 3 (M/C

app-1-8

2D and M/C 3D, respectively) as a backup 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 fail, the emergency
power supplies (M/C 2D and 3D) can receive electricity from DGs (B) of Units
2 and 3 because they are in a usable condition.
Moreover, Unit 1’s DG (B) was also restored on July 15. Thus all theemergency power supplies required for cooling the reactor and the SFP havebeen secured.

app-1-9

Table Appendix 1-1

Fukushima Daini Nuclear Power Station Unit 1 Situation in
Chronological Order After the Earthquake

Friday, March 11, 2011
14:46
Great East Japan Earthquake occurs.
14:48
The reactor automatically shuts down. (The alarm goes off detecting
the state of “high seismic acceleration trip.”) All control rods are fully
inserted.
14:48
One of the 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. (From then the tsunami iscontinuously confirmed until 17:14.)
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 theearthquake because it was undergoing an 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 of the same day. Based on these facts, wecannot deny the possibility that the cause of the pressure increase
could be a leakage of reactor coolant in the PCV. The Station therefore
determines that the specified events (leakage of reactor coolant)
stipulated in Article 10, Paragraph 1 of the Special Law for Nuclear
Emergency have occurred. (As a result of the inspection on therelevant parameters subsequently conducted, no reactor coolant
leakage is observed. Therefore, at around 18:33 on the same day, theStation determined 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 seawater pump that has thefunction to remove heat from a reactor cannot be confirmed, theStation determines that the specified event (loss of the function toremove heat from a reactor) stipulated in Article 10, Paragraph 1 of theSpecial Law for Nuclear Emergency has occurred.
Saturday, March 12, 2011
00:00
MUWC alternative water injection is initiated.
03:50
Rapid depressurization of the reactor is initiated.
(This is because the situation has turned into the operation prohibition
status regarding thermal capacity limit.)
04:56
Rapid depressurization of the reactor is complete.
app-1-10

04:58The RCIC is manually shut down (operation prohibition due to thereactor pressure decrease).
05:22
Because the S/C temperature has exceeded 100°C, the Station
determines that the specified event (loss of the pressure suppressionfunction) stipulated in Article 15, Paragraph 1 of the Special Law for
Nuclear Emergency has occurred.
05:58
The alarm goes off detecting the abnormality in control rod 10-51 PIP
06:20
The S/C cooling operation using the FCS coolant (MUWC) is
conducted.
07:10
The D/W spraying operation using MUWC is conducted. (From thispoint forward, it has been conducted according to the situation.)
07:37
The S/C spraying operation using MUWC is conducted. (From thispoint forward, it has been conducted according to the situation.)
07:45
The S/C cooling operation using the FCS coolant (MUWC) is
terminated.
10:21
Creation of the PCV pressure-resistant vent line is initiated.
10:30
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. (The restoration of
Unit 2 has been completed.)
18:30
Creation of the PCV pressure-resistant vent line is complete.
Sunday, March 13, 201105:15 (approx.)
Two Iwaido circuits begin receiving electricity. (The restoration of Unit
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, the Station
determines that the situation does not fall under the specified event(loss of the function to remove heat from a reactor) stipulated in Article
10, Paragraph 1 of the Special Law for Nuclear Emergency.
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 onto the reactor is conducted under the
RHR (B) LPCI mode.
10:15
Because the S/C water temperature has dropped to lower than 100°C,
the Station determines that the situation has recovered from thespecified event (loss of the pressure suppression function) stipulated
in Article 15, Paragraph 1 of the Special Law for Nuclear Emergency.
app-1-11

16:30
17:00
20:26
22:07

The water pouring operation through the FPMUW system is initiated tocool down the SFP.
Because the reactor water 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 5 μGy/h is measured at the
MP (No. 1), the Station determines that the specified event (increasein the site boundary radiation dose) stipulated in Article 10, Paragraph
1 of the Special Law for Nuclear Emergency has occurred. (It can besurmised that the increase in the radiation dose was caused byradioactive materials discharged into the atmosphere upon theoccurrence of the accident at Fukushima Daiichi.)Tuesday, March 15, 201100:12
Because a radiation level exceeding 5 μGy/h is measured at the MP(No. 3), the Station determines that the specified event (increase in thesite boundary radiation dose) stipulated in Article 10, Paragraph 1 of
the Special Law for Nuclear Emergency has occurred. (It can besurmised that the increase in the radiation dose was caused byradioactive materials 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 earthquake.)app-1-12

(3)Unit 2a.

Station behavior upon occurrence of the earthquake
The reactor was running at normal rated heat output when an earthquake
centered on the Sanriku-coast occurred at 14:46 on March 11, 2011. The
reactor automatically shut down its operation at 14:48 on the same day with a“high seismic acceleration trip” alert. Immediately thereafter, full-insertion of allcontrol rods (CR) and the reactor’s subcriticality were confirmed. The Station
also confirmed that equipment necessary for the reactor’s cold shutdown andcooling of SFP were in a sound and stable condition.
However, due to the tsunami after the earthquake (1st tsunami was visuallyconfirmed at 15:22, on the same day), these pieces of equipment, which arenecessary for cold shutdown of the reactor and SFP cooling, became disabledbecause of water intrusion and such like. Also, S/C cooling was not possible
due to loss of the reactor heat removal function, and S/C water temperature
gradually rose up to over 100°C.
Later, to enable part of the equipment for reactor cold shutdown and SFP
cooling, the workers inspected and repaired the submerged equipment. Also,
power supply was obtained from a temporary power supply. After recovery of
the reactor’s heat removal function, S/C cooling was performed, and the S/Cwater temperature fell below 100°C. Thereafter, by RHR1 system, the reactor
maintained its cold shutdown with the water temperature below 100°C until
18:00 on March 14. At the same time, SFP was being continuously cooled
down, and the Station has currently been maintaining its stable condition.

b.

State of the “Stopping” function
The earthquake triggered a “high seismic 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 coreof the reactor decreased due to the sharp decline of the reactor output. The
water level of the reactor dropped down to “reactor vessel low water level(L3).” After that, with the water supply from the reactor’s feedwater system, thewater level was recovered and did not drop to the point at which the ECCS
pump and RCIC would automatically start.
At 15:34 on March 11, the Main Steam Isolation Valve (MSIV) was fully closed
manually, and the reactor’s pressure control was implemented by SRV. The
workers took this measure to prepare for the situation where the condenser
fails to condense main steam, following CWP shutdown due to the tsunami,
and in the case of losing turbine ground seal steam, following shutdown of theauxiliary boiler due to impact of the earthquake.
With MSIV fully-closed, RCIC was manually activated at 15:43, and theworkers injected water into the reactor. After RCIC’s automatic shutdown at
15:46 on the same day by “reactor vessel low water level (L8),” the reactor’swater level was adjusted by manual start-up and automatic shutdown of RCIC.
app-1-13

Based on indication of the operation and shut off lamp displays and the factthat seawater heat exchanger was flooded by the tsunami, the Station
determined that RHRC pump (A, B, C, D), RHRS pump (A, B, C, D), EECW
pump (A, B) and HPCSC pump could not be started (the Station confirmed
later that pumps were inoperable due to water exposure on a part of themonitors and emergency power supply (P/C 2C-2, 2D-2). Thus, all ECCS
pumps became unusable, and the function of removing residual heat from thereactor was lost.
Initially, the workers injected water into the reactor by RCIC. However, with the
opening of SRV, the reactor pressure lowered. Accordingly, at 04:50 on March
12, the workers began to inject water by MUWC based on the operation
procedure manual. This was implemented as a severe accident
countermeasure. RCIC automatically shut down at 04:53 on the same day,due to lower steam pressure for the RCIC turbine caused by depressurizationof the reactor. Subsequently, the workers adjusted the reactor’s water level byconducting an alternative water injection with MUWC.
With the operation of RCIC and opening of SRV, the temperature and pressure
inside PCV increased. However, as cooling with RHR pump (A,B) was notpossible, an alarm signal was triggered at 18:50 on March 11, indicating “highD/W pressure” (design value: 13.7 kPa [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 pump were disabled, workers conducted a manual shutdown after
startup. After this, the auto-start prevention measure (hold pull of control
switch) was implemented.
Later on at 05:32 on March 12, the S/C water temperature rose to over 100°C.
The temperature rose up to approximately 139°C at 07:00 on March 14.
To cool down the S/C, from 06:30 on March 12, the workers injected MUWP,
which is cooling water, into the S/C using the cooling water drain line from the
FCS’s cooler to the S/C. Alternative injection into the reactor by MUWC waschanged into D/W spray from 07:11 on the same day, and changed into S/Cspray from 07:35 as necessary to perform alternative cooling of PCV.
The workers inspected and repaired the RHRC pump (B), RHRS pump (B)and EECW pump (B) along with conducting an alternative injection into thereactor by MUWC, PCV alternative cooling and S/C cooling by FCS coolingwater (MUWP). The seawater heat exchanger building and emergency power
supply unit (P/C 2C-2, 2D-2) were submerged in water. Therefore, temporary
cables that had been obtained offsite were used to feed emergency power.
Power was supplied from the Radioactive Waste Disposal Building (P/C
1WB-1) receiving power from the external power system. Power was also
received from the Unit 3 heat exchanger building emergency power (P/C3D-2)
by installation 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 03:20 on March 14.
Later on, RHR pump (B) started from 07:13 on March 14. Also, S/C cooling
performed by RHR pump (B) resulted in a gradual decline in the S/C watertemperature. By 15:52 on the same day, the S/C water temperature had fallen
to below 100°C.
To promptly cool down the reactor water as well as S/C water, the Station
created an implementing procedure manual using a previously formulated
app-1-14

accident operation manual as reference. From 10:48 on the same day, theworkers injected S/C water into the reactor by RHR pump (B) via LPCI line. At
the same time, the reactor water flowed into S/C via SRV, and the S/C water
was cooled by RHR heat exchanger (B) to re-inject into the reactor via LPCI
line again. Such circulation line (S/C → RHR pump (B) → RHR heatexchanger (B) → LPCI line → Reactor → SRV → S/C) was implemented as
an emergency cooling measure. The Station confirmed that this measurecooled down the reactor temperature to below 100°C at 18:00 on the sameday.
The above indicates that despite a temporary loss of the reactor’s cooling
function, water injection into the reactor was continued. Later sampling of thereactor water showed that iodine 131 was below detection limit. Thus, no fueldamage occurred.
d. State of the “Confining” function
With “reactor vessel low water level (L3)” alert triggered at the reactor’s
automatic shutdown, PCIS and SGTS operated normally, and PCV wasisolated, and reactor building subatomospheric pressure was maintained. The
PCV pressure increased up to approximately 279 kPa [gage] (on S/C), but it
did not reach the maximum operating pressure of 310 kPa [gage].
The Station confirmed that there was no abnormal change in the stackradiation monitor or MP values, or radiation impact to the outside.
Assuming PCV pressure’s tendency to increase and the long recovery time forthe reactor’s heat removal function, the workers used a line configuration for
PCV pressure vent (one action remained to be done 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 the
SFP water temperature at about 32.5°C and the water level higher than thelevel of overflow water prior to the earthquake. Due to the impact of theearthquake, the FPC pump tripped (“low water level of skimmer surge tank” or“low pump pressure”). The tsunami flooded the SW pump (A, B, C) in the
non-safety service water system near the outside water intake. The RCWpump on the first basement level of the seawater heat exchanger 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 notpossible.
This raised the SFP water temperature up to about 56°C, but workers started
cooling the SFP by RHP pump (B) from 01:28 on March 16, and by 10:30 on
the same day, the temperature was down to about 32.5°C, the same level asthat 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; watertemperature; 65°C or lower) specified under the Safety Preservation Rules of
Nuclear Facilities were met.

app-1-15

f.

State of the “Power Supply” function
Immediately after automatic shutdown of the reactor, the facility’s powersystem was usable. However, as the seawater heat exchanger building was
submerged due to the tsunami, the emergency power supply (P/C 2C-2 and2D-2) became unusable.
Emergency DGs (Systems A, B and HPCS) were all usable right after thereactor’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 cableinstallation (performed on March 14): Power for RHRC pump (B) and RHRS
pump (B) necessary for cooling the reactor and SFP was supplied from thepower supply (P/C 1WB-1) of the Radioactive Waste Disposal Building. EECW
pump (B) received power from the emergency power supply (P/C 3D-2) of theUnit 3 heat exchanger building.
As the above measure enabled use of RHRC pump (B), RHRS pump (B) and
EECW pump (B), emergency power (M/C 2D) became available via DG (B)even if external power supply was lost. From April 2, DG (HPCS) became
usable, thus emergency power supply for cooling of the reactor and SFP has
been secured.

app-1-16

Table Appendix 1-2

Fukushima Daini Nuclear Power Station Unit 2 Situation in
Chronological Order After the Occurrence of the Earthquake

Friday, March, 11, 2011
14:46
Great East Japan Earthquake occurred.
14:48
Automatic shutdown of reactor (“high seismic 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 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 / immediately thereafter shut down
due to tsunami impact
15:41
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 seawater pump for equipment having capability
to remove heat from the reactor could not be confirmed, the Station
determined that an event specified under Article 10, Paragraph 1 ofSpecial Law for Nuclear Emergency had occurred (loss of reactor heatremoval capability).
18:50“High D/W pressure” alert was issued.
20:02
Drywell 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, the Station
determined that an event specified under Article 15, Paragraph 1 ofSpecial 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
10:58
Configuration of PCV hardened venting line completed
app-1-17

13:38 (approx.)
Power received from Iwaido line No. 1 (No. 2 restored)
Sunday, March 13, 201105:15 (approx.)
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, Paragraph 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, the Station
determined that the Station had recovered from the event specified
under Article 15, Paragraph 1 of Special Law for Nuclear Emergency(loss of pressure suppression capability)
18:00
Reactor water temperature had fallen below 100°C, and cold
shutdown of reactor achieved.
22:07
As radiation dose measured exceeded 5 μGy/h at MP (No. 1), theStation determined that a special event (increase in radiation dose insite boundary) under the provision of Article 10, Paragraph 1 ofSpecial Law for Nuclear Emergency took place (the cause of the dose
increase is surmised 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, 201100:12
As radiation dose measured exceeded 5 μGy/h at MP (No. 3), theStation determined that a special event (increase in radiation dose insite boundary) under the provision of Article 10, Paragraph 1 ofSpecial Law for Nuclear Emergency had taken place (the cause of thedose increase is surmised 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 the earthquake)app-1-18

(4)Unit 3a.

Station behavior upon occurrence of the earthquake
The Station was in operation at a constant rate thermal power when theearthquake occurred at 14:46 on March 11, 2011. The earthquake’s epicenter
was offshore of Sanriku. At 14:48 on the same day, the reactor automaticallyshut down due to “high seismic acceleration trip” alert. Immediately thereafter,
all control rods were fully inserted, and the reactor’s subcriticality wasconfirmed. The Station also confirmed that equipment necessary for thereactor’s cold shutdown and cooling of SFP were steadily operating in soundcondition.
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. A
cold shutdown of the reactor was confirmed on the same day.

b.

State of the “Stopping” function
Due to the earthquake, “high seismic acceleration trip” alert (reactor building
underground 2nd floor, operational set point value in horizontal direction: 135gal) 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, the void in the reactor
core decreased, following drastic decline of reactor output. The water level ofthe reactor decreased to “reactor vessel low water level (L3).” With watersupply from the reactor’s feedwater system, the water level of the reactor waslater recovered and did not drop to the point at which the ECCS pump and
RCIC would automatically start. On March 11, at 15:37, the Main Steam
Isolation Valve (MSIV) was fully closed manually, and the reactor’s pressure
control was implemented by SRV. The workers took this measure to prepare
for the situation where the condenser fails to condense main steam, following
CWP shutdown due to the tsunami, and loss of turbine ground seal steam,following shutdown of auxiliary boiler due to earthquake impact.
With full opening of MSIV, Reactor Core Isolation Cooling (RCIC) was
manually started at 16:06 on the same day, and water was injected into thereactor.
Due to the fact that the seawater heat exchanger building was flooded by thetsunami along with indication by operation on/off lamp, the Station determinedthat RHRC pump (A, C), RHRS pump (A, C) and EECW pump (A) could not be
started (the Station confirmed later at the Station site that these pumps were
not usable because the motors and emergency power supply (P/C 3C-2) werepartially flooded). For this reason, it was not possible to start LPCS pump andRHR pump (A).
As for the emergency power supply (P/C 3D-2) and its load, RHRC pump (B,D), RHRS pump (B, D) and EECW pump (B) as well as HPCSC pump and
HPCSS pump, the amount of seawater exposure to the seawater heatapp-1-19exchanger building was smaller compared with other units. Based on thisreason, these pumps were presumed to be usable as there was little impact ofinundation on this equipment.
Also, RHR pump (B, C) and HPCS pump were usable because there was nowater intrusion caused by the tsunami in the 2nd underground floor of thereactor ward of reactor building.
Water injection into the reactor was initially performed by RCIC. However, from
22:53 on March 11, alternative water injection by MUWC introduced as asevere accident countermeasure, was also implemented. Later, lower reactor
pressure due to SRV opening reduced steam pressure 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, workers implemented water injection andcooling by the usable RHR pump (B). At 12:15 on March 12, the reactor’swater temperature fell below 100°C, and its cold shutdown was confirmed.
At 19:46 on March 11, “high D/W pressure (design value: 13.7 kPa [gage])alert was issued due to temperature and pressure rise inside PCV, following
RCIC operation and SRV opening. This was followed by the automatic start-up
signal for all ECCS pumps. However, HPCS pump, LPCS pump and RHRpump (A,C) were not automatically started because the automatic start
prevention measure was in place (hold pulling control switch) due to thedisabled cooling system (RHRC (A, C), RHRS (A, C) and EECW(A)). RHRpump (B) was in operation for cooling of S/C at “high D/W pressure” alert(started at 15:36 on March 11).
Based on the above, we found that fuel was not damaged as the reactor’scooling function was maintained. Later sampling of the reactor water
confirmed that iodine-131 was below detection limits.
d. State of the “Confining” function
With “reactor vessel low water level (L3)” alert issued at automatic shutdownof the reactor, Primary Containment Isolation System (PCIS) and Standby Gas
Treatment System (SGTS) were operating normally, and PVC isolation andmaintenance of the reactor’s subatomospheric pressure were conducted. PCV
pressure rose up to about 38 kPa [gage] at D/W, but it did not reach PCV’s
maximum operating pressure, 310 kPa [gage].
Also, the Station confirmed that there was no abnormal change in stackradiation monitor and MP values, or radiation impact to the outside.
To prevent rising PCV pressure, the workers implemented a line configurationof PCV hardened venting line (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 the overflow level andkept SFP’s water temperature at about 34°C. Yet, with the impact of theearthquake, FPC pump tripped (“low water level of skimmer surge” tank orapp-1-20

“low pump pressure”). At the same 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 first
basement level of the seawater heat exchanger building were submerged anddisabled. For these reasons, cooling water was not supplied to the FPC heatexchanger, thereby disabling SFP cooling by FPC.
These events raised the SFP water temperature to about 51°C. However, from
17:42 on March 15, the workers switched the cooling water of FPC heatexchanger from RCW to RHRC. By this measure, SFP cooling by FPC was
implemented. At 22:30 on 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, we found that while SFP suffered a 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 of 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 seawater 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
the tsunami reached the facility, it was not possible to start RHRS (A, C) pumpand 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 the event of losing external
power supply.
Based on the above, we found that emergency power necessary for cooling of
the reactor and SFP were secured.

app-1-21

Table Appendix 1-3

Fukushima Daini Nuclear Power Station Unit 3 Situation in
Chronological Order After the Occurrence of the Earthquake

Friday, March 11, 2011
14:46
Great East Japan Earthquake occurred
14:48
Automatic shutdown of the reactor (“high seismic acceleration trip”
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 tsunami was confirmed (thereafter, tsunami waves wereconfirmed 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 opened and closed both automaticallyand manually to control 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 (approx.)
Power received by Iwaido line No. 1 (No. 2 restored)
Sunday, March 13, 201105:15 (approx.)
Power received by Iwaido line No. 2 (No. 1 restored)
app-1-22

Monday, March 14, 2011
22:07
As the radiation dose exceeded 5 μGy/h at MP (No. 1), the Station
determined that a special event (increase in radiation dose in siteboundary) under the provision of Article 10, Paragraph 1 of Special
Law for Nuclear Emergency had occurred (the cause of the dose
increase is surmised 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, 201100:12
As the radiation dose exceeded 5 μGy/h at MP (No. 3), the Station
determined that a special event (increase in radiation dose in siteboundary) under the provision of Article 10, Paragraph1 of Special
Law for Nuclear Emergency had occurred (the cause of the dose
increase is surmised to be due to the effects of radioactive material
released into the atmosphere, 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 of that before the earthquake)app-1-23

(5)Unit 4a.

Station behavior upon occurrence of the earthquake
During the constant thermal output rated operation, due to the Great East
Japan Earthquake that occurred at 14:46 on March 11, 2011 with its epicenter
off the coast of the Sanriku region, the reactor automatically shut down by“high seismic acceleration trip” 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 wereconfirmed to have integrity and be in a stable condition.
However, because of the tsunami after the quake (the first wave was visuallyobserved 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
function of the reactor, and then the S/C water temperature gradually rose andexceeded 100°C.
After those events, in order to recover part of the facilities for cold shutdown of
the reactor and SFP cooling, the workers checked and repaired thesubmerged facilities, and power supply from the temporary source was started.
By cooling S/C after the recovery of heat removal function of the reactor, theS/C water temperature dropped under 100°C. From that time on, by using
RHR1 system the water temperature of the reactor was lowered to less than
100°C and kept in a cold shutdown state by 07:15 on March 15, SFP has been
continuously cooled down, and the Station is currently kept stable.

b.

State of the “Stopping” 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 rods 10 to 19 was issued at 12:43 on March 13, which
was cleared at 20:19 on March 14 but was issued again at 21:07 on March 14.
The control rod position was readable both with the indicator of “fully inserted”
and the display of position itself. 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 inorder to stop it the countermeasure was taken by isolating the control rod(valve out) at 16:56 on March 15. 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 dueto the steep drop of reactor output. Later, the reactor water level recovered byapp-1-24

supply from the reactor feedwater system without dropping to the level toautomatically start 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 the
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 March 11 to control the pressure of the reactor by SRV.
Along with a full close of MSIV, RCIC was manually started at 15:54 on thesame 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 water level was controlled by manual start and automatic stop of
RCIC.
Judging 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 as being inoperable becauseof submersion). In that situation, LPCS pumps and RHR pumps (A, B, C) were
inoperable, and therefore the function to remove the residual heat of thereactor was lost.
The Station estimated that HPCSC pumps and HPCSS pumps were available
because they suffered less damage from water submersion as there was lessamount of seawater in the pump area inside the seawater heat exchanger
building in comparison to other pumps. In addition, as there was no inundationon the second basement floor of the reactor building, HPCS pumps were in an
operable condition.
Workers initially injected water to the reactor by RCIC, but because the steampressure to drive RCIC turbine decreased due to the reactor pressure drop
caused by the opened SRV, after the automatic stop of RCIC at 00:16 on
March 12, alternative water injection was started according to the operation
manual by MUWC. The Station had created this manual as a countermeasure
to severe accidents. From that time on, the reactor water level was adjusted bystarting and stopping the HPCS pumps which were operable without damage
from the tsunami.
The temperature and pressure inside the PCV increased along with theoperation of RCIC and opening of SRV. Because the cooling by RHR pumps(A, B) was not available, “high D/W pressure” (setting: 13.7 kPa [gage]) was
issued on 19:02 on March 11.
The signal to automatically start all of the ECCS pumps was generated alongwith that alarm, but the workers injected the water into the reactor by RCIC
and the automatic start was prevented (the control switch was pulled and hold)because the cooling system (RHRC, RHRS and EECW) was not operable,
each ECCS pump was not automatically started.
After those events, S/C water temperature exceeded 100°C at 06:07 on March
12. The S/C water temperature has reached a maximum of 137°C (12:30 on
March 14).
In order to cool down S/C, the workers started to inject cooling water (MUWP)
to S/C at 07:23 on March 12 by using cooling water drain lines from FCS

app-1-25

cooler to S/C. In addition, they performed alternative cooling of PCV at 07:35
by switching over to S/C spray from alternative water injection by MUWC.
In conjunction 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 replaced for RHRC pump (B)) were checked and repaired. Because theseawater heat exchanger building was submerged and its emergency power
supplies (PC 4C-2, 4D-2) were inundated, high voltage power supply vehicles
and temporary power cords were urgently prepared from outside the site andused. By installing temporary power cords and receiving power from theemergency power supply (P/C 3D-2) of the heat exchanger building of Unit 3
whose power was supplied from the electric system outside and by receiving
power from the high voltage power supply vehicles, RHRC pump (B), RHRS
pump (D) and EECW pump (B) were recovered to operable conditions, andthen they were started one by one from 11:00 on March 14.
After that, RHR pump (B) was started at 15:42 on March 14.
As a result of S/C cooling by RHR pump (B), the S/C water temperaturestarted gradually dropping and fell under 100°C at 07:15 on March 15.
Furthermore, in order to quickly cool down the reactor water in addition to thecooling of S/C water, the Station improvised an implementation procedure byreferring to the predefined emergency operation manual. Along with the startof 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 by flowing the reactor water into S/C via SRV, cooling S/Cwater by RHR heat exchanger (B), and injecting the water to the reactor fromLPCI line again (S/C → RHR pump (B) → RHR heat exchanger (B) → LPCI
line → the reactor → SRV → S/C). With this configuration the reactor watertemperature dropped under 100°C and the reactor was confirmed to be in acold shutdown state at 07:15 on March 15.
The mentioned above suggests that the water injection to the reactor was
successfully continued although the cooling functions of the reactor were
temporarily lost, and no fuel damage occurred because the level of iodine-131
was found to be lower than the detection limit in the reactor water sampled in alater investigation.
d. State of the “Confining” 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 toisolate PCV and maintain reactor building subatomospheric pressure. The
pressure of PCV has reached a maximum value of about 245 kPa [gage] (S/Cside), but not reached its maximum operating pressure of 310 kPa [gage].
No unusual changes in the stack radiation monitor or MP values wereobserved and no outside radioactive influence was confirmed.
Because there was a rising trend in the pressure of PCV and it could possibly
take a long time to recover the heat removal function of the reactor, the line for
PCV pressure control vent was configured (so that it could vent by opening theoutlet valve in S/C side).

app-1-26e.

State of the spent fuel pool cooling system
The facilities for SFP cooling maintained the water level of SFP at more thanits 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 theNon-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 theseawater heat exchanger building, all of them became inoperable and unable
to supply water to FPC heat exchanger, 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 for
SFP cooling at 16:35 on March 15, the water temperature of SFP dropped toaround 35°C at 17:00 on March 16, as low as it was before the earthquake.
The items mentioned above suggest that although SFP temporarily lost its
cooling function, it successfully satisfied the operation limits and conditions(SFP water level; 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 supply (P/C 4C-2, 4D-2) became
unavailable because the seawater heat exchanger building was submerged
due to the 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
arrived, 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 supply (P/C 4D-2), RHRC pump (B) and RHRS pump (D) wereallocated power to cool down the reactor and SFP, by the installment of
temporary cords and the power supply from the emergency power supply (P/C
3D-2) of the seawater heat exchanger of Unit 3, and to EECW pump (B) by thehigh voltage power supply vehicle promptly prepared from outside the site
(performed on March 14).
By this procedure, the emergency DG (B) was in operable condition, and theemergency power supply (M/C 4D) became ready to receive power from theemergency DG (B) even if the external power supply was lost.
Later, the temporary power supply for the EECW pump (B) was switched fromthe high voltage power supply vehicle to the emergency power supply (P/C
4D-1) (switched on March 29).
The emergency DG (HPCS) was in operable condition from the time of theautomatic shutdown of the reactor, and therefore sufficient emergency power
supplies to cool down the reactor and SFP had been secured.

app-1-27

Table Appendix 1-4

Fukushima Daini Nuclear Power Station Unit 4 Situation in
Chronological Order After the Occurrence of the Earthquake

Friday, March 11, 2011
14:46
Great East Japan Earthquake occurred.
14:48
Reactor automatically shut down (“high seismic acceleration trip” wasissued), 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
The first tsunami arrives (tsunami waves were observed intermittently
from this time to 17:14).
15:33
CWP (C) was stopped by manual operation.
15:34 (approx.)
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 open/close 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, the Station judged that a particular
event (loss of the heat removal function of the reactor) defined by
Article 10, Paragraph 1 of Special Law for Nuclear Emergency had
occurred.
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, the Station judgedthat a particular event (loss of the function to control pressure) as
defined by Article 15, Paragraph 1 of Special Law for Nuclear
Emergency had occurred.
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 (approx.)
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,
app-1-28this procedure was done as required).
Sunday, March 13, 201105:15 (approx.)
Received power from two lines of Iwaido lines (Line 1 was recovered).
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, the Station decided to cancel theoccurrence of particular event (loss of the heat removal function of thereactor) defined by Article 10, Paragraph 1 of Special Law for Nuclear
Emergency.
16:02
RHR (B) S/C 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 radiation dose greater than 5 μGy/h was detected at MP (No.
1), the Station judged that a particular event (rise of radiation dose atsite boundary) as defined by Article 10, Paragraph 1 of Special Law for
Nuclear Emergency had occurred. (The rise in radiation dose wasestimated to be due to the radioactive release to the air caused by theaccident in the Fukushima Daiichi Nuclear Power Station.)
Tuesday, March 15, 201100:12
Because a radiation dose greater than 5 μGy/h was detected at MP(No. 3), the Station judged that a particular event (rise of radiationdose at site boundary) as defined by Article 10, Paragraph 1 of Special
Law for Nuclear Emergency had occurred. (The rise in radiation dose
was estimated to be due to the radioactive release to the air caused by
the accident in Fukushima Daiichi Nuclear Power Station.)
07:15
Because the temperature of S/C dropped under 100°C, the Station
decided that it recovered from the particular event (loss of the function
to control pressure) defined by Article 15, Paragraph 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-1-29

Summary of the events on Onagawa Nuclear Power Station(1)

Influence of the earthquake and tsunami
All of Onagawa’s reactors from Units 1 to 3 automatically shut down due to theGreat East Japan Earthquake. Some damage was found to part of the facilities
due to the influence of the quake and tsunami. But there was no influence to thecooling 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 units quickly
halted and entered a cold shutdown state.
The series of events on Onagawa are shown in Figure Appendix 1-2 with theevent tree.

app-1-30

Events
occurred
Earthquake andtsunamiReactorshutdown
Reactorscram

DC power
AC power supply
supplyDC power External
Emergenc
supplypower
y DG
supplyElectric power
Ensuring long-term cold shutdown
recoveryIC, turbineExternal
RPV
Reactor
RHR
RHR
PCV vent
driven coolant power supply, depressuri coolant
recovery (before
injection
emergency
zationinjectioncore
system
DG, power(including (including
damage)(RCIC, HPCI) interchange alternative alternative
for which AC
sources) sources)
supply is not
requiredCore cooling

Status of reactor core

Degraded core coolin

Cold
shutdown,
core
damage,
PCV damage
etc.

Depressu Reactor
rization of coolant
nuclearinjection
reactor

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

Units 2, 3

Cold shutdown (No power supply)(Continuation)

PCV integrity or PCV leakage (excessive
temperature)
PCV leakage (excessive temperature) andhydrogen 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](Successful)[Tsunami][Earthquake][Tsunami]

Diagram Appendix 1-2 Onagawa Nuclear Power Station disaster identification events tree
app-1-31

a.

Impacts of earthquake
The seismic intensity observed at the site of the Station during the Great East
Japan Earthquake was slightly lower than Level 6. Even on a seismographarranged for safety purposes on the 2nd basement level in the Unit 1 reactor
building, 567.5 gal was observed.
The maximum acceleration observed on each level of the Onagawa Units 1 to
3 reactor buildings was almost equivalent, while some levels indicated amaximum acceleration that exceeded the maximum response acceleration forbasic 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 site ground recorded in the
instrumental earthquake data, every location indicated an almost equivalent
response spectrum, while some values exceeded the response spectrum forbasic design earthquake ground motion Ss (reflecting the impact of groundstatus 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 to 3 reactor buildings as well as the shear force
applied on the earthquake resisting wall of each level, maintenance of thefunctions of reactor buildings was confirmed.
Five circuits supply power to Onagawa Power Station as external power
supplies (No. 1 and No. 2 lines of Ojika Main Line [275 kV system], No. 1 andNo. 2 lines of the Matsushima Main Line [275 kV system], and Tsukahama
Branch Line [66 kV system]). Only the No. 2 line of the Matsushima Main Line
was available immediately after the earthquake due to the actuation of thesystem protection circuit accompanying the accident in the power transmission
line in the jurisdiction area of TEPCO. However, the No. 1 line of the Ojika
Main Line was recovered at 20:12 on March 12, and then the No. 2 line of the
Ojika Main Line at 20:15 on the same day. This was followed by the No. 1 lineof the Matsushima Main Line at 10:47 on March 17 and Tsukahama BranchLine on 15:41 on March 26.

b.

Impacts of tsunami
The maximum height of the tsunami confirmed on a current meter immediately
after the earthquake was O.P. + approx. 13 m* (at 15:29 on March 11) at the
maximum. This height did not exceed the altitude of the Onagawa site (O.P.+
approx. 13.8 m*) (before the earthquake: O.P.+ approx. 14.8 m).
While a trace of seawater intrusion was observed due to runup in part of plantsite along the coastline, the waves did not reach the main buildings.
* This value reflects the diastrophism (approx. -1 m: based on the value of top
line report) in the vicinity of Onagawa announced by the Geospatial
Information Authority of Japan after the earthquake.

app-1-32

(2)Unit 1a.

Station behavior after the earthquake
The earthquake occurred at 14:46 on March 11 during the constant rated
thermal power operation, and a seismic acceleration high signal wastransmitted and immediately control rods were fully inserted, which resulted in
an automatic shutdown of the reactor.
The starting transformers were shut down due to a short-circuit or earth fault
inside non-essential metal clad switchgear 6 – 1A that occurred during theearthquake, which led to a temporary loss of non-essential on-site power.
While this caused a shutdown of entire pump units in the Condensate andFeed Water System, prompt actuation of RCIC enabled water to be fed to thereactor.
Because the on-site non-essential power was lost temporarily through thestarting transformer 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 thereactor by using the control rod drive hydraulic control system (hereafter called“CRD”).
The workers cooled the reactor without any problem by RHR and a cold
shutdown was achieved at 00:58 on March 12.

b.

Status of “Stopping” function
Under the impact of the earthquake, a seismic acceleration high signal (“O.P.
15.00 m vertical direction seismic sensor motion trip”: Set point 100 gal) wastransmitted and immediately after that all control rods were fully inserted (at14:46 on March 11, 2011). Then the reactors were shutdown automatically as
designed and subcriticality was confirmed at 15:05 on the same day.

c.

Status of “Cooling” function
The starting transformers were shut down due to a short-circuit or earth fault
inside non-essential metal clad switchgear 6 – 1A that occurred in theearthquake, 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 thestarting transformer 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.
The workers cooled the reactor without any problem by RHR and a cold
shutdown was achieved at 00:58 on March 12.

d.

Status of “Confining” function
PCIS motions accompanying the reactor vessel low water level (L – 3) signal
were normal and no abnormal value was observed on the stack monitor,

app-1-33

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 on March 11, it was
restarted at 19:30 on the same day upon confirming that there was noabnormality in the equipment. During the shutdown, no significant elevation
was seen in the temperature of the spent fuel storage pool. The cause of the
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 pumpsuction pressure through a temporary drop of the spent fuel storage pool level
due to the seismic vibration.

f.

Status of “Power Supply” function
With regard to the power supply, the Station was receiving external powerfrom the No. 2 line of the Matsushima Main Line via starting transformers.
However, at 14:55 on March 11, on-site power was lost due to shutdown of thestarting transformers. Accordingly, power was supplied to emergency bus by
DG (A) and DG (B) based on the design.
The cause of the starting transformer shutdown was the earth fault or
short-circuit that occurred inside the non-essential metal clad switchgear 6 –1A (this resulted in a fire later), which actuated the overcurrent relay of thestarting transformers.
Thereafter, the workers confirmed the integrity of the starting transformers
based on an appearance inspection and a measurement of the insulation
resistance and they restored the starting transformers at 02:05 on March 12.
After the starting transformers were restored, power supply of non-essential
bus was recovered one by one excluding the non-essential metal cladswitchgear 6 – 1A.
The emergency power supply that was needed to cool down the reactor andthe spent fuel storage pool until the restoration of the starting transformers
was secured.

app-1-34

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
Great East Japan 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)
00:57
Reactor coolant temperature reached 100C
00:58
Reactor status “Cold shutdown”
02:05
After the power supply (restoration) of starting transformers, non-essential
buses were recovered excluding M/C6-1A which was damaged by fire.

app-1-35

(3)Unit 2a.

Station 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,a seismic acceleration high signal was transmitted. Then, all control rods were
fully inserted immediately and the reactor was shut down.
Because Unit 2 was in the status immediately after starting reactor operationand the status immediately before the earthquake was of subcriticality with thereactor water temperature lower than 100°C, a cold shutdown was
implemented at 14:49 on March 11 by operating the reactor mode switch“Shutdown.”b.

Status of “Stopping” 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,a seismic acceleration high signal (“R/B bottom horizontal direction highseismic acceleration trip”: set point 200 gal) was transmitted. Then, all control
rods were fully inserted immediately and the reactor was automatically shut
down as designed. The status of subcriticality was maintained.

c.

Status of “Cooling” function
Because 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, a 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 seawater pump area andentered part of the reactor building via an underground trench, which resulted
in the functional loss of 2 systems (RCW-B and HPCW). However, because
the RCWA system was sound, there was no negative impact on the cooling
function of the reactors by RHR.

d.

Confining
No abnormal value was observed on the stack monitor, radioactive waste
discharge water monitor and MP. There was no radioactive impact on theexternal environments.

e.

Status of spent fuel storage pool
While the earthquake caused FPC shutdown at 14:47 on March 11, it was
restarted at 20:29 of the same day upon confirming that there was noabnormality in the equipment. During the shutdown, no significant elevation
was seen in the temperature of spent fuel storage pool.
The cause of FPC shutdown seems to be the actuation of level switch for“skimmer surge tank level low” due to the seismic vibration, or the drop of FPC

app-1-36

pump suction pressure through a temporary drop in the spent fuel storage poollevel due to the seismic vibration.
Under the impact of tsunami accompanying the earthquake, the seawater
intruded from the cooling water intake canal of the seawater pump area andentered part of the reactor building via an underground trench, which resulted
in the functional loss of 2 systems (RCW-B and HPCW). However, because
the RCW-A system was sound, there was no negative impact on the cooling
function of spent fuel storage pool by FPC similar to the case with reactor
cooling function by RHR.
f.

Status of “Power Supply” function
With regard to the power supply, the Station was receiving external powerfrom the No. 2 line of the Matsushima Main Line via starting transformers andon-site power has been secured constantly.
Under the impact of vibration caused by the earthquake, a generator loss offield signal was transmitted, which resulted in the automatic start-up of DGs(A) / (B) and the high pressure core spay system DG (hereafter called“DG(H)”) followed by a standby status with no load operation. Thereafter, theseawater intruded from the cooling water intake canal of the seawater pumparea and entered part of the reactor building via an underground trench, which
resulted in the functional loss of 2 systems (RCW-B and HPCW). While thisevent triggered the automatic shutdown of DG(B) and DG(H), DG(A) was
sound and, even if the external power was lost, the emergency power supply
that was needed to cool down the reactor and the spent fuel storage 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, a generator loss offield signal 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 ofcooling water supply 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 of cooling water supply for emergency DG (H) and it
was shut down automatically at 15:42.
Inspection of the site revealed that the seawater entered the RCW heatexchanger (B) area, HPCW heat exchanger area and the staircase thataccessed the elevator in the uncontrolled area on the 3rd basement level ofthe reactor building (submergence depth: approx. 2.5 m), and that the RCW
pumps (B) / (D) and HPCW pump were submerged and the seawater intruded
also into the RCW heat exchanger (A) area (submergence depth: approx. 0.5m). Therefore, the workers began to drain seawater in the submerged area
outside the building at 20:25 by using temporary pumps. The draining process
was completed at 10:30 on March 16.
Further, a patrol after the earthquake revealed that the seawater system(hereafter called “RSW”) pump (B) area in the outdoor seawater pump area

app-1-37

was submerged and the RSW pumps (B) / (D) installed in this area might havebeen submerged, too.
We inspected the motors of RCW pumps (B) / (D), RSW pumps (B) / (D) and
HPCW pump affected by seawater intrusion (or suspected seawater intrusion)as well as the submerged actuator of the motor operated valves in the factory.
As a result of an overhaul inspection, rust was observed on all the components,
while the insulation resistance value of some components satisfied the criteria.
Accordingly, these pumps were judged as being unable to operate sufficiently
to secure the safety of the reactor facility.
This event belongs to a type of event for which reporting is mandatory basedon the Rules for the Installation, Operation, etc. of Commercial Power
Reactors, Article 19-17, Item 1, and No. 3.
To protect the area against a back wave of a tsunami, level instrumentation for
CWP automatic shutdown (hereafter called “this level instrumentation”) was
installed additionally in the RSW pump (B) section of the seawater pump area(in 2002). However, the Station did not make sufficient consideration in
selecting the location for installation or water shielding measures to protect thearea from the impacts of the waves that push forward.
Therefore, we estimate the following process occurred: Under the impact of
the tsunami accompanying the earthquake, the seawater intruded from thecooling water intake canal and entered the seawater pump area via this level
instrumentation box, submerging the RSW pump (B) area and entering part of
the reactor building via an underground trench. This event resulted in thefunctional loss of RCW-B system, RSW-B system and HPCW system.
While this type of level instrumentation is installed also in Units 1 and 3, there
was no damage to the RCW system and such like because they are installed
in a different area (dust extractor area).
Although there was a trace of seawater intrusion also into the dust extractorarea, the cover of this level instrumentation was not detached. Therefore, weestimate that the seawater must have entered from the opening of the dust
extractor installed in front of this level instrumentation.
The following countermeasures were implemented to prevent recurrence ofthe loss of function in RCW (B), RSW (B) and HPCW systems:• This level instrumentation was detached and water shielding work was
carried out on its opening.
The location to install this level instrumentation is to be changed to an arealess susceptible to seawater intrusion.
• Repair work was conducted on the section from the seawater pump area tothe pipe penetration to trench and the cable tray penetration.
•

As mid-term and long-term countermeasures, we will improve the watersealing performance of the building doors and the installation of tideembankment and tide wall as part of countermeasures to prevent
submergence by tsunami. These measures are implemented within theframework of emergency safety countermeasures for the Onagawa
Nuclear Power Station complied based on the instructions of the Minister of
Economy, Trade and Industry dated March 30, 2011, “On the
Implementation of Emergency Safety Measures at Other Power Stations

app-1-38

drawn from the 2011 Accident at Fukushima Daiichi and Daini Nuclear
Power Stations (Minister’s Instructions)”
With regard to the submerged pump motors and motor operated valve
actuators, repair work (cleaning, maintenance etc.) was conducted based on
the result of an overhaul inspection.

app-1-39

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
Great East Japan 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-1-40

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
Great East Japan Earthquake occurred (seismic intensity observed onsite: 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 first tsunami arrives (an operator confirms 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 seawater.
20:25
Temporary pumps were installed and work to drain the seawater that flew
into the 3rd basement level of the reactor building (uncontrolled area) tooutside the building was started.
March 16, 2011
10:30
Draining of the seawater from the building was completed.

app-1-41

(4)Unit 3a.

Station behavior after the earthquake
Upon the earthquake that occurred at 14:46 on March 11 during the constant
rated thermal power operation, the seismic acceleration high signal wastransmitted and all control rods were fully inserted, which resulted in theautomatic shutdown of the reactor.
Under the impact of the tsunami, a “seawater pump area level low” signal wastransmitted, which resulted in the shutdown of CWP. At the same time,
submergence under the seawater that entered the seawater pump area of theheat exchanger building caused the shutdown of the turbine auxiliary sea
water system (hereafter called “TSW”) pumps. Due to the loss of cooling watersupply caused in this event, all the reactor feed water pumps were manually
shut down. Then, RCIC was started up to feed water to the reactor, andreactor pressure control was carried out by SRVs upon fully closing MSIVs,
because the condensation 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, the workers cooled thereactor by RHR without any problem and a cold shutdown of the unit wasachieved at 01:17 on March 12.

b.

Status of “Stopping” function
Upon the earthquake that occurred at 14:46 on March 11 during the constant
rated thermal power operation, the seismic acceleration high signal (“R/Bbottom vertical direction high seismic acceleration trip”: Setpoint 100 gal) wastransmitted and all control rods were fully inserted, which resulted in theautomatic shutdown of the reactor as designed. Status of subcriticality wasconfirmed at 14:57 on the same day.

c.

Status of “Cooling” function
Under the impact of a back wave of the tsunami, a “seawater pump area levellow” signal was transmitted, which resulted in the shutdown of CWP as
designed. At the same time, submergence under the seawater that entered
the seawater pump area of the heat exchanger building due to the tsunami
caused the shutdown of TSW pumps. Due to the loss of cooling water supply
caused by this event, all the 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
the condensation of main steam by using condenser failed.
To feed water to the reactor after the shutdown of RCIC accompanying thereactor depressurization, multiple options could be considered; i.e. the ECCS
by using suppression pool water as water source, the MUWC by using
condensate storage tank as water source and so on. We selected MUWC
using condensate storage tank as water source to feed water to the reactor
with giving consideration to the decrease of decay heat through plant
shutdown and the maintenance of reactor water quality.
The workers cooled the reactor without any problem by RHR and a cold
shutdown was achieved at 01:17 on March 12.
app-1-42d.

Status of “Confining” function
No abnormal value was observed on the stack monitor, radioactive waste
discharge water monitor and MP. There was no radioactive impact on theexternal environments.

e.

Status of spent fuel storage pool
While the earthquake caused FPC shutdown at 14:47 on March 11, it was
restarted at 15:23 on the same day upon confirming that there was noabnormality in the equipment. During the shutdown, no significant elevation
was seen in the temperature of spent fuel storage pool.
The cause of FPC shutdown seems to be the actuation of level switch for“skimmer surge tank level low” due to the seismic vibration, or the drop in FPC
pump suction pressure through a temporary drop in the spent fuel storage poollevel due to the seismic vibration.

f.

Status of “Power Supply” function
With regard to the power supply, the Station was receiving external powerfrom the No. 2 line of the Matsushima Main Line via starting transformer, andon-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 that was needed to cool down the reactor and the spent fuel storage
pool could be secured.

app-1-43

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
Great East Japan 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)
01:17
Reactor coolant temperature lower than 100°C (Reactor status “Coldshutdown”)

app-1-44

Summary of Events in Tokai Daini Nuclear Power Station(1)

Impacts of earthquake and tsunami
The earthquake struck 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 a turbine trip caused by a large vibration of the turbine bearing
under the impact of the earthquake. Immediately after the earthquake, all threecircuits for external power were lost. However, the power supply 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, seawater pumps for cooling Emergency
DG2C were automatically shut down, which resulted in the Emergency DG2C
becoming unusable. Nevertheless, emergency power could be secured byoperating the remaining 2 emergency DGs without causing any disturbance to thecooling function for the reactor and SFP. At 00:40 on March 15, the cold shutdownof the reactor was achieved.
The course of events in the Tokai Daini Nuclear Power Station is illustrated as an
event tree in Figure Appendix 1-3.

app-1-45

Events
occurred

Reactorshutdown

Earthquake
and tsunami

Directpower
supplyDC power
supplyReactorscram

AC supply
External
power
supplyEmergency
DG

Core coolingElectric
power
recovery

Ensuring long-term cold shutdown

RPV
ReactorHigh pressure External
power supply, depressuriza coolant
core coolinginjectionsystem (RCIC, emergency tion
DG, power(including(including
HPCS)
interchange

alternative
sources)RHR

PCV vent
Cold shutdown,(before core core damage, PCV
damage)damage etc.

alternative
sources)(Successful)
External power supply

RHR
recovery

Status of reactor
core

Cold shutdown
Cold shutdown
Cold shutdown (NoRHR)
PCV damage(over-(Integrity)
RCIC/HPCS[2/3 integrity]
DG-2D,HPCS(Successful)[Earthquake]

Figure Appendix 1-3

Fukushima Daini Nuclear Power Station disaster identification events tree

app-1-46

a.

Impacts of earthquake
Seismic intensity observed on site (Tokai in Tokai Village) was slightly lower
than Level 6, and the maximum accelerations recorded on a seismographarranged on the 2nd basement level of the reactor building was 214 gal in the
North-South direction, 225 gal in the East-West direction and 189 gal in thevertical direction.
The Station confirmed that the maximum acceleration observed on each levelof the reactor building was maintained below the maximum acceleration for thebasic 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 some records indicated that they locally exceeded the
response spectrum for the basic design earthquake ground motion Ss, theanalysis 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 (275 kV systems
and 154 kV system) were lost. Thereafter, 154 kV system was restored on
March 13 and one circuit of 275 kV systems was restored on March 17.
Another 275 kV system was recovered on April 27. With this, all circuits of theexternal power were recovered.

b.

Impacts of tsunami
The status of the flood caused by the tsunami during the earthquake wasinspected at the intake area. As a result, the Station confirmed that, at around
15:35, the intake area located at altitude* +3.3 m (approx.) (altitude before theearthquake) suffered flooding with approx. 1 m depth (depth of submergenceis estimated based on images from monitor cameras in every case).
Further, the Station confirmed that the submergence of approx. 2 m occurredin the intake area also at around 16:51. This was followed by submergencethat occurred several times (depth  1 m) 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 isestimated as altitude* +5.3 m (approx.) (altitude after the diastrophism
investigation). Because this height did not exceed the height of the Tokai Daini
Nuclear Power Station site (altitude* +8.0 m (altitude before the earthquake)),
the tsunami did not reach the main buildings of the Station.
The tsunami reached the point around the seawater pump area in the plantsite along the coastline (altitude* +3.3 m (altitude before the earthquake)).
While there was no water intrusion from above the North-South side walls of
the seawater pump area because they had altitude * +6.1 m height (altitude
before the earthquake) as a countermeasure against tsunami, some water
intrusion from penetration hole was observed in the north pump area, since
the area was under construction 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).

app-1-47

(2)Status of “Stopping” function
Under the impact of the earthquake, vibration of the turbine bearing increased anda “turbine bearing vibration high” signal was transmitted, which resulted in aturbine 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 a subcriticality status.

(3)

Status of “Power Supply” 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.
The Emergency DG seawater pump (2C) shut down due to the intrusion ofseawater into the north pump tank under the impact of tsunami. This event caused
the Emergency DG2C to become unusable, which led to the power loss of theEmergency 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, the workers supplied power to the batteries
and to the DC power load from the emergency power bus. While the Emergency
AC Power Bus 2C had lost power, the DC power supply connected to this bus was
switched to the power supply from the sound emergency AD power bus and thepower was fed to the batteries without any problem. Therefore, the site did not lose
DC power.

(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 a high pressure status, which helped
keep a normal water level in the reactor. Thereafter, the reactor water level wasmaintained by RCIC (at first CST, and then the suppression pool was used as thewater source) and the pressure control for the reactor 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 diesel generator seawater pump (2C) installed in thenorth pump tank, no negative impact on the function was observed on the RHR
seawater pumps (A) / (C) and the seawater pumps (A) / (C), although they were
submerged up to the bottom level of the generators.

(5)

Status of “Confining” function
The water level fluctuation (level drop) that occurred immediately after the reactor
scram caused normal actuation of the containment isolation system, leading toisolation of reactor containment.

app-1-48

Similarly, the water level fluctuation (level drop) that occurred immediately after thereactor scram caused automatic isolation of the reactor building and the normal
switching of the reactor building ventilation system from normal ventilation system
to the Standby Gas Treatment System (SGTS).
(6)

Status of spent fuel pool cooling
Sloshing of water by the earthquake actuated an alarm for the spent fuel pool level(“FUEL POOL LEVEL HI/LO”). At the same time, the water level decreased byapprox. 20 cm from the normal level, because flooding from the pool occurred.
To address this event, water was filled in the spent fuel pool by using CST water.
While the water level dropped, spent fuel stored in the spent fuel pool were
sufficiently covered by water continuously (fuel top + approx. 7 m).
While the Fuel Pool Cooling System was shut down due to loss of external power,
cooling was resumed by using a power supply from the sound Emergency DG2D.

(7)

Cause of seawater intrusion into north pump tank under the impact oftsunami and countermeasures
With regard to the emergency seawater pump tanks, separation walls of altitude*+6.1 m height (altitude before the earthquake) had been installed to protect thearea against tsunami of altitude* +5.7 m height (approx.) (altitude before theearthquake). At this time on March 11, construction work for water sealing thepump tanks was going on.
The height of the tsunami observed in the current disaster was altitude* + approx.
4.8 to 5.3 m (altitude after the diastrophism investigation). While this height did notexceed the height of the separation walls of altitude* +6.1 m (altitude before theearthquake), seawater entered the north pump tank from the following 2 points
during the water sealing work.
The water sealing work on the south pump tank had been completed then.
i) Opening between the north pump tank and ASW strainer area (discharge
channel)ii) Unsealed structure of the cable pit
Therefore, these 2 points were blocked by placing concrete over them.
* This altitude is based on Tokyo Peil (T.P.).

app-1-49

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
Great East Japan 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 diesel generator seawater 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 power
supplies one after another
March 14, 2011 (Monday)
23:43
Shutdown cooling mode operation started by Residual Heat Removal A System
March 15, 2011 (Tuesday)
00:40
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 standby (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-1-50

Appendix 2

Agenda for Future Review

In this report, we summarized the results of investigating the initial phase of theaccident at the Fukushima Daiichi Nuclear Power Station, the analysis of theaccident causes and the countermeasures considered effective based on thelessons we learned from the accident.
However, because there are many aspects that require further collection andanalysis of information regarding the international approaches for improving safety,
further approaches to control the impacts of this accident, and consultations withnational and local governments, we still have a series of agenda that are not includedin the scope of the current review. Therefore, we would like to identify the items
requiring examination in future by the nuclear industry community so as to further
improve safety in nuclear power plants.
In the current review, we did not cover the analysis of the root cause of the accident
including the organization factors, because it was impossible for TEPCO workers incharge of each area to share their time for interviews that may last a long time due totheir intensive efforts and the time pressure they are under in controlling this accident.
If additional information that is not accessible to us at this moment becomes unveiledin 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 fires 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 the the
Chuetsu Offshore Earthquake in Niigata Prefecture. However, we want toproceed with our examination on items, if any, that need to be addressed basedon the findings from the current earthquake.

•

Countermeasure against the facility destruction by terrorist attacks
Since 9/11, 2001, the United States has promoted diverse commitments toprevent terrorist attacks on nuclear power plants. It is necessary for Japan topromote examinations on how to secure functions of nuclear power plants(“stopping,” “cooling” and “confining”) to address the large-scale destruction offacilities by terrorist attacks.

•

Reinforcement of disaster preparedness measures
In the current accident at the Fukushima Daiichi Nuclear Power Station, theextent of discharge and diffusion of radioactive materials exceeded the scopeof impacts estimated in the conventional disaster preparedness plans. A radical
review of the disaster preparedness plans is needed while reflecting in them theapp-2-1

findings of the current disaster. Elaborate countermeasures should be soughtfrom the standpoint of nuclear power plant owners while reflecting them in thecontents to be reviewed.
•

Reconsideration of approaches for safety culture
Industry-wide serious reconsideration is needed on the cause of the Station’s
inability to prevent this accident including the aspects such as “Why waspreventive measure not implemented in anticipation of tsunami in the scale far
beyond the estimation?” and “Why was countermeasure not taken in
anticipation of long time power loss?” The results of the reconsideration should
be reflected in the approaches to nurture the safety culture with the purpose toprevent recurrence of 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 the safety of nuclear power
plants 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 andeducation with a wide outreach. Further, at least education to hand down thelessons from this accident and the learning of the new countermeasures are
essential and it is imperative to establish a mechanism to keep this cycle
running on a continuous basis.

•

Approaches to the agenda related to multiple-reactor siting
At present, the scope of impact and problems associated with specificmeasures against the accident in a multiple-reactor site are yet to be identified
except the hydrogen explosion that occurred in Unit 4 caused by the hydrogenthat was generated in Unit 3 and migrated to Unit 4 via an exhaust line.
Based on the experience of the explosion in Unit 4, we plan to screen out
similar types of systems from the standpoint of ensuring independency as oneof 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

app-2-2

the necessity through the training based on the assumption of simultaneous
accidents in multiple units.
•

Approaches to the findings related to accident handling
While many kinds of work are going on currently to terminate the accident, wehave learned various lessons from each of them. For instance, with regard to
the handling of retained water, there are various lessons in a series ofprocesses including the generation of polluted water, prevention of its leakage
to the environment, management and disposal. Approaches to issues related tothe accident handling should be continued on a mid-term and 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 severe accident conditions, the impact of discharged
radioactive materials on the surrounding environments is a concern.
To address this issue, some overseas countries (in Europe) install containment
filter vents to suppress any discharge of a large amount of radioactive materials
to the atmosphere as one of the countermeasures for severe accidents. This
practice has 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 thelarge amount of radioactive materials. Nevertheless, the appropriateness of
installing a containment filter vent should be examined as part of acountermeasure to mitigate impacts on the surrounding environment.

app-2-3

Appendix 3

Comparison with Government Report and 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 guesswork as much as possible. Further, we
manifested our continued commitment to the review by identifying the agenda
requiring further examination in future besides the lessons perceived from the courseof the accident at the Fukushima Daiichi Nuclear Power Station.
Meanwhile, in the report of the Japanese Government to the IAEA Ministerial
Conference on Nuclear Safety held in June 2011, 28 lessons were compiled that
require commitments by workers, 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 the US NRC task force as a report of investigation and review on theaccident at the Fukushima Daiichi Nuclear Power Station, 12 recommendations arelisted including the requests to regulatory bodies. We also made a comparison withthese recommendations and listed the comparison result in Appendix 3.2 to identify
any additional issues to be addressed.

app-3-1

Appendix-3.1 Comparison with the training of 28 items of Government Report to IAEA and the measure against this report on the accident ofFukushima Daiichi Nuclear Power Station
Group name

Measure toprevent a
severeaccident

Items(1) Measure to mitigate anearthquake and tsunami

Contents

Additional requirements in attachment data

Comparison with the measures of this report
The following evaluation conditions to evaluate an earthquake andtsunami are added based on the knowledge acquired from this data.
• In the present view, Ss measures for basic design earthquake
ground motion are considered to be almost satisfactory. But inorder to be assured of further safety, interlocking of two or more
epicenters should be considered.
• When evaluating the height of tsunami caused by an earthquake,consideration should be given to the interlocking of multiple
epicenters as a wave source at a plate barrier and, the necessityof assuming the epicenter fault has a sliding quantity on a large
scale should be examined.

This earthquake was a very large-scale earthquake due to the interlocking of multiple
epicenters. As a result, in the Fukushima Daiichi Nuclear Power Station, theacceleration response spectrum of the earthquake motion observed on the reactor
building basic board exceeded some periodic belts as compared with the acceleration
response spectrum of the basic earthquake ground motion of its design. Due to theearthquake, damage was observed in the external power supply.
Although the major damage to the reactor facilities caused by the earthquake has notbeen confirmed in terms of important safety-related system and equipment. The
detailed situation is still not known and further investigation about it is required.
The height of the tsunami that struck the Fukushima Daiichi Nuclear Power Station was
14 to 15 m, exceeding well over the assumed height of the construction permit designand valuation. As a result of this tsunami there was major damage to seawater pumps
and such like, making it impossible for Station to ensure and reserve an emergency
diesel power supply and reactor cooling function. In the manual, the intrusion of atsunami was not assumed but only the measures against an undertow were defined.
Thus, the Station failed to assume the frequency of the tsunami occurrence and its
height sufficiently, the measures were not good enough for handling an extensive
tsunami.
From the viewpoint of design, in the seismic design of the nuclear power plant the time
range of an active fault is considered to be between 120,000 and 130,000 years (in theold guidelines it is 50,000 years). The recurrence cycle of a big earthquake and residual
risk should be considered. On the other hand, the Station is designed to handle tsunami
based on past traditions and clear traces of tsunami. To attain safety objectives, theStation made no efforts that considered an appropriate recurrence cycle.
Due to this, while assuming an earthquake would occur, multiple epicenters interlocking
should be considered and at the same time the external power supply’s resistance todisasters should be improved. The suitable generating frequency of the tsunami inconsideration of a sufficient recurrence cycle for achieving safety and sufficient height
are assumed from the viewpoint of preventing a severe accident. For the tsunami
having an appropriate height, safety design of construction which prevents water fromflooding the site should be executed with consideration given to the destructive force ofthat tsunami. From the viewpoint of defense in depth, risks should be fully recognizedby observing the exceeded tsunami’s coverage on site as against the planned design
for a tsunami. Countermeasures should be taken so that important safety functions canbe preserved even with consideration given to the site flood or destruction force ofwaves.

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 measures(circuit connection, strengthening of a steel tower, measure against
inundation of switchyard) for reliability reservation is performed.
The assumed tsunami height is estimated and, prevention measures
should be taken to prevent the tsunami from entering the site.
Moreover, the following multi-level countermeasures should be
performed to ensure functions of safety equipment.
Countermeasures should be used to prevent the tsunami fromentering the site. Examples are as follows.
• Care should be taken for arranging the safety-related power
supply system. While preparing for a flood from a tsunami, acoastal levee (storm surge barrier) or breakwater should beinstalled, or from the viewpoint of multilevel security, a protection
barrier should be installed for important systems.
• There should be protection against tsunami for items such as
tanks which may become flotsam and interfere with emergency
responses (protective barrier etc.). Heavy industrial machines
should be used to remove flotsam.
Moreover, countermeasures should be taken to prevent seawaterfrom entering the building. Illustrative examples are as follows.
• For tsunamis which exceed the site height, tsunami preventive
measures such as improvement of the external gate of thebuilding or the sealing of air supply opening and penetration
should be executed according to the inundation height.
• For tsunamis which exceed the site height, safety-related facilities
should be protected from inundation by countermeasures for
improving watertightness of the ground stage gate and the gate
which passes through the main ground stage. Thus, the impact ofinundation should be minimized.
• To promptly restore the station, portable water discharging pumps
should be deployed in the building for discharging the seawaterthat has entered.
• Regarding the impact of the wave power of a tsunami, attention
should be paid to places where a direct impact of tsunami isobserved such as external gates facing the sea.
Moreover, tsunami countermeasures should be taken against
seawater pumps.
■ Items mentioned in this report is
4.6 Preparation against earthquake and tsunami■ Countermeasure Examples
Preparation against earthquake and tsunami (1) to (9)(2) Assurance of power
supplyThe major factor of this accident is that the required power supply was not assured.
This cause indicated that there was an insufficient variety of power supplies toovercome the Station’s vulnerability to a common cause failure due to external events.

app-3.1-1• From the viewpoint of improving the Station’s
reliability to ensure an external power supply,
countermeasures such as connecting each unit

From the viewpoint of ensuring a power supply, an external power
supply should be assured, and when it is not possible, equipment
should be installed for supplying power from emergency DG.

Group name

Items

Contents

Additional requirements in attachment data

Comparison with the measures of this reportand all power lines, Strengthening a transmission However, these preparations did not work this time.
tower (power lines) and avoiding the switchyard Therefore, the reliability of an external power supply should befrom being soaked were indicated.
improved.
Entrepreneurs were asked about the measure(1) External power supply is based on the NISA directions [Aboutrequired for the power supply reservation in an
reliability reservation of the external power supply of a nuclear
emergency. These measures include ensurance
power plant and a reprocessing facility]. It corresponds to theof high capacity of a rechargeable battery or
measures (circuit connection, strengthening of a steel tower,
battery charging from the existing backup power
measure against inundation of switchyard) to ensure a reliablesupply, safety-related equipment or power supply
power supply.
boards having a higher evaluation so that all
The following details are based on the assumption thatfunctions will not be lost in a flood andemergency DG functioning has failed.
decentralized geometry such as installing on(2) As a backup power supply of an emergency DG, various types ofhigh ground, earthquake-proof tempering of
power supplies should be secured by deploying a power supply
switchyard facilities, maintenance of the systemvehicle or high capacity power supply (including necessary fuel).
to procure fuel oil, which is required for securing
Regarding power panels, preparations must be made for aemergency power supply.
tsunami by taking inundation countermeasures. Moreover, thepower supply vehicle is installed on high ground which is not[September 11, 2011, attached Government Report]
affected by a tsunami. Separately, measures for the loss of DC
power supply must be given .
As a future measure, rechargeable battery having ahigh capacity or earthquake-proof strengthening of (3) A rechargeable battery ensures a power route from a power
the fuel tank of emergency power supply, etc. aresupply vehicle and it must have inundation measures to combat aplanned.
tsunami.

Setting up a power board or such like was not sufficient to withstand a severe
environment. Moreover, the battery life ended before the AC power supply came back;this indicates that the objective of having a battery is not clear. The Station did not know
the time needed for the external power supply to be restored.
•
For this, a power supply should be able to be assured for a long time in a severe
situation by installing various emergency power supply such as air cooled diesel
generators and gas turbine generators, diversifying types of power supply by deploying
a power supply vehicle, distributing a switchboard having high environmental tolerance,or installing a battery charging generator.

Secure power supply reservation is achieved by these multilevelmeasures.
■ Items mentioned in this report
4.7.1 Station blackout and loss of DC power supply■ Counter-measure examples
Preparation of power supply (1) to (6)(3) Assurance of secured
cooling function of reactor
and containment

In this accident, there was a functional loss of a seawater pump, which resulted the loss •of the ultimate heat escaping place. Although the nuclear reactor cooling function was
operated by injection, depletion of the water source for injection or the loss of power
supply made it impossible to prevent core damage. The Station’s container cooling
function was not operated sufficiently. After that, reactor depressurization also tooktime. After depressurization, the reactor injection by heavy machines such as fireengines was not set as an accident management countermeasure, which made thesituation very difficult. Thus, the reactor and container lost their cooling functionality
and, the accident escalated.
Due to this, alternate cooling function of reactor and container should be assured byhaving various alternative injection functions, various water sources for injection andincreasing the capacity of such injection, introducing an air cooling system, andensuring an alternative ultimate heat sink that lasts for a long time.

(4) Securing cooling
function of spent fuel pool
without a failThis time, the cooling of the spent fuel pool became impossible because there was aloss of power supply. Such cooling could have prevented a severe accident so it isnecessary to have a method to cool the spent fuel pool while dealing with the accident
at the nuclear reactor. Until now, the risk of major accident at the spent fuel pool was
considered minor compared with the risk of a reactor accident. Because of this,measures like alternate injection were not considered.
For this reason, even when the power supply is lost, cooling must be secured byintroducing an alternative cooling function of a natural circulation cooling system or air

app-3.1-2

Entrepreneurs were asked about the necessary In this accident, the external power supply was lost due to themeasures required to ensure a means of water
earthquake, and the seawater pump was flooded by the tsunami.
injection. Necessary countermeasures includes This resulted in a station blackout and loss of seawater coolingthe following;
function. Regarding RCIC, HPCI, and IC, the reactor core coolingensuring a water source for injection to thesystem can be used even at the time of a station blackout, but thisreactor and container, such as seismic retrofit for function was lost due to the loss or depletion of a DC power supply.
an intake pit or large fresh water tank;
For this reason, the following measures are considered necessary.
intensifying an inspection of components of a
These measure examples can be considered as multilevel measurescontainment spray ring; installing a suction pump by combining them with the conventional equipment.
to a reservoir, seawater pit or pump which can(1) Injection cooling function of reactor is secured as follows;
perform external injection to the reactor andensuring means of injecting water into the reactor by a portablesteam generator without a power supply;
power pump which is independent from the existing power supply;
maintenance of injection equipment such as DG
ensuring a water source for injection including seawater; ensuring
drive pump, high-pressure piping.
a backup power supply necessary for SRV operations because it• The entrepreneurs were also asked about theis imperative to securely depressurize the reactor by SRV during
necessary measures required to ensure the heat
injection.
sink used for removing decay heat and waste(2) The ultimate heat sink must be ensured by promptly restoring aheat from auxiliary equipment. These measures
seawater pump and seawater system by arranging spare parts.
include an intake pit installation for cooling by(3) An alternative ultimate heat sink must be ensured to prevent heatseawater, deployment of intake pump sparefrom escaping to the atmosphere through containment vent.
parts, diversification of intake parts, anddevelopment and maintenance of an air- cooled■ Items mentioned in this reportsystem.
4.8.1 Injection to reactor
4.8.2 Seawater cooling loss
[September 11, 2011 attached government report]■ Counter-measures example
As a future measure, seismic retrofit of aHeat sink loss correspondence (1), (2), (8) to (11)large-scale fresh water tank etc. are planned.
The plant operators were asked to have the
To secure the cooling function of a spent fuel pool, it is extremely
measures required to ensure a cooling function of a important to maintain water level so that the spent fuel is not
spent fuel pool. This includes seismic retrofit of the exposed even at the loss of cooling function. For that purpose, ancooling system piping of a spent fuel storage pool; alternative injection means must be secured. For example, ensure ameans of injection with fire engines or pump cars and ensure there isensuring the power supply from an emergency
power supply to the water gauge and thermometer a corresponding injection route and water source.
of a spent fuel pool; strengthening checks on the
Moreover, in order to check the validity of an alternative injection
cooling pump of a spent fuel pool; strengthening the measure, the Station must enhance the monitoring function of themonitoring of a spent fuel pool (ITV etc.);

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Additional requirements in attachment datacooling system, and having an alternative injection function so that cooling of a spent
fuel pool can be maintained.

introducing dry cask storage.
[September 11, 2011 attached government report]
As a future measure, seismic retrofit of coolingpiping of a spent fuel pool, etc. are planned.

Comparison with the measures of this reportpool water level and pool water temperature. For example, a power
supply to level instrumentation and thermometer from emergency
power source, or status monitoring enhancement of pool using ITV ofbattery drive measures can be considered.

Thus, there is possibility of securing integrity of the spent fuel byperforming injection to the spent fuel pool and maintaining thepool water level. For further reliability, an example of a measureis one which maintains the Station’s cooling function.
■ Items mentioned in this report
4.11 Ensuring integrity of spent fuel■ Measure example
Ensuring integrity of spent fuel (1) to (3)Power plant regulations are the jurisdiction of the country and a
subject are not raised in this report made by the industrial world.
However, the validity of using PSA would be recognized from this
accident, and here onward the use of PSA will be considered.
Measures are examined in multiple ways. Valid measures for severe
accidents such as preventing the outbreak of an accident arestrongly recommended in [4.5 chapter measures collection].

(5) Accident management(AM) measurescomplianceThis accident was severe. To minimize the possibility of a severe accident or mitigate
the influence of a severe accident, the accident management measures wereintroduced also in the Fukushima Nuclear Power Station. From the viewpoint of this
accident, part of the alternate injection system for the reactor from the Fire Service
Water System was working, but various power supplies or reactor cooling functions
failed and accident management measures were insufficient. Moreover, these arefundamentally considered to be a Station operator’s independent measure, and there
was no demand for laws and regulations but maintenance was lacking. Furthermore,since accident management guidelines were designed in 1992, they had not beenreexamined, improved and reinforced.
For this reason, accident management measures should be performed for preventing
severe accidents effectively and efficiently. These measures are, self-control safety
precautions re-modified by a plant operator. This should be done at the demand of laws
and regulations. Probabilistic valuation techniques need to be applied while
re-modifying design requirement items.

(6) Handling problems of
multiple furnace locations

This time, accidents occurred simultaneously at multiple furnaces, and the resources
required for handling the accidents were distributed thinly. Moreover, the two nuclear
reactors shared equipment and the physical gap between them was very small, so
emergency response to at the adjoining reactor was affected by the progress of theother reactor accident
For this reason, when multiple nuclear reactors are located in one plant, and if an
accident occurs in one reactor, the operations of the other reactor should be separated
and executed independently. Also, the engineering independency of each nuclear
reactor should be ensured and care should be taken so that the impact of the reactor
where the accident occurred should not extend to the adjoining reactor. Moreover, it isrecommended that a chief person should be assigned for each unit to ensure reactor
safety, and he or she should handle multiple accidents independently.

(7) Consideration on basic
designs of nuclear power
station arrangement

This time, it was hard to handle the accident as the spent fuel pool was located high up Plant operators were asked for rationalization ofin the reactor building. Moreover, the Station could not stop the contaminated water of arrangement of a reactor building and a turbine
the reactor building from going between the turbine building and another building.
building, etc.
For this reason, regarding basic designs of nuclear power station arrangement, thefacilities and buildings should be suitably arranged from now on, which will make it[September 11, 2011 attached government
possible to carry out secure cooling even when a serious accident is occurred andreport]prevent expansion of an accident. In that case, the additional measure for having an
Materialization of examination is planned byequivalent function is taken for the existing facilities.

At present, the level or problem of influence on specificallyhandling accidents in multiple furnace locations is not clear, otherthan the influence on the adjacent unit work in which thehydrogen that occurred at Unit 3 started an explosion when it
leaked from the exhaust line to Unit 4. If more information on
multiple furnace locations comes to light in the process ofaccident investigation here onward, we will consider examining
concrete measures. For the time being, training will be conductedby considering the occurrence of simultaneous accidents at two[September 11, 2011 attached government
report]or more furnaces. In it, we will reexamine the procedures and the
Hereafter the verification of the policy for making necessity for reexamination of staff disposition.
engineering independency of each nuclear
reactor in multiple furnace locations in a moresecure way, is scheduled.

Plant operators were asked about the necessary
measures for accidents that occurred in the same
site, to ensure the period necessary for safety,
thoroughly isolate the building period, ensure theengineering independency in the location ofmultiple units etc (rationalization of arrangement ofa reactor building and a turbine building).

considering suitable arrangement of facilities of
the nuclear power stations and buildings, etc. in
the basic design for nuclear reactor
establishment etc.

app-3.1-3

In a newly constructed plant, about arrangement of apparatus,
designs should be made in such a way that they reflect this accident
perception. For the existing plant, concrete examples of measures
must be followed to fill in the gap between the ideal situation and thepresent reality.
When examining an injection route to a spent fuel pool, it isnecessary to consider countermeasures if the pool is located at highaltitude.
• Injection of water by fire engines, pump trucks and such like, as
well as water injection route and water source should be ensured.
Countermeasures must be taken such as watertightening of abreakwater and compartment, or sealing the penetration area, withrespect to the installation location or installation conditions.
• Build a sea wall or breakwater and also build a protective wall for
important installations.
• Evaluate the impact of tsunami that exceeds the site height. ,buildings outdoors, air supply opening aperture & penetration withrespect to flood height. For external gates of the building and air
supply openings and penetration area, implement flood

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Additional requirements in attachment data

Comparison with the measures of this report
•

•
•

•
•
•

prevention countermeasures, such as sealing improvement.
Evaluate the impact of tsunami that exceeds the site height, andimprove watertightening for basement gates and gates that leadto the basement floors, where there are safety-related equipmentthat require flood prevention measures. This way flooding impact
can be minimized.
Deploy portable drainage pumps to drain seawater inside thebuilding for early restoration.
Give consideration to the impact of tsunami wave when externalgates facing the sea could be exposed to direct impact of atsunami.
Take flood prevention measures like a water prevention wallaround a seawater pump.
Deploy motor spare parts for early restoration
Deploy movable seawater pumps or waterproof pumps.

■ Items mentioned in this report
4.6.2 Tsunami flood prevention within the premises
4.6.3 (1) Watertightening of important facilities
4.6.3 (2) Flood countermeasures of seawater system
4.11 Ensure integrity of spent fuel■ Counter-measures
Provisions against earthquake and tsunami (1) – (9)Ensure integrity of spent fuel (1) – (2)(8) Ensuring
watertightening ofimportant facilities

One of the reasons this accident occurred was that a lot of important equipment
facilities, such as seawater pumps for auxiliary cooling, Emergency Diesel Generator
and switchboard, were flooded in the tsunami. As a result there were difficulties inensuring an electricity supply and cooling system.
From the viewpoint of achieving the desired level of safety, assume that a tsunami thatexceeds the design comes. Ensure the important safety even if there is a flood thatexceeds the design assumptions for sites adjacent to rivers. Specifically, install gatesthat can handle the destructive force of a tsunami or flood, put blocks in the piping or
flood pathway, use waste water pumps, and ensure the watertightening of importantequipment.

Move and distribute safety-related control devices First, make scientifically reasonable assumptions to evaluate theto upper floors or high ground, order the workers to tsunami height. For that 1) “ensuring the necessary sight height” istake countermeasures to ensure water tightening of the top priority countermeasure. But from a multi-layered protection
important facilities.
viewpoint, assume a tsunami that exceeds the site height, andaccording to the pushing and pulling force of a tsunami, investigate
how to prevent important safety facilities from being destroyed. Carryout 2) “defense of a breakwater or such like.” Moreover, assuming atsunami that is exceedingly large, 3) Countermeasures such as“installing watetightening gates” or “improving sealing of the
openings” should be taken for places where seawater intrusion cancause the functional loss of safety-related facilities. Those places,such as openings of the building, can become pathways for seawater
to infiltrate. Assume that the Station cannot prevent a tsunami fromentering the building even after all those measures were taken, andensure safety functions by performing 4 “water tightening of eachcompartment,” ensure the safety functions. Also assume that thewater may still enter the water tightening compartments. Havecountermeasures such as 5) “deployment of an drainage pump” toremove the flood water as early as possible.
For temporary power supply system, 6) “move and deploy at uplandwhere tsunami cannot reach.” In the situation where possibilities oftsunami impact are totally eliminated, a back-up system is expected
to work even when the existing power supply fails.
Like this, multi-layered countermeasures must be developed fortsunami. Assuming these countermeasures, the loss of function that
may cause a power supply failure, heat sink failure and othercommon-factor failures can be prevented and the accident can also
be prevented from escalating.
■ 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 Station blackout and loss of DC power supply■ Countermeasures
Provisions for earthquake and tsunami (1)-(3), (5)Power supply preparation (1)-(3), (5), (6)Heat sink failure measures (12)Responding (9) Reinforce theto incidents in countermeasures forprevention of hydrogensevereIn the present accident, there was a hydrogen explosion in the Unit 1 of reactor building • With respect to BWR, decisions must be madeat 15:36 on March 12, and in Unit 3 of the reactor building at 11:01 on March 14.
and measures taken with the objective ofMoreover, there was an explosion which seemed to be due to hydrogen in Unit 4 atpreventing a hydrogen explosion and hydrogenapp-3.1-4

Based on the design standards up till now, with respect to thehydrogen which can be generated due to acid reaction of nuclear
reactor coolant with fuel cladding tube, hydrogen is prevented from

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Additional requirements in attachment data

15:06 on March 15. That is, no help was received from the initial explosion of Unit 1 andthe conditions were such that the explosions occurred in continuation, and that was amajor factor that made the present accident worse. In order to maintain the integrity of
the containment vessel in a boiling water reactor, as per the design standards, the innerpart of the containment vessel was inactivated, and a flammable gas control system issetup. However, it was not assumed that there would be a leakage of hydrogen gas in
the nuclear reactor building that would lead to conditions that can create an explosion.
Hence, hydrogen countermeasures were not implemented in the nuclear reactor
building.
In order to precisely reduce or release generated hydrogen, along with thehydrogen-related countermeasures for the containment vessel, it is necessary toreinforce the hydrogen explosion prevention countermeasures for the flammable gas
control system setup in the nuclear reactor, which is used in the event of severe•
accidents and completion of facilities in order to let the hydrogen escape outside.

outlet should be ensured by securing means of
blowout panel opening.

Comparison with the measures of this reportbeing generated by cooling the reactor with an emergency reactor
cooling system or in the case of boiling water reactor (BWR), theFor BWR, measures must be taken to prevent deactivation is done by including nitrogen in the nuclear reactor
hydrogen from accumulating in the flammable containment vessel and hence even if hydrogen leaks by somechance, the atmosphere inside the nuclear reactor containment
gas control system in the nuclear reactor
vessel remains below combustible limits.
building. Based on the analysis of the
However, in the case of a BWR, due to circumstances which wereconditions in Fukushima Daiichi (leakage),outside the design scope, there was hydrogen leakage from theimportant hydrogen explosion
countermeasures include having a hydrogen same container to the inside of the nuclear reactor building. For this,detector device within the building, or setup in following countermeasures:
the nuclear reactor building for hydrogen vent (1) “Countermeasures for releasing the hydrogen stagnated in the
nuclear reactor building or reducing its concentration,” (2) “Based on
to release hydrogen.
the analysis of the path where hydrogen leakage had occurred,
For PWR, the power supply for the annulus
setting up of the hydrogen gas detector system, which will carefully
exhaust system by a power supply vehicle to letmonitor the accumulation of hydrogen gas inside the nuclear reactor
hydrogen escape at the time of leakage, powerbuilding” are proposed.
supply for the igniter for the power supply vehicle
Moreover, in order to prevent the collection of hydrogen inside the(hydrogen ignition device) should be ensured.
Important countermeasures against hydrogen nuclear reactor building, at the time of containment vessel venting
corresponding to severe accident, the following countermeasures:
explosion include installing a Passive(3) “In case junction is present in the containment vessel vent line,
Auto-Catalytic Recombiner on a containment
prevention of circulation of hydrogen from that exhaust pipe,” (4)vessel.
“Prevention of circulation of hydrogen in the other units sharing theexhaust pipes” are proposed and the prevention of accumulation of[Refer September 11, 2011 government report]hydrogen inside the reactor building is done.
About the medium- to long-term efforts hereafterThus, for the hydrogen which may be generated in the reactor core,• For BWR, it is planned that the hydrogen ventthe explosion due to hydrogen accumulation in the reactor building isdevice should be installed at the top of theprevented by securely implementing these countermeasures, andnuclear reactor building. A hydrogen detector
the severity of the accident can be prevented.
should be set up inside the nuclear reactor
Furthermore, if the countermeasures for prevention of generationbuilding.
and release of hydrogen are certainly implemented, it is possible to• For PWR, it is planned to set up a device which prevent hydrogen explosion, and the setup of flammable gas control
will reduce the concentration of hydrogen inside system inside the reactor building for BWR is not considered
the containment vessel of Passive Auto-Catalytic essential.
Recombiner, which does not use a power supply. ■ Items mentioned in this report
4.9 hydrogen countermeasures■ Examples of countermeasures
Hydrogen countermeasures (1) to (4)accident(10) Reinforce containment In this accident, there were problems in operating the containment vessel vent system •vessel vent system
when the severe accident occurred. Also the containment vessel vent system could notremove radioactive materials sufficiently. Hence accident management could not makeeffective use of countermeasures. Also the vent line could not be isolated and so theconnecting possibly had a bad impact on other areas.
Hence hereafter, the Station must improve operability of containment vessel ventsystem and secure its independency, reinforce the radioactive material removal
management and hence reinforce the containment vessel vent system.

Operators will be asked to take necessary
countermeasures at reinforce of containment
vessel vent, such as installation of filter at vent,design of rupture disk, evaluation of operating
condition or review, installation of accumulator atvent AO valve, independence of vent exhaustline which was assumed at accident, andreinforcing leakage prevention at adjacent Unit.

Contentment vessel vent has post-accident over pressure
prevention and heat release functionality as severe accident
countermeasures. To ensure the actual vent functionality, thefollowing is proposed as line configuration countermeasure;1) Prepare for existing source failure, ensuring necessary backup
driving source (electricity / pump) for operation of containment vesselvent valve.
Considering the possibility of damage due to excessive
temperature, procedure for vent operation should also be reviewed.
[September 11, 2011 Government Report]
2) “Optimization of operating conditions for vent,” Investigation
about“leakage (wraparound) prevention to adjacent unit throughBy doing a broader study of domestic technicalknowledge, reinforce radioactive material and work exhaust line.”out reinforcement of containment vessel vent3) Moreover, cooling by containment vessel alternative spray after
systemthe core damage should be investigated as a countermeasure.

■ Items mentioned in this report
4.8.3 Containment vessel vent
4.9 Hydrogen countermeasures■ Countermeasures
Measures for heat sink failure (3) – (5)Hydrogen countermeasures (3)(11) Strengthening theenvironment to handle theaccident

At the time of this accident, the radiation dose of the central control room wasincreased, once the conditions were such that the workers could not even enter and it
was difficult to do work for a long time. Hence, habitability of the central control room
decreased. Moreover, there was a radiation dose increase even in a nuclear power
plant emergency support facility, which became a central location for emergency
countermeasures implementation. It became more and more difficult to perform theactivities to handle this accident due to worsening of communication environment orapp-3.1-5• Ensure that the emergency power supply isavailable for in campus PHS communication
system, ensure communication tools such astransceivers, ensure portable lighting devices,maintain radiation shielding function of central
control room corresponding to ventilation and air
conditioning system installation for emergency

To ensure habitability inside the central control room, set up a central
control room ventilation system containing charcoal filter for
removing iodine while designing shielding, ensuring that theconditions are livable for workers even at the time of an accident.
However, due to the station blackout, the central control room
ventilation system containing charcoal filter for removing iodine could
not function and hence the following countermeasures are required:Group name

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lighting conditions.
Hence, the protection of central control room or emergency support facility from theradiation, exclusive ventilation and air conditioning system on the site should be
reinforced. The communication which will not depend on AC power supply, setup oflighting-related systems should be reinforced. Even if a severe accident occurs, theactivities to handle the accident should be able to be continued and such anenvironment should be reinforced.

Additional requirements in attachment data

Comparison with the measures of this report

power supply vehicle and completion of suchinfrastructure of the environment corresponding
to the accident situation is demanded.
• Reinforcement of communication system(diversification of electric supply) and reinforce
the functions of emergency room (ensureisolation, shielding, important personnel skills).
The aseismic reinforce for administrationbuilding, countermeasures to reinforce theenvironment corresponding to accident shouldbe implemented by operators.

(1) “Ensure the availability of power supply to the central control
room ventilation system so as to perform functions in case of
emergency. Moreover, complete the operation procedures for this.”Also, (2) “Investigate the main cause for rise of radiation dose in thecentral control room, reinforce the countermeasures for radiation
protection of the central control room” are proposed.
Hence it is possible to improve the environment corresponding to theaccident, by implementing the countermeasures for improving thehabitability of the central control room at the time of accident.

[Refer September 11, 2011 government report]

Along with shifting the PHS etc. devices within
the site, to higher elevation, reinforcing thefunctions of emergency room or aseismic
reinforce of administration building is planned.

Regarding communication system, which is an important device forsupport in unusual conditions of class 3 importance classification, it
is ensured that the authenticity is maintained at least equal or above
the general industry facility.
However, if natural phenomenon much beyond the assumptions
occur, similar to this time, it is feared that such communication
systems become unusable and hence regarding communication
facility within the power plant as well as with the outside, thefollowing countermeasures are proposed.:(1) Ensure a self-reliant backup supply system which is not affectedby earthquake, tsunami etc.
(2) Implement the facilities such that they are not cut off easily due toearthquake, tsunami etc.
(3) Deploy multiple means of communication
Also, from a viewpoint that the information sharing with the actualsite should take place smoothly without a fail at the time of accident(4) Necessary number of communication devices should be set up in
the places including the actual site.
(5) It is proposed that the countermeasures such as mastering theoperation by conveying the definition of the method of practical use
and practice etc.
Such important countermeasures for maintaining the communication
environment at the time of accident have been proposed andbecause of these countermeasures, the communication within the
power plant as well as with the outside is maintained even if unusual
natural phenomenon exceeding all the assumptions occur. This
makes it possible to conduct incident response activities moresmoothly and securely.
In case of severe natural phenomenon, such as earthquake ortsunami, station blackout for a long duration, and severe conditions
such as release of large amounts of radioactive material in theenvironment, the functions of emergency support facility become
important. Hence arrangements should be made such that thesefunctions should be served even when assuming these conditions.
(1) The structure should be resistant to the assumed earthquake ortsunami.
(2) Ensure an emergency power supply which is independent fromthe plant.
(3) Ensure monitoring important plant parameters and means forcommunication with the central control room and actual site.
(4) Radiation protection countermeasures.
(5) Taking into consideration the things such as ensuring habitabilityof necessary person corresponding to emergency conditions andsecuring important materials and equipment, complete theemergency plant.
Regarding the emergency support facility, all the aspects areextensively considered, if the environment corresponding to theaccident is reinforced by implementing these countermeasures, theincident response activities can be carried out more smoothly andsecury.
■ Items mentioned in this report
4.10.2 Central control room air conditioning, shielding
4.10.7 Communication

app-3.1-6

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Additional requirements in attachment data

Comparison with the measures of this report
4.10.4 Emergency support facility■ Example of countermeasures
Arrangements for emergency situations. (6), (7),(13), (22) to(28),(30) to (33)(12) Reinforcement ofradiation exposureadministration system atthe time of accident

In this accident, many individual dosimeters or radiation reading devices were•
immersed in seawater due to the tsunami and become unusable. Hence adequate
radiation measurement control was difficult; personnel engaged in radiation work wereforced to get involved in field works. Also air radioactive material measurement was•delayed, increasing the risk of internal exposure.
Hence, at the time of accident, individual dosimeter or radiation protection equipment
should be kept in abundance. Make a system that allows expansion of radiation control
personnel at the time of accident. Develop a system and facility to promptly measureradiation workers’ exposure, and reinforce radiation exposure control system at the timeof accident.

Ensure seeking of more individual dosimeterbased on number of people at emergency workat the time of accident
Seek system completion for expandable
radiation control personnel at the time of
accident.

Based on this accident, when a severe accident occurs, incident
response activities in high radiation environment become necessary.
Besides, there could be a situation in which radiation control may
become essential in areas where in normal times radiation control is
not required. Furthermore, severity of natural phenomena and power
failure of power plant can be considered as the factors of the severe
accident. They can lead to considerable confusion.
To perform adequate radiation control and prevent workers fromexcess exposure in such a situation, the following countermeasures
are under proposal.
(1) Ensure materials and equipment necessary for radiation control.
(2) Ensure adequate radiation control personnel.
(3) Measures to avoid unwanted radiation exposure.
Specifically about (1), considering station blackout, preparation ofradiation measurement instrument, accommodating the radiation
measurement instrument for alternate power supply, reviewing thenumber of protection equipment based on the accident, (2) is about
deployment of radiation control support system, (3) is about
information sharing on radiation etc.
Apart from these countermeasures, deployment of emergency
support facility, which has taken measures for radioactive protection
as a completion of support environment at the time of accident, isdemanded. Also, it is pursued to take measures to prevent each typeof accident and furthermore, considering occurrence of accident andthen taking countermeasures to reduce radiation exposure■ Items mentioned in this report
4.10.5 Radiation control / Work management■ Example of countermeasures
Preparations corresponding to emergency (1) to (5)(13) Reinforce practice for
dealing with a severe
accident

In case of severe accidents, to deal with nuclear power plant accidents, effective• It is pursued that workers should carry out thepractice for implementing convergence of related facilities was not much in place tillpractice to deal with emergency conditions withnow. In this accident, for instance, time was needed to establish cooperation between
assumptions of occurrence of severe accident
due to breaking of primary coolant etc., as wellthe emergency support facility, Nuclear Emergency Response Headquarters andas its prolonged duration and severe conditions.
Local Nuclear Emergency Response Headquarters inside the power plant,
cooperation between the Self-Defense Forces, police, fire department which played an • Important countermeasures are demanded fromimportant role in dealing with an accident. This could have been also prevented by
the workers regarding practice for dealing with
implementing precise practice.
terrorism.
Hence, when a severe accident occurs, smoothly carry out incident response activities,
grasp the situations of both inside and outside of the power plant, conduct emergency [Refer September 11, 2011 Government Report]
assembling of skilled persons who are necessary for ensuring the safety of local
The country will ask workers to carry out theresidents. Practice for severe accidents must be reinforced so that related
practice to deal with emergency conditions withadministrative agencies can work in coorperation.
assumptions of occurrence of severe accident due
to breaking of primary coolant etc., as well as its
prolonged duration and severe conditions.

It is assumed that, when a severe accident takes place, then theincident response activities are mainly focused on field works. Also
the field works are assumed to be executed under severe working
conditions and hence, take the following countermeasures:(1) The knowledge and abilities required from each necessary
person should be sorted out and practice should be done basedon this.
(2) Execute practice of activities on actual site related to accident
management.
(3) It is proposed to execute counter-measurescountermeasures bycarrying out 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 countermeasures as below should
be executed:(4) High level executions such as blind practice, real time practiceetc. is carried out.
(5) Accessing the actual site or prior confirmation of the required 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 of a severe accident
should be completed and carried out continuously; due to which it isthought that it is possible to perform incident response 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-governing organizations, but the workers
should deal appropriately on the basis of this analysis result andshould cooperate positively.

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Comparison with the measures of this report■ Items mentioned in this report
4.10.1 Practice■ Examples of countermeasures
Arrangements related to emergency (14) to (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 andaccurately, because the instrumentation system of nuclear reactor and containment
vessel did not work under severe accident conditions.
Hence, reinforce the instrumentation system of the nuclear reactor and containment
vessel which performs sufficient functions at the time of severe accident.

• It is demanded that the business workers should Regarding the instrumentation system of the nuclear reactor andreinforce development as well as completion of containment vessel, monitoring within the estimated scope for
instrumentation system of pressure vessel andfluctuations was possible at the normal operation/operation whencontainment vessel, which performs sufficient
there was abnormal change till now; To take measures at the time offunctions at the time of severe accident, and that accident after knowing about the accident, the design is capable ofof spent fuel pool and the instrumentation system monitoring important parameters by distributing over a sufficientfor understanding the situation.
scope using proper methods.
However, the supply of the meter is lost due to loss of functionality of[Refer September 11, 2011 Government Report] DC power supply system and station blackout for long duration, theimportant parameters, the measurement or recording of important
Planning the preparation and development of theparameters was not possible and hence, the following measures are
instrumentation system of the nuclear reactor andproposed:containment vessel which performs sufficient
functions at the time of severe accident as well as (1) “Install temporary storage battery or connection cable in the
instrumentation system of spent fuel pool.
vicinity of central control room for electricity backup.”(2) “Reconsider conservation of electric power of the meter or
non-failure of electricity as per requirement.”
Also, under the conditions at the time of occurrence of severe
accident, it was not assumed that there will be circumstances
which lower the range of measurement of the water level meterand hence, the following measure is proposed.
(3) “Consider the occurrence of severe accident, development of themeter system with wide range of measurement of reactor waterlevel or investigate the means to understand the condition of thenuclear reactor from containment vessel meter, even if the reactor
meter became nonfunctional.”
Also, there was no measurement requirement till now, for thethings such that hydrogen concentration of the nuclear reactor
building and hence, support cannot be given to those items whichwere other than the requirements for measurements. Considering
this, the following action is proposed.
(4) “Establish a measurement system for the parameters such asconcentration 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 of occurrence ofsevere 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 countermeasures
Preparations to deal with emergency (8) to (12)(15) Focused management In this accident, around the J village, all the concerned persons handling the accident or •of equipment fordisaster victim concentrated the materials and equipment and are providing earnest
emergency operations and support from behind. Initially, though people were there in the surrounding area, thedeployment of rescue team mobilization of the part of rescue team forces, supporting the activities of accident
forces.
management and emergency support materials and equipment, could not be carriedout fast as earthquake or tsunami disaster had occurred. Therefore incident responseon the site was not sufficient.
For this, the completion of the rescue team supporting the management of emergency
support materials and equipment and their practical use, is promoted so that the•
emergency operations can be smoothly carried out even in case of severe conditions.

app-3.1-8

In order to carry out restoration rapidly at the time To have various means of transport by assuming conditions such asof accident, sufficient heavy equipment toblocking of access route or simultaneous occurrence of accident inprocess rubble should be deployed. Ensure
multiple units, and about the preparation of materials and equipment,
sufficient protection clothes, protection mask
the lessons from the accident this time, are:under high radiation dose, the procedures for(1) Bringing of necessasry heavy equipment and materials and their
doing transactions between the enterprises
deployment
during a period of dealing with accident, should (2) Establish the means of air transport as well as transport throughbe defined.
sea, instead of land transport, completion of its setup
Complete the management system of collection (3) Reconfirm and arrange the means of a flexible system betweenof all the materials and equipment for emergency
enterprises.
operations including robots or unmanned(4) Establish equipment or devices (robots, unmanned helicoptershelicopters. Complete one part of the rescueetc.) for remote operation under high radiation conditions asteam forces having high level of skills of
nuclear disaster countermeasures. Cooperation for the
supporting emergency conditions by practicallyinvestigation committee concerning the development and practicalusing these pieces of equipment, and reinforce
use of such equipment, as well as, for actual bringing in of thethe tie with related administrative agencies.
equipment, perform setup of cooperation between the enterprises
and related administrative agencies.

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Comparison with the measures of this report
Investigation is done in corporation to promote completion of one
part of the rescue forces, while using the existing resources from thecountry or self-governing body. Also, it is considered that there canbe conditions when transportation by normal means is not possible.
The cooperation is planned beforehand such that requests forcooperation of Self-Defense Forces etc. is possible.
■ Items mentioned in this report
4.10.9 Provision for emergency countermeasures (heavyequipment, rescue), cooperation set-up in case of emergency■ Examples of countermeasures
Preparations to deal with emergency (20), (21),(41) to (43)(16) Support for complex
circumstances of nuclear
reactor disaster and large
scale natural calamity

This 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. Also, due to prolonged period of reactor
accident the measures for evacuation oflocal residents have been forced to prolong
though it was initially assumed to be for a short duration.
For this, support for the conditions when a large-scale natural calamity as well as
nuclear reactor accident simultaneously occurred, the system and environment shouldbe established which will ensure appropriate means of communication and method of
supply of materials. Also, it should be assumed that the nuclear reactor accident canoccur for a long time and reinforce the support for mobilization of important persons
from every field related to support for accident or disaster.

Presently, it is being investigated how a country and self-governing
bodies will support basic disaster prevention plan, and also how
enterprises will support the plan in response. Corresponding to thecomplex conditions, it is necessary to review this basic disasterprevention plan. Afterwards, enterprises will need to review theirsupport corresponding to the nation’s investigation. However, thisreport proposes to carry out the reinforcement of arrangement ofcommunication devices and materials and equipment. Hereafter, we
aim to support investigation of added countermeasures and ensuremeans of communication with the country, in wider investigations
carried out with the nation.
• Backup power supply such as battery which is independent of thenuclear reactor facility and does not have any impact ofearthquake, tsunami etc., is allowed.
• The facility should be done such that the communication setup forserver and converter etc. which are used in case of emergency,
cable communication circuit and wireless relay base, should not
cut off easily due to natural calamities such as earthquake,tsunami etc.
• Deploying various communication equipment such as exclusive
circuit, satellite phone and, wireless devices etc.，and clarify themethod of use of these devices.
Assuming the conditions such that, due to natural calamity(earthquake, tsunami), when the access route inside the power plantfrom outside is blocked, the consolidation of heavy equipment toremove rubble should be done. The diversification of methods oftransportation of people and materials and equipment is essential,
hence considering the cooperation with the related administrative
agencies, basically, the following support is thought of :• Identifying and deploying essential heavy equipment other than
foil holder which is deployed for severe accident
countermeasures.
• Completion of establishment and setup of means of air transport,
sea transport which can replace the current land transport• Based on the support for this accident, identification anddeployment of essential materials and equipment as well asreconfirmation of necessary quantity of them.
• Reconfirm the flexible system between enterprises for emergency
operations and deploy its means• Establish equipment and devices (robots, unmanned helicoptersetc.) for remote operation under high radiation conditions, of thereactor in case of emergency. Cooperation for the investigationcommittee concerning the development and practical use of such
equipment, as well as, for actual bringing in of the equipment,
perform setup of cooperation between the enterprises and related
administrative agencies.

Support for
nuclear hazard■ Items mentioned in this report
4.10.7 Communication
4.10.9 Preparation of emergency countermeasures (heavyequipment, rescue), cooperation setup for emergency

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Comparison with the measures of this report■ Examples of countermeasures
Preparation with respect to emergency (13), (30) to (33), (20),(21), (41) to (43)(17) Reinforcing
environment monitoring

Currently, emergency environment monitoring is the duty of local government.
However, as the environment monitoring equipment and facilities were damaged byearthquake and tsunami, and forced to evacuate from base facility for emergency
response and countermeasures, it was not possible to perform adequate environment
monitoring initially. To compensate this, Ministry of Education, Culture, Sports, Scienceand Technology have had corporation from related administrative agencies to perform
monitoring activity.
Hence, in the case of emergency, it is the nation’s responsibility to build up the system
to perform environment monitoring both securely and systematically.

According to the Basic Plan for Emergency Preparedness,environment monitoring in case of nuclear power disaster is to be
implemented as follows: site boundary and its internal is monitoredby operator and outside the site boundary is monitored mainly by
local government.
Due to the nuclear power disaster, cooperation support forenvironment monitoring of site boundary and within premises ofpower plant, which was supposed to be implemented by business
operator, and similarly, environment monitoring outside the siteboundary, which was supposed to be carried out by local
government, was proposed as feasible countermeasures taken bybusiness operator.
It is essential to redeploy the industry’s coorporation and support
system for environment monitoring implemented by local
government, and similary, support structure between business
operators. The contents were thought to be as below.
• Monitoring equipment necessary to accommodate in emergency,
reconfirmation of instrument and deploying necessary numbers ofthem.
• Review of necessary person for support for emergency
operations, completion of rules related with that dispatch
Essential utility needs to be reinforced on constant set monitoring
post, which is based on this natural disaster (earthquake, tsunami).
The contents of reinforcement are thought as below• Reinforce electric and communication line.
• Tolerance reinforce against earthquake, tsunami
It is necessary to define alternative monitoring procedure whenconstant set monitoring post is not usable. The contents are
considered as below• Review of alternate measurement procedure such as constant set
monitoring instrument• Deploy equipment and facilities that are required for measurment
by alternative means and confirm how many they are.
■ Items mentioned in this report
4.10.8 Environment monitoring■ Examples of countermeasures
Preparation for emergency operations (34)-(39)(18) Definition of roles of
local and central
administrative agencies

During the initial period of the accident, ensuring means of communication was difficult. [Refer September 11, 2011 Government Report]
From this it is seen that, starting from local and central agencies, the communication
Regarding the television conference used at theand cooperation between the related administrative agencies were not sufficient, also time of nuclear reactor disaster, the plan is tothe sharing of roles as well as responsibility relationship was not always clear.
promote consolidation of the government
Specifically speaking, regarding the relationship between Nuclear Emergency
administrative agencies and all the electric supply
Response Headquarters and Local Nuclear Emergency Response Headquarters,
enterprises so that emergency instructions as wellrelationship between government and TEPCO, relationship between TEPCO and theas information collection can be carried out clearly
actual nuclear power plant site, sharing of roles among the internal departments of the and rapidly, by connecting with the nuclear power
government etc., the structure of rights and responsibilities had some uncertain things. plant.
Particularly, the mutual understanding between the government and TEPCO was not
sufficient during the initial period of the accident.
Hence, for this, the following are promoted: reviewing and defining responsibility
relationship and sharing of roles among the related administrative agencies such as
Nuclear Emergency Response Headquarters; defining responsibilities, roles andmeans of information contact; system deployment.

app-3.1-10

As for the items necessary to be investigated by enterprises,
considering the organization / command, order within theenterprises, instructions were given for the following
countermeasures.
Among the support operations in case of emergency, one is to define
the time for doing judgment regarding the things which exert bad
impact for the convergence of the accident if judgment delays.
Preparations should be made such that analysis is done fast andoperations can be carried out rapidly.
About the basic conditions at the time of accident or the judgment,
they can be clarified by interview of the concerned people made by
the committee on Fukushima Daiichi Nuclear Power Station accident
investigation and review. If new facts are identified, investigation
should be done to decide whether the countermeasures and lessons
should be added or not.
Furthermore, regarding nuclear reactor disaster prevention
countermeasures, country, local self-governing body, related
administrative agencies and enterprises should implement as one
body, for relation between the other agencies, enterprises should

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Comparison with the measures of this reporttake measures based on result of analysis made by the committeeon accident investigation and review.
■ Items mentioned in this report
4.10.6 Organization / command, order■ Examples of countermeasures
Arrangements corresponding to emergency (29)(19) Reinforce thecommunication related tothe accident

Regarding sharing of information with the local residents, in the initial period when
accident has occurred, difficulties came due to damage of means of communication
owing to the occurrence of large scale earthquake disaster. After this, even information
communication with the local residents or with the self-governing bodies could not be
achieved at proper timing. Moreover, there was no sufficient, easy-to-understandexplanation regarding thinking of International Commission on Radiological Protection(ICRP) related to radiation protection, or health impact of radiation or radioactive
materials for the local residents. Also, regarding official announcement of information to
the nationals, the precise facts have always been announced to the public till now but
the forecast of risks etc. have never been described in details and hence, there wasanxiety regarding future happenings.
Hence, we strengthen sharing of accurate information on the accident
situation/response it with the local residents, giving proper explanation regarding impact
of radiation. Regarding the information announcement when accident was progressing,attention should be given to the point that information should include the description of
future risks etc.

In this large-scale accident, it was under the jurisdiction to give
instructions regarding evacuation of local residents, hence this topic
is not raised in this report which is for the industry.
However, press was carried out on enterprise side also, and lessons
should be extracted from the press conditions this time. The
examples of countermeasures related to official announcement ofenvironment monitoring data etc., are specified.
It is required to define the means of communication or the contentsof the necessary information to be communicated to general people,
related administrative agencies, persons related to the actual siteactual site. Following things are thought regarding this.
• Contents of information to be coveyed, the method of deciding
this, sorting out necessary persons• Means for transmission (press, HP etc.) and study of official
announcement timing■ Items mentioned in this report
4.10.8 Environment monitoring■ Examples of countermeasures
Arrangements related to emergency (40)(20) Strengthening of
dealing with aid from eachnation and information
sharing with the global
community

After the occurrence of this accident, the proposals regarding aid of materials andequipment from each nation could not be dealt sufficiently as there was no system inplace within the government which will associate this with the needs in the country. Also
the communication was not done beforehand with the neighboring countries and withthe community about the release of water contaminated with low radiation levels. Thus
there was no sufficient information sharing with the international community.
Hence, an inventory list of materials and equipment should be prepared beforehand forinternational cooperation. In case of accident, the contact points of each nation should
be clearly kept in advance. Strengthen the setup of information sharing by improving
the international reporting system. Provide information that enables scientifically-soundresponse more rapidly and accurately. A proper structure should be formed for effective
dealing with international world.

There is no topic raised regarding the government support offered byeach country and information sharing with them in this industrial
report. However, from now on, the investigation should be maderegarding the coorperation between nuclear power industries ofother countries such as WANO and INPO, and the national nuclear
reactor related administrative agencies.

(21) Accurate
System for Prediction of Environmental Emergency Dose Information network system
understanding and(SPEEDI) could not obtain information about the source of release of radiation. Hence
prediction of impact caused impact of radiation based on the information about the release source could not beby release of radioactive
predicted eventhough that was the originally what SPEEDI should have done. Evenmaterials
under such constraints, certain assumptions should have been made, and the
spreading tendency of the radioactive material should have been guessed using
SPEEDI for reference for evacuation implementation. The calculation results ofSPEEDI have currently been officially announced, but they should have beenannounced from initial stage.
Hence, strengthen the measuring equipment which will clearly obtain the information
about the source of release at the time of accident. Also, while making a plan toeffectively utilize SPEEDI in various situations, the result of actions should also be
officially announced from initial stage.

Effective use of SPEEDI is under the jurisdiction of government andhence no topic is raised in this industrial report.
Countermeasures of strengthening the environment monitoring
utilities of the nuclear power plant site boundary implemented by theenterprises are specified. They are not information about the sourceof release at the time of accident, but used to understand the impact
of radioactive material release.
It is necessary to strengthen important utilities for constant set
monitoring post, based on this natural disaster (earthquake,tsunami). 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 the outside,
following countermeasures are specified.
• The facility should be done such that the communication setup forserver and converter etc. which are used in case of emergency,
cable communication circuit and wireless relay base, should not
cut off easily due to natural calamities such as earthquake,tsunami etc.

app-3.1-11

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Comparison with the measures of this report■ Items mentioned in this report
4.10.8 Environment monitoring
4.10.7 Communication■ Examples of countermeasures
Arrangements related to emergency (37),(38),(31)(22) Define the wide area
evacuation and radiation
protection standards at thetime of nuclear reactor
disaster

In case of this accident, from the start of accident occurrence, evacuation zone and the
sheltering zone were set. With the cooperation from all the related people such aspolice as well as local residents and local self-governing bodies etc., the evacuationand sheltering were carried out rapidly. On the other hand, due to the prolonged
duration of the accident, the evacuation and sheltering were also prolonged. After that,to set the planned evacuation zone and emergency evacuation preparation zone, adecision to utilize the guidance of ICRP and IAEA was made quickly. Furthermore, thescope of the protection area set at this accident was considered as the area whichshould be improved as per the protection countermeasures and exceeded well over 8
to 10 km.
For this, based on the experience of this accident, a structure should be set up to define
the scope of wide area of evacuation at the time of nuclear disaster and guidance forradiological protection standards.

About the way of having a complete system for disaster prevention, it
is under the jurisdiction of the government and hence no topic israised in this industrial report.
However, accompanying the review of plan of disaster prevention,
the review of incident response program at the TEPCO side is to beconsidered as next investigation topic. Also, for, we would like toinvestigate how to handle those items which need adjustment withthe government as well as self-governing bodies, and how tocooperate with them as enterprises, while checking the related
points.

(23) Reinforce safety
regulation administration
systemSince the administrative organizations, which are involved in nuclear safety assurance,
are divided, it was difficult to indentify those who had the primary responsibility toensure sufficient safety work for the public to prevent disaster. Such safety operations
include safety regulations as the primary regulatory body by the Nuclear and Industrial
Safety Agency (NISA) of the Ministry of Economy, Trade and Industry, monitoring
regulation of the primary administrative agencies by the Nuclear Safety Commission of
the Cabinet Office, and the implementation of environmental monitoring by each
Ministry and local governments involved in emergency operations. In addition, we must
admit that the current system had a problem on swiftly responding a large-scale nuclear
accident like this in a cooperative manner.
For this, Nuclear and Industrial Safety Agency should be made independent of Ministryof Economy, Trade and Industry, and we should start the investigation of systems to
implement environment monitoring and nuclear safety regulation administration with
Nuclear Safety Commission and all the ministries.

Regulatory framework with respect to safety is under the jurisdictionof the government and hence no topic is raised in this industrial
report.
However, if there is a chance, we would like to participate in a
discussion about the method for the desirable regulation, based on
the lessons from this accident.
About the way of having an organization on TEPCO side, for theincident response, the following countermeasures are specified.
• For some of the incident response operations of which judgment
delay exert bad impact on the convergence of the accident,
preparations should be made such as clarifying when to makejudgment. This makes it possible to perform adjustment promptlywith those outside the company and carry out operations rapidly.

Ensure basic
safety■ Items mentioned in this report
4.10.6 organization/command, order■ Examples of countermeasures
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 the nuclear
reactor safety and nuclear reactor disaster prevention and the related standards andguidelines is under progress. In this case, the countermeasures for high ageing of theexisting facilities will have to be re-examined not only from the viewpoint of structuralauthenticity, but also from the viewpoint of the latest findings that include theadvancements in system concepts. Furthermore, the orientation of regulation system
backfitting, i.e., technical demands based on latest laws and new findings related toexisting authorized facilities should become clear. Also, maximum contributions have to
be made by providing related data so that the IAEA can reinforce its standards andguidelines.

(25) Ensuring well-qualified In case of an accident along the lines that happened this time, along with experts who •personnel for nuclear
can respond to severe accidents, it is also necessary to have experts in the fields ofreactor safety and nuclear nuclear reactor safety, nuclear reactor disaster prevention, crisis-management, andreactor disaster prevention 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 also extremely important to develop and promote personnel who are competentand well-qualified in nuclear reactor safety and nuclear reactor disaster prevention inorder 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 also need tobe reinforced.
(26) Ensuring impartiality

Discussion on the reviewing guidelines is a basically regulator’s task
and hence no topic is raised in this industrial report. However, he or
she should participate in discussing the guideline review by making
positive recommendations of the items which needs to be reflected
in industrial field. Also, going a step ahead of discussion, he or sheshould independently make feedbacks for those items that requiremeasures. Furthermore, the establishment of industry guidelines is
expected in the future, to ensure a constant level of voluntary
support by the private sector.

Aim for reinforcing the cooperation within the
country as well as with educational
establishments in order to build up a network of
nuclear reactor experts. Reinforce thedevelopment of qualified personnel in regulatory
agencies and specialized agencies, arrange foractive employment of experts including
public-private exchange, etc., and regularize themethods of experts in on-site and off-site
response.

For ensuring the authenticity of safety system, multiplicity was been pursued so far, but [Refer September 11, 2011 Government Report]

app-3.1-12

Until now, each company was engaged in ensuring the talent for its
own company and the work of the entire industry as a whole waslimited to some activities related to operating in-charge systems thatwere implemented by the Nuclear Power Technical Society. On thebasis of the response conditions in the current accident and from theviewpoint of improving the nuclear power plant safety and nuclear
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 talent development
system.

Based on the lessons learned from this accident, countermeasures

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Comparison with the measures of this report

there was lack of response to avoid common cause failure, and sufficient impartialityand diversity could not be ensured.
Therefore, for accurate handling of the common cause failure and further improvementof authenticity of safety functions, ensuring impartiality and diversity of safety system isreinforced.

For ensuring impartiality and diversity in the types
and installation sites of emergency generators andseawater cooling systems, the plan is to reinforce
ensuring impartiality and diversity of safety system
in addition to the accurate handling of the common
cause multifailure and further improvement ofauthenticity.

are examined from the point of multilayered protection, and concrete
countermeasures are presented by taking into account impartialityand diversity.
Loss of power and loss of heat sink are considered to be thecommon factors for failure. Although examples of concrete
countermeasures against such factors are given, they are thecountermeasures that have taken multiplicity and diversity into
consideration.
To be more specific, the countermeasures are described in ‘4.12
Summary of Countermeasures’ as follows.
- Injection and Cooling• Flood prevention measures such as bulkhead etc. around theseawater pump
• Deployment of seawater pump motor spare parts• Flood prevention countermeasures such as core spray system
etc. of the establishment area related to the safety system• Deployment of movable seawater pump
• Preparation of backup power for SRV drive and extra air container• Deployment of backup power for vent operation and driving
source• Improvement in the authenticity of existing injection system bydeployment of backup power supply vehicle or mass power
supply• Flood prevention countermeasures of severe accident
countermeasures system• Improvement in the authenticity with the flood countermeasures ofsevere accident countermeasures system• Portable power pump and hose etc. that are independent ofexisting systems• Ensuring water supply• Heat escape in air through containment vessel vent
- Power supply• Install sea wall and breakwater in important safety equipment.
• Flood prevention countermeasures of the important safety
equipment in the establishment area
• Flood prevention countermeasures such as improvement in thesealing of the opening such as air supply opening and penetration
corresponding to the flood height• Water-tightness of the placement location of receiving transformer
and switchyard or flood countermeasures of the equipment• Water-tightness of the installation area of the direct current power
supply system or flood countermeasures of the equipment• Deployment of backup power supply vehicle or mass power
supply, and improvement in the emergency procedures• Deployment of the direct power supply charging route with the
backup power■ Items mentioned in this report
4.12 Summary of countermeasures■ Examples of countermeasures
Provisions against earthquake and tsunami (1)(2)(5)(7)(9)Corresponding to the loss of heat sink (1) to (3),(6),(8),(10) to (12)Power supply preparations (1) to (4)(27) Effective utilization of
the Probabilistic Safety
Assessment Methodology(PAS) in the risk
management

It was found after systematic examination of the efforts taken to reduce the risks of thenuclear reactor power generation facility that PSA has not always been effectively used.
In addition, quantitative evaluation of risks 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 theuncertainty of such risks.
Hereafter, apply the PSA again actively and promptly based on the opinion related toapp-3.1-13

Till now the deterministic plan and operations have been conducted,
and it has been concluded that the accident will not occur byconfirming that the accident will not occur with sufficient conservative
conditions. As for this review, it is necessary to evaluate by the PSA
though the probability is very low and it is also necessary to improvesafety by checking for any event that can come to the cliff edge. Oneof them is the stress tests conducted under the guidelines of theGroup name

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the uncertainty and develop the safety improvement measures including effective
accident management countermeasures based on that.

Bring about aculture ofsafety(28) Bringing about aculture of safety

All those involved in nuclear power generation should inculcate a safety culture.
According to IAEA, ‘Nuclear Safety Culture’ means, ‘The organization and individual
should be prepared with the attitude of an integrated recognition, temperament andattitude 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 nuclear reactor safety, if safety
culture is not adopted.
However, when compared with this accident, as nuclear operators of our country arethose who bear the primary responsibility to ensure the safety along with theorganizations and individuals, they have to look through all the new opinions, which
confirms that whether it means vulnerability of own plant. When it was found to beaffecting the hope of reducing the risk related to public safety of our plant, it isnecessary to review if the sincere efforts had been taken to improve the safetymeasures.
Similarly, as personnel who have the responsibility for ensuring nuclear safety for bothorganizations as well as individuals, the people involved in setting the nuclear power
regulations for the country must show sensitivity and agility towards latest findings andopinions, without downplaying even the smallest of doubts related to ensuring safety.
Therefore, it is essential to put the basic focus back on the absolute necessity forpursuing defense in depth for ensuring nuclear reactor safety in the future. The
personnel involved in nuclear reactor safety must never neglect learning and gaining
expert knowledge related to safety, and must work towards building a complete safety
culture by always working to find out any weak points in the nuclear reactor safety andto find room for improving the safety.

app-3.1-14

Comparison with the measures of this reportcountry, 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 work on
establishing safety objectives of theorganizations, and promotion, evaluation,
improvement of activities for building a safety
culture in individuals and organizations,reinforcing the cooperation with educational
establishments and working towards developing
qualified personnel for regulatory bodies.

[Government Report dated September 11, 2011]
Both organizations and individuals involved innuclear reactor enterprises and safety regulations
are sincerely working on restructuring theprofessional attitude towards understanding andgaining new knowledge.
Steadfast acquisition of a nuclear reactor safety
culture by all organizations and individuals is thestarting point for personnel involved in nuclear
reactor safety and is their obligation andresponsibility. Constant improvement in nuclear
reactor safety in places where there is no safety
culture is the starting point for ensuring future safetyof our country and it needs to be reconfirmed andimplemented through various formats.

Following the JCO accident and concealment of troubles by Tokyo
Electric Power Company, the nuclear power industry has beenworking honestly towards reinforcing the activities for developing asafety culture. The industry needs to reflect sincerely on why theaccident at Fukushima Daiichi Nuclear Power Station could not be
prevented and to review the methods related to safety culture. The
industry should work on the future problems in having an ideal safety
culture.

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

Recommendation(1) A logical, systematic and consistent
regulatory framework should be established tocoordinate the defense in depth and risk
consideration in a right way for adequate
protection.

Guarantee of protection(2) NRC should re-evaluate the protection ofeach active reactor’s SSC for earthquake andflood of design standards, and ask business
operators 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 notraised as an issue in this report for industry.

 According to the existing requirements and guidelines of NRC, earthquake For the evaluation of earthquake and tsunami, the following evaluation conditions
are added based on the knowledge acquired from this data.
and flood hazards on the premises should be re-evaluated and wheneverrequired, the important security design standards and SSC which must be• Standard earthquake motion Ss design is considered asprotected from updated hazards should be updated.

almost satisfactory as per the present way of thinking, but A walk down preventive measure for earthquake and flood should beworking of multiple earthquake centers is considered for
implemented, along with the specification and improvement in the plant
further safety ensure.
specific vulnerability, the appropriateness of monitoring and preservation of
the protection function such as temporary waterproof barrier and sheet• At the time of evaluation of tsunami height which arrives along
should be verified until the long-term activity to update the design standards
with the earthquake, hereafter the working of multiple
for outside events is completed.

earthquake centers is considered as a wave source at plateboundary and the necessity of setting the slip amount ofearthquake source fault to a larger value is also verified at thesame time.
A possible tsunami height is evaluated, and protection isprovided for the intrusion of tsunami on the premises.
Further, the following multilayered measures are taken
and security for the functions of safety equipment isprovided.
Measures are taken for the intrusion on the premises due toinvasion of tsunami. The concrete example is as follows.

• Attention is ensured for the arrangement of power-supply
system important to safety, and provided with the intrusion
due to tsunami, the storm surge barrier (flood barrier) or
breakwater should be set or the protection barrier should be
set on important equipment from the viewpoint of multi-tiered
protection.
• Protection against tsunami such as tank responsible forproblem like a sea wrack at emergency response (protection
barrier etc.). Deployment of heavy equipment to remove a sea
wreck.
Further, measures are taken for the protection against intrusion in the building. The
concrete example is as follows.

• For a tsunami which exceeds the assumption, intrusion
prevention measure such as improvement in the sealing of
opening and penetration area of the outside door or air supply
opening of a building as per tsunami height is implemented.
• For a tsunami which exceeds the assumption, measures aretaken such as improvement in water-tightness of the door
necessary to prevent the intrusion in equipment important tosafety, and the impact of instruction is determined.
• For the early restoration, a portable drainage pump is
app-3.2-1

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this reportarranged to pump out the seawater 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 seawater pump.
■ Relevant section in this report
4.6 Preparation for earthquake or tsunami■ Example of countermeasure
Preparation for earthquake tsunami (1) to (5), (7), (10), (11)(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 have already been 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 isnecessary to continue working on the investigation of measures for composite
events.

 Rulemaking to revise 10 CFR 50.63 should be started, and for the
From a power supply security viewpoint, first when the secured external powerbusiness operators of active reactors as well as new reactor, (1) Response supply was not good, the equipment was already installed to supply the power bytime of minimum 8 hours should be verified for the station blackout, (2) The emergency DG. However, this time the equipment did not work due to the loss ofequipment, procedure and training required to implement the response time function.
of 72 hours during “Station blackout extension” for the reactor core and
Therefore, first it is necessary to improve the reliability of the external power supply.
cooling of spent fuel pool, or integrity of reactor cooling system and primary [1] The external power supply is based on NISA instructions document “About
containment as required, should be established, and (3) External traffic
ensuring reliability of external power supply for nuclear power plant andinfrastructure function related to the large-scale natural disasters should
reprocessing plant,” and corresponds to the measures to ensure the reliability ofinclude the capacity to acquire equipment in the given time for response
power supply (circuit connection, steel tower strengthening, switchyard intrusion
capacity building under significantly reduced conditions, and persistentmeasure).
cooling capability of the reactor core and spent fuel pool on the premises orthe external resource to support the integrity of reactor cooling system andprimary containment system as required, should be planned and prepared in Next, preparation is done for DC power supply by assuming the loss of function of
emergency DG.
advance.
[2] As a backup power supply of emergency D/G, diversification of power supply is According to 10 CFR 50.54(h)(2), currently provided equipment should be
provided by deploying the power supply vehicle or power supply with large
rationally protected from outside events in terms of design standards which
capacity (including the required fuel). The power panel is prepared for a tsunami
together with other requirements are to be revised and implemented. In thisinvasion by taking intrusion countermeasures. Further, a power supply vehicle isway, the Station shall handle events at multiple reactors by adding
placed in an elevated location where there is no tsunami impact.
equipment as required.
And for DC power supply, countermeasures are taken separately to avoid any loss of
power.

[3] Battery secures the charging route from backup power supply and also is readyfor a tsunami invasion by intrusion countermeasures.
Good power supply security is achieved by taking these multilayered countermeasures.

The following countermeasures are taken to secure the functions of coolingsystem.
[1] Security of measures to inject water into the reactor with a portable power pumpindependent of the existing power supply, security of water source for injection
including seawater, and depressurization of a good reactor by SRV is necessary
for injection. In this way, the Station secures a good injection cooling function to
the reactor by securing the backup drive force necessary for SRV operation.
[2] Security of alternate ultimate heat sink to handle heat escape to the air bycontainer vent.
[3] Early restoration of seawater pump or arrangement of spare parts makes it
possible to secure ultimate heat sink by restoration of seawater system.
It is not necessary to set a time by which to take these countermeasures. Training
is conducted by considering the time from functional loss of equipment of safetysystem to the core damage. And the countermeasures are deemed to be
acceptable by confirming whether an alternative measure is ready before the coreis damaged.
Further, basically all the necessary equipment are placed inside the power plant.
app-3.2-2

Item

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this report
Refer to item (7) for cooling of spent fuel pool.
■ Relevant section in this report
4.7 Preparation for power supply
4.8 Heat sink loss response■ Recommended example of countermeasure
Preparation for power supply [1] to [6]Heat sink loss response [1], [2], [8] to [11](5) Pressure vent design should be used for a A reliable pressure vent should be installed in a BWR provided with Mark I Not only for the installation of a pressure vent, but also to secure a good vent
BWR provided with Mark type I and Mark type II type and Mark II type containers.
function, following can be suggested as an example of a countermeasure for line
containers.
configuration.
[1] In case of loss of the existing drive source, it is advised to secure a backup drive
source (power supply or pump) which is required to operate the container ventvalve.
And the vent operating procedure will be considered hereafter, by taking into
consideration the possibility of overtemperature failure.

[2] Investigation will proceed for “Optimization of operating conditions of vent” and“Preventive measures for the leakage (wraparound) to neighboring unit fromvent exhaust line.”[3] Further, investigation is carried out as a countermeasure for cooling by container
alternate spray after core damage.
■ Relevant section in this report
4.8.3 Container vent
4.9 Hydrogen measure■ Example of countermeasure
Heat sink loss response [1], [3] to [5]Hydrogen measure [3](6) Hereafter with the disclosure of additional
information by future investigation of an accident
occurred in Fukushima Daiichi Nuclear Power
Station, a vision should be determined for
mitigation and hydrogen control in the container
and in the building as a part of long-term
re-examination.

Hydrogen may be generated due to the oxidizing reaction between the reactor
coolant and fuel cladding. So, 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 core cooling system
or by creating inert gas environment by nitrogen sealing to reactor containment in
the boiling water reactor (BWR) even though the hydrogen is generated suddenly.
However, when hydrogen is generated in the same container of BWR and design
conditions are exceeded, the hydrogen will leak inside the reactor building. Hence,[1] “Countermeasure to release or reduce the hydrogen retained inside the reactor
building” and [2] “Installation of hydrogen gas detector to properly monitor theretention of hydrogen in the reactor building, along with the investigation of achannel where the hydrogen leaked inside the reactor building” are recommended.
Moreover, during container vent in severe accident management response, [3]“Prevention of hydrogen wraparound from the ventilation pipe when a branch pipe
is placed in the container vent line” and [4] “Prevention of hydrogen wraparound toother units sharing the vent stack” are recommended to prevent the wraparound ofhydrogen inside the reactor building, and we are trying to prevent the accumulationof hydrogen inside the reactor building.
In this way, for hydrogen which may be generated in the reactor core, an explosionin the reactor building due to the accumulation of hydrogen can be prevented,
stopping extension of the accident, by taking the above countermeasures.
Further, a hydrogen explosion can be prevented by ensuring the Station takes
action to prevent hydrogen generation and takes release countermeasures without
fail. Hence it is not necessary to install a flammable gas control system in thereactor building in BWR.
■ Relevant section in this report
4.9 Hydrogen countermeasure■ Example of countermeasure
Hydrogen countermeasure [1] to [4](7) Condenser capacity and instrumentation of Enough safety system instrumentation which can withstand natural
app-3.2-3

In the security of a reliable cooling function of a spent fuel pool, it is extremely

Item

Recommendation
spent fuel pool should be strengthened.

Detailed requirements for electric power
phenomenon of design standards, and can be monitored from the control
room of important parameters (water level, temperature and area radiationlevel) of spent fuel pool should be provided.

Comparison with the countermeasures of this reportimportant that the water level in the pool is maintained even if the cooling function islost and the spent fuel is not exposed. For that, securing an alternative injection
measure is important. For example, countermeasures such as injection by fireengines or pump cars and together the security of injection route and water source Safety system AC power supply should be provided to the makeup system
can be considered.
of spent fuel pool.
Moreover, it is necessary to intensify the monitoring function of pool water level and Technical specifications should be revised so that the demand to provide pool water temperature to check the validity of alternate injection measure. For
operable one house emergency power supply train for the makeup andexample, a power supply to the level instrumentation and thermometer from
instrumentation of spent fuel pool is fulfilled, irrespective of the operation
emergency power supply or monitoring enhancement of pool using battery drivemode of a reactor, when there is irradiated fuel in the spent fuel pool.
ITV can be considered as countermeasures.
In this way, the integrity of the spent fuel can be secured by maintaining the pool
 Already installed equipment to spray the water on the spent fuel poolwater level by injection to the spent fuel pool, but this can also be raised as anwhich is confirmed after considering the earthquake should be provided
example of a countermeasure to further improve reliability or a measure to maintain
including the connection where the access to water supply is easy on thesame plane as external building (by using the portable pump or pump train). the cooling function.
In the Fukushima Daiichi Nuclear Power Station, where all the power supply is lost,
it is thought good enough from the viewpoint of fuel damage prevention, if water
can certainly supplied by temporary equipment, rather than making a power supply
of a permanent makeup water system as safety system. (Therefore, it is notnecessary to revise the technical specifications.)■ Relevant section in this report
4.11 Integrity security of spent fuel■ Example of countermeasure
Integrity security of spent fuel [1] to [3](8) The ability to respond during emergency in Technical guidelines (supplement 1 “Requirements for emergency•
the facilities such as plant Emergency Operation response ability” for NUREG-0737 in GL 82-33) for plant emergency
Procedure (EOP), Severe Accident
operation procedure must be revised so as to (1) include the plant
Management Guideline (SAMG) and EDMG
emergency operation procedure, severe accident management guidancemust be strengthened and integrated.
and EDMG in an integrated format, (2) specify precise command and control
strategy related to the implementation and (3) grant itable qualifications and •provide training to the staff who make a decision during emergency.
 Section 5.0 “Administrative control” of standard technical specifications torefer to technical guidelines for plant emergency operation procedure
approved for plant in each reactor operation design must be revised, and thetechnical specifications of each plant must be revised so as to comply withthose changes.

Countermeasure is taken such as installation of equipment byconsidering all power supply loss or heat sink loss, andmeasures are also raised for making the procedure for that.
As a countermeasure related to the organization/command ororder, for items that are badly affected by accident
convergence due to delay in the judgment, the timing of
judgment is clarified, and external adjustment is carried sothat the operation can be done quickly. And a countermeasure
to build up a communication facility is also raised. In theemergency station, securing a means to collect the plantparameters required for command is also raised as acountermeasure.

Qualifications are not demanded officially, but Station workers should seek tosecure aknowledge and techniques as a part of their training.
■ Relevant 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 order
4.10.7 Communication■ Example of countermeasure
Preparation for power supply [1], [4]Heat sink loss response [1], [3], [4], [6], [8] to [10]Preparation for emergency [14] to [19], [24], [28], [29], [31] to [33]Strengthening emergency
response(9) NRC should request the summary to respond  The following points should be implemented until rule making activity is
to long-term station blackout (SBO) and multiple completed.
unit events in facility emergency• Determination of workers to fulfill all the positions
countermeasures.

required to respond to multiple unit events and its
implementation.
app-3.2-4• Countermeasure is taken such as installation of equipment byconsidering all power supply loss or heat sink loss, andmeasures are also raised for making the procedure for that.
• Countermeasures are raised for strengthening ofcommunication equipment or emergency response forItem

Recommendation

Detailed requirements for electric power

Comparison with the countermeasures of this reportsystem, instrument and monitoring. Its validity will be checked• Addition of guidelines in the documented emergency
through training.
plan for the implementation method of multiple unit dose
evaluation (including the discharge from spent fuel pool) • Implementation of training is raised as a countermeasure.
using the dose evaluation software and approach
Whether the preparation to respond to multiple units is done
specialized for the facility given by business operator.
will be checked through training, and as a result,re-examination such as strengthening of system will be• Implementation of periodical training and practice forcarried out if required.
multiple units and long-term SBO scenario. Exercise
related to the identification and acquisition of
non-institutional resources within the possible range(simulate).

• Provision of disaster control measures and building acooperative system during emergencies are taken as
measures for resources.

• Assurance about the fact that facilities and equipment for ■ Relevant section in this report
emergency response will properly deal with the scenario
4.7 Preparation for power supplyof multiple units and long-term SBO.
4.8 Heat sink loss response
4.10.1 Training• Power supply measures to communication equipment
4.10.7 Communication
required for the communication within the plant (wireless 4.10.8 Environmental monitoring
communication between the response team and the plant ■ Example of countermeasure
Preparation for power supply [1], [4]etc.) as well as outside the plant (cellular phone, satellite
Heat sink loss response [1], [3], [4], [6], [8] to [10]phone etc.) during long-term SBO.
• Maintenance of capacity of emergency response datasystem throughout the accident occurrence period.

Preparation for emergency [13] to [20], [30] to [43] To secure the monitoring capability of multiple units, initiative to bring the
emergency response data system to the latest state needs to be completed
before June 2012.
(10) As a part of long-term review, NRC shouldsearch for additional themes to emergency
response, which are related to multiple unit
events and long-term SBO.

 Based on the insights taken from Fukushima Daiichi Nuclear Power• As a countermeasure related to the organization/command ororder, for items that are badly affected by accident
convergence due to delay in the judgment, the timing of
judgment is clarified, and external adjustment is carried in an About long-term SBO or multiple unit events or both, to secure theexistence of proper level authority and monitoring system in a proper plant,
early stage so that the operation can be done quickly. And aevaluation should be carried out for command and control formulation as wellcountermeasure to build up a communication facility is also
as qualifications of the decision-maker.
raised. In the emergency station, securing a means to collect Emergency Response Data System (ERDS) is evaluated by using thethe plant parameters required for command is also raised as afollowing methods.
countermeasure.
•
Qualifications are not demanded officially, but Station workers should seek to secure• Alternative methods (such as satellite connection) area knowledge and techniques as a part of their training.
determined to send the data of emergency response data
■ Relevant section in this reportsystem, which is independent of the fixed-line
4.10.1 Training
infrastructure which may not be useful during large scale 4.10.7 Communication
natural disaster.
■ Example of countermeasure
Station accident, analysis should be carried out for emergency responder
about the requirements and guidelines related to the current security
equipment.

• Currently received data set in various locations
determines whether the conditions necessary in the
updated evaluation are fulfilled.

Preparation for emergency [14] to [19], [24], [28], [29], [31] to [33]• During emergency, whether the emergency response
data system should continuously send the data so thatthe operator need not perform it is determined.
(11) As a part of long-term review, NRC shouldsearch for emergency response related todecision-making, radiation monitoring andpromotion 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 capacity to transport equipment to the site,when there is no support from outside the plant or the support is delayedsince the infrastructure outside the plant is destroyed due to the extensive
natural disaster or there are other priorities for response resource.

 Insights obtained from the implementation of emergency response inapp-3.2-5

Country and the local government have carried out the monitoring at the time of
accident and publicized the results, and power companies will cooperate with thatsystem. 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 companies.
Specifically, countermeasures are implemented for the following items.

Item

Recommendation

Detailed requirements for electric power
Fukushima should be evaluated.

 Validity (including consideration about independence from AC power
supply, availability of real-time internet) of radiation monitoring within theplant and in the range of emergency planning zone in real-time should be
investigated.

 Training about radioactivity, its safety and correct dose method of KI
should be implemented in a local community around each nuclear power
plant.

Comparison with the countermeasures of this report- Monitoring equipment required in advance for emergency,
reconfirmation of equipment and their deployment as required- Re-examination of the supporting staff necessary during emergency
and upgrading 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 arrangementof the required staff- Review of means of communication (press, HP etc.) or
announcement timing■ Relevant section in this report
4.10.8 Environmental monitoring■ Example of countermeasure
Preparation for emergency time [34] to [40]Efficiency improvement of
NRC program(12) NRC should strengthen the regulatory
 To include the consideration of defense in depth completely, annual
monitoring for a safety record (that is, reactor
self-assessment of reactor oversight process and reorganization of reactor
oversight process) of a business operator, byoversight process every 2 years should be implemented.
keeping the focus on requirements to defense indepth which conform to the recommended
defense in depth framework.

(Note) Text written in italic is the countermeasure which may relate to both regulations as well as business operator.

app-3.2-6

This 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 four 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 a 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

MARK-I

Improved MARK-I

MARK-II

Improved MARK-II
Figure Appendix 4-1

app-4-2

MARK-III

ABWR

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 RCCV. While the capacity of the MARK-I Containmentis smaller, it shows an equivalent level of energy absorption to the energy
absorption in other models in terms of absorption of energies discharged from thecore in the event 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 space volume versus thermal output

MARK-I

Averagecontainment
space volume [m3]6,558

Average reactor
thermal output[MW]
1,681.8

Containment spacevolume ÷ thermal
output [m3/MW]
4.10

Improved MARK-I

12,5002,581.2

4.95

MARK-II

Improved MARK-II

9,775
14,406

3,293
3,293

2.96
4.37

RCCV

13,355

3,926

3.40

Containment type3.

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(i) Earthquake
As indicated below, dynamic load by an earthquake is not a MARK-I
Containment-specific problem.
Following two items are the key points in the arguments:• Sloshing of pool water caused by an earthquake generates a dynamic loadon 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 thisevent occurs concurrently with a loss of reactor coolant accident, steam
from RPVs is discharged directly to the suppression chamber space. This
may elevate the pressure 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 theapp-4-3

suppression pool is the lowest and considering the point where amplitude of
the pool water sloshing is the maximum. Therefore, it is confirmed that theevent which may be raised in the arguments never occurs.
(ii) Impact of steam discharge from SRVs
A countermeasure has been implemented against the dynamic load throughthe SRV steam applied on internal wall of the suppression chamber. Therefore,
it does 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 internal
walls of suppression chamber has been pointed out due to local discharge ofthe 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 straightnozzle. In this configuration, the steam is distributed and discharged into thewater (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), identified andimplemented 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 tomaintain integrity 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

Station Overview

Station overviews for the Units 1 to 3 of Fukushima Daiichi Nuclear Power Station are
indicated in Table Reference 1, Figure Reference 1-1 and Figure Reference 1-2.
Further, the outlines of severe accident countermeasures already implemented in the
Units 1 to 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 Station overview
StationReactor 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 coolingsystem (Pump #)Shutdown coolingsystem (Pump #)Emergency DG(Unit #)
Containment
capacity

Unit 1
BWR3
Mark-I
1,380

Unit 2BWR4Mark-I
2,381

Unit 3BWR4Mark-I
2,381

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: 3,410 m3
WW: 2,620 m3
SP: 1,750 m3

DW: 4,240 m3
WW: 3,160 m3
SP: 2,980 m3

DW: 4,240 m3
WW: 3,160 m3
SP: 2,980 m3

ref-1-2

Table Reference 1-2 Countermeasures implemented for severe accident
Function
Reactor shutdown
function

Coolant injection toreactor andcontainment

Function to remove
heat from containment

Function to support
safety features

Unit 1Units 2and 3

Recirculation pump trip (RPT)Alternative control rod insertion (ARI)









—



























Countermeasure for severe accident

Alternative coolant injection(Coolant injection to reactor andcontainment by using condensate watermakeup system and fire extinguishingpump etc.)
Automatic actuation of reactor
depressurization(Addition of ADS interlock)
Alternative heat removal(Positive use of DW cooler and 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 supply(480 V is made available by adjacent
plant)
Restoration of failed component ofemergency D/Gs
(Procedure)ref-1-3

What is the countermeasure against severe accident (i.e., accident management)?
A countermeasure against severe accident (i.e., accident management) means acountermeasure to prevent development of an event that may lead to a severe
accident (i.e., 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 additionalsafety regulations are necessary because the safety in the domestic nuclear power
plants has been sufficiently guaranteed by currently available safety measures.
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 STATIONS
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 defense in depth at the fourth levelref-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)Core spray system(CS)-B system

Containment cooling system (CCS)–B system

Control rod drive
hydraulic control system(2 units)

Containment
coolingsystem heatexchanger

Containment cooling system (CCS)–A system

Core spray system(CS)-A system

Figure Reference1-1 Station overview (Unit 1)

Feed water pump
(2 turbine operated
units)High Pressure
Cooolant
Injection
System (HPCI) Reactor Core
Isolation Cooling(RCIC)

Main turbine
Condensate pump
3 high pressure 3 low pressure
units
units

Condenser

2 motor operated units

Circulating
water
pump
(2 units)

To suppression
poolTo
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

Control rod drive
hydraulic control
system (2 units)

Residual heat removal
system (RHR)

Core spray system (CS)

Figure Reference1-2 Station overview (Units 2 and 3)

ref-1-5Sea

Emergency
condenser B

Reactor pressure vessel

Reactor containment

PLR pump

Figure Reference1-3 Emergency condenser

ref-1-6

Emergency
condenser A

Pressure vessel
Dry well

PCV spray(D/W)

Condensate watermakeup system

Condensate
water makeup
systemCore spray system

Fire extinguishingsystemDiesel pump

Motor operated
pump

Filtered watertank

Fire extinguishingsystemMotor operated
pump (Stand-by)

PCV spray(S/C)

Containment
coolingsystemCondensate
watermakeupsystemPressure control
chamber

Condensate
storage tank
Motor operated
pump
Condensate watermakeup system

Core spray system

Containment cooling system

Figure Reference 1-4 Alternative coolant injection system (Unit 1)
Diesel pumpPressure vessel head sprayFire
extinguishingsystemCondensate
water makeup
systemMotor operated
pump

Filteredwater tankPressure vessel

Fire extinguishingsystemDry wellMotor 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 and 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 and 3)

ref-1-8

Starting transformer

Common bus (6.9KV)

Normal bus (6.9KV)Emergency bus (6.9KV)Emergency bus (480V)

Emergency bus (480V)
Battery
cell

Breaker(M/C)

Breaker(MCC)

Spare battery
charger
Dedicated battery charger
Unit 1 (3, 5)Transformer

125V DC bus

Unit 2 (4, 5)<A> Route: Interchanges 6.9KV AC power supply.
(Operation at M/C is possible only if DC
power is available)<B> Route: Interchanges 480V AC power supply.
(MCC is manually operated. MCC isnormally open and its access is controlled
by key lock.)

Figure Reference 1-8 Power supply interchange

ref-1-9